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dhcpd.conf(5)		      File Formats Manual		 dhcpd.conf(5)

NAME
       dhcpd.conf - dhcpd configuration	file

DESCRIPTION
       The  dhcpd.conf	file contains configuration information	for dhcpd, the
       Internet	Systems	Consortium DHCP	Server.

       The dhcpd.conf file is a	free-form ASCII	text file.  It	is  parsed  by
       the  recursive-descent  parser  built into dhcpd.  The file may contain
       extra tabs and newlines for formatting purposes.	 Keywords in the  file
       are  case-insensitive.  Comments	may be placed anywhere within the file
       (except within quotes).	Comments begin with the	# character and	end at
       the end of the line.

       The file	essentially consists of	a list of statements.  Statements fall
       into two	broad categories - parameters and declarations.

       Parameter statements either say how to do something (e.g., how  long  a
       lease  to  offer),  whether to do something (e.g., should dhcpd provide
       addresses to unknown clients), or what parameters  to  provide  to  the
       client (e.g., use gateway 220.177.244.7).

       Declarations  are  used to describe the topology	of the network,	to de-
       scribe clients on the network, to provide addresses  that  can  be  as-
       signed to clients, or to	apply a	group of parameters to a group of dec-
       larations.  In any group	of parameters and declarations,	all parameters
       must be specified before	any declarations which depend on those parame-
       ters may	be specified.

       Declarations about network topology include the shared-network and  the
       subnet  declarations.   If  clients  on a subnet	are to be assigned ad-
       dresses dynamically, a range declaration	must appear within the	subnet
       declaration.   For  clients  with statically assigned addresses,	or for
       installations where only	known clients will be served, each such	client
       must  have  a  host  declaration.  If parameters	are to be applied to a
       group of	declarations which are not related strictly  on	 a  per-subnet
       basis, the group	declaration can	be used.

       For  every  subnet  which will be served, and for every subnet to which
       the dhcp	server is connected, there must	 be  one  subnet  declaration,
       which  tells  dhcpd how to recognize that an address is on that subnet.
       A subnet	declaration is required	for each subnet	even if	 no  addresses
       will be dynamically allocated on	that subnet.

       Some  installations  have  physical  networks on	which more than	one IP
       subnet operates.	 For example, if there is a site-wide requirement that
       8-bit  subnet  masks  be	 used, but a department	with a single physical
       ethernet	network	expands	to the point where it has more than 254	nodes,
       it may be necessary to run two 8-bit subnets on the same	ethernet until
       such time as a new physical network can be added.  In  this  case,  the
       subnet  declarations  for  these	 two  networks	must  be enclosed in a
       shared-network declaration.

       Note that even when the shared-network declaration is absent, an	 empty
       one  is created by the server to	contain	the subnet (and	any scoped pa-
       rameters	included in the	subnet).  For practical	purposes,  this	 means
       that  "stateless"  DHCP	clients,  which	are not	tied to	addresses (and
       therefore subnets) will receive	the  same  configuration  as  stateful
       ones.

       Some  sites  may	 have  departments which have clients on more than one
       subnet, but it may be desirable to offer	those clients a	uniform	set of
       parameters  which  are  different than what would be offered to clients
       from other departments on the same subnet.  For clients which  will  be
       declared	 explicitly  with host declarations, these declarations	can be
       enclosed	in a group declaration along with  the	parameters  which  are
       common to that department.  For clients whose addresses will be dynami-
       cally assigned, class declarations and conditional declarations may  be
       used  to	 group	parameter  assignments based on	information the	client
       sends.

       When a client is	to be booted, its boot parameters  are	determined  by
       consulting that client's	host declaration (if any), and then consulting
       any class declarations matching the client, followed by the pool,  sub-
       net  and	shared-network declarations for	the IP address assigned	to the
       client.	Each of	these declarations itself  appears  within  a  lexical
       scope,  and  all	 declarations at less specific lexical scopes are also
       consulted for client option declarations.  Scopes are never  considered
       twice,  and  if parameters are declared in more than one	scope, the pa-
       rameter declared	in the most specific scope is the one that is used.

       When dhcpd tries	to find	a host declaration  for	 a  client,  it	 first
       looks for a host	declaration which has a	fixed-address declaration that
       lists an	IP address that	is valid for the subnet	or shared  network  on
       which  the  client  is  booting.	 If it doesn't find any	such entry, it
       tries to	find an	entry which has	no fixed-address declaration.

EXAMPLES
       A typical dhcpd.conf file will look something like this:

       global parameters...

       subnet 204.254.239.0 netmask 255.255.255.224 {
	 subnet-specific parameters...
	 range 204.254.239.10 204.254.239.30;
       }

       subnet 204.254.239.32 netmask 255.255.255.224 {
	 subnet-specific parameters...
	 range 204.254.239.42 204.254.239.62;
       }

       subnet 204.254.239.64 netmask 255.255.255.224 {
	 subnet-specific parameters...
	 range 204.254.239.74 204.254.239.94;
       }

       group {
	 group-specific	parameters...
	 host zappo.test.isc.org {
	   host-specific parameters...
	 }
	 host beppo.test.isc.org {
	   host-specific parameters...
	 }
	 host harpo.test.isc.org {
	   host-specific parameters...
	 }
       }

				      Figure 1

       Notice that at the beginning of the file, there's a  place  for	global
       parameters.  These might	be things like the organization's domain name,
       the addresses of	the name servers (if they are common to	the entire or-
       ganization), and	so on.	So, for	example:

	    option domain-name "isc.org";
	    option domain-name-servers ns1.isc.org, ns2.isc.org;

				      Figure 2

       As  you	can see	in Figure 2, you can specify host addresses in parame-
       ters using their	domain names rather than their numeric	IP  addresses.
       If  a given hostname resolves to	more than one IP address (for example,
       if that host has	two ethernet interfaces), then	where  possible,  both
       addresses are supplied to the client.

       The  most obvious reason	for having subnet-specific parameters as shown
       in Figure 1 is that each	subnet,	of necessity, has its own router.   So
       for the first subnet, for example, there	should be something like:

	    option routers 204.254.239.1;

       Note  that  the address here is specified numerically.  This is not re-
       quired -	if you have a different	domain name for	each interface on your
       router,	it's  perfectly	legitimate to use the domain name for that in-
       terface instead of the numeric address.	However, in many  cases	 there
       may  be only one	domain name for	all of a router's IP addresses,	and it
       would not be appropriate	to use that name here.

       In Figure 1 there is also a group statement, which provides common  pa-
       rameters	for a set of three hosts - zappo, beppo	and harpo.  As you can
       see, these hosts	are all	in the test.isc.org domain, so it  might  make
       sense  for  a group-specific parameter to override the domain name sup-
       plied to	these hosts:

	    option domain-name "test.isc.org";

       Also, given the domain they're in, these	are  probably  test  machines.
       If we wanted to test the	DHCP leasing mechanism,	we might set the lease
       timeout somewhat	shorter	than the default:

	    max-lease-time 120;
	    default-lease-time 120;

       You may have noticed that while some parameters start with  the	option
       keyword,	some do	not.  Parameters starting with the option keyword cor-
       respond to actual DHCP options, while parameters	that do	not start with
       the  option  keyword  either  control  the  behavior of the DHCP	server
       (e.g., how long a lease dhcpd will give out), or	specify	client parame-
       ters  that  are not optional in the DHCP	protocol (for example, server-
       name and	filename).

       In Figure 1, each host had host-specific	parameters.  These  could  in-
       clude  such things as the hostname option, the name of a	file to	upload
       (the filename parameter)	and the	address	of the server  from  which  to
       upload the file (the next-server	parameter).  In	general, any parameter
       can appear anywhere that	parameters are allowed,	and  will  be  applied
       according to the	scope in which the parameter appears.

       Imagine that you	have a site with a lot of NCD X-Terminals.  These ter-
       minals come in a	variety	of models, and you want	to  specify  the  boot
       files  for each model.  One way to do this would	be to have host	decla-
       rations for each	server and group them by model:

       group {
	 filename "Xncd19r";
	 next-server ncd-booter;

	 host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
	 host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
	 host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }
       }

       group {
	 filename "Xncd19c";
	 next-server ncd-booter;

	 host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
	 host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }
       }

       group {
	 filename "XncdHMX";
	 next-server ncd-booter;

	 host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
	 host ncd4 { hardware ethernet 0:c0:c3:91:a7:8;	}
	 host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f;	}
       }

ADDRESS	POOLS
       The pool	and pool6 declarations can be used to specify a	 pool  of  ad-
       dresses	that  will  be	treated	 differently  than another pool	of ad-
       dresses,	even on	the same network segment or subnet.  For example,  you
       may  want  to  provide a	large set of addresses that can	be assigned to
       DHCP clients that are registered	to your	DHCP server, while providing a
       smaller	set  of	 addresses,  possibly with short lease times, that are
       available for unknown clients.  If you have a firewall, you may be able
       to  arrange for addresses from one pool to be allowed access to the In-
       ternet, while addresses in another pool are not,	thus encouraging users
       to register their DHCP clients.	To do this, you	would set up a pair of
       pool declarations:

       subnet 10.0.0.0 netmask 255.255.255.0 {
	 option	routers	10.0.0.254;

	 # Unknown clients get this pool.
	 pool {
	   option domain-name-servers bogus.example.com;
	   max-lease-time 300;
	   range 10.0.0.200 10.0.0.253;
	   allow unknown-clients;
	 }

	 # Known clients get this pool.
	 pool {
	   option domain-name-servers ns1.example.com, ns2.example.com;
	   max-lease-time 28800;
	   range 10.0.0.5 10.0.0.199;
	   deny	unknown-clients;
	 }
       }

       It is also possible to set up entirely different	subnets	for known  and
       unknown	clients	- address pools	exist at the level of shared networks,
       so address ranges within	pool declarations can be on different subnets.

       As you can see in the preceding example,	pools can  have	 permit	 lists
       that  control  which  clients  are allowed access to the	pool and which
       aren't.	Each entry in a	pool's permit list is introduced with the  al-
       low  or	deny  keyword.	 If  a pool has	a permit list, then only those
       clients that match specific entries on the permit list will be eligible
       to  be  assigned	 addresses  from the pool.  If a pool has a deny list,
       then only those clients that do not match any entries on	the deny  list
       will  be	 eligible.    If  both permit and deny lists exist for a pool,
       then only clients that match the	permit list and	do not match the  deny
       list will be allowed access.

       The pool6 declaration is	similar	to the pool declaration.  Currently it
       is only allowed within a	subnet6	declaration, and may not  be  included
       directly	 in  a	shared network declaration.  In	addition to the	range6
       statement it allows the prefix6 statement to be included.  You may  in-
       clude range6 statements for both	NA and TA and prefixy6 statements in a
       single pool6 statement.

DYNAMIC	ADDRESS	ALLOCATION
       Address allocation is actually only done	when a client is in  the  INIT
       state and has sent a DHCPDISCOVER message.  If the client thinks	it has
       a valid lease and sends a DHCPREQUEST to	initiate or renew that	lease,
       the server has only three choices - it can ignore the DHCPREQUEST, send
       a DHCPNAK to tell the client it should stop using the address, or  send
       a  DHCPACK,  telling  the  client to go ahead and use the address for a
       while.

       If the server finds the address the client is requesting, and that  ad-
       dress  is  available to the client, the server will send	a DHCPACK.  If
       the address is no longer	available, or the client  isn't	 permitted  to
       have  it,  the server will send a DHCPNAK.  If the server knows nothing
       about the address, it will remain silent, unless	the address is	incor-
       rect  for the network segment to	which the client has been attached and
       the server is authoritative for that network segment, in	which case the
       server  will  send  a DHCPNAK even though it doesn't know about the ad-
       dress.

       There may be a host declaration matching	the  client's  identification.
       If  that	 host  declaration  contains  a	fixed-address declaration that
       lists an	IP address that	is valid for the network segment to which  the
       client  is connected, the DHCP server will never	do dynamic address al-
       location.  In this case,	the client is required	to  take  the  address
       specified  in  the host declaration.  If	the client sends a DHCPREQUEST
       for some	other address, the server will respond with a DHCPNAK.

       When the	DHCP server allocates a	new address for	 a  client  (remember,
       this  only  happens  if	the  client has	sent a DHCPDISCOVER), it first
       looks to	see if the client already has a	valid lease on an IP  address,
       or  if there is an old IP address the client had	before that hasn't yet
       been reassigned.	 In that case, the server will take that  address  and
       check  it  to  see  if the client is still permitted to use it.	If the
       client is no longer permitted to	use it,	the  lease  is	freed  if  the
       server  thought it was still in use - the fact that the client has sent
       a DHCPDISCOVER proves to	the server that	the client is no longer	 using
       the lease.

       If no existing lease is found, or if the	client is forbidden to receive
       the existing lease, then	the server will	look in	the  list  of  address
       pools  for  the	network	 segment to which the client is	attached for a
       lease that is not in use	and that the client is permitted to have.   It
       looks through each pool declaration in sequence (all range declarations
       that appear outside of pool declarations	are grouped into a single pool
       with  no	 permit	 list).	  If  the  permit list for the pool allows the
       client to be allocated an address from that pool, the pool is  examined
       to  see	if  there  is an address available.  If	so, then the client is
       tentatively assigned that address.  Otherwise, the next pool is tested.
       If  no  addresses  are found that can be	assigned to the	client,	no re-
       sponse is sent to the client.

       If an address is	found that the client is permitted to have,  and  that
       has  never  been	 assigned to any client	before,	the address is immedi-
       ately allocated to the client.  If the address is available for alloca-
       tion but	has been previously assigned to	a different client, the	server
       will keep looking in hopes of finding an	address	that has never	before
       been assigned to	a client.

       The  DHCP  server  generates  the list of available IP addresses	from a
       hash table.  This means that the	addresses are not sorted in  any  par-
       ticular	order, and so it is not	possible to predict the	order in which
       the DHCP	server will allocate IP	addresses.  Users of previous versions
       of  the	ISC  DHCP server may have become accustomed to the DHCP	server
       allocating IP addresses in ascending order, but this is no longer  pos-
       sible, and there	is no way to configure this behavior with version 3 of
       the ISC DHCP server.

IP ADDRESS CONFLICT PREVENTION
       The DHCP	server checks IP addresses to see if they are  in  use	before
       allocating  them	 to clients.  It does this by sending an ICMP Echo re-
       quest message to	the IP address being allocated.	 If no ICMP Echo reply
       is  received  within a second, the address is assumed to	be free.  This
       is only done for	leases that have been specified	in  range  statements,
       and  only  when	the  lease  is thought by the DHCP server to be	free -
       i.e., the DHCP server or	its failover peer has not listed the lease  as
       in use.

       If  a response is received to an	ICMP Echo request, the DHCP server as-
       sumes that there	is a configuration error - the IP address is in	use by
       some  host  on the network that is not a	DHCP client.  It marks the ad-
       dress as	abandoned, and will not	assign it to clients. The  lease  will
       remain abandoned	for a minimum of abandon-lease-time seconds.

       If  a  DHCP  client tries to get	an IP address, but none	are available,
       but there are abandoned IP addresses, then the DHCP server will attempt
       to  reclaim  an abandoned IP address.  It marks one IP address as free,
       and then	does the same ICMP Echo	request	 check	described  previously.
       If there	is no answer to	the ICMP Echo request, the address is assigned
       to the client.

       The DHCP	server does not	cycle through abandoned	IP  addresses  if  the
       first  IP  address  it tries to reclaim is free.	 Rather, when the next
       DHCPDISCOVER comes in from the client, it will attempt a	new allocation
       using  the  same	method described here, and will	typically try a	new IP
       address.

DHCP FAILOVER
       This version of the ISC DHCP server supports the	DHCP failover protocol
       as  documented  in draft-ietf-dhc-failover-12.txt.  This	is not a final
       protocol	document, and we have not done interoperability	 testing  with
       other vendors' implementations of this protocol,	so you must not	assume
       that this implementation	conforms to the	standard.  If you wish to  use
       the  failover  protocol,	make sure that both failover peers are running
       the same	version	of the ISC DHCP	server.

       The failover protocol allows two	DHCP servers (and no more than two) to
       share  a	 common	address	pool.  Each server will	have about half	of the
       available IP addresses in the pool at any given	time  for  allocation.
       If one server fails, the	other server will continue to renew leases out
       of the pool, and	will allocate new addresses out	of the roughly half of
       available  addresses  that  it  had  when communications	with the other
       server were lost.

