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dhcpd.conf(5)		      File Formats Manual		 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 de-
       scribe clients on the network, to provide addresses  that  can  be  as-
       signed to clients, or to	apply a	group of parameters to a group of dec-
       larations.   In any group of parameters and declarations,  all  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 ad-
       dresses dynamically, a range declaration	must appear within the	subnet
       declaration.    For  clients with statically assigned addresses,	or for
       installations where only	known clients will be served, each such	client
       must  have  a  host declaration.	  If parameters	are to be applied to a
       group of	declarations which are not related strictly  on	 a  per-subnet
       basis, the group	declaration can	be used.

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

       Some  installations  have  physical  networks on	which more than	one IP
       subnet operates.	  For example, if there	 is  a	site-wide  requirement
       that  8-bit subnet masks	be used, but a department with a single	physi-
       cal 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	ether-
       net until such time as a	new physical network can be added.    In  this
       case,  the  subnet declarations for these two networks must be enclosed
       in a shared-network declaration.

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

       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	dynam-
       ically 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 pa-
       rameter declared	in the most specific scope is the one that is used.

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

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

       In Figure 1 there is also a group statement, which provides common  pa-
       rameters	for a set of three hosts - zappo, beppo	and harpo.  As you can
       see, these hosts	are all	in the domain, so it  might  make
       sense  for  a group-specific parameter to override the domain name sup-
       plied 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
       correspond to actual DHCP options, while	parameters that	do  not	 start
       with  the option	keyword	either control the behavior of the DHCP	server
       (e.g., how long a lease dhcpd will give out), or	specify	client parame-
       ters  that  are not optional in the DHCP	protocol (for example, server-
       name and	filename).

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

       Imagine	that  you  have	 a site	with a lot of NCD X-Terminals.	 These
       terminals 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	declaration 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 providing a smaller  set  of  ad-
       dresses,	 possibly  with	 short lease times, that are available for un-
       known 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 ad-
       dresses in another pool are not,	thus  encouraging  users  to  register
       their  DHCP clients.   To do this, you would set	up a pair of pool dec-

       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  al-
       low  or	deny  keyword.	  If a pool has	a permit list, then only those
       clients that match specific entries on the permit list will be eligible
       to  be  assigned	 addresses from	the pool.   If a pool has a deny list,
       then only those clients that do not match any entries on	the deny  list
       will  be	 eligible.     If both permit and deny lists exist for a pool,
       then only clients that match the	permit list and	do not match the  deny
       list will be allowed access.

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

       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  dy-
       namic 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

       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	 allo-
       cation  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  ver-
       sions  of  the  ISC  DHCP server	may have become	accustomed to the DHCP
       server allocating IP addresses in  ascending  order,  but  this	is  no
       longer  possible,  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 re-
       quest message to	the IP address being allocated.	  If no	ICMP Echo  re-
       ply  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 as-
       sumes that there	is a configuration error - the IP address is in	use by
       some  host on the network that is not a DHCP client.   It marks the ad-
       dress as	abandoned, and will not	assign it to clients.

       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-07.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  peer
       state  declaration  in  the lease file, and restarting the server.   If
       you use this last method, be sure to leave the date  and	 time  of  the
       start of	the state blank:

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

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

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

       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
       service 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 de-
	 scribed earlier under DHCP FAILOVER.

       The address statement

	 address address;

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

       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
	 not currently be omitted, because the failover	protocol does not  yet
	 have a	reserved TCP port number.

       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 not be omitted because the	failover protocol does
	 not yet have a	reserved TCP port number.   The	port  number  declared
	 in  the  peer	port  statement	may be the same	as the port number de-
	 clared	in the port statement.

       The max-response-delay statement

	 max-response-delay seconds;

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

       The max-unacked-updates statement

	 max-unacked-updates count;

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

       The mclt	statement

	 mclt seconds;

	 The mclt statement defines the	Maximum	Client Lead Time.   It must be
	 specified  on the primary, and	may not	be specified on	the secondary.
	 This is the length of time for	which a	lease may be renewed by	either
	 failover  peer	 without  contacting  the  other.   The	longer you set
	 this, the longer it will take for the running server  to  recover  IP
	 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 index;

	 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 255,  of	 which
	 the most reasonable is	128.

