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dhcpd.conf(5)							 dhcpd.conf(5)

       dhcpd.conf - dhcpd configuration	file

       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

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

       Declarations about network topology include the shared-network and  the
       subnet  declarations.   If  clients  on	a  subnet  are	to be assigned
       addresses dynamically, a	range declaration must appear within the  sub-
       net  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-sub-
       net 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
       parameters 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

       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

       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
       parameter 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.

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

       global parameters...

       subnet netmask {
	 subnet-specific parameters...

       subnet netmask {
	 subnet-specific parameters...

       subnet netmask {
	 subnet-specific parameters...

       group {
	 group-specific	parameters...
	 host {
	   host-specific parameters...
	 host {
	   host-specific parameters...
	 host {
	   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
       organization), and so on.  So, for example:

	    option domain-name "";
	    option domain-name-servers,;

				      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;

       Note that the address here  is  specified  numerically.	 This  is  not
       required	 -  if	you have a different domain name for each interface on
       your router, it's perfectly legitimate to use the domain	name for  that
       interface 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
       parameters for a	set of three hosts - zappo, beppo and harpo.   As  you
       can  see,  these	 hosts are all in the domain, so it might
       make sense for a	group-specific parameter to override the  domain  name
       supplied	to these hosts:

	    option domain-name "";

       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
       include 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;	}

       The  pool  and  pool6  declarations  can	 be  used to specify a pool of
       addresses that  will  be	 treated  differently  than  another  pool  of
       addresses,  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 provid-
       ing 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
       Internet,  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 netmask {
	 option	routers;

	 # Unknown clients get this pool.
	 pool {
	   option domain-name-servers;
	   max-lease-time 300;
	   allow unknown-clients;

	 # Known clients get this pool.
	 pool {
	   option domain-name-servers,;
	   max-lease-time 28800;
	   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
       allow  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
       include range6 statements for both NA and TA and	prefixy6 statements in
       a single	pool6 statement.

       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

       If the server finds the address the  client  is	requesting,  and  that
       address 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

       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.	In  this  case,	 the DHCP server will never do
       dynamic address allocation.  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
       response	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.

       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
       request	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	state-
       ments, 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
       assumes	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
       address as abandoned, and will not assign it to clients.

       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

       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

       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

       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.

       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" {
	 port 519;
	 peer address;
	 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
	 described 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

       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
	 before	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
	 addresses  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:

	 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:

	   hba 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.

       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
	 directly  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
	 default),  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
	 describe shortly.

	 The  first  thing  calculated	on  any	 pool  balance	run is a value
	 referred 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
	 approach,  marked  by the presence of the max-lease-misbalance	state-
	 ment.	This parameter configures what used to be a 10%	fixed value in
	 previous  versions:  if lts is	less than free+backup *	max-lease-mis-
	 balance 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,  how-
	 ever,	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-balance  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  protocol POOLREQ message, in	the hopes that the remote sys-
	 tem will be woken up into action.

	 Once the lts value exceeds  the  max-lease-misbalance	percentage  of
	 total	free  state leases as described	above, leases are moved	to the
	 remote	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
	 applies 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
	 reduce	 the lts underneath the	total number of	free state leases mul-
	 tiplied by the	max-lease-ownership percentage.	  In  this  pass,  the
	 oldest	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

	 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
	 default to values 60 and 3600 respectively (to	place  balance	events
	 between 1 minute and 1	hour).

       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 addded 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";

       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" {

       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 netmask {
	 pool {
	   allow members of "allocation-class-1";
	 pool {
	   allow members of "allocation-class-2";

       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-

       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
       allocation 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.

       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,
       either 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.

       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.

       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-

       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.

       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 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:


       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-
       updates;.  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
       ""  domain,  whose  hostname is "jschmoe".  The server	is for
       the "" domain.  The DHCP client indicates in the FQDN	option
       that  its  FQDN	is  "".   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   Once  the	DHCP  client has an IP address,	it can
       update its own A	record,	assuming that the "" 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
       example above, "jschmoe"	instead	of "".

       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

       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
       operator	 intervention,	and  once  this	has been done, the DNS will be
       updated 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 he 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.

       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
       updates	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
       therefore provide information to	the DHCP server	which the DHCP	server
       will  then  use	in  its	updates, with the constraints described	previ-

       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	domain	will  be  assigned  addresses  on  the  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 "" {
	    type master;
	    file "";
	    allow-update { key DHCP_UPDATER; };

       zone "" {
	    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. {

       zone {

       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  ""  and	for "".  For example, if
       there were a subdomain ""	 with  no  separate  SOA,  you
       could not write a zone declaration for ""  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-


       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/";
		 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/	 and  /var/log/update-
       debug.log  files	before starting	the name server.  For more information
       on configuring ISC BIND,	consult	the documentation that accompanies it.

       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.