       It is possible during a prolonged failure to tell the remaining	server
       that  the other server is down, in which	case the remaining server will
       (over time) reclaim all the addresses the other	server	had  available
       for  allocation,	 and  begin to reuse them.  This is called putting the
       server into the PARTNER-DOWN state.

       You can put the server into the PARTNER-DOWN state either by using  the
       omshell	(1)  command  or  by  stopping	the  server,  editing the last
       failover	state declaration  in  the  lease  file,  and  restarting  the
       server.	If you use this	last method, change the	"my state" line	to:

       failover	peer name state	{
       my state	partner-down;.
       peer state state	at date;
       }

       It is only required to change "my state"	as shown above.

       When the	other server comes back	online,	it should automatically	detect
       that it has been	offline	and request a complete update from the	server
       that  was running in the	PARTNER-DOWN state, and	then both servers will
       resume processing together.

       It is possible to get into a dangerous situation: if you	put one	server
       into  the PARTNER-DOWN state, and then *that* server goes down, and the
       other server comes back up, the other server will  not  know  that  the
       first  server  was  in  the PARTNER-DOWN	state, and may issue addresses
       previously issued by the	other server to	different  clients,  resulting
       in  IP  address	conflicts.   Before putting a server into PARTNER-DOWN
       state, therefore, make sure that	the other server will not restart  au-
       tomatically.

       The  failover  protocol	defines	 a primary server role and a secondary
       server role.  There are some differences	in how	primaries  and	secon-
       daries  act, but	most of	the differences	simply have to do with provid-
       ing a way for each peer to behave in the	opposite way from  the	other.
       So one server must be configured	as primary, and	the other must be con-
       figured as secondary, and it doesn't  matter  too  much	which  one  is
       which.

FAILOVER STARTUP
       When  a	server	starts	that  has not previously communicated with its
       failover	peer, it must establish	communications with its	failover  peer
       and  synchronize	 with it before	it can serve clients.  This can	happen
       either because you have just configured your DHCP  servers  to  perform
       failover	 for  the  first time, or because one of your failover servers
       has failed catastrophically and lost its	database.

       The initial recovery process  is	 designed  to  ensure  that  when  one
       failover	 peer  loses  its database and then resynchronizes, any	leases
       that the	failed server gave out before it failed	will be	honored.  When
       the  failed  server starts up, it notices that it has no	saved failover
       state, and attempts to contact its peer.

       When it has established contact,	it asks	the peer for a	complete  copy
       its  peer's lease database.  The	peer then sends	its complete database,
       and sends a message indicating that it is done.	The failed server then
       waits until MCLT	has passed, and	once MCLT has passed both servers make
       the transition back into	normal operation.  This	waiting	period ensures
       that  any leases	the failed server may have given out while out of con-
       tact with its partner will have expired.

       While the failed	server is recovering, its partner remains in the part-
       ner-down	state, which means that	it is serving all clients.  The	failed
       server provides no service at all to DHCP clients until it has made the
       transition into normal operation.

       In  the case where both servers detect that they	have never before com-
       municated with their partner, they both come up in this recovery	 state
       and follow the procedure	we have	just described.	 In this case, no ser-
       vice will be provided to	DHCP clients until MCLT	has expired.

CONFIGURING FAILOVER
       In order	to configure failover, you need	to write  a  peer  declaration
       that  configures	the failover protocol, and you need to write peer ref-
       erences in each pool declaration	for which you  want  to	 do  failover.
       You  do	not  have to do	failover for all pools on a given network seg-
       ment.   You must	not tell one server it's doing failover	on a  particu-
       lar  address  pool and tell the other it	is not.	 You must not have any
       common address pools on which you are not doing failover.  A pool  dec-
       laration	that utilizes failover would look like this:

       pool {
	    failover peer "foo";
	    pool specific parameters
       };

       The   server currently  does very  little  sanity checking,  so if  you
       configure it wrong, it will just	 fail in odd ways.  I would  recommend
       therefore  that you either do  failover or don't	do failover, but don't
       do any mixed pools.  Also,  use the same	master configuration file  for
       both   servers,	and  have  a  separate file  that  contains  the  peer
       declaration and includes	the master file.  This will help you to	 avoid
       configuration   mismatches.  As our  implementation evolves,  this will
       become  less of	a  problem.  A	basic  sample dhcpd.conf  file for   a
       primary server might look like this:

       failover	peer "foo" {
	 primary;
	 address anthrax.rc.example.com;
	 port 519;
	 peer address trantor.rc.example.com;
	 peer port 520;
	 max-response-delay 60;
	 max-unacked-updates 10;
	 mclt 3600;
	 split 128;
	 load balance max seconds 3;
       }

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

	 [ primary | secondary ];

	 This  determines  whether  the	server is primary or secondary,	as de-
	 scribed earlier under DHCP FAILOVER.

       The address statement

	 address address;

	 The address statement declares	the IP address or DNS  name  on	 which
	 the  server should listen for connections from	its failover peer, and
	 also the value	to use for the DHCP Failover Protocol  server  identi-
	 fier.	 Because  this	value  is used as an identifier, it may	not be
	 omitted.

       The peer	address	statement

	 peer address address;

	 The peer address statement declares the IP address  or	 DNS  name  to
	 which	the  server  should  connect  to  reach	 its failover peer for
	 failover messages.

       The port	statement

	 port port-number;

	 The port statement declares the TCP port on which the	server	should
	 listen	for connections	from its failover peer.	 This statement	may be
	 omitted, in which case	the IANA assigned port number 647 will be used
	 by default.

       The peer	port statement

	 peer port port-number;

	 The  peer  port  statement  declares the TCP port to which the	server
	 should	connect	to reach its  failover	peer  for  failover  messages.
	 This  statement  may be omitted, in which case	the IANA assigned port
	 number	647 will be used by default.

       The max-response-delay statement

	 max-response-delay seconds;

	 The max-response-delay	statement tells	the DHCP server	how many  sec-
	 onds  may pass	without	receiving a message from its failover peer be-
	 fore it assumes that connection has failed.  This  number  should  be
	 small enough that a transient network failure that breaks the connec-
	 tion will not result in the servers being out of communication	for  a
	 long  time,  but large	enough that the	server isn't constantly	making
	 and breaking connections.  This parameter must	be specified.

       The max-unacked-updates statement

	 max-unacked-updates count;

	 The max-unacked-updates statement tells the remote  DHCP  server  how
	 many BNDUPD messages it can send before it receives a BNDACK from the
	 local system.	We don't have enough  operational  experience  to  say
	 what  a good value for	this is, but 10	seems to work.	This parameter
	 must be specified.

       The mclt	statement

	 mclt seconds;

	 The mclt statement defines the	Maximum	Client Lead Time.  It must  be
	 specified  on the primary, and	may not	be specified on	the secondary.
	 This is the length of time for	which a	lease may be renewed by	either
	 failover peer without contacting the other.  The longer you set this,
	 the longer it will take for the running  server  to  recover  IP  ad-
	 dresses  after	 moving	 into PARTNER-DOWN state.  The shorter you set
	 it, the more load your	servers	will experience	when they are not com-
	 municating.   A  value	of something like 3600 is probably reasonable,
	 but again bear	in mind	that we	have no	 real  operational  experience
	 with this.

       The split statement

	 split bits;

	 The  split statement specifies	the split between the primary and sec-
	 ondary	for the	purposes of load balancing.  Whenever a	client makes a
	 DHCP  request,	 the DHCP server runs a	hash on	the client identifica-
	 tion, resulting in value from 0 to 255.  This is  used	 as  an	 index
	 into  a  256 bit field.  If the bit at	that index is set, the primary
	 is responsible.  If the bit at	that index is not set,	the  secondary
	 is  responsible.   The	split value determines how many	of the leading
	 bits are set to one.  So, in practice,	higher split values will cause
	 the  primary  to  serve more clients than the secondary.  Lower split
	 values, the converse.	Legal values are between 0 and 256  inclusive,
	 of  which  the	 most reasonable is 128.  Note that a value of 0 makes
	 the secondary responsible for all clients and a value	of  256	 makes
	 the primary responsible for all clients.

       The hba statement

	 hba colon-separated-hex-list;

	 The  hba  statement  specifies	the split between the primary and sec-
	 ondary	as a bitmap rather than	a cutoff, which	 theoretically	allows
	 for  finer-grained  control.	In practice, there is probably no need
	 for such fine-grained control,	however.  An example hba statement:

	   hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:
	       00:00:00:00:00:00:00:00:00:00:00:00:00:00:00:00;

	 This is equivalent to a split 128;  statement,	 and  identical.   The
	 following two examples	are also equivalent to a split of 128, but are
	 not identical:

	   hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:
	       aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa;

	   hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:
	       55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55;

	 They are equivalent, because half the bits are	set to 0, half are set
	 to  1	(0xa and 0x5 are 1010 and 0101 binary respectively) and	conse-
	 quently this would roughly divide the	clients	 equally  between  the
	 servers.  They	are not	identical, because the actual peers this would
	 load balance to each server are different for each example.

	 You must only have split or hba defined, never	both.  For most	cases,
	 the  fine-grained  control that hba offers isn't necessary, and split
	 should	be used.

       The load	balance	max seconds statement

	 load balance max seconds seconds;

	 This statement	allows you to configure	a cutoff after which load bal-
	 ancing	 is  disabled.	 The  cutoff is	based on the number of seconds
	 since the client sent its first DHCPDISCOVER or DHCPREQUEST  message,
	 and only works	with clients that correctly implement the secs field -
	 fortunately most clients do.  We recommend setting this to  something
	 like 3	or 5.  The effect of this is that if one of the	failover peers
	 gets into a state where it is responding to failover messages but not
	 responding to some client requests, the other failover	peer will take
	 over its client load automatically as the clients retry.

	 It is possible	to disable load	balancing  between  peers  by  setting
	 this  value  to  0  on	both peers.  Bear in mind that this means both
	 peers will respond to all DHCPDISCOVERs or DHCPREQUESTs.

       The auto-partner-down statement

	 auto-partner-down seconds;

	 This statement	instructs the server to	initiate a  timed  delay  upon
	 entering the communications-interrupted state (any situation of being
	 out-of-contact	with the remote	failover peer).	 At the	conclusion  of
	 the  timer,  the  server  will	 automatically	enter the partner-down
	 state.	 This permits the server to allocate leases from the partner's
	 free  lease  pool after an STOS+MCLT timer expires, which can be dan-
	 gerous	if the partner is in fact  operating  at  the  time  (the  two
	 servers will give conflicting bindings).

	 Think	very carefully before enabling this feature.  The partner-down
	 and communications-interrupted	states	are  intentionally  segregated
	 because there do exist	situations where a failover server can fail to
	 communicate with its peer, but	still has the ability to  receive  and
	 reply to requests from	DHCP clients.  In general, this	feature	should
	 only be used in those deployments where the failover servers are  di-
	 rectly	 connected  to	one  another, such as by a dedicated hardwired
	 link ("a heartbeat cable").

	 A zero	value disables the auto-partner-down  feature  (also  the  de-
	 fault),  and any positive value indicates the time in seconds to wait
	 before	automatically entering partner-down.

       The Failover pool balance statements.

	  max-lease-misbalance percentage;
	  max-lease-ownership percentage;
	  min-balance seconds;
	  max-balance seconds;

	 This version of the DHCP Server evaluates pool	balance	on a schedule,
	 rather	 than  on demand as leases are allocated.  The latter approach
	 proved	to be slightly klunky when pool	misbalanced reach total	 satu-
	 ration	 --  when any server ran out of	leases to assign, it also lost
	 its ability to	notice it had run dry.

	 In order to understand	pool balance, some elements of	its  operation
	 first	need  to  be  defined.	 First,	 there are 'free' and 'backup'
	 leases.  Both of these	 are  referred	to  as	'free  state  leases'.
	 'free'	 and  'backup'	are  'the free states' for the purpose of this
	 document.  The	difference is that only	the primary may	allocate  from
	 'free'	 leases	 unless	under special circumstances, and only the sec-
	 ondary	may allocate 'backup' leases.

	 When pool balance is performed, the only plausible expectation	is  to
	 provide  a  50/50  split  of  the  free  state	leases between the two
	 servers.  This	is because no one can predict which server will	 fail,
	 regardless  of	the relative load placed upon the two servers, so giv-
	 ing each server half the leases gives both servers the	same amount of
	 'failure  endurance'.	 Therefore,  there  is no way to configure any
	 different behaviour, outside of some very small windows we  will  de-
	 scribe	shortly.

	 The  first  thing  calculated	on any pool balance run	is a value re-
	 ferred	to as 'lts', or	"Leases	To Send".  This, simply, is  the  dif-
	 ference  in the count of free and backup leases, divided by two.  For
	 the secondary,	it is the difference in	the backup  and	 free  leases,
	 divided  by  two.   The resulting value is signed: if it is positive,
	 the local server is expected to hand out leases  to  retain  a	 50/50
	 balance.   If	it  is	negative, the remote server would need to send
	 leases	to balance the pool.  Once the lts  value  reaches  zero,  the
	 pool  is perfectly balanced (give or take one lease in	the case of an
	 odd number of total free state	leases).

	 The current approach is still something of a hybrid of	 the  old  ap-
	 proach, marked	by the presence	of the max-lease-misbalance statement.
	 This parameter	configures what	used to	be a 10% fixed value in	previ-
	 ous  versions:	if lts is less than free+backup	* max-lease-misbalance
	 percent, then the server will skip balancing a	given pool  (it	 won't
	 bother	 moving	 any  leases,  even if some leases "should" be moved).
	 The meaning of	this value is also somewhat  overloaded,  however,  in
	 that it also governs the estimation of	when to	attempt	to balance the
	 pool (which may then also be skipped over).  The oldest leases	in the
	 free  and  backup states are examined.	 The time they have resided in
	 their respective queues is used as an estimate	to indicate  how  much
	 time it is probable it	would take before the leases at	the top	of the
	 list would be consumed	(and thus, how long it would take to  use  all
	 leases	 in  that  state).   This percentage is	directly multiplied by
	 this time, and	fit into the schedule if it falls within the  min-bal-
	 ance  and  max-balance	 configured  values.  The scheduled pool check
	 time is only moved in a downwards direction, it is  never  increased.
	 Lastly,  if  the  lts is more than double this	number in the negative
	 direction, the	local server will 'panic' and transmit a Failover pro-
	 tocol	POOLREQ	 message,  in the hopes	that the remote	system will be
	 woken up into action.

	 Once the lts value exceeds the	max-lease-misbalance percentage	of to-
	 tal free state	leases as described above, leases are moved to the re-
	 mote server.  This is done in two passes.

	 In the	first pass, only leases	whose most recent bound	 client	 would
	 have been served by the remote	server - according to the Load Balance
	 Algorithm (see	above split and	hba configuration  statements)	-  are
	 given	away  to  the  peer.  This first pass will happily continue to
	 give away leases, decrementing	the lts	value by one for  each,	 until
	 the  lts value	has reached the	negative of the	total number of	leases
	 multiplied by the max-lease-ownership percentage.  So it  is  through
	 this  value that you can permit a small misbalance of the lease pools
	 - for the purpose of giving the peer  more  than  a  50/50  share  of
	 leases	 in  the hopes that their clients might	some day return	and be
	 allocated by the peer (operating normally).  This process is referred
	 to  as	 'MAC Address Affinity', but this is somewhat misnamed:	it ap-
	 plies equally to DHCP Client  Identifier  options.   Note  also  that
	 affinity  is  applied to leases when they enter the state 'free' from
	 'expired' or 'released'.  In this case	also, leases will not be moved
	 from free to backup if	the secondary already has more than its	share.

	 The  second  pass is only entered into	if the first pass fails	to re-
	 duce the lts underneath the total number of free state	leases	multi-
	 plied	by the max-lease-ownership percentage.	In this	pass, the old-
	 est leases are	given over to the peer without	second	thought	 about
	 the  Load  Balance  Algorithm,	and this continues until the lts falls
	 under this value.  In this way, the local server  will	 also  happily
	 keep  a  small	percentage of the leases that would normally load bal-
	 ance to itself.

	 So, the  max-lease-misbalance	value  acts  as	 a  behavioural	 gate.
	 Smaller values	will cause more	leases to transition states to balance
	 the pools over	time, higher values will decrease the amount of	change
	 (but may lead to pool starvation if there's a run on leases).

	 The  max-lease-ownership  value  permits a small (percentage) skew in
	 the lease balance of a	percentage of the total	number of  free	 state
	 leases.