       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 Failover pool balance statements.

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

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

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

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

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

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

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

	 In the	first pass, only leases	whose most recent bound	 client	 would
	 have been served by the remote	server - according to the Load Balance
	 Algorithm (see	above split and	hba configuration  statements)	-  are
	 given	away  to  the  peer.  This first pass will happily continue to
	 give away leases, decrementing	the lts	value by one for  each,	 until
	 the  lts value	has reached the	negative of the	total number of	leases
	 multiplied by the max-lease-ownership percentage.  So it  is  through
	 this  value that you can permit a small misbalance of the lease pools
	 - for the purpose of giving the peer  more  than  a  50/50  share  of
	 leases	 in  the hopes that their clients might	some day return	and be
	 allocated by the peer (operating normally).  This process is referred
	 to  as	 'MAC Address Affinity', but this is somewhat misnamed:	it ap-
	 plies equally to DHCP Client  Identifier  options.   Note  also  that
	 affinity is applied to	leases when they enter the state be moved from
	 free to backup	if the secondary already has more than its share.

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

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

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

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

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

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

       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.

       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

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

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

       class "limited-1" {
	 lease limit 4;

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

       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

       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 opton	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.

       Two DNS update  schemes	are  currently	implemented,  and  another  is
       planned.	   The two that	are currently available	are the	ad-hoc DNS up-
       date mode and the interim DHCP-DNS interaction draft update  mode.   If
       and  when  the  DHCP-DNS	 interaction draft and the DHCID draft make it
       through the IETF	standards process, there will be a third  mode,	 which
       will  be	the standard DNS update	method.	  The DHCP server must be con-
       figured to use one of the two currently-supported methods, or not to do
       dns  updates.	This can be done with the ddns-update-style configura-
       tion parameter.

       The ad-hoc Dynamic DNS update scheme is now  deprecated	and  does  not
       work.   In future releases of the ISC DHCP server, this scheme will not
       likely be available.  The interim scheme	works,	allows	for  failover,
       and should now be used.	The following description is left here for in-
       formational purposes only.

       The ad-hoc Dynamic DNS update scheme implemented	in this	version	of the
       ISC  DHCP  server is a prototype	design,	which does not have much to do
       with the	standard update	method that is being standardized in the  IETF
       DHC  working  group, but	rather implements some very basic, yet useful,
       update capabilities.   This mode	does not work with the failover	proto-
       col  because  it	 does not account for the possibility of two different
       DHCP servers updating the same set of DNS records.

       For the ad-hoc DNS update method, the client's FQDN is derived  in  two
       parts.	 First,	the hostname is	determined.   Then, the	domain name is
       determined, and appended	to the hostname.

       The DHCP	server determines the client's hostname	by first looking for a
       ddns-hostname  configuration  option,  and using	that if	it is present.
       If no such option is present, the server	looks for a valid hostname  in
       the  FQDN option	sent by	the client.  If	one is found, it is used; oth-
       erwise, if the client sent a host-name option, that  is	used.	Other-
       wise,  if  there	 is a host declaration that applies to the client, the
       name from that declaration will be used.	 If none of these applies, the
       server will not have a hostname for the client, and will	not be able to
       do a DNS	update.

       The domain name is determined from  the	ddns-domainname	 configuration
       option.	The default configuration for this option is:

	 option	server.ddns-domainname = config-option domain-name;

       So  if this configuration option	is not configured to a different value
       (over-riding the	above default),	or if a	 domain-name  option  has  not
       been  configured	 for  the client's scope, then the server will not at-
       tempt to	perform	a DNS update.