       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

       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,
       enclosed	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
       addresses  may be dynamically allocated to clients booting on that sub-
       net.  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
       addresses may be	dynamically allocated to clients booting on that  sub-

       The  subnet6-number  should be an IPv6 network identifier, specified as

       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-

       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

       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-

       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

       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 delcaration 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

       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.

       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
       address	allocation pools.  In other contexts, the keywords simply con-
       trol 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.

       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
       restrict	 access	 on your network to known clients, you should use deny
       unknown-clients;	inside of your address pool, as	 described  under  the

       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,
       duplicates 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
       address,	 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-

       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
       address	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.  This is
       only relevant when doing	interim	DNS updates.   See  the	 documentation
       under the heading THE INTERIM 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
       information 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-

       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
       respond 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
       allocation 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

       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
       allow 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:


       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.


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

	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
       authentication 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

       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
	 exhaustion  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
	 Microsoft  DHCP  client is known not to have this problem, as are the
	 OpenTransport 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


	 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
	 until their old lease has expired, which  could  take	quite  a  long

	 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
	 authority 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-

       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 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 selects
	 one of	two output formats.  The default format	prints the day,	 date,
	 and  time  in	UTC, while the local format prints the system seconds-
	 since-epoch, and helpfully provides the day and time  in  the	system
	 timezone  in  a comment.  The time formats are	described 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  set-
	 ting up the client's A	and PTR	records.  If no	ddns-hostname is spec-
	 ified in scope, then the server will derive  the  hostname  automati-
	 cally,	 using	an  algorithm  that  varies  for each of the different
	 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 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

       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 "", 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,
	 reversed - for	example, if the	IP address assigned to the  client  is,  then  the  reversed  IP	address	 is  So a
	 client	with that IP address would, by default,	be given a PTR	record

       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

       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  certain
	 scope.	  The ddns-updates parameter is	on by default.	To disable 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 specific	 expi-
	 ration	 time.	 This is used for both DHCPv4 and DHCPv6 leases	(it is
	 also known as the "valid lifetime" in DHCPv6).	 The default is	 43200

       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
	 28.  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 penalty) will be made, and
	 the reply packets will	be transmitted in a  batch  afterwards.	  This
	 preserves  the	 RFC2131  direction  that  "stable 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 transmitted.

	 Similarly, microseconds indicates how many microseconds are permitted
	 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).

	 Please	 note  that  as	 delayed-ack  is  currently  experimental, the
	 delayed-ack feature is	not  compiled  in  by  default,	 but  must  be
	 enabled at compile time with './configure --enable-delayed-ack'.

       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.

	 Clients that attempt renewal  frequently  can	cause  the  server  to
	 update	 and  write the	database frequently resulting in a performance
	 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  assigns  the  same  lease	 (i.e.
	 reuses	 it) with no modifications except for CLTT (Client Last	Trans-
	 mission Time) which does not require disk  operations.	 This  feature
	 applies to IPv4 only.

	 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
	     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

       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	effect
	 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  default  and
	 if  the flag is set to	false the server will call fsync().  Suppress-
	 ing the call to fsync() may increase the performance  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 example, the end of a	school
	 term, or the time at night when a facility 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 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 Coordinated  Uni-
	 versal	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 number of seconds.
	 If a client reboots using BOOTP during	the timeout 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  option  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 initial
	 boot  file which is to	be loaded by a client.	The filename should be
	 a filename recognizable to whatever file transfer protocol 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  declaration.
	 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	state-
	 ment are valid	for the	network	to which the client is connected, that
	 client	 will  not  match  the host declaration	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
	 addresses  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	prefix
	 to a client.  It should only appear in	a host declaration, but	multi-
	 ple fixed-prefix6 statements may appear in a single host declaration.

	 The low-address specifies the start of	the prefix and the bits	speci-
	 fies 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 delcarations 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

	 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  provide  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	state-
	 ment.	 hardware-type	must be	the name of a physical hardware	inter-
	 face 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;


	 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 examine 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) indicate 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	state-
	 ment 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 RESERVED
	 LEASES	below.	If this	flag is	 on,  the  server  will	 automatically
	 reserve  leases  allocated  to	 clients  which	 requested an infinite
	 (0xffffffff) lease-time.

	 The default is	off.

       The lease-file-name statement

	 lease-file-name name;

	 Name should be	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.  Furthermore,	it has no effect if overridden
	 by the	-lf flag or the	PATH_DHCPD_DB environment variable.

       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 dhcpv6-lease-file-name statement

	 dhcpv6-lease-file-name	name;

	 Name is the name of the lease file to use if and only if  the	server
	 is  running in	DHCPv6 mode.  By default, this is DBDIR/dhcpd6.leases.
	 This statement, like lease-file-name, must appear in the outer	 scope
	 of the	configuration file.  It	has no effect if overridden by the -lf
	 flag or the PATH_DHCPD6_DB environment	 variable.   If	 dhcpv6-lease-
	 file-name  is not specified, but lease-file-name is, the latter value
	 will be used.