	 Finally,  the	min-balance and	max-balance make certain that a	sched-
	 uled rebalance	event happens within a reasonable timeframe (not to be
	 thrown	off by,	for example, a 7 year old free lease).

	 Plausible  values  for	 the percentages lie between 0 and 100,	inclu-
	 sive, but values over 50 are indistinguishable	from one another (once
	 lts  exceeds  50% of the free state leases, one server	must therefore
	 have 100% of the leases in its	respective free	state).	 It is	recom-
	 mended	 to  select a max-lease-ownership value	that is	lower than the
	 value selected	for the	max-lease-misbalance value.   max-lease-owner-
	 ship defaults to 10, and max-lease-misbalance defaults	to 15.

	 Plausible values for the min-balance and max-balance times also range
	 from 0	to (2^32)-1 (or	the limit of your local	time_t value), but de-
	 fault to values 60 and	3600 respectively (to place balance events be-
	 tween 1 minute	and 1 hour).

CLIENT CLASSING
       Clients can be separated	into classes, and treated differently  depend-
       ing on what class they are in.  This separation can be done either with
       a conditional statement,	or with	a match	 statement  within  the	 class
       declaration.   It is possible to	specify	a limit	on the total number of
       clients within a	particular class or subclass that may hold  leases  at
       one  time, and it is possible to	specify	automatic subclassing based on
       the contents of the client packet.

       Classing	support	for DHCPv6 clients was added in	4.3.0.	It follows the
       same  rules  as	for DHCPv4 except that support for billing classes has
       not been	added yet.

       To add clients to classes based	on  conditional	 evaluation,  you  can
       specify a matching expression in	the class statement:

       class "ras-clients" {
	 match if substring (option dhcp-client-identifier, 1, 3) = "RAS";
       }

       Please  note  that  the	values used in match expressions may only come
       from data or options that are part of the client	packet.	It is not pos-
       sible  to  use values constructed through one or	more executable	state-
       ments.  This stems from the fact	that client classification occurs  be-
       fore  any statements are	executed. Attempting to	do so will yield inde-
       terminate results.

       Note that whether you use matching expressions or  add  statements  (or
       both)  to  classify  clients, you must always write a class declaration
       for any class that you use.  If there will be no	match statement	and no
       in-scope	statements for a class,	the declaration	should look like this:

       class "ras-clients" {
       }

SUBCLASSES
       In  addition  to	classes, it is possible	to declare subclasses.	A sub-
       class is	a class	with the same name as a	regular	class, but with	a spe-
       cific  submatch expression which	is hashed for quick matching.  This is
       essentially a speed hack	- the main  difference	between	 five  classes
       with  match  expressions	 and one class with five subclasses is that it
       will be quicker to find the subclasses.	Subclasses work	as follows:

       class "allocation-class-1" {
	 match pick-first-value	(option	dhcp-client-identifier,	hardware);
       }

       class "allocation-class-2" {
	 match pick-first-value	(option	dhcp-client-identifier,	hardware);
       }

       subclass	"allocation-class-1" 1:8:0:2b:4c:39:ad;
       subclass	"allocation-class-2" 1:8:0:2b:a9:cc:e3;
       subclass	"allocation-class-1" 1:0:0:c4:aa:29:44;

       subnet 10.0.0.0 netmask 255.255.255.0 {
	 pool {
	   allow members of "allocation-class-1";
	   range 10.0.0.11 10.0.0.50;
	 }
	 pool {
	   allow members of "allocation-class-2";
	   range 10.0.0.51 10.0.0.100;
	 }
       }

       The data	following the class name in the	subclass declaration is	a con-
       stant  value  to	 use  in  matching the match expression	for the	class.
       When class matching is done, the	server will evaluate the match expres-
       sion  and  then	look  the  result up in	the hash table.	 If it finds a
       match, the client is considered a member	of both	the class and the sub-
       class.

       Subclasses  can	be declared with or without scope.  In the above exam-
       ple, the	sole purpose of	the subclass is	to allow some  clients	access
       to  one address pool, while other clients are given access to the other
       pool, so	these subclasses are declared without scopes.  If part of  the
       purpose	of  the	subclass were to define	different parameter values for
       some clients, you might want to declare some subclasses with scopes.

       In the above example, if	you had	a single client	that needed some  con-
       figuration parameters, while most didn't, you might write the following
       subclass	declaration for	that client:

       subclass	"allocation-class-2" 1:08:00:2b:a1:11:31 {
	 option	root-path "samsara:/var/diskless/alphapc";
	 filename "/tftpboot/netbsd.alphapc-diskless";
       }

       In this example,	we've used subclassing as a way	to control address al-
       location	 on  a	per-client  basis.  However, it's also possible	to use
       subclassing in ways that	are not	specific to clients - for example,  to
       use  the	 value of the vendor-class-identifier option to	determine what
       values to send in the vendor-encapsulated-options option.   An  example
       of  this	 is  shown  under  the VENDOR ENCAPSULATED OPTIONS head	in the
       dhcp-options(5) manual page.

PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION
       You may specify a limit to the number of	clients	in a class that	can be
       assigned	leases.	 The effect of this will be to make it difficult for a
       new client in a class to	get an address.	 Once  a  class	 with  such  a
       limit  has  reached  its	limit, the only	way a new client in that class
       can get a lease is for an existing client to relinquish its lease,  ei-
       ther by letting it expire, or by	sending	a DHCPRELEASE packet.  Classes
       with lease limits are specified as follows:

       class "limited-1" {
	 lease limit 4;
       }

       This will produce a class in which a maximum of four members may	hold a
       lease at	one time.

SPAWNING CLASSES
       It  is  possible	 to  declare  a	spawning class.	 A spawning class is a
       class that automatically	produces subclasses based on what  the	client
       sends.	The  reason  that spawning classes were	created	was to make it
       possible	to create lease-limited	classes	on the	fly.   The  envisioned
       application  is	a cable-modem environment where	the ISP	wishes to pro-
       vide clients at a particular site with more than	one  IP	 address,  but
       does  not  wish to provide such clients with their own subnet, nor give
       them an unlimited number	of IP addresses	from the  network  segment  to
       which they are connected.

       Many  cable  modem  head-end  systems  can be configured	to add a Relay
       Agent Information option	to DHCP	packets	when relaying them to the DHCP
       server.	 These	systems	typically add a	circuit	ID or remote ID	option
       that uniquely identifies	the customer site.  To take advantage of this,
       you can write a class declaration as follows:

       class "customer"	{
	 spawn with option agent.circuit-id;
	 lease limit 4;
       }

       Now  whenever  a	 request comes in from a customer site,	the circuit ID
       option will be checked against the class's hash table.  If  a  subclass
       is  found that matches the circuit ID, the client will be classified in
       that subclass and treated accordingly.  If no subclass is found	match-
       ing  the	 circuit  ID,  a  new  one  will  be created and logged	in the
       dhcpd.leases file, and the client will be classified in this new	class.
       Once  the  client  has been classified, it will be treated according to
       the rules of the	class, including, in this case,	being subject  to  the
       per-site	limit of four leases.

       The  use	 of the	subclass spawning mechanism is not restricted to relay
       agent options - this particular example is given	only because it	 is  a
       fairly straightforward one.

COMBINING MATCH, MATCH IF AND SPAWN WITH
       In  some	 cases,	 it  may  be  useful to	use one	expression to assign a
       client to a particular class, and a second expression to	put it into  a
       subclass	of that	class.	This can be done by combining the match	if and
       spawn with statements, or the match if and match	statements.  For exam-
       ple:

       class "jr-cable-modems" {
	 match if option dhcp-vendor-identifier	= "jrcm";
	 spawn with option agent.circuit-id;
	 lease limit 4;
       }

       class "dv-dsl-modems" {
	 match if option dhcp-vendor-identifier	= "dvdsl";
	 spawn with option agent.circuit-id;
	 lease limit 16;
       }

       This  allows you	to have	two classes that both have the same spawn with
       expression without getting the clients in the two classes confused with
       each other.

DYNAMIC	DNS UPDATES
       The  DHCP  server has the ability to dynamically	update the Domain Name
       System.	Within the configuration files,	you can	define	how  you  want
       the  Domain Name	System to be updated.  These updates are RFC 2136 com-
       pliant so any DNS server	supporting RFC 2136 should be able  to	accept
       updates from the	DHCP server.

       There  are two DNS schemes implemented.	The interim option is based on
       draft revisions of the DDNS documents  while  the  standard  option  is
       based on	the RFCs for DHCP-DNS interaction and DHCIDs.  A third option,
       ad-hoc, was deprecated and has now been removed	from  the  code	 base.
       The DHCP	server must be configured to use one of	the two	currently-sup-
       ported methods, or not to do DNS	updates.

       New installations should	use the	standard option.  Older	 installations
       may want	to continue using the interim option for backwards compatibil-
       ity with	the DNS	database until the database can	be updated.  This  can
       be done with the	ddns-update-style configuration	parameter.

THE DNS	UPDATE SCHEME
       the interim and standard	DNS update schemes operate mostly according to
       work from the IETF.  The	interim	version	was based  on  the  drafts  in
       progress	at the time while the standard is based	on the completed RFCs.
       The standard RFCs are:

			    RFC	4701 (updated by RF5494)
				      RFC 4702
				      RFC 4703

       And the corresponding drafts were:

			  draft-ietf-dnsext-dhcid-rr-??.txt
			  draft-ietf-dhc-fqdn-option-??.txt
			draft-ietf-dhc-ddns-resolution-??.txt

       The basic framework for the two schemes is similar with the main	 mate-
       rial  difference	 being that a DHCID RR is used in the standard version
       while the interim versions uses a TXT RR.  The format of	the TXT	record
       bears  a	 resemblance  to the DHCID RR but it is	not equivalent (MD5 vs
       SHA2, field length differences etc).

       In these	two schemes the	DHCP server does not necessarily always	update
       both the	A and the PTR records.	The FQDN option	includes a flag	which,
       when sent by the	client,	indicates that the client wishes to update its
       own  A  record.	 In  that case,	the server can be configured either to
       honor the client's intentions or	ignore them.  This is  done  with  the
       statement  allow	 client-updates;  or  the  statement ignore client-up-
       dates;.	By default, client updates are allowed.

       If the server is	configured to allow client updates, then if the	client
       sends a fully-qualified domain name in the FQDN option, the server will
       use that	name the client	sent in	the FQDN  option  to  update  the  PTR
       record.	 For example, let us say that the client is a visitor from the
       "radish.org" domain, whose hostname is "jschmoe".  The  server  is  for
       the "example.org" domain.  The DHCP client indicates in the FQDN	option
       that its	FQDN is	"jschmoe.radish.org.".	 It  also  indicates  that  it
       wants  to  update its own A record.  The	DHCP server therefore does not
       attempt to set up an A record for the client, but does  set  up	a  PTR
       record  for  the	 IP  address  that  it assigns the client, pointing at
       jschmoe.radish.org.  Once the DHCP client has an	IP address, it can up-
       date  its  own A	record,	assuming that the "radish.org" DNS server will
       allow it	to do so.

       If the server is	configured not to allow	 client	 updates,  or  if  the
       client doesn't want to do its own update, the server will simply	choose
       a name for the client. By default, the server will choose from the fol-
       lowing three values:

	    1. fqdn option (if present)
	    2. hostname	option (if present)
	    3. Configured hostname option (if defined).

       If  these  defaults  for	choosing the host name are not appropriate you
       can write your own statement to set the ddns-hostname variable  as  you
       wish.  If none of the above are found the server	will use the host dec-
       laration	name (if one) and use-host-decl-names is on.

       It will use its own domain name for the client.	It  will  then	update
       both the	A and PTR record, using	the name that it chose for the client.
       If the client sends a fully-qualified domain name in the	 fqdn  option,
       the  server uses	only the leftmost part of the domain name - in the ex-
       ample above, "jschmoe" instead of "jschmoe.radish.org".

       Further,	if the ignore client-updates;  directive  is  used,  then  the
       server  will  in	addition send a	response in the	DHCP packet, using the
       FQDN Option, that implies to the	client that it should perform its  own
       updates	if it chooses to do so.	 With deny client-updates;, a response
       is sent which indicates the client may not perform updates.

       Both the	standard and interim options also include a  method  to	 allow
       more  than  one DHCP server to update the DNS database without acciden-
       tally deleting A	records	that shouldn't be deleted nor failing to add A
       records that should be added.  For the standard option the method works
       as follows:

       When the	DHCP server issues a client a new lease,  it  creates  a  text
       string  that  is	an SHA hash over the DHCP client's identification (see
       RFCs 4701 & 4702	for details).  The update attempts to add an A	record
       with the	name the server	chose and a DHCID record containing the	hashed
       identifier string (hashid).  If this update  succeeds,  the  server  is
       done.

       If  the update fails because the	A record already exists, then the DHCP
       server attempts to add the A record with	the  prerequisite  that	 there
       must  be	 a DHCID record	in the same name as the	new A record, and that
       DHCID record's contents must be equal to	hashid.	 If this  update  suc-
       ceeds,  then  the client	has its	A record and PTR record.  If it	fails,
       then the	name the client	has been assigned (or requested)  is  in  use,
       and  can't  be used by the client.  At this point the DHCP server gives
       up trying to do a DNS update for	the client until the client chooses  a
       new name.

       The  server  also  does not update very aggressively.  Because each DNS
       update involves a round trip to the DNS server, there is	a cost associ-
       ated  with  doing  updates  even	if they	do not actually	modify the DNS
       database.  So the DHCP server tracks whether or not it has updated  the
       record  in  the past (this information is stored	on the lease) and does
       not attempt to update records that it thinks it has already updated.

       This can	lead to	cases where the	DHCP server adds a  record,  and  then
       the  record  is	deleted	 through  some other mechanism,	but the	server
       never again updates the DNS because  it	thinks	the  data  is  already
       there.  In this case the	data can be removed from the lease through op-
       erator intervention, and	once this has been done, the DNS will  be  up-
       dated the next time the client renews.

       The  interim  DNS update	scheme was written before the RFCs were	final-
       ized and	does not quite follow them.  The RFCs call  for	 a  new	 DHCID
       RRtype while the	interim	DNS update scheme uses a TXT record.  In addi-
       tion the	ddns-resolution	draft called for the  DHCP  server  to	put  a
       DHCID  RR  on the PTR record, but the interim update method does	not do
       this.  In the final RFC this requirement	was relaxed such that a	server
       may add a DHCID RR to the PTR record.

DDNS IN	DUAL STACK ENVIRONMENTS
       As described in RFC 4703, section 5.2, in order to perform DDNS in dual
       stack environments, both	IPv4 and IPv6 servers would need to be config-
       ured  to	 use  the standard update style	and participating IPv4 clients
       MUST convey DUIDs as described in RFC  4361,  section  6.1.,  in	 their
       dhcp-client-identifiers.

       In  a nutshell, this mechanism is intended to use globally unique DUIDs
       to idenfity both	IPv4 and IPv6 clients, and where  a  device  has  both
       IPv4  and IPv6 leases it	is identified by the same DUID.	 This allows a
       dual stack client to use	the same FQDN for both mappings,  while	 being
       protected  from	updates	for other clients by the rules of conflict de-
       tection.

       However,	not all	IPv4 clients implement this behavior which makes  sup-
       porting	them dual stack	environments problematic.  In order to address
       this issue ISC DHCP (as of 4.4.0) supports a new	mode of	DDNS  conflict
       resolution referred to as Dual Stack Mixed Mode (DSMM).

       The  concept behind DSMM	is relatively simple.  All dhcp	servers	of one
       protocol	(IPv4 or v6) use one ddns-update-style (interim	 or  standard)
       while all servers of the	"other"	protocol will use the "other" ddns-ud-
       pate-style.  In this way, all servers of	a given	protocol are using the
       same  record  type (TXT or DHCID) for their DHCID RR entries.  This al-
       lows conflict detection to be enforced within each protocol without in-
       terferring with the other's entries.

       DSMM modifications now ensure that IPv4 DSMM servers only ever modify A
       records,	their associated PTR records and  DHCID	 records,  while  DSMM
       IPv6 severs only	modify AAAA records, their associated PTR records, and
       DHCID records.

       Note that DSMM is not a perfect solution, it is a compromise  that  can
       work  well  provided all	participating DNS updaters play	by DSMM	rules.
       As with anything	else in	life, it only works as well as those who  par-
       ticpate behave.

       While conflict detection	is enabled by default, DSMM is not.  To	enable
       DSMM,  both  update-conflict-detection  and  ddns-dual-stack-mixed-mode
       must be true.

