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

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

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

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

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

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

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

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

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

       global parameters...

       subnet netmask {
	 subnet-specific parameters...

       subnet netmask {
	 subnet-specific parameters...

       subnet netmask {
	 subnet-specific parameters...

       group {
	 group-specific	parameters...
	 host {
	   host-specific parameters...
	 host {
	   host-specific parameters...
	 host {
	   host-specific parameters...

				      Figure 1

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

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

				      Figure 2

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

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

	    option routers;

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

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

	    option domain-name "";

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

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

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

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

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

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

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

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

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

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

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

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

       subnet netmask {
	 option	routers;

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

	 # Known clients get this pool.
	 pool {
	   option domain-name-servers,;
	   max-lease-time 28800;
	   deny	unknown-clients;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       pool {
	    failover peer "foo";
	    pool specific parameters

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

       failover	peer "foo" {
	 port 519;
	 peer address;
	 peer port 520;
	 max-response-delay 60;
	 max-unacked-updates 10;
	 mclt 3600;
	 split 128;
	 load balance max seconds 3;

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

	 [ primary | secondary ];

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

       The address statement

	 address address;

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

       The peer	address	statement

	 peer address address;

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

       The port	statement

	 port port-number;

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

       The peer	port statement

	 peer port port-number;

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

       The max-response-delay statement

	 max-response-delay seconds;

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

       The max-unacked-updates statement

	 max-unacked-updates count;

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

       The mclt	statement

	 mclt seconds;

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

       The split statement

	 split bits;

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

       The hba statement

	 hba colon-separated-hex-list;

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

	   hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:

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

	   hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:

	   hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:

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

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

       The load	balance	max seconds statement

	 load balance max seconds seconds;

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

       The auto-partner-down statement

	 auto-partner-down seconds;

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

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

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

       The Failover pool balance statements.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       class "ras-clients" {

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

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

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

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

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

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

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

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

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

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

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

       class "limited-1" {
	 lease limit 4;

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

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

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

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

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

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

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

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

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

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

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

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

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

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

			    RFC	4701 (updated by RF5494)
				      RFC 4702
				      RFC 4703

       And the corresponding drafts were:


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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       zone "" {
	    type master;
	    file "";
	    allow-update { key DHCP_UPDATER; };

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

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

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

       zone EXAMPLE.ORG. {

       zone {

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

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

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

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

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


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

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

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

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

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

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

       The include statement

	include	"filename";

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

       The shared-network statement

	shared-network name {
	  [ parameters ]
	  [ declarations ]

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

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

       The subnet statement

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

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

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

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

       The subnet6 statement

	subnet6	subnet6-number {
	  [ parameters ]
	  [ declarations ]

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

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

       The range statement

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

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

       The range6 statement

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

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

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

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

       The prefix6 statement

       prefix6 low-address high-address	/ bits;

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

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

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

       The host	statement

	host hostname {
	  [ parameters ]
	  [ declarations ]

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

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

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

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

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

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

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

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

       The group statement

	group {
	  [ parameters ]
	  [ declarations ]

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

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

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

       The unknown-clients keyword

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

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

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

       The bootp keyword

	allow bootp;
	deny bootp;
	ignore bootp;

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

       The booting keyword

	allow booting;
	deny booting;
	ignore booting;

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

       The duplicates keyword

	allow duplicates;
	deny duplicates;

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

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

       The declines keyword

	allow declines;
	deny declines;
	ignore declines;

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

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

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

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

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

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

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

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

       The client-updates keyword

	allow client-updates;
	deny client-updates;

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

       The leasequery keyword

	allow leasequery;
	deny leasequery;

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

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

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

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

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


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


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

	members	of "class";

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

	dynamic	bootp clients;

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

	authenticated clients;

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

	unauthenticated	clients;

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

	all clients;

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

	after time;

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

       The abandon-lease-time statement

	 adandon-lease-time time;

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

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

       The adaptive-lease-time-threshold statement

	 adaptive-lease-time-threshold percentage;

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

       The always-broadcast statement

	 always-broadcast flag;

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

       The always-reply-rfc1048	statement

	 always-reply-rfc1048 flag;

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

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

       The authoritative statement


	 not authoritative;

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

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

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

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

       The boot-unknown-clients	statement

	 boot-unknown-clients flag;

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

       The db-time-format statement

	 db-time-format	[ default | local ] ;

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

       The ddns-hostname statement

	 ddns-hostname name;

	 The  name  parameter should be	the hostname that will be used in set-
	 ting up the client's A	and PTR	records.  If no	ddns-hostname is spec-
	 ified	in  scope,  then the server will derive	the hostname automati-
	 cally,	using an algorithm that	 varies	 for  each  of	the  different
	 update	methods.