       The client's fully-qualified domain name, derived as we have described,
       is  used	 as  the  name	on  which an "A" record	will be	stored.	 The A
       record will contain the IP address that the client was assigned in  its
       lease.	 If there is already an	A record with the same name in the DNS
       server, no update of either the A or PTR	records	will occur - this pre-
       vents a client from claiming that its hostname is the name of some net-
       work  server.	For  example,  if  you	have   a   fileserver	called
       "", and	the client claims its hostname is "fs",	no DNS
       update will be done for that client,  and  an  error  message  will  be

       If  the	A record update	succeeds, a PTR	record update for the assigned
       IP address will be done,	pointing to the	A record.   This update	is un-
       conditional  -  it  will	be done	even if	another	PTR record of the same
       name exists.   Since the	IP address  has	 been  assigned	 to  the  DHCP
       server, this should be safe.

       Please note that	the current implementation assumes clients only	have a
       single network interface.   A client with two network  interfaces  will
       see  unpredictable  behavior.	This  is considered a bug, and will be
       fixed in	a later	release.   It may be helpful to	enable the  one-lease-
       per-client  parameter  so that roaming clients do not trigger this same

       The DHCP	protocol normally involves a four-packet exchange - first  the
       client sends a DHCPDISCOVER message, then the server sends a DHCPOFFER,
       then the	client sends a DHCPREQUEST, then the server sends  a  DHCPACK.
       In  the	current	version	of the server, the server will do a DNS	update
       after it	has received the DHCPREQUEST, and before it has	sent the  DHC-
       PACK.	It  only  sends	 the DNS update	if it has not sent one for the
       client's	address	before,	in order to minimize the impact	 on  the  DHCP

       When the	client's lease expires,	the DHCP server	(if it is operating at
       the time, or when next it operates) will	remove the client's A and  PTR
       records	from  the  DNS database.   If the client releases its lease by
       sending a DHCPRELEASE message, the server will likewise	remove	the  A
       and PTR records.

       The  interim  DNS  update  scheme  operates mostly according to several
       drafts that are being considered	by the IETF and	are expected to	become
       standards,  but	are not	yet standards, and may not be standardized ex-
       actly as	currently proposed.   These are:


       Because our implementation is slightly different	than the standard,  we
       will briefly document the operation of this update style	here.

       The  first  point  to understand	about this style of DNS	update is that
       unlike the ad-hoc style,	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 config-
       ured 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 op-
       tion 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 from either the fqdn option (if present)  or  the
       hostname	 option	(if present).  It will use its own domain name for the
       client, just as in the ad-hoc update scheme.  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.

       Also, if	the use-host-decl-names	configuration option is	enabled,  then
       the  host  declaration's	hostname will be used in place of the hostname
       option, and the same rules will apply as	described above.

       The other difference between the	ad-hoc scheme and the  interim	scheme
       is that with the	interim	scheme,	a method is used that allows more than
       one DHCP	server to update the DNS database without accidentally	delet-
       ing  A  records	that shouldn't be deleted nor failing to add A records
       that should be added.   The scheme works	as follows:

       When the	DHCP server issues a client a new lease,  it  creates  a  text
       string  that  is	an MD5 hash over the DHCP client's identification (see
       draft-ietf-dnsext-dhcid-rr-??.txt for details).	 The update adds an  A
       record  with  the name the server chose and a TXT record	containing the
       hashed identifier string	 (hashid).    If  this	update	succeeds,  the
       server is done.

       If  the update fails because the	A record already exists, then the DHCP
       server attempts to add the A record with	the  prerequisite  that	 there
       must be a TXT record in the same	name as	the new	A record, and that TXT
       record's	contents must be equal to hashid.   If this  update  succeeds,
       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

       The  interim  DNS  update  scheme  is  called  interim for two reasons.
       First, it does not quite	follow the drafts.   The current  versions  of
       the  drafts call	for a new DHCID	RRtype,	but this is not	yet available.
       The interim DNS update scheme uses a TXT	record	instead.    Also,  the
       existing	ddns-resolution	draft calls for	the DHCP server	to put a DHCID
       RR on the PTR record, but the interim update method does	not  do	 this.
       It is our position that this is not useful, and we are working with the
       author in hopes of removing it from the next version of the  draft,  or
       better understanding why	it is considered useful.