PROTECTING DNS ENTRIES FOR STATIC CLIENTS
       Built  into  conflict resolution	is the protection of manually made en-
       tries for static	clients.  Per the rules	of conflict resolution,	 a DNS
       updater	may  not  alter	forward	DNS entries unless there is a DHCID RR
       which matches for whom the update is being made.	 Therefore,  any  for-
       ward  DNS entries without a corresponding DHCID RR cannot be altered by
       such an updater.

       In some environments, it	may be desirable to use	only  this  aspect  of
       conflict	resolution and allow DNS updaters to overwrite entries for dy-
       namic clients regardless	of what	client owns them.  In other words, the
       presence	or lack	of a DHCID RR is used to determine whether entries may
       or may not be overwritten.  Whether or not the client matches the  data
       value  of  the DHCID RR is irrelevant.	This behavior, off by default,
       can be configured through the parameter,	ddns-guard-id-must-match.   As
       with  DSMM, this	behavior is can	only be	enabled	if conflict resolution
       is enabled.   This  behavior  should  be	 considered  carefully	before
       electing	to use it.

       There is	an additional parameter	that can be used with DSMM ddns-other-
       guard-is-dynamic.  When enabled along with DSMM,	a server  will	regard
       the  presence  of a DHCID RR of the other style type as indicating that
       the forward DNS entries for that	FQDN should  be	 dynamic  and  may  be
       overwritten.   For  example,  such  a  server using interim style could
       overwrite the DNS entries for an	FQDN if	there is only a	DHDID type DH-
       DID RR for the FQDN.  Essentially, if there are dynamic entries for one
       protocol, that is enough	to overcome the	static protection  of  entries
       for  the	 other protocol.  This behavior	warrants careful consideration
       before electing to use it.

DYNAMIC	DNS UPDATE SECURITY
       When you	set your DNS server up to allow	updates	from the DHCP  server,
       you  may	 be  exposing  it to unauthorized updates.  To avoid this, you
       should use TSIG signatures - a method of	cryptographically signing  up-
       dates using a shared secret key.	 As long as you	protect	the secrecy of
       this key, your updates should also be secure.  Note, however, that  the
       DHCP  protocol itself provides no security, and that clients can	there-
       fore provide information	to the DHCP server which the DHCP server  will
       then use	in its updates,	with the constraints described previously.

       The  DNS	 server	 must be configured to allow updates for any zone that
       the DHCP	server will be updating.  For example, let us say that clients
       in  the	sneedville.edu	domain	will  be  assigned  addresses  on  the
       10.10.17.0/24 subnet.  In that case, you	will need  a  key  declaration
       for  the	 TSIG  key you will be using, and also two zone	declarations -
       one for the zone	containing A records that will be updates and one  for
       the zone	containing PTR records - for ISC BIND, something like this:

       key DHCP_UPDATER	{
	 algorithm HMAC-MD5.SIG-ALG.REG.INT;
	 secret	pRP5FapFoJ95JEL06sv4PQ==;
       };

       zone "example.org" {
	    type master;
	    file "example.org.db";
	    allow-update { key DHCP_UPDATER; };
       };

       zone "17.10.10.in-addr.arpa" {
	    type master;
	    file "10.10.17.db";
	    allow-update { key DHCP_UPDATER; };
       };

       You will	also have to configure your DHCP server	to do updates to these
       zones.  To do  so,  you	need  to  add  something  like	this  to  your
       dhcpd.conf file:

       key DHCP_UPDATER	{
	 algorithm HMAC-MD5.SIG-ALG.REG.INT;
	 secret	pRP5FapFoJ95JEL06sv4PQ==;
       };

       zone EXAMPLE.ORG. {
	 primary 127.0.0.1;
	 key DHCP_UPDATER;
       }

       zone 17.127.10.in-addr.arpa. {
	 primary 127.0.0.1;
	 key DHCP_UPDATER;
       }

       The primary statement specifies the IP address of the name server whose
       zone information	is to be updated.  In addition to the  primary	state-
       ment there are also the primary6	, secondary and	secondary6 statements.
       The primary6 statement specifies	an IPv6	address	for the	 name  server.
       The secondaries provide for additional addresses	for name servers to be
       used if the primary does	not respond.  The number of name  servers  the
       DDNS  code  will	attempt	to use before giving up	is limited and is cur-
       rently set to three.

       Note that the zone declarations have to correspond to authority records
       in your name server - in	the above example, there must be an SOA	record
       for "example.org." and for "17.10.10.in-addr.arpa.".  For  example,  if
       there  were  a  subdomain  "foo.example.org"  with no separate SOA, you
       could not write a zone declaration for "foo.example.org."  Also keep in
       mind  that  zone	 names in your DHCP configuration should end in	a ".";
       this is the preferred syntax.  If you do	not end	your zone  name	 in  a
       ".",  the  DHCP	server will figure it out.  Also note that in the DHCP
       configuration, zone names are not encapsulated in  quotes  where	 there
       are in the DNS configuration.

       You  should choose your own secret key, of course.  The ISC BIND	9 dis-
       tribution comes with  a	program	 for  generating  secret  keys	called
       dnssec-keygen.	If you are using BIND 9's dnssec-keygen, the above key
       would be	created	as follows:

	    dnssec-keygen -a HMAC-MD5 -b 128 -n	USER DHCP_UPDATER

       The key name, algorithm,	and secret must	match that being used  by  the
       DNS  server.  The  DHCP	server	currently supports the following algo-
       rithms:

	       HMAC-MD5
	       HMAC-SHA1
	       HMAC-SHA224
	       HMAC-SHA256
	       HMAC-SHA384
	       HMAC-SHA512

       You may wish to enable logging of DNS updates on	your DNS  server.   To
       do so, you might	write a	logging	statement like the following:

       logging {
	    channel update_debug {
		 file "/var/log/update-debug.log";
		 severity  debug 3;
		 print-category	yes;
		 print-severity	yes;
		 print-time	yes;
	    };
	    channel security_info    {
		 file "/var/log/named-auth.info";
		 severity  info;
		 print-category	yes;
		 print-severity	yes;
		 print-time	yes;
	    };

	    category update { update_debug; };
	    category security {	security_info; };
       };

       You  must  create  the /var/log/named-auth.info and /var/log/update-de-
       bug.log files before starting the name server.  For more	information on
       configuring ISC BIND, consult the documentation that accompanies	it.

REFERENCE: EVENTS
       There  are three	kinds of events	that can happen	regarding a lease, and
       it is possible to declare statements  that  occur  when	any  of	 these
       events  happen.	These events are the commit event, when	the server has
       made a commitment of a certain lease to a client,  the  release	event,
       when  the  client  has released the server from its commitment, and the
       expiry event, when the commitment expires.

       To declare a set	of statements to execute when an  event	 happens,  you
       must  use the on	statement, followed by the name	of the event, followed
       by a series of statements to execute when the event  happens,  enclosed
       in braces.

REFERENCE: DECLARATIONS
       The include statement

	include	"filename";

       The  include statement is used to read in a named file, and process the
       contents	of that	file as	though it were entered in place	of the include
       statement.

       The shared-network statement

	shared-network name {
	  [ parameters ]
	  [ declarations ]
	}

       The  shared-network  statement  is  used	to inform the DHCP server that
       some IP subnets actually	share the same physical	network.  Any  subnets
       in  a  shared network should be declared	within a shared-network	state-
       ment.  Parameters specified in the  shared-network  statement  will  be
       used  when  booting clients on those subnets unless parameters provided
       at the subnet or	host level override them.  If any subnet in  a	shared
       network has addresses available for dynamic allocation, those addresses
       are collected into a common pool	for that shared	network	 and  assigned
       to  clients  as needed.	There is no way	to distinguish on which	subnet
       of a shared network a client should boot.

       Name should be the name of the shared network.  This name is used  when
       printing	debugging messages, so it should be descriptive	for the	shared
       network.	 The name may have the syntax of a valid domain	name (although
       it  will	 never	be used	as such), or it	may be any arbitrary name, en-
       closed in quotes.

       The subnet statement

	subnet subnet-number netmask netmask {
	  [ parameters ]
	  [ declarations ]
	}

       The subnet statement is used to provide dhcpd with  enough  information
       to tell whether or not an IP address is on that subnet.	It may also be
       used to provide subnet-specific parameters  and	to  specify  what  ad-
       dresses may be dynamically allocated to clients booting on that subnet.
       Such addresses are specified using the range declaration.

       The subnet-number should	be an IP address or domain name	which resolves
       to the subnet number of the subnet being	described.  The	netmask	should
       be an IP	address	or domain name which resolves to the  subnet  mask  of
       the  subnet being described.  The subnet	number,	together with the net-
       mask, are sufficient to determine whether any given IP  address	is  on
       the specified subnet.

       Although	 a  netmask must be given with every subnet declaration, it is
       recommended that	if there is any	variance in subnet masks at a site,  a
       subnet-mask  option statement be	used in	each subnet declaration	to set
       the desired subnet mask,	since any subnet-mask  option  statement  will
       override	the subnet mask	declared in the	subnet statement.

       The subnet6 statement

	subnet6	subnet6-number {
	  [ parameters ]
	  [ declarations ]
	}

       The  subnet6 statement is used to provide dhcpd with enough information
       to tell whether or not an IPv6 address is on that subnet6.  It may also
       be  used	 to provide subnet-specific parameters and to specify what ad-
       dresses may be dynamically allocated to clients booting on that subnet.

       The subnet6-number should be an IPv6 network identifier,	 specified  as
       ip6-address/bits.

       The range statement

       range [ dynamic-bootp ] low-address [ high-address];

       For  any	 subnet	on which addresses will	be assigned dynamically, there
       must be at least	one range statement.  The range	 statement  gives  the
       lowest  and  highest  IP	addresses in a range.  All IP addresses	in the
       range should be in the subnet in	which the range	statement is declared.
       The  dynamic-bootp  flag	may be specified if addresses in the specified
       range may be dynamically	assigned to BOOTP  clients  as	well  as  DHCP
       clients.	  When	specifying a single address, high-address can be omit-
       ted.

       The range6 statement

       range6 low-address high-address;
       range6 subnet6-number;
       range6 subnet6-number temporary;
       range6 address temporary;

       For any IPv6 subnet6 on which addresses will be	assigned  dynamically,
       there  must  be at least	one range6 statement. The range6 statement can
       either be the lowest and	highest	IPv6 addresses in  a  range6,  or  use
       CIDR  notation,	specified as ip6-address/bits. All IP addresses	in the
       range6 should be	in the subnet6 in which	the range6  statement  is  de-
       clared.

       The  temporary  variant makes the prefix	(by default on 64 bits)	avail-
       able for	temporary (RFC 4941) addresses.	A new address  per  prefix  in
       the  shared  network  is	computed at each request with an IA_TA option.
       Release and Confirm ignores temporary addresses.

       Any IPv6	addresses given	to hosts with fixed-address6 are excluded from
       the range6, as are IPv6 addresses on the	server itself.

       The prefix6 statement

       prefix6 low-address high-address	/ bits;

       The  prefix6 is the range6 equivalent for Prefix	Delegation (RFC	3633).
       Prefixes	of bits	length are assigned between low-address	 and  high-ad-
       dress.

       Any  IPv6  prefixes  given to static entries (hosts) with fixed-prefix6
       are excluded from the prefix6.

       This statement is currently global but it should	have a	shared-network
       scope.

       The host	statement

	host hostname {
	  [ parameters ]
	  [ declarations ]
	}

       The  host  declaration provides a way for the DHCP server to identify a
       DHCP or BOOTP client.  This allows the server to	provide	 configuration
       information including fixed addresses or, in DHCPv6, fixed prefixes for
       a specific client.

       If it is	desirable to be	able to	boot a DHCP or BOOTP  client  on  more
       than  one  subnet with fixed v4 addresses, more than one	address	may be
       specified in the	fixed-address  declaration,  or	 more  than  one  host
       statement may be	specified matching the same client.

       The  fixed-address6 declaration is used for v6 addresses.  At this time
       it only works with a single address.  For  multiple  addresses  specify
       multiple	host statements.

       If  client-specific boot	parameters must	change based on	the network to
       which the client	is attached, then multiple host	declarations should be
       used.   The  host declarations will only	match a	client if one of their
       fixed-address statements	is viable on the subnet	 (or  shared  network)
       where  the  client  is attached.	 Conversely, for a host	declaration to
       match a client being allocated a	dynamic	address, it must not have  any
       fixed-address  statements.   You	 may  therefore	need a mixture of host
       declarations for	any given client...some	 having	 fixed-address	state-
       ments, others without.

       hostname	 should	 be a name identifying the host.  If a hostname	option
       is not specified	for the	host, hostname is used.

       Host declarations are matched to	actual DHCP or BOOTP clients by	match-
       ing the dhcp-client-identifier option specified in the host declaration
       to the one supplied by the client, or, if the host declaration  or  the
       client  does  not  provide a dhcp-client-identifier option, by matching
       the hardware parameter in the host declaration to the network  hardware
       address	supplied by the	client.	 BOOTP clients do not normally provide
       a dhcp-client-identifier, so the	hardware address must be used for  all
       clients that may	boot using the BOOTP protocol.

       DHCPv6 servers can use the host-identifier option parameter in the host
       declaration, and	specify	any option with	 a  fixed  value  to  identify
       hosts.

       Please  be  aware  that	only the dhcp-client-identifier	option and the
       hardware	address	can be used to match a host declaration, or the	 host-
       identifier option parameter for DHCPv6 servers.	For example, it	is not
       possible	to match a host	declaration to a host-name  option.   This  is
       because	the host-name option cannot be guaranteed to be	unique for any
       given client, whereas both the hardware address and dhcp-client-identi-
       fier  option  are  at  least theoretically guaranteed to	be unique to a
       given client.

       The group statement

	group {
	  [ parameters ]
	  [ declarations ]
	}

       The group statement is used simply to apply one or more parameters to a
       group of	declarations.  It can be used to group hosts, shared networks,
       subnets,	or even	other groups.

REFERENCE: ALLOW AND DENY
       The allow and deny statements can be used to control  the  response  of
       the  DHCP server	to various sorts of requests.  The allow and deny key-
       words actually have different meanings depending	on the context.	 In  a
       pool context, these keywords can	be used	to set up access lists for ad-
       dress allocation	pools.	In other contexts, the keywords	simply control
       general	server	behavior with respect to clients based on scope.  In a
       non-pool	context, the ignore keyword can	be used	in place of  the  deny
       keyword to prevent logging of denied requests.

ALLOW DENY AND IGNORE IN SCOPE
       The following usages of allow and deny will work	in any scope, although
       it is not recommended that they be used in pool declarations.

       The unknown-clients keyword

	allow unknown-clients;
	deny unknown-clients;
	ignore unknown-clients;

       The unknown-clients flag	is used	to tell	dhcpd whether or not to	dynam-
       ically assign addresses to unknown clients.  Dynamic address assignment
       to unknown clients is allowed by	default.  An unknown client is	simply
       a client	that has no host declaration.

       The  use	 of  this  option is now deprecated.  If you are trying	to re-
       strict access on	your network to	known clients, you should use deny un-
       known-clients;  inside  of  your	 address  pool,	as described under the
       heading ALLOW AND DENY WITHIN POOL DECLARATIONS.

       The bootp keyword

	allow bootp;
	deny bootp;
	ignore bootp;

       The bootp flag is used to tell dhcpd whether or not to respond to bootp
       queries.	 Bootp queries are allowed by default.

       The booting keyword

	allow booting;
	deny booting;
	ignore booting;

       The  booting  flag  is  used to tell dhcpd whether or not to respond to
       queries from a particular client.  This keyword only has	 meaning  when
       it  appears in a	host declaration.  By default, booting is allowed, but
       if it is	disabled for a particular client, then that client will	not be
       able to get an address from the DHCP server.

       The duplicates keyword

	allow duplicates;
	deny duplicates;

       Host  declarations  can	match client messages based on the DHCP	Client
       Identifier option or based on the client's network  hardware  type  and
       MAC  address.   If  the	MAC address is used, the host declaration will
       match any client	with that MAC address -	even  clients  with  different
       client identifiers.  This doesn't normally happen, but is possible when
       one computer has	more than one operating	system installed on it	-  for
       example,	Microsoft Windows and NetBSD or	Linux.

       The duplicates flag tells the DHCP server that if a request is received
       from a client that matches the MAC address of a host  declaration,  any
       other  leases  matching	that  MAC  address  should be discarded	by the
       server, even if the UID is not the same.	 This is a  violation  of  the
       DHCP  protocol, but can prevent clients whose client identifiers	change
       regularly from holding many leases at the same time.  By	 default,  du-
       plicates	are allowed.