       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 attached net-
	 works;	it is only  realistically  useful  for	a  server  whose  only
	 clients are reached via unicasts, such	as via DHCP relay 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 com-
	 pile-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 specified
	 address, that only one	address	may be supported  in  a	 daemon	 at  a
	 given time.

       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	local0
	 through local7.  Not all of these facilities  are  available  on  all
	 systems,  and	there  may be other facilities available on other sys-

	 In addition to	setting	this value, you	may need to modify  your  sys-
	 log.conf  file	to configure logging of	the DHCP server.  For example,
	 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 operating
	 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	syslogd.  Some systems support
	 log rollover using a shell script  or	program	 called	 newsyslog  or
	 logrotate, and	you may	be able	to configure this as well so that your
	 log file doesn't grow uncontrollably.

	 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 prevent
	 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 configura-
	 tion 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  pro-
	 duced.	  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	generated each
	 time a	lease is acknowledged when the pool usage is  above  the  high

	 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:

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

       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-

       The min-secs statement

	 min-secs seconds;

	 Seconds should	be the minimum number of seconds since a client	 began
	 trying	 to acquire a new lease	before the DHCP	server will respond 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 sec-
	 onds.	Generally, setting this	to one will result in the DHCP	server
	 not  responding  to the client's first	request, but always responding
	 to its	second request.

	 This can be used to set up a secondary	DHCP server which never	offers
	 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-allocat-
	 able 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  numeric  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  required  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 DHCPRE-
	 QUEST,	 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 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/  Like the
	 lease-file-name statement, this statement must	appear	in  the	 outer
	 scope	of  the	configuration file.  It	has no effect if overridden by
	 the -pf flag or the 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 DBDIR/
	    This statement, like pid-file-name,	must appear in the outer scope
	    of	the configuration file.	 It has	no effect if overridden	by the
	    -pf	 flag  or  the	PATH_DHCPD6_PID	 environment   variable.    If
	    dhcpv6-pid-file-name  is  not specified, but pid-file-name is, the
	    latter value will be used.

	 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.

	    This  ping check introduces	a default one-second delay in respond-
	    ing	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  parame-
	    ter	 can  be  used to control checking - if	its value is false, no
	    ping check is done.

	 The ping-timeout statement

	    ping-timeout seconds;

	    If the DHCP	server determined it should send an ICMP echo  request
	    (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	abandoned, and
	    the	server does not	respond	to the client.	If no  value  is  set,
	    ping-timeout defaults to 1 second.

	 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 the renew+rebind	timers,	or 3/4
	    the	default	lease time if none were	specified.

	 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 conveyed	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 select prefix(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 ignore, prefer, exact,  minimum,
	    or maximum where:

	    1. ignore -	The requested length is	ignored. The server will offer
	    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.

	    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.   This  is
	    the	default	behavior.

	    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 prefixes	avail-
	    able.  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 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 configura-
	    tion value.	 Should	the DHCP  Relay	 happen	 to  be	 addressed  as,	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	gener-
	    ally 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 particu-
	    lar	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
	    default 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
	    request 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 equiva-
	    lent 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 asso-
	    ciated with	the physical network interface on  which  the  request

	    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 reboots
	    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

	    If you choose EN, you must include the enterprise number  and  the

	    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 override 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 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-specific
	    uses, but are frequently used by vendors of	embedded hardware that
	    contains  DHCP  clients.   Because site-specific options are allo-
	    cated 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-specific 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	RENEW-
	    ING	state.	This works around a problem with relay agent  informa-
	    tion 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 bindings
	    to install the new binding without question.  The default is true.

	 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 attempting 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 leav-
	    ing	 this  option enabled, which is	the default.  This option only
	    affects the	behavior of the	interim	DNS update scheme, and has  no
	    effect  on the ad-hoc DNS update scheme.  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 statement  -  that
	    is,	 the client is being given a static assignment.	 This can only
	    work with the interim DNS update scheme.  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  perfor-
	    mance  impact in environments that place heavy demands on the DHCP

	 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;

	    is equivalent to

		  host joe {
		    hardware ethernet 08:00:2b:4c:29:32;
		    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 dis-
	    cussed in more detail under	DNS updates.

	    An option host-name	statement within a host	declaration will over-
	    ride 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

	 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

       Sometimes  it's	helpful	 to  be	able to	set the	value of a DHCP	server
       parameter 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 evalua-
       tion to an option, define the option as follows:

	 my-parameter =	expression ;

       For example:

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

       It's  often  useful to allocate a single	address	to a single client, in
       approximate perpetuity.	Host  statements  with	fixed-address  clauses
       exist  to  a  certain  extent  to  serve	this purpose, but because host
       statements 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
       released,  and  any  events or services that would normally be supplied
       during 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
       'infinite-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'.

       DHCP option statements are documented  in  the  dhcp-options(5)	manual

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

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

       dhcpd.conf(5) is	maintained by ISC.  Information	about Internet Systems
       Consortium can be found at



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