       In  addition to these differences, the server also does not update very
       aggressively.  Because each DNS update involves a round trip to the DNS
       server,	there  is a cost associated 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 up-
       dated the next time the client renews.

       When you	set your DNS server up to allow	updates	from the DHCP  server,
       you  may	 be  exposing  it to unauthorized updates.  To avoid this, you
       should use TSIG signatures - a method of	cryptographically signing  up-
       dates  using  a shared secret key.   As long as you protect the secrecy
       of this key, your updates should	also be	secure.	  Note,	however,  that
       the  DHCP  protocol  itself  provides no	security, and that clients can
       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.

       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 8 and 9
       distributions  come  with  a  program for generating secret keys	called
       dnssec-keygen.  The version that	comes with BIND	9 is likely to produce
       a  substantially	more random key, so we recommend you use that one even
       if you are not using BIND 9 as your DNS server.	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

       If  you	are  using the BIND 8 dnskeygen	program, the following command
       will generate a key as seen above:

	    dnskeygen -H 128 -u	-c -n DHCP_UPDATER

       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-de-
       bug.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.    Events  are used to implement DNS	updates, so you	should
       not define your own event handlers if you are using  the	 built-in  DNS
       update mechanism.

       The  built-in  version  of the DNS update mechanism is in a text	string
       towards the top of server/dhcpd.c.   If you  want  to  use  events  for
       things  other than DNS updates, and you also want DNS updates, you will
       have to start out by copying this code into your	 dhcpd.conf  file  and
       modifying it.

       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  (al-
       though 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  ad-
       dresses may be dynamically allocated to clients booting on that subnet.
       Such addresses are specified using the range declaration.

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

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

       The subnet6 statement

	subnet6	subnet6-number {
	  [ parameters ]
	  [ declarations ]

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

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

       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  de-

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

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

       The prefix6 statement

       prefix6 low-address high-address	/ bits;

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

       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 scope in	which to provide configuration
       information about a specific client, and	also provides a	way to	assign
       a  client a fixed address.  The host declaration	provides a way for the
       DHCP server to identify a DHCP or BOOTP client, and also	a way  to  as-
       sign the	client a static	IP address.

       If  it  is  desirable to	be able	to boot	a DHCP or BOOTP	client on more
       than one	subnet with fixed 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.

       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-
       identifier 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  net-
       works, 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 ad-
       dress allocation	pools.	In other contexts, the keywords	simply control
       general server behavior with respect to clients based on	scope.	 In  a
       non-pool	 context,  the ignore keyword can be used in place of the deny
       keyword to prevent logging of denied requests.

       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  assign-
       ment  to	 unknown  clients is allowed by	default.  An unknown client is
       simply a	client that has	no host	declaration.

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

       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.

       This option does	not satisfy the	 requirement  of  failover  peers  for
       denying	dynamic	bootp clients.	The deny dynamic bootp clients;	option
       should be used instead. See the ALLOW AND DENY WITHIN POOL DECLARATIONS
       section of this man page	for more details.

       The booting keyword

	allow booting;
	deny booting;
	ignore booting;

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

       The duplicates keyword

	allow duplicates;
	deny duplicates;

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

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

       The declines keyword

	allow declines;
	deny declines;
	ignore declines;

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

       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 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 in-
       formation about a specific lease, such as when it was issued  and  when
       it  will	expire.	By default, the	server will not	respond	to these pack-

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

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

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


       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  au-
       thentication protocol.	This is	not yet	supported.

	unauthenticated	clients;

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

	all clients;

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

	after time;

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

       The adaptive-lease-time-threshold statement

	 adaptive-lease-time-threshold percentage;

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

       The always-broadcast statement

	 always-broadcast flag;