       The declines keyword

	allow declines;
	deny declines;
	ignore declines;

       The  DHCPDECLINE	 message  is used by DHCP clients to indicate that the
       lease the server	has offered is not valid.  When	the server receives  a
       DHCPDECLINE  for	 a  particular	address, it normally abandons that ad-
       dress, assuming that some unauthorized system is	 using	it.   Unfortu-
       nately,	a  malicious  or buggy client can, using DHCPDECLINE messages,
       completely exhaust the DHCP server's allocation pool.  The server  will
       eventually  reclaim  these  leases, but not while the client is running
       through the pool. This may cause	serious	thrashing in the DNS,  and  it
       will also cause the DHCP	server to forget old DHCP client address allo-
       cations.

       The declines flag tells the DHCP	server whether or not to honor DHCPDE-
       CLINE  messages.	 If it is set to deny or ignore	in a particular	scope,
       the DHCP	server will not	respond	to DHCPDECLINE messages.

       The declines flag is only supported by DHCPv4 servers.  Given the large
       IPv6  address space and the internal limits imposed by the server's ad-
       dress generation	mechanism we don't think it is	necessary  for	DHCPv6
       servers at this time.

       Currently, abandoned IPv6 addresses are reclaimed in one	of two ways:
	   a) Client renews a specific address:
	   If a	client using a given DUID submits a DHCP REQUEST containing
	   the last address abandoned by that DUID, the	address	will be
	   reassigned to that client.

	   b) Upon the second restart following	an address abandonment.	 When
	   an address is abandoned it is both recorded as such in the lease
	   file	and retained as	abandoned in server memory until the server
	   is restarted. Upon restart, the server will process the lease file
	   and all addresses whose last	known state is abandoned will be
	   retained as such in memory but not rewritten	to the lease file.
	   This	means that a subsequent	restart	of the server will not see the
	   abandoned addresses in the lease file and therefore have no record
	   of them as abandoned	in memory and as such perceive them as free
	   for assignment.

       The total number	addresses in a pool, available for a given DUID	value,
       is internally limited by	the server's address generation	mechanism.  If
       through	mistaken  configuration,  multiple  clients are	using the same
       DUID they will competing	for the	same addresses causing the  server  to
       reach  this internal limit rather quickly.  The internal	limit isolates
       this type of activity such that address	range  is  not	exhausted  for
       other  DUID  values.  The appearance of the following error log,	can be
       an indication of	this condition:

	   "Best match for DUID	<XX> is	an abandoned address, This may be a
	    result of multiple clients attempting to use this DUID"

	   where <XX> is an actual DUID	value depicted as colon	separated
	   string of bytes in hexadecimal values.

       The client-updates keyword

	allow client-updates;
	deny client-updates;

       The client-updates flag tells the DHCP server whether or	not  to	 honor
       the  client's  intention	to do its own update of	its A record.  See the
       documentation under the heading THE DNS UPDATE SCHEME for details.

       The leasequery keyword

	allow leasequery;
	deny leasequery;

       The leasequery flag tells the DHCP server whether or not	to answer DHC-
       PLEASEQUERY packets. The	answer to a DHCPLEASEQUERY packet includes in-
       formation about a specific lease, such as when it was issued  and  when
       it  will	expire.	By default, the	server will not	respond	to these pack-
       ets.

ALLOW AND DENY WITHIN POOL DECLARATIONS
       The uses	of the allow and deny keywords shown in	the  previous  section
       work  pretty much the same way whether the client is sending a DHCPDIS-
       COVER or	a DHCPREQUEST message -	an address will	be  allocated  to  the
       client  (either	the old	address	it's requesting, or a new address) and
       then that address will be tested	to see if it's okay to let the	client
       have  it.   If  the  client requested it, and it's not okay, the	server
       will send a DHCPNAK message.  Otherwise,	the server will	simply not re-
       spond  to the client.  If it is okay to give the	address	to the client,
       the server will send a DHCPACK message.

       The primary motivation behind pool declarations is to have address  al-
       location	 pools	whose allocation policies are different.  A client may
       be denied access	to one pool, but allowed access	to another pool	on the
       same network segment.  In order for this	to work, access	control	has to
       be done during address allocation,  not	after  address	allocation  is
       done.

       When a DHCPREQUEST message is processed,	address	allocation simply con-
       sists of	looking	up the address the client is requesting	and seeing  if
       it's  still  available  for the client.	If it is, then the DHCP	server
       checks both the address pool permit lists and the relevant in-scope al-
       low  and	 deny  statements to see if it's okay to give the lease	to the
       client.	In the case of a DHCPDISCOVER message, the allocation  process
       is done as described previously in the ADDRESS ALLOCATION section.

       When declaring permit lists for address allocation pools, the following
       syntaxes	are recognized following the allow or deny keywords:

	known-clients;

       If specified, this statement either allows or prevents allocation  from
       this  pool  to any client that has a host declaration (i.e., is known).
       A client	is known if it has a host declaration in any scope,  not  just
       the current scope.

	unknown-clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to any	client that has	no  host  declaration  (i.e.,  is  not
       known).

	members	of "class";

       If  specified, this statement either allows or prevents allocation from
       this pool to any	client that is a member	of the named class.

	dynamic	bootp clients;

       If specified, this statement either allows or prevents allocation  from
       this pool to any	bootp client.

	authenticated clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to any	client that has	been authenticated using the DHCP  au-
       thentication protocol.  This is not yet supported.

	unauthenticated	clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to any	client that has	not been authenticated using the  DHCP
       authentication protocol.	 This is not yet supported.

	all clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to all	clients.  This can be used when	you want  to  write  a
       pool  declaration  for some reason, but hold it in reserve, or when you
       want to renumber	your network quickly, and  thus	 want  the  server  to
       force  all clients that have been allocated addresses from this pool to
       obtain new addresses immediately	when they next renew.

	after time;

       If specified, this statement either allows or prevents allocation  from
       this  pool  after  a given date.	This can be used when you want to move
       clients from one	pool to	another. The server adjusts the	regular	 lease
       time  so	 that  the  latest expiry time is at the given time+min-lease-
       time.  A	short min-lease-time enforces a	step change, whereas a	longer
       min-lease-time  allows  for  a  gradual	change.	 time is either	second
       since epoch, or a UTC time string e.g.	4  2007/08/24  09:14:32	 or  a
       string  with  time  zone	 offset	 in seconds e.g. 4 2007/08/24 11:14:32
       -7200

REFERENCE: PARAMETERS
       The abandon-lease-time statement

	 abandon-lease-time time;

	 Time should be	the maximum amount of time (in seconds)	that an	 aban-
	 doned	IPv4  lease  remains  unavailable  for assignment to a client.
	 Abandoned leases will only be offered to clients if there are no free
	 leases.  If not defined, the default abandon lease time is 86400 sec-
	 onds (24 hours).  Note	the abandoned lease time for a given lease  is
	 preserved  across  server restarts.  The parameter may	only be	set at
	 the global scope and is evaluated only	once during server startup.

	 Values	less than sixty	seconds	are not	recommended as this  is	 below
	 the  ping  check  threshold  and  can cause leases once abandoned but
	 since returned	to the free state to not be pinged  before  being  of-
	 fered.	  If  the  requested time is larger than 0x7FFFFFFF - 1	or the
	 sum of	the current  time  plus	 the  abandoned	 time  isgreater  than
	 0x7FFFFFFF it is treated as infinite.

       The adaptive-lease-time-threshold statement

	 adaptive-lease-time-threshold percentage;

	 When  the  number  of	allocated leases within	a pool rises above the
	 percentage given in this statement, the  DHCP	server	decreases  the
	 lease	length for new clients within this pool	to min-lease-time sec-
	 onds. Clients renewing	an already valid (long)	leases	get  at	 least
	 the  remaining	 time  from the	current	lease. Since the leases	expire
	 faster, the server may	either recover more quickly or avoid pool  ex-
	 haustion  entirely.   Once  the number	of allocated leases drop below
	 the threshold,	the server reverts back	to normal lease	times.	 Valid
	 percentages are between 1 and 99.

       The always-broadcast statement

	 always-broadcast flag;

	 The  DHCP  and	BOOTP protocols	both require DHCP and BOOTP clients to
	 set the broadcast bit in the flags field of the BOOTP message header.
	 Unfortunately,	some DHCP and BOOTP clients do not do this, and	there-
	 fore may not receive responses	from the DHCP server.  The DHCP	server
	 can  be  made to always broadcast its responses to clients by setting
	 this flag to 'on' for the relevant scope; relevant  scopes  would  be
	 inside	 a  conditional	statement, as a	parameter for a	class, or as a
	 parameter for a host declaration.  To avoid creating excess broadcast
	 traffic  on  your  network, we	recommend that you restrict the	use of
	 this option to	as few clients as possible.  For example,  the	Micro-
	 soft  DHCP client is known not	to have	this problem, as are the Open-
	 Transport and ISC DHCP	clients.

       The always-reply-rfc1048	statement

	 always-reply-rfc1048 flag;

	 Some BOOTP clients expect RFC1048-style responses, but	do not	follow
	 RFC1048  when	sending	their requests.	 You can tell that a client is
	 having	this problem if	it is not getting the options you have config-
	 ured  for  it	and  if	 you  see in the server	log the	message	"(non-
	 rfc1048)" printed with	each BOOTREQUEST that is logged.

	 If you	want to	send rfc1048 options to	such a client, you can set the
	 always-reply-rfc1048  option  in  that	client's host declaration, and
	 the DHCP server will respond with an  RFC-1048-style  vendor  options
	 field.	  This	flag  can  be  set  in	any scope, and will affect all
	 clients covered by that scope.

       The authoritative statement

	 authoritative;

	 not authoritative;

	 The DHCP server will normally assume that the configuration  informa-
	 tion  about a given network segment is	not known to be	correct	and is
	 not authoritative.  This is so	that if	a naive	user installs  a  DHCP
	 server	 not fully understanding how to	configure it, it does not send
	 spurious DHCPNAK messages to clients  that  have  obtained  addresses
	 from a	legitimate DHCP	server on the network.

	 Network  administrators  setting  up  authoritative  DHCP servers for
	 their networks	should always write authoritative; at the top of their
	 configuration file to indicate	that the DHCP server should send DHCP-
	 NAK messages to misconfigured clients.	 If this is not	done,  clients
	 will be unable	to get a correct IP address after changing subnets un-
	 til their old lease has expired, which	could take quite a long	time.

	 Usually, writing authoritative; at the	top level of the  file	should
	 be  sufficient.  However, if a	DHCP server is to be set up so that it
	 is aware of some networks for which it	is authoritative and some net-
	 works	for which it is	not, it	may be more appropriate	to declare au-
	 thority on a per-network-segment basis.

	 Note that the most specific scope for which the concept of  authority
	 makes	any  sense  is the physical network segment - either a shared-
	 network statement or a	subnet statement that is not contained	within
	 a shared-network statement.  It is not	meaningful to specify that the
	 server	is authoritative for some subnets within a shared network, but
	 not  authoritative  for  others, nor is it meaningful to specify that
	 the server is authoritative for some host declarations	and  not  oth-
	 ers.

	 In  order for DHCPINFORMs to be responded to by the server, they must
	 match to subnets over which the server	has authority; otherwise  they
	 will  be  ignored and logged.	To minimize the	impact on logging vol-
	 ume, only the first and every subsequent 100th	occurrence of  an  ig-
	 nored DHCPINFORM is logged.

       The boot-unknown-clients	statement

	 boot-unknown-clients flag;

	 If  the  boot-unknown-clients statement is present and	has a value of
	 false or off, then clients for	which there  is	 no  host  declaration
	 will not be allowed to	obtain IP addresses.  If this statement	is not
	 present or has	a value	of true	or on, then clients without host  dec-
	 larations  will  be  allowed to obtain	IP addresses, as long as those
	 addresses are not restricted by  allow	 and  deny  statements	within
	 their pool declarations.

       The check-secs-byte-order statement

	 check-secs-byte-order flag;

	 When  check-secs-byte-order  is  enabled,  the	 server	will check for
	 DHCPv4	clients	that do	the byte ordering on  the  secs	 field	incor-
	 rectly.  This	field should be	in network byte	order but some clients
	 get it	wrong. When this parameter is enabled the server will  examine
	 the secs field	and if it looks	wrong (high byte non zero and low byte
	 zero) swap the	bytes.	The default is	disabled.  This	 parameter  is
	 only  useful  when  doing  load balancing within failover. (Formerly,
	 this behavior had to be enabled during	compilation configuration  via
	 --enable-secs-byteorder).

	 The db-time-format statement

	    db-time-format [ default | local ] ;

	    The	 DHCP  server software outputs several timestamps when writing
	    leases to persistent storage.  This	 configuration	parameter  se-
	    lects  one	of  two	output formats.	 The default format prints the
	    day, date, and time	in UTC,	while the local	format prints the sys-
	    tem	 seconds-since-epoch,  and helpfully provides the day and time
	    in the system timezone in a	comment.  The  time  formats  are  de-
	    scribed in detail in the dhcpd.leases(5) manpage.

	 The ddns-hostname statement

	    ddns-hostname name;

	    The	 name  parameter  should  be the hostname that will be used in
	    setting up the client's A and PTR records.	If no ddns-hostname is
	    specified in scope,	then the server	will derive the	hostname auto-
	    matically, using an	algorithm that varies for each of the  differ-
	    ent	update methods.

	 The ddns-domainname statement

	    ddns-domainname name;

	    The	name parameter should be the domain name that will be appended
	    to the client's hostname to	 form  a  fully-qualified  domain-name
	    (FQDN).

	 The ddns-dual-stack-mixed-mode	statement

	    ddns-dual-stack-mixed-mode flag;

	    The	 ddns-dual-stack-mixed-mode  parameter controls	whether	or not
	    the	server applies Dual Stack Mixed	Mode rules  during  DDNS  con-
	    flict resolution.  This parameter is off by	default, has no	effect
	    unless update-conflict-detection is	enabled, and may only be spec-
	    ified at the global	scope.

	 The ddns-guard-id-must-match statement

	    ddns-guard-id-must-match flag;

	    The	 ddns-guard-id-must-match  parameter controls whether or not a
	    the	client id within a DHCID RR must match that  of	 the  DNS  up-
	    date's  client to permit DNS entries associated with that DHCID RR
	    to be ovewritten.  Proper conflict resolution requires ID matching
	    and	 should	 only  be  disabled after careful consideration.  When
	    disabled, it is allows any DNS updater to replace DNS entries that
	    have  an  associated DHCID RR, regardless of client	identity. This
	    parameter is on by default,	has no effect unless  update-conflict-
	    detection  is  enabled,  and  may  only be specified at the	global
	    scope.

	 The dns-local-address4	and dns-local-address6 statements

	    ddns-local-address4	address;

	    ddns-local-address6	address;

	    The	address	parameter should be the	local IPv4 or IPv6 address the
	    server should use as the from address when sending DDNS update re-
	    quests.

	 The ddns-other-guard-is-dynamic statement

	    ddns-other-guard-is-dynamic	flag;

	    The	ddns-other-guard-is-dynamic parameter controls whether or  not
	    a  a  server  running DSMM will consider the presence of the other
	    update style DHCID RR as an	indcation that a DNS  entries  may  be
	    overwritten.  It  should only be enabled after careful study as it
	    allows DNS entries that would otherwise be protected as static, to
	    be overwritten in certain cases. This paramater is off by default,
	    has	no effect unless ddns-dual-stack-mixed-mode  is	 enabled,  and
	    may	only be	specified at the global	scope.

	 The ddns-rev-domainname statement

	    ddns-rev-domainname	name;

	    The	name parameter should be the domain name that will be appended
	    to the client's reversed IP	address	to produce a name for  use  in
	    the	client's PTR record.  By default, this is "in-addr.arpa.", but
	    the	default	can be overridden here.

	    The	reversed IP address to which this domain name is  appended  is
	    always  the	IP address of the client, in dotted quad notation, re-
	    versed - for example, if the IP address assigned to	the client  is
	    10.17.92.74,  then	the  reversed IP address is 74.92.17.10.  So a
	    client with	that IP	address	would, by  default,  be	 given	a  PTR
	    record of 10.17.92.74.in-addr.arpa.