	 The DHCP and BOOTP protocols both require DHCP	and BOOTP  clients  to
	 set the broadcast bit in the flags field of the BOOTP message header.
	 Unfortunately,	some DHCP and BOOTP clients do not do this, and	there-
	 fore  may  not	 receive  responses  from  the DHCP server.   The DHCP
	 server	can be made to always broadcast	its responses  to  clients  by
	 setting  this	flag  to  'on' for the relevant	scope; relevant	scopes
	 would be inside a conditional statement, as a parameter for a	class,
	 or  as	a parameter for	a host declaration.   To avoid creating	excess
	 broadcast traffic on your network, we recommend that you restrict the
	 use  of this option to	as few clients as possible.   For example, the
	 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 un-
	 til their old lease has expired, which	could take quite a long	time.

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

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

       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
	 declarations 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
	 specified in scope, then the server will derive the hostname automat-
	 ically,  using	an algorithm that varies for each of the different up-
	 date methods.

       The ddns-domainname statement

	 ddns-domainname name;

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

       The ddns-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  al-
	 ways  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 ad-hoc, 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 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  de-
	 layed-ack  feature is not compiled in by default, but must be enabled
	 at compile time with './configure --enable-delayed-ack'.

       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 un-
	 less DNS updates are enabled and ddns-update-style is set to interim.
	 Forward updates are enabled by	default.   If this statement  is  used
	 to disable forward updates, the DHCP server will never	attempt	to up-
	 date 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 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 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-ad-
	 dress	declaration.   Each  address  in the fixed-address declaration
	 should	be either an IP	address	or a domain name that resolves to  one
	 or more IP addresses.

       The fixed-address6 declaration

	 fixed-address6	ip6-address ;

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

       The 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;

	 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.

       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 re-
	 serve	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 de-
	 fault,	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 vari-

       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  ad-
	 dresses.   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- 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 be-
	 fore parsing it are logged to the default log facility.   To  prevent
	 this,	see the	README file included with this distribution, which de-
	 scribes how to	change the default log facility.  When this  parameter
	 is used, the DHCP server prints its startup message a second time af-
	 ter parsing the configuration file, so	that the log will be  as  com-
	 plete as possible.

       The max-lease-time statement

	 max-lease-time	time;

	 Time should be	the maximum length in seconds that will	be assigned to
	 a lease.   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 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 re-
	 ports,	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-
	 allocatable addresses.

       The next-server statement

	 next-server server-name;

	 The  next-server statement is used to specify the host	address	of the
	 server	from which the initial boot file (specified  in	 the  filename
	 statement)  is	to be loaded.	Server-name should be a	numeric	IP ad-
	 dress 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
	 DHCPREQUEST, it has forgotten any lease not mentioned in the  DHCPRE-
	 QUEST	-  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  re-
	    sponse  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 ad-
	    visory  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 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 de-
	    fault value	to be sent on occasions	where the default value	 would
	    be	incorrect.   The default value is the first IP address associ-
	    ated with the physical network interface on	which the request  ar-

	    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 ad-
	    dress 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-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

	    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  op-
	    tion  can  be  used	to assign a different set of site-specific op-
	    tions 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  in-
	    cluded in all subsequent DHCPREQUEST messages sent in the RENEWING
	    state.   This works	around a problem with relay agent  information
	    options, which is that they	usually	not appear in DHCPREQUEST mes-
	    sages sent by the client in	the RENEWING state, because such  mes-
	    sages  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 de-
	    tection.  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
	    leaving 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  as-
	    signed 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";

	    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 be-
	    yond the scope of this document to describe	how to make  this  de-

	 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 pa-
       rameter based on	some value that	the client has sent.   To do this, you
       can use expression evaluation.	The dhcp-eval(5) manual	page describes
       how to write expressions.   To assign the result	of an evaluation to an
       option, define the option as follows:

	 my-parameter =	expression ;

       For example:

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

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

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

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

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

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

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

       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) was written by Ted	Lemon  under  a	 contract  with	 Vixie
       Labs.	Funding	for this project was provided by Internet Systems Con-
       sortium.	 Information about Internet Systems Consortium can be found at



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