	 The ddns-update-style parameter

	    ddns-update-style style;

	    The	style parameter	must be	one of standard, interim or none.  The
	    ddns-update-style statement	is only	meaningful in the outer	 scope
	    -  it  is evaluated	once after reading the dhcpd.conf file,	rather
	    than each time a client is assigned	an IP address, so there	is  no
	    way	 to use	different DNS update styles for	different clients. The
	    default is none.

	 The ddns-updates statement

	     ddns-updates flag;

	    The	ddns-updates parameter controls	whether	or not the server will
	    attempt to do a DNS	update when a lease is confirmed.  Set this to
	    off	if the server should not attempt to do updates within  a  cer-
	    tain scope.	 The ddns-updates parameter is on by default.  To dis-
	    able DNS updates in	all scopes, it is preferable to	use the	 ddns-
	    update-style statement, setting the	style to none.

	 The default-lease-time	statement

	    default-lease-time time;

	    Time  should  be  the length in seconds that will be assigned to a
	    lease if the client	requesting the lease does not ask for  a  spe-
	    cific  expiration  time.   This is used for	both DHCPv4 and	DHCPv6
	    leases (it is also known as	the "valid lifetime" in	DHCPv6).   The
	    default is 43200 seconds.

	 The delayed-ack and max-ack-delay statements

	    delayed-ack	count;

	    max-ack-delay microseconds;

	    Count  should be an	integer	value from zero	to 2^16-1 and defaults
	    to 0, which	means that the feature is disabled.  Otherwise,	28 may
	    be	a  sensible  starting point for	many configurations (SO_SNDBUF
	    size / 576 bytes.)	The count represents how many  DHCPv4  replies
	    maximum will be queued pending transmission	until after a database
	    commit event.  If this number is reached, a	database commit	 event
	    (commonly resulting	in fsync() and representing a performance pen-
	    alty) will be made,	and the	reply packets will be transmitted in a
	    batch afterwards.  This preserves the RFC2131 direction that "sta-
	    ble	storage" be updated prior to replying to clients.  Should  the
	    DHCPv4 sockets "go dry" (select() returns immediately with no read
	    sockets), the commit is made and any queued	packets	are  transmit-
	    ted.

	    Similarly,	microseconds  indicates	how many microseconds are per-
	    mitted to pass inbetween queuing a packet pending  an  fsync,  and
	    performing	the  fsync.   Valid values range from 0	to 2^32-1, and
	    defaults to	250,000	(1/4 of	a second).

	    The	delayed-ack feature is compiled	in by default, but can be dis-
	    abled  at  compile	time with './configure --disable-delayed-ack'.
	    Please note	that the delayed-ack feature is	not currently compati-
	    ble	with support for DHPCv4-over-DHCPv6 so when a 4to6 port	ommand
	    line argument enables this in the server the delayed-ack value  is
	    reset to 0.

	 The dhcp-cache-threshold statement

	    dhcp-cache-threshold percentage;

	    The	 dhcp-cache-threshold  statement  takes	 one integer parameter
	    with allowed values	between	0 and 100. The	default	 value	is  25
	    (25%  of  the lease	time). This parameter expresses	the percentage
	    of the total lease time, measured from the beginning, during which
	    a  client's	 attempt to renew its lease will result	in getting the
	    already assigned lease, rather than	an extended lease.  This  fea-
	    ture  is  supported	 for  both  IPv4 and IPv6 and down to the pool
	    level and for IPv6 all three pool types: NA, TA and	PD.

	    Clients that attempt renewal frequently can	cause  the  server  to
	    update  and	 write	the database frequently	resulting in a perfor-
	    mance impact on the	server.	  The  dhcp-cache-threshold  statement
	    instructs  the DHCP	server to avoid	updating leases	too frequently
	    thus avoiding this behavior.  Instead the server replies with  the
	    same  lease	(i.e. reuses it) with no modifications except for CLTT
	    (Client Last Transmission Time) and	for IPv4:

		the lease time sent to the client is shortened by the age of
		the lease

	    while for IPv6:

		the preferred and valid	lifetimes sent to the client are
		shortened by the age of	the lease.

	    None of these changes require writing the lease to disk.

	    When an existing lease is matched to a renewing client, it will be
	    reused if all of the following conditions are true:
		1. The dhcp-cache-threshold is larger than zero
		2. The current lease is	active
		3. The percentage of the lease time that has elapsed is	less than
		dhcp-cache-threshold
		4. The client information provided in the renewal does not alter
		any of the following:
		   a. DNS information and DNS updates are enabled
		   b. Billing class to which the lease is associated (IPv4 only)
		   c. The host declaration associated with the lease (IPv4 only)
		   d. The client id - this may happen if a client boots	without
		      a	client id and then starts using	one in subsequent
		      requests.	(IPv4 only)

	    While  lease  data	is not written to disk when a lease is reused,
	    the	server will still execute any on-commit	statements.

	    Note that the lease	can be reused if the options the client	or re-
	    lay	 agent	sends are changed.  These changes will not be recorded
	    in the in-memory or	on-disk	databases until	the client renews  af-
	    ter	the threshold time is reached.

	 The do-forward-updates	statement

	    do-forward-updates flag;

	    The	 do-forward-updates  statement instructs the DHCP server as to
	    whether it should attempt to update	a DHCP client's	A record  when
	    the	 client	acquires or renews a lease.  This statement has	no ef-
	    fect unless	DNS updates are	enabled.  Forward updates are  enabled
	    by default.	 If this statement is used to disable forward updates,
	    the	DHCP server will  never	 attempt  to  update  the  client's  A
	    record,  and  will	only  ever  attempt to update the client's PTR
	    record if the client supplies an FQDN that should be placed	in the
	    PTR	record using the fqdn option.  If forward updates are enabled,
	    the	DHCP server will still honor the setting of the	client-updates
	    flag.

	 The dont-use-fsync statement

	    dont-use-fsync flag;

	    The	 dont-use-fsync	 statement  instructs  the  DHCP  server if it
	    should call	fsync()	when writing leases to the lease file.	By de-
	    fault  and	if  the	 flag  is  set	to  false the server will call
	    fsync().  Suppressing the call to fsync() may increase the perfor-
	    mance  of the server but it	also adds a risk that a	lease will not
	    be properly	written	to the disk after it  has  been	 issued	 to  a
	    client  and	 before	 the server stops.  This can lead to duplicate
	    leases being issued	to different clients.  Using  this  option  is
	    not	recommended.

	 The dynamic-bootp-lease-cutoff	statement

	    dynamic-bootp-lease-cutoff date;

	    The	 dynamic-bootp-lease-cutoff statement sets the ending time for
	    all	leases assigned	dynamically to BOOTP clients.	Because	 BOOTP
	    clients  do	 not  have  any	way of renewing	leases,	and don't know
	    that their leases could expire, by default dhcpd assigns  infinite
	    leases  to	all BOOTP clients.  However, it	may make sense in some
	    situations to set a	cutoff date for	all BOOTP leases -  for	 exam-
	    ple,  the end of a school term, or the time	at night when a	facil-
	    ity	is closed and all machines are required	to be powered off.

	    Date should	be the date on which all assigned  BOOTP  leases  will
	    end.  The date is specified	in the form:

				  W YYYY/MM/DD HH:MM:SS

	    W  is the day of the week expressed	as a number from zero (Sunday)
	    to six (Saturday).	YYYY is	the year, including the	 century.   MM
	    is the month expressed as a	number from 1 to 12.  DD is the	day of
	    the	month, counting	from 1.	 HH is the hour, from zero to 23.   MM
	    is the minute and SS is the	second.	 The time is always in Coordi-
	    nated Universal Time (UTC),	not local time.

	 The dynamic-bootp-lease-length	statement

	    dynamic-bootp-lease-length length;

	    The	dynamic-bootp-lease-length statement is	used to	set the	length
	    of	leases	dynamically assigned to	BOOTP clients.	At some	sites,
	    it may be possible to assume that a	lease is no longer in  use  if
	    its	 holder	has not	used BOOTP or DHCP to get its address within a
	    certain time period.  The period is	specified in length as a  num-
	    ber	 of seconds.  If a client reboots using	BOOTP during the time-
	    out	period,	the lease duration is reset  to	 length,  so  a	 BOOTP
	    client  that  boots	 frequently  enough will never lose its	lease.
	    Needless to	say, this parameter should be  adjusted	 with  extreme
	    caution.

	 The echo-client-id statement

	    echo-client-id flag;

	    The	echo-client-id statement is used to enable or disable RFC 6842
	    compliant behavior.	 If the	echo-client-id	statement  is  present
	    and	 has  a	value of true or on, and a DHCP	DISCOVER or REQUEST is
	    received which contains the	client identifier option (Option  code
	    61),  the  server will copy	the option into	its response (DHCP ACK
	    or NAK) per	RFC 6842.  In other words if the client	sends the  op-
	    tion  it  will  receive  it	back. By default, this flag is off and
	    client identifiers will not	echoed back to the client.

	 The filename statement

	    filename "filename";

	    The	filename statement can be used to specify the name of the ini-
	    tial  boot	file  which is to be loaded by a client.  The filename
	    should be a	filename recognizable to whatever file transfer	proto-
	    col	the client can be expected to use to load the file.

	 The fixed-address declaration

	    fixed-address address [, address ... ];

	    The	 fixed-address declaration is used to assign one or more fixed
	    IP addresses to a client.  It should only appear in	a host	decla-
	    ration.   If  more	than  one  address  is supplied, then when the
	    client boots, it will be assigned the address that corresponds  to
	    the	 network  on which it is booting.  If none of the addresses in
	    the	fixed-address statement	are valid for the network to which the
	    client  is connected, that client will not match the host declara-
	    tion containing that fixed-address declaration.  Each  address  in
	    the	 fixed-address declaration should be either an IP address or a
	    domain name	that resolves to one or	more IP	addresses.

	 The fixed-address6 declaration

	    fixed-address6 ip6-address ;

	    The	fixed-address6 declaration is used to assign a fixed IPv6  ad-
	    dresses to a client.  It should only appear	in a host declaration.

	 The fixed-prefix6 declaration

	    fixed-prefix6 low-address /	bits;

	    The	 fixed-prefix6 declaration is used to assign a fixed IPv6 pre-
	    fix	to a client.  It should	only appear in a host declaration, but
	    multiple fixed-prefix6 statements may appear in a single host dec-
	    laration.

	    The	low-address specifies the start	of the	prefix	and  the  bits
	    specifies the size of the prefix in	bits.

	    If	there  are multiple prefixes for a given host entry the	server
	    will choose	one that matches the requested prefix size or, if none
	    match, the first one.

	    If	there  are  multiple  host declarations	the server will	try to
	    choose a declaration where the fixed-address6 matches the client's
	    subnet.   If  none	match  it  will	choose one that	doesn't	have a
	    fixed-address6 statement.

	    Note Well: Unlike the fixed	address	the fixed prefix does not need
	    to	match a	subnet in order	to be served.  This allows you to pro-
	    vide a prefix to a client that is outside of the subnet  on	 which
	    the	client makes the request to the	the server.

	 The get-lease-hostnames statement

	    get-lease-hostnames	flag;

	    The	get-lease-hostnames statement is used to tell dhcpd whether or
	    not	to look	up the domain name corresponding to the	IP address  of
	    each  address  in the lease	pool and use that address for the DHCP
	    hostname option.  If flag is true, then this lookup	 is  done  for
	    all	 addresses  in	the  current scope.  By	default, or if flag is
	    false, no lookups are done.

	 The hardware statement

	    hardware hardware-type hardware-address;

	    In order for a BOOTP client	to be recognized, its network hardware
	    address  must  be  declared	 using	a  hardware clause in the host
	    statement.	hardware-type must be the name of a physical  hardware
	    interface type.  Currently,	only the ethernet and token-ring types
	    are	recognized, although support for a  fddi  hardware  type  (and
	    others) would also be desirable.  The hardware-address should be a
	    set	of hexadecimal octets (numbers from 0 through ff) separated by
	    colons.  The hardware statement may	also be	used for DHCP clients.

	 The host-identifier option statement

	    host-identifier option option-name option-data;

	    or

	    host-identifier v6relopt number option-name	option-data;

	    This  identifies a DHCPv6 client in	a host statement.  option-name
	    is any option, and option-data is the value	for  the  option  that
	    the	 client	 will  send. The option-data must be a constant	value.
	    In the v6relopts case the additional number	is the relay to	 exam-
	    ine	 for  the specified option name	and value.  The	values are the
	    same as for	the v6relay option.  0 is a  no-op,  1	is  the	 relay
	    closest  to	 the client, 2 the next	one in and so on.  Values that
	    are	larger than the	maximum	number of relays (currently 32)	 indi-
	    cate the relay closest to the server independent of	number.

	 The ignore-client-uids	statement

	    ignore-client-uids flag;

	    If	the ignore-client-uids statement is present and	has a value of
	    true or on,	the UID	for clients will not  be  recorded.   If  this
	    statement  is  not	present	 or  has a value of false or off, then
	    client UIDs	will be	recorded.

	 The infinite-is-reserved statement

	    infinite-is-reserved flag;

	    ISC	DHCP now supports 'reserved' leases.  See the section  on  RE-
	    SERVED LEASES below.  If this flag is on, the server will automat-
	    ically reserve leases allocated to clients which requested an  in-
	    finite (0xffffffff)	lease-time.

	    The	default	is off.

	 The lease-file-name statement

	    lease-file-name name;

	    Name  Where	 name  is the name of the DHCP server's	lease file. By
	    default, this is DBDIR/dhcpd.leases.  This statement  must	appear
	    in	the  outer  scope of the configuration file - if it appears in
	    some other scope, it will have no effect.  The value must  be  the
	    absolute  path  of	the  file to use.  The order of	precedence the
	    server uses	for the	lease file name	is:

		1. lease-file-name configuration file statement.
		2. -lf command line flag.
		3. PATH_DHCPD_DB environment variable.

	 The dhcpv6-lease-file-name statement

	    dhcpv6-lease-file-name name;

	    Where name is the name of the DHCP server's	lease  file  when  the
	    server is running DHCPv6. By default, this is DBDIR/dhcpd6.leases.
	    This statement must	appear in the outer scope of the configuration
	    file  - if it appears in some other	scope, it will have no effect.
	    The	value must be the absolute path	of the file to use.  The order
	    of precedence the server uses for the lease	file name is:

		1. dhcpv6-lease-file-name configuration	file statement.
		2. -lf command line flag.
		3. PATH_DHCPD6_DB environment variable.

	 The lease-id-format parameter

	    lease-id-format format;

	    The	 format	parameter must be either octal or hex.	This parameter
	    governs the	format used to write certain values  to	 lease	files.
	    With  the  default	format,	 octal,	 values	 are written as	quoted
	    strings in which non-printable characters are represented as octal
	    escapes  -	a  backslash character followed	by three octal digits.
	    When the hex format	is specified, values are  written  as  an  un-
	    quoted series of pairs of hexadecimal digits, separated by colons.

	    Currently, the values written out based on lease-id-format are the
	    server-duid, the uid (DHCPv4 leases), and  the  IAID_DUID  (DHCPv6
	    leases).  Note the server automatically reads the values in	either
	    format.

	 The limit-addrs-per-ia	statement

	    limit-addrs-per-ia number;

	    By default,	the DHCPv6 server will limit clients to	one IAADDR per
	    IA	option,	meaning	one address.  If you wish to permit clients to
	    hang onto multiple addresses at a time, configure a	larger	number
	    here.

	    Note  that	there  is no present method to configure the server to
	    forcibly configure the client with one IP address per each	subnet
	    on a shared	network.  This is left to future work.

	 The local-port	statement

	    local-port port;

	    This  statement causes the DHCP server to listen for DHCP requests
	    on the UDP port specified in port, rather than on port 67.

	 The local-address statement

	    local-address address;

	    This statement causes the DHCP server to listen for	DHCP  requests
	    sent  to  the  specified address, rather than requests sent	to all
	    addresses.	Since serving directly attached	DHCP  clients  implies
	    that  the  server must respond to requests sent to the all-ones IP
	    address, this option cannot	be used	if clients are on directly at-
	    tached  networks;  it  is  only  realistically useful for a	server
	    whose only clients are reached via unicasts, such as via DHCP  re-
	    lay	agents.

	    Note:  This	statement is only effective if the server was compiled
	    using the USE_SOCKETS #define statement, which  is	default	 on  a
	    small  number  of operating	systems, and must be explicitly	chosen
	    at compile-time for	all others.  You can be	sure if	your server is
	    compiled  with  USE_SOCKETS	 if  you  see  lines of	this format at
	    startup:

	     Listening on Socket/eth0

	    Note also that since this bind()s all DHCP sockets to  the	speci-
	    fied  address,  that only one address may be supported in a	daemon
	    at a given time.

	 The local-address6 and	bind-local-address6 statements

	    local-address6 address;

	    bind-local-address6	flag;

	    The	local-address6 statement causes	the DHCP server	to  send  IPv6
	    packets  as	 originating  from  the	specified IPv6 address,	rather
	    than leaving the kernel to fill in the source address field.

	    When bind-local-address6 is	present	and has	a value	of true	or on,
	    service sockets are	bound to address too.

	    By default address is the undefined	address	and the	bind-local-ad-
	    dress6 is disabled,	both may only be set at	the global scope.

	 The log-facility statement

	    log-facility facility;

	    This statement causes the DHCP server to do	all of its logging  on
	    the	specified log facility once the	dhcpd.conf file	has been read.
	    By default the DHCP	server logs to the daemon facility.   Possible
	    log	 facilities  include  auth, authpriv, cron, daemon, ftp, kern,
	    lpr, mail, mark, news, ntp,	security, syslog, user,	uucp, and  lo-
	    cal0 through local7.  Not all of these facilities are available on
	    all	systems, and there may be other	facilities available on	 other
	    systems.

	    In	addition  to  setting  this value, you may need	to modify your
	    syslog.conf	file to	configure logging of the DHCP server.  For ex-
	    ample, you might add a line	like this:

		 local7.debug /var/log/dhcpd.log

	    The	 syntax	of the syslog.conf file	may be different on some oper-
	    ating systems - consult the	syslog.conf manual page	 to  be	 sure.
	    To	get  syslog  to	 start logging to the new file,	you must first
	    create the file with correct ownership and	permissions  (usually,
	    the	 same  owner  and  permissions	of  your  /var/log/messages or
	    /usr/adm/messages file should be fine) and send a SIGHUP  to  sys-
	    logd.   Some  systems support log rollover using a shell script or
	    program called newsyslog or	logrotate, and you may be able to con-
	    figure  this as well so that your log file doesn't grow uncontrol-
	    lably.

	    Because the	log-facility setting is	controlled by  the  dhcpd.conf
	    file,  log	messages  printed while	parsing	the dhcpd.conf file or
	    before parsing it are logged to the	default	log facility.  To pre-
	    vent  this,	 see  the README file included with this distribution,
	    which describes BUG: where is that mentioned in  README?   how  to
	    change the default log facility.  When this	parameter is used, the
	    DHCP server	prints its startup message a second time after parsing
	    the	 configuration	file,  so  that	the log	will be	as complete as
	    possible.

	 The log-threshold-high	and log-threshold-low statements

	    log-threshold-high percentage;

	    log-threshold-low percentage;

	    The	log-threshold-low and log-threshold-high statements  are  used
	    to	control	 when a	message	is output about	pool usage.  The value
	    for	both of	them is	the percentage of the pool  in	use.   If  the
	    high threshold is 0	or has not been	specified, no messages will be
	    produced.  If a high threshold is given, a message is output  once
	    the	 pool  usage  passes that level.  After	that, no more messages
	    will be output until the pool usage	falls below the	low threshold.
	    If the low threshold is not	given, it default to a value of	zero.

	    A  special	case  occurs when the low threshold is set to be higer
	    than the high threshold.  In this case, a message will  be	gener-
	    ated  each	time  a	 lease	is acknowledged	when the pool usage is
	    above the high threshold.

	    Note that threshold	logging	will  be  automatically	 disabled  for
	    shared  subnets  whose  total  number  of addresses	is larger than
	    (2^64)-1.  The server will emit a log statement  at	 startup  when
	    threshold logging is disabled as shown below:

		"Threshold  logging disabled for shared	subnet of ranges: <ad-
	    dresses>"

	    This is likely to have no practical	runtime	effect as CPUs are un-
	    likely  to	support	a server actually reaching such	a large	number
	    of leases.

	 The max-lease-time statement

	    max-lease-time time;

	    Time should	be the maximum length in seconds that will be assigned
	    to	a  lease.   If	not defined, the default maximum lease time is
	    86400.  The	only exception to this is  that	 Dynamic  BOOTP	 lease
	    lengths, which are not specified by	the client, are	not limited by
	    this maximum.

	 The min-lease-time statement

	    min-lease-time time;

	    Time should	be the minimum length in seconds that will be assigned
	    to	a  lease.   The	 default is the	minimum	of 300 seconds or max-
	    lease-time.

	 The min-secs statement

	    min-secs seconds;

	    Seconds should be the minimum number of seconds since a client be-
	    gan	 trying	to acquire a new lease before the DHCP server will re-
	    spond to its request.  The number of seconds is based on what  the
	    client  reports,  and the maximum value that the client can	report
	    is 255 seconds.  Generally,	setting	this to	one will result	in the
	    DHCP  server not responding	to the client's	first request, but al-
	    ways responding to its second request.

	    This can be	used to	set up a secondary DHCP	server which never of-
	    fers  an  address  to  a  client until the primary server has been
	    given a chance to do so.  If  the  primary	server	is  down,  the
	    client  will  bind	to the secondary server, but otherwise clients
	    should always bind to the primary.	Note that this	does  not,  by
	    itself,  permit a primary server and a secondary server to share a
	    pool of dynamically-allocatable addresses.

	 The next-server statement

	    next-server	server-name;

	    The	next-server statement is used to specify the host  address  of
	    the	 server	 from  which  the  initial boot	file (specified	in the
	    filename statement)	is to be loaded.  Server-name should be	a  nu-
	    meric IP address or	a domain name.

	 The omapi-port	statement

	    omapi-port port;

	    The	 omapi-port  statement	causes	the  DHCP server to listen for
	    OMAPI connections on the specified port.  This  statement  is  re-
	    quired  to enable the OMAPI	protocol, which	is used	to examine and
	    modify the state of	the DHCP server	as it is running.

	 The one-lease-per-client statement

	    one-lease-per-client flag;

	    If this flag is enabled, whenever a	client sends a DHCPREQUEST for
	    a  particular  lease, the server will automatically	free any other
	    leases the client holds.  This presumes that when the client sends
	    a  DHCPREQUEST,  it	 has  forgotten	any lease not mentioned	in the
	    DHCPREQUEST	- i.e.,	the client has only a single network interface
	    and	 it does not remember leases it's holding on networks to which
	    it is not currently	attached.  Neither of  these  assumptions  are
	    guaranteed	or  provable,  so  we  urge caution in the use of this
	    statement.

	 The persist-eui-64-leases statement

	    persist-eui-64-leases flag;

	    When this flag is enabled, the  server  will  write	 EUI-64	 based
	    leases  to	the  leases  file. Since such leases can only, ever be
	    valid for a	single DUID value it can be argued that	 writing  them
	    to	the leases file	isn't essential	and not	doing so may have per-
	    fomance advantages.	 See use-eui-64	statement for more details  on
	    EUI-64  based  address allocation.	The flag is enabled by default
	    and	may only be set	at the global scope.

	 The pid-file-name statement

	    pid-file-name name;

	    Name should	be the name of the  DHCP  server's  process  ID	 file.
	    This  is  the file in which	the DHCP server's process ID is	stored
	    when the server starts.  By	 default,  this	 is  RUNDIR/dhcpd.pid.
	    Like  the lease-file-name statement, this statement	must appear in
	    the	outer scope of the configuration file. The order of precedence
	    used by the	server is:

		1. pid-file-name configuration file statement.
		2. -lf command line flag.
		3. PATH_DHCPD_PID environment variable.

	    The	dhcpv6-pid-file-name statement

	      dhcpv6-pid-file-name name;

	      Name  is	the  name  of  the  pid	file to	use if and only	if the
	      server is	running	in DHCPv6  mode.   By  default,	 this  is  DB-
	      DIR/dhcpd6.pid.  This statement, like pid-file-name, must	appear
	      in the outer scope of the	 configuration	file.	The  order  of
	      precedence used by the server is:

		  1. dhcpv6-pid-file-name configuration	file statement.
		  2. -lf command line flag.
		  3. PATH_DHCPD6_PID environment variable.

	    The	ping-check statement

	      ping-check flag;

	      When the DHCP server is considering dynamically allocating an IP
	      address to a client, it first sends  an  ICMP  Echo  request  (a
	      ping) to the address being assigned.  It waits for a second, and
	      if no ICMP Echo response has been	heard, it assigns the address.
	      If  a  response is heard,	the lease is abandoned,	and the	server
	      does not respond to the client.  The lease will remain abandoned
	      for a minimum of abandon-lease-time seconds.

	      If a there are no	free addressses	but there are abandoned	IP ad-
	      dresses, the DHCP	server will attempt to reclaim an abandoned IP
	      address regardless of the	value of abandon-lease-time.

	      This  ping  check	 introduces  a default one-second delay	in re-
	      sponding to DHCPDISCOVER messages, which can be  a  problem  for
	      some clients.  The default delay of one second may be configured
	      using the	ping-timeout parameter.	 The ping-check	 configuration
	      parameter	 can  be  used	to  control checking - if its value is
	      false, no	ping check is done.

	    The	ping-cltt-secs statement

	      ping-cltt-secs seconds;

	      The server will conduct a	ping check if all the following	condi-
	      tions are	true:

	      1. Ping checking is enabled.

	      2. The server is responding to a DISCOVER.

	      3.  The  lease  to be offered is neither static nor active (i.e.
	      still a valid lease).

	      4. And any of the	following are true:
		  a. This will be the first offer of this lease	(CLTT is 0).
		  b. The lease is being	offered	to a  client  other  than  its
	      previous owner
		  c. The lease is being	offered	to its previous	owner and more
	      than
		  ping-cltt-secs have  elapsed	since  CLTT  of	 the  original
	      lease.
		  d.  The  lease was abandoned and the server is attempting to
	      reclaim it.

	      The ping-cltt-secs statement allows  the	user  to  specify  the
	      amount  of  time that must elaspe	after CLTT before a ping check
	      will be conducted.  The default value is sixty seconds.

	    The	ping-timeout statement

	      ping-timeout seconds;

	      If the DHCP server determined it should send an  ICMP  echo  re-
	      quest  (a	 ping) because the ping-check statement	is true, ping-
	      timeout allows you to configure how many seconds the DHCP	server
	      should  wait  for	 an ICMP Echo response to be heard, if no ICMP
	      Echo response has	been received before the timeout  expires,  it
	      assigns the address.  If a response is heard, the	lease is aban-
	      doned, and the server does not respond to	 the  client.	If  no
	      value is set, ping-timeout defaults to 1 second. (See also ping-
	      timeout-ms below)

	    The	ping-timeout-ms	statement

	      ping-timeout-ms milliseconds;

	      Allows you to specify the	ping timeout  in  milliseconds	rather
	      than  seconds.   If  this	value is greater than zero, the	server
	      will use it in place of  ping-timeout.   The  default  value  is
	      zero.

	    The	preferred-lifetime statement

	      preferred-lifetime seconds;

	      IPv6  addresses  have  'valid'  and  'preferred' lifetimes.  The
	      valid lifetime determines	at what	point at lease might  be  said
	      to have expired, and is no longer	useable.  A preferred lifetime
	      is an advisory condition to help applications move  off  of  the
	      address  and  onto currently valid addresses (should there still
	      be any open TCP sockets or similar).

	      The preferred lifetime defaults to 5/8 the default lease time.

	    The	prefix-length-mode statement

	      prefix-length-mode mode;

	      According	to RFC 3633, DHCPv6 clients  may  specify  preferences
	      when  soliciting	prefixes  by  including	an IA_PD Prefix	option
	      within the IA_PD option. Among the preferences that may be  con-
	      veyed  is	 the  "prefix-length".	When  non-zero	it indicates a
	      client's desired length for offered prefixes.   The  RFC	states
	      that servers "MAY	choose to use the information...to select pre-
	      fix(es)" but does	not specify any	particular rules for doing so.
	      The  prefix-length-mode  statement can be	used to	set the	prefix
	      selection	rules employed by the server, when clients send	a non-
	      zero  prefix-length value. The mode parameter must be one	of ig-
	      nore, prefer, exact, minimum, or maximum where:

	      1. ignore	- The requested	length is ignored. The server will of-
	      fer the first available prefix.

	      2.  prefer  -  The  server will offer the	first available	prefix
	      with the same length as the requested length.  If	none are found
	      then  it	will  offer  the first available prefix	of any length.
	      This is the default behavior.

	      3. exact - The server will offer the first available prefix with
	      the  same	length as the requested	length.	 If none are found, it
	      will return a status indicating no prefixes available.

	      4. minimum - The server will offer the  first  available	prefix
	      with  the	 same  length  as  the	requested length.  If none are
	      found, it	will return the	first available	prefix whose length is
	      greater  than  (e.g. longer than), the requested value.  If none
	      of those are found, it will return a status indicating  no  pre-
	      fixes  available.	  For  example,	if client requests a length of
	      /60, and the server has available	prefixes of  lengths  /56  and
	      /64, it will offer prefix	of length /64.

	      5.  maximum  -  The server will offer the	first available	prefix
	      with the same length as  the  requested  length.	 If  none  are
	      found, it	will return the	first available	prefix whose length is
	      less than	(e.g. shorter than), the requested value.  If none  of
	      those  are found,	it will	return a status	indicating no prefixes
	      available.  For example, if client requests a length of /60, and
	      the  server  has	available  prefixes of lengths /56 and /64, it
	      will offer a prefix of length /56.

	      In general "first	available" is determined by the	order in which
	      pools  are  defined in the server's configuration.  For example,
	      if a subnet is defined with three	prefix pools A,B, and C:

	      subnet 3000::/64 {
		   # pool A
		   pool6 {
			:
		   }
		   # pool B
		   pool6 {
			:
		   }
		   # pool C
		   pool6 {
			:
		   }
	      }

	      then the pools will be checked in	the order A, B,	C.  For	 modes
	      prefer, minimum, and maximum this	may mean checking the pools in
	      that order twice.	 A first pass through is made looking  for  an
	      available	 prefix	 of exactly the	preferred length.  If none are
	      found, then a second pass	is performed starting with pool	A  but
	      with appropriately adjusted length criteria.

	    The	release-on-roam	statement

	      release-on-roam flag;

	      When  enabled  and the dhcpd server detects that a DHCPv6	client
	      (IAID+DUID) has roamed to	a new network,	it  will  release  the
	      pre-existing  leases on the old network and emit a log statement
	      similiar to the following:

		    "Client: <id> roamed to new	network, releasing lease: <ad-
	      dress>"

	      The  server will carry out all of	the same steps that would nor-
	      mally occur when a client	explicitly releases a lease.  When re-
	      lease-on-roam  is	 disabled  (the	default) the server makes such
	      leases unavailable until they expire or the server is restarted.
	      Clients  that  need  leases  in  multiple	networks must supply a
	      unique IAID in each IA.  This parameter may only be specified at
	      the global level.

	    The	remote-port statement

	      remote-port port;

	      This statement causes the	DHCP server to transmit	DHCP responses
	      to DHCP clients upon the UDP port	specified in port, rather than
	      on  port	68.  In	the event that the UDP response	is transmitted
	      to a DHCP	Relay, the server generally uses the  local-port  con-
	      figuration  value.  Should the DHCP Relay	happen to be addressed
	      as 127.0.0.1, however, the DHCP Server transmits its response to
	      the  remote-port	configuration  value.	This is	generally only
	      useful for testing purposes, and this configuration value	should
	      generally	not be used.

	    The	server-identifier statement

	      server-identifier	hostname;

	      The  server-identifier statement can be used to define the value
	      that is sent in the DHCP Server Identifier option	 for  a	 given
	      scope.   The  value specified must be an IP address for the DHCP
	      server, and must be reachable by all clients served by a partic-
	      ular scope.

	      The  use of the server-identifier	statement is not recommended -
	      the only reason to use it	is to force a value other than the de-
	      fault  value  to	be  sent  on occasions where the default value
	      would be incorrect.  The default value is	the first  IP  address
	      associated  with the physical network interface on which the re-
	      quest arrived.

	      The usual	case where the server-identifier statement needs to be
	      sent  is when a physical interface has more than one IP address,
	      and the one being	sent by	default	isn't appropriate for some  or
	      all  clients  served  by that interface.	Another	common case is
	      when an alias is defined for the purpose of having a  consistent
	      IP  address  for	the  DHCP  server,  and	it is desired that the
	      clients use this IP address when contacting the server.

	      Supplying	a  value  for  the  dhcp-server-identifier  option  is
	      equivalent to using the server-identifier	statement.

	    The	server-id-check	statement

	      server-id-check flag;

	      The  server-id-check statement is	used to	control	whether	or not
	      a	server,	participating in failover, verifies that the value  of
	      the  dhcp-server-identifier  option  in  received	 DHCP REQUESTs
	      match the	server's id before processing the request.  Server  id
	      checking	is  disabled by	default.  Setting this flag enables id
	      checking and thereafter the server will  only  process  requests
	      that  match.  Note the flag setting should be consistent between
	      failover partners.

	      Unless overridden	by use of the server-identifier	statement, the
	      value the	server uses as its id will be the first	IP address as-
	      sociated with the	physical network interface on  which  the  re-
	      quest arrived.

	      In order to reduce runtime overhead the server only checks for a
	      server id	option in the global and subnet	 scopes.   Complicated
	      configurations may result	in different server ids	for this check
	      and when the server id for a reply packet	is  determined,	 which
	      would prohibit the server	from responding.

	      The  primary  use	 for this option is when a client broadcasts a
	      request but requires that	the  response  come  from  a  specific
	      failover	peer.	An  example of this would be when a client re-
	      boots while its lease is	still  active  -  in  this  case  both
	      servers  will  normally  respond.	  Most	of the time the	client
	      won't check the server id	and can	use either of  the  responses.
	      However if the client does check the server id it	may reject the
	      response if it came from the wrong peer.	If the timing is  such
	      that the "wrong" peer responds first most	of the time the	client
	      may not get an address for some time.

	      Care should be taken before enabling this	option.

	    The	server-duid statement

	      server-duid LLT [	hardware-type timestamp	hardware-address ] ;

	      server-duid EN enterprise-number enterprise-identifier ;

	      server-duid LL [ hardware-type hardware-address ]	;

	      The server-duid statement	configures the server  DUID.  You  may
	      pick either LLT (link local address plus time), EN (enterprise),
	      or LL (link local).

	      If you choose LLT	or LL, you may specify the exact  contents  of
	      the  DUID.   Otherwise  the  server  will	generate a DUID	of the
	      specified	type.

	      If you choose EN,	you must include the enterprise	number and the
	      enterprise-identifier.

	      If  there	 is  a server-duid statement in	the lease file it will
	      take precedence over the server-duid statement from  the	config
	      file  and	a dhcp6.server-id option in the	config file will over-
	      ride both.

	      The default server-duid type is LLT.

	    The	server-name statement

	      server-name name ;

	      The server-name statement	can be used to inform  the  client  of
	      the name of the server from which	it is booting.	Name should be
	      the name that will be provided to	the client.

	    The	dhcpv6-set-tee-times statement

	      dhcpv6-set-tee-times flag;

	      The dhcpv6-set-tee-times statement enables setting T1 and	T2  to
	      the values recommended in	RFC 3315 (Section 22.4).  When setting
	      T1 and T2, the server will use  dhcp-renewal-time	 and  dhcp-re-
	      binding-time, respectively.  A value of zero tells the client it
	      may choose its own value.

	      When those options are not defined then values will  be  set  to
	      zero  unless  the	 global	dhcpv6-set-tee-times is	enabled.  When
	      this option is enabled the times are calculated  as  recommended
	      by RFC 3315, Section 22.4:

		    T1	will  be set to	0.5 times the shortest preferred life-
	      time
		    in the reply.  If the "shortest" preferred lifetime	is
		    0xFFFFFFFF,	 T1 will set to	0xFFFFFFFF.

		    T2 will be set to 0.8 times	the shortest  preferred	 life-
	      time
		    in the reply.  If the "shortest" preferred lifetime	is
		    0xFFFFFFFF,	 T2 will set to	0xFFFFFFFF.

	      Keep  in mind that given sufficiently small lease	lifetimes, the
	      above calculations will result in	the two	 values	 being	equal.
	      For  example,  a 9 second	lease lifetime would yield T1 =	T2 = 4
	      seconds, which would cause clients to issue  rebinds  only.   In
	      such  a  case it would likely be better to explicitly define the
	      values.

	      Note that	dhcpv6-set-tee-times is	intended  to  be  transitional
	      and will likely be removed in a future release. Once removed the
	      behavior will be to use the configured values  when  present  or
	      calculate	 them  per  the	RFC. If	you want zeros,	define them as
	      zeros.

	    The	site-option-space statement

	      site-option-space	name ;

	      The site-option-space statement can be used  to  determine  from
	      what option space	site-local options will	be taken.  This	can be
	      used in much the same way	as the vendor-option-space  statement.
	      Site-local options in DHCP are those options whose numeric codes
	      are greater than 224.  These options are intended	for  site-spe-
	      cific uses, but are frequently used by vendors of	embedded hard-
	      ware that	contains DHCP clients.	Because	site-specific  options
	      are  allocated on	an ad hoc basis, it is quite possible that one
	      vendor's DHCP client might use the same option code that another
	      vendor's	client uses, for different purposes.  The site-option-
	      space option can be used to assign a different set of  site-spe-
	      cific options for	each such vendor, using	conditional evaluation
	      (see dhcp-eval (5) for details).

	    The	stash-agent-options statement

	      stash-agent-options flag;

	      If the stash-agent-options parameter is true for a given client,
	      the  server will record the relay	agent information options sent
	      during the client's initial DHCPREQUEST message when the	client
	      was  in  the  SELECTING state and	behave as if those options are
	      included in all subsequent DHCPREQUEST messages sent in the  RE-
	      NEWING  state.  This works around	a problem with relay agent in-
	      formation	options, which is that	they  usually  not  appear  in
	      DHCPREQUEST  messages  sent by the client	in the RENEWING	state,
	      because such messages are	unicast	directly to the	server and not
	      sent through a relay agent.

	    The	update-conflict-detection statement

	      update-conflict-detection	flag;

	      If  the  update-conflict-detection parameter is true, the	server
	      will perform standard DHCID multiple-client,  one-name  conflict
	      detection.  If the parameter has been set	false, the server will
	      skip this	check and instead simply tear down any previous	 bind-
	      ings  to	install	the new	binding	without	question.  The default
	      is true and this parameter may only be specified at  the	global
	      scope.

	    The	update-optimization statement

	      update-optimization flag;

	      If  the  update-optimization  parameter  is  false  for  a given
	      client, the server will attempt a	DNS  update  for  that	client
	      each time	the client renews its lease, rather than only attempt-
	      ing an update when it appears to be necessary.  This will	 allow
	      the  DNS	to heal	from database inconsistencies more easily, but
	      the cost is that the DHCP	server must do many more DNS  updates.
	      We  recommend leaving this option	enabled, which is the default.
	      If this parameter	is not specified, or is	true, the DHCP	server
	      will only	update when the	client information changes, the	client
	      gets a different lease, or the client's lease expires.

	    The	update-static-leases statement

	      update-static-leases flag;

	      The update-static-leases	flag,  if  enabled,  causes  the  DHCP
	      server  to  do DNS updates for clients even if those clients are
	      being assigned their IP address using a fixed-address or	fixed-
	      address6 statement - that	is, the	client is being	given a	static
	      assignment.  It is not recommended because the DHCP  server  has
	      no way to	tell that the update has been done, and	therefore will
	      not delete the record when it is not in use.  Also,  the	server
	      must  attempt  the update	each time the client renews its	lease,
	      which could have a significant performance  impact  in  environ-
	      ments that place heavy demands on	the DHCP server.  This feature
	      is supported for both DHCPv4 and DHCPv6, and update modes	 stan-
	      dard or interim. It is disabled by default.

	    The	use-eui-64 statement

	      use-eui-64 flag;

	      (Support for this	must be	enabled	at compile time, see EUI_64 in
	       includes/site.h)

	      The  use-eui-64  flag,  if enabled, instructs the	server to con-
	      struct an	address	using the client's EUI-64  DUID	 (Type	3,  HW
	      Type  EUI-64), rather than creating an address using the dynamic
	      algorithm.  This means that a given DUID	will  always  generate
	      the  same	 address for a given pool and further that the address
	      is guaranteed to be unique to that DUID.	The IPv6 address  will
	      be  calculated from the EUI-64 link layer	address, conforming to
	      RFC 2373,	unless there is	a host declaration for the client-id.

	      The range6 statement for EUI-64 must define full /64 bit ranges.
	      Invalid  ranges  will be flagged during configuration parsing as
	      errors.  See the following example:

		  subnet6 fc00:e4::/64 {
		      use-eui-64 true;
		      range6 fc00:e4::/64;
		  }

	      The statement may	be specified down to the pool level,  allowing
	      a	mixture	of dynamic and EUI-64 based pools.

	      During  lease  file  parsing,  any leases	which map to an	EUI-64
	      pool, that have a	non-EUI-64 DUID	or for which the lease address
	      is  not  the  EUI-64 address for that DUID in that pool, will be
	      discarded.

	      If a host	declaration exists for the DUID, the server grants the
	      address  (fixed-prefix6,	fixed-address6)	 according to the host
	      declaration, regardless of the DUID type of the client (even for
	      EUI-64 DUIDs).

	      If  a  client request's an EUI-64	lease for a given network, and
	      the resultant address conflicts with a  fixed  address  reserva-
	      tion, the	server will send the client a "no addresses available"
	      response.

	      Any client with a	non-conforming DUID (not type 3	or not hw type
	      EUI-64) that is not linked to a host declaration,	which requests
	      an address from an EUI-64	enabled	pool will be ignored  and  the
	      event will be logged.

	      Pools that are configured	for EUI-64 will	be skipped for dynamic
	      allocation.  If there are	no pools in the	 shared	 network  from
	      which  to	 allocate,  the	 client	 will  get back	a no addresses
	      available	status.

	      On an EUI-64 enabled pool, any client with a  DUID  3,  HW  Type
	      EUI-64,  requesting  a solicit/renew and including IA_NA that do
	      not match	the EUI-64 policy, they	will be	treated	as though they
	      are "outside" the	subnet for a given client message:

		  Solicit  -  Server  will advertise with EUI-64 ia suboption,
	      but with rapid
		  commit off
		  Request - Server will	send "an address not on	link  status",
	      and no ia
		  suboption  Renew/Rebind - Server will	send the requested ad-
	      dress ia
		  suboption with lifetimes of 0, plus an EUI-64	ia

	      Whether or not  EUI-64 based leases are written out to the lease
	      database may be controlled by persist-eui-64-leases statement.

	    The	use-host-decl-names statement

	      use-host-decl-names flag;

	      If  the  use-host-decl-names parameter is	true in	a given	scope,
	      then for every host declaration within that scope, the name pro-
	      vided for	the host declaration will be supplied to the client as
	      its hostname.  So, for example,

		  group	{
		    use-host-decl-names	on;

		    host joe {
		      hardware ethernet	08:00:2b:4c:29:32;
		      fixed-address joe.example.com;
		    }
		  }

	      is equivalent to

		    host joe {
		      hardware ethernet	08:00:2b:4c:29:32;
		      fixed-address joe.example.com;
		      option host-name "joe";
		    }

	      Additionally, enabling use-host-decl-names instructs the	server
	      to use the host declaration name in the the forward DNS name, if
	      no other values are available.  This value selection process  is
	      discussed	in more	detail under DNS updates.

	      An  option  host-name  statement	within a host declaration will
	      override the use of the name in the host declaration.

	      It should	be noted here that most	DHCP clients completely	ignore
	      the  host-name  option  sent by the DHCP server, and there is no
	      way to configure them not	to do this.  So	you generally  have  a
	      choice  of  either  not having any hostname to client IP address
	      mapping that the client will recognize, or  doing	 DNS  updates.
	      It  is beyond the	scope of this document to describe how to make
	      this determination.

	    The	use-lease-addr-for-default-route statement

	      use-lease-addr-for-default-route flag;

	      If the use-lease-addr-for-default-route parameter	is true	 in  a
	      given  scope, then instead of sending the	value specified	in the
	      routers option (or sending no value at all), the IP  address  of
	      the lease	being assigned is sent to the client.  This supposedly
	      causes Win95 machines to ARP for all IP addresses, which can  be
	      helpful  if your router is configured for	proxy ARP.  The	use of
	      this feature is not recommended, because it won't	work for  many
	      DHCP clients.

	    The	vendor-option-space statement

	      vendor-option-space string;

	      The  vendor-option-space	parameter  determines from what	option
	      space vendor options are taken.  The use of  this	 configuration
	      parameter	 is illustrated	in the dhcp-options(5) manual page, in
	      the VENDOR ENCAPSULATED OPTIONS section.

SETTING	PARAMETER VALUES USING EXPRESSIONS
       Sometimes it's helpful to be able to set	the value of a DHCP server pa-
       rameter	based on some value that the client has	sent.  To do this, you
       can use expression evaluation.  The dhcp-eval(5)	manual page  describes
       how  to write expressions.  To assign the result	of an evaluation to an
       option, define the option as follows:

	 my-parameter =	expression ;

       For example:

	 ddns-hostname = binary-to-ascii (16, 8, "-",
					  substring (hardware, 1, 6));

RESERVED LEASES
       It's often useful to allocate a single address to a single  client,  in
       approximate perpetuity.	Host statements	with fixed-address clauses ex-
       ist to a	certain	extent to serve	this purpose, but because host	state-
       ments  are  intended to approximate 'static configuration', they	suffer
       from not	being referenced in a littany of other Server  Services,  such
       as dynamic DNS, failover, 'on events' and so forth.

       If  a  standard	dynamic	 lease,	as from	any range statement, is	marked
       'reserved', then	the server will	only allocate this lease to the	client
       it is identified	by (be that by client identifier or hardware address).

       In practice, this means that the	lease follows the normal state engine,
       enters ACTIVE state when	the client is bound to it, expires, or is  re-
       leased, and any events or services that would normally be supplied dur-
       ing these events	are processed normally,	 as  with  any	other  dynamic
       lease.	The  only  difference  is that failover	servers	treat reserved
       leases as special when they enter the FREE  or  BACKUP  states  -  each
       server  applies the lease into the state	it may allocate	from - and the
       leases are not placed on	the queue for  allocation  to  other  clients.
       Instead	they  may  only	 be 'found' by client identity.	 The result is
       that the	lease is only offered to the returning client.

       Care should probably be taken to	ensure that the	client	only  has  one
       lease within a given subnet that	it is identified by.

       Leases  may be set 'reserved' either through OMAPI, or through the 'in-
       finite-is-reserved' configuration option	(if this is applicable to your
       environment and mixture of clients).

       It  should  also	be noted that leases marked 'reserved' are effectively
       treated the same	as leases marked 'bootp'.

REFERENCE: OPTION STATEMENTS
       DHCP option statements are documented  in  the  dhcp-options(5)	manual
       page.

REFERENCE: EXPRESSIONS
       Expressions used	in DHCP	option statements and elsewhere	are documented
       in the dhcp-eval(5) manual page.

SEE ALSO
       dhcpd(8),  dhcpd.leases(5),  dhcp-options(5),  dhcp-eval(5),   RFC2132,
       RFC2131.

AUTHOR
       dhcpd.conf(5) is	maintained by ISC.  Information	about Internet Systems
       Consortium can be found at https://www.isc.org.

								 dhcpd.conf(5)

NAME | DESCRIPTION | EXAMPLES | ADDRESS POOLS | DYNAMIC ADDRESS ALLOCATION | IP ADDRESS CONFLICT PREVENTION | DHCP FAILOVER | FAILOVER STARTUP | CONFIGURING FAILOVER | CLIENT CLASSING | SUBCLASSES | PER-CLASS LIMITS ON DYNAMIC ADDRESS ALLOCATION | SPAWNING CLASSES | COMBINING MATCH, MATCH IF AND SPAWN WITH | DYNAMIC DNS UPDATES | THE DNS UPDATE SCHEME | DDNS IN DUAL STACK ENVIRONMENTS | PROTECTING DNS ENTRIES FOR STATIC CLIENTS | DYNAMIC DNS UPDATE SECURITY | REFERENCE: EVENTS | REFERENCE: DECLARATIONS | REFERENCE: ALLOW AND DENY | ALLOW DENY AND IGNORE IN SCOPE | ALLOW AND DENY WITHIN POOL DECLARATIONS | REFERENCE: PARAMETERS | SETTING PARAMETER VALUES USING EXPRESSIONS | RESERVED LEASES | REFERENCE: OPTION STATEMENTS | REFERENCE: EXPRESSIONS | SEE ALSO | AUTHOR

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