       The ddns-domainname statement

	 ddns-domainname name;

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

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

	 ddns-local-address4 address;

	 ddns-local-address6 address;

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

       The ddns-rev-domainname statement

	 ddns-rev-domainname name;

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

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

       The ddns-update-style parameter

	 ddns-update-style style;

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

       The ddns-updates	statement

	  ddns-updates flag;

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

       The default-lease-time statement

	 default-lease-time time;

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

       The delayed-ack and max-ack-delay statements

	 delayed-ack count;

	 max-ack-delay microseconds;

	 Count should be an integer value from zero to 2^16-1, and defaults to
	 28.   The  count  represents  how many	DHCPv4 replies maximum will be
	 queued	pending	transmission until after a database commit event.   If
	 this  number  is reached, a database commit event (commonly resulting
	 in fsync() and	representing a performance penalty) will be made,  and
	 the  reply  packets  will be transmitted in a batch afterwards.  This
	 preserves the RFC2131 direction  that	"stable	 storage"  be  updated
	 prior	to  replying  to  clients.  Should the DHCPv4 sockets "go dry"
	 (select() returns immediately with no read sockets),  the  commit  is
	 made and any queued packets are transmitted.

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

	 The delayed-ack feature is not	compiled in by default,	 but  must  be
	 enabled  at  compile  time  with  './configure	--enable-delayed-ack'.
	 While we no longer consider it	experimental and we don't know of  any
	 issues	 with it, in order to minimize problems	with existing configu-
	 ration	files we have left it disabled by default.

       The dhcp-cache-threshold	statement

	 dhcp-cache-threshold percentage;

	 The dhcp-cache-threshold statement takes one integer  parameter  with
	 allowed values	between	0 and 100. The default value is	25 (25%	of the
	 lease time). This parameter expresses the  percentage	of  the	 total
	 lease	time,  measured	 from  the  beginning, during which a client's
	 attempt to renew  its	lease  will  result  in	 getting  the  already
	 assigned lease, rather	than an	extended lease.

	 Clients  that	attempt	 renewal  frequently  can  cause the server to
	 update	and write the database frequently resulting in	a  performance
	 impact	 on  the server.  The dhcp-cache-threshold statement instructs
	 the DHCP server to avoid updating leases too frequently thus avoiding
	 this  behavior.   Instead  the	 server	 assigns  the same lease (i.e.
	 reuses	it) with no modifications except for CLTT (Client Last	Trans-
	 mission  Time)	 which	does not require disk operations. This feature
	 applies to IPv4 only.

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

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

       The do-forward-updates statement

	 do-forward-updates flag;

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

       The dont-use-fsync statement

	 dont-use-fsync	flag;

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

       The dynamic-bootp-lease-cutoff statement

	 dynamic-bootp-lease-cutoff date;

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

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


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

       The dynamic-bootp-lease-length statement

	 dynamic-bootp-lease-length length;

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

       The echo-client-id statement

	 echo-client-id	flag;

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

       The filename statement

	 filename "filename";

	 The filename statement	can be used to specify the name	of the initial
	 boot  file which is to	be loaded by a client.	The filename should be
	 a filename recognizable to whatever file transfer protocol the	client
	 can be	expected to use	to load	the file.

       The fixed-address declaration

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

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

       The fixed-address6 declaration

	 fixed-address6	ip6-address ;

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

       The fixed-prefix6 declaration

	 fixed-prefix6 low-address / bits;

	 The  fixed-prefix6  declaration is used to assign a fixed IPv6	prefix
	 to a client.  It should only appear in	a host declaration, but	multi-
	 ple fixed-prefix6 statements may appear in a single host declaration.

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

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

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

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

       The get-lease-hostnames statement

	 get-lease-hostnames flag;

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

       The hardware statement

	 hardware hardware-type	hardware-address;

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

       The host-identifier option statement

	 host-identifier option	option-name option-data;


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

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

       The ignore-client-uids statement

	 ignore-client-uids flag;

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

       The infinite-is-reserved	statement

	 infinite-is-reserved flag;

	 ISC DHCP now supports 'reserved' leases.  See the section on RESERVED
	 LEASES	below.	If this	flag is	 on,  the  server  will	 automatically
	 reserve  leases  allocated  to	 clients  which	 requested an infinite
	 (0xffffffff) lease-time.

	 The default is	off.

       The lease-file-name statement

	 lease-file-name name;

	 Name should be	the name of the	DHCP server's lease file.  By default,
	 this  is DBDIR/dhcpd.leases.  This statement must appear in the outer
	 scope of the configuration file - if it appears in some other	scope,
	 it  will have no effect.  Furthermore,	it has no effect if overridden
	 by the	-lf flag or the	PATH_DHCPD_DB environment variable.

       The limit-addrs-per-ia statement

	 limit-addrs-per-ia number;

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

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

       The dhcpv6-lease-file-name statement

	 dhcpv6-lease-file-name	name;

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

       The lease-id-format parameter

	 lease-id-format format;

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

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

       The local-port statement

	 local-port port;

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

       The local-address statement

	 local-address address;

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

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

	  Listening on Socket/eth0

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

       The log-facility	statement

	 log-facility facility;

	 This statement	causes the DHCP	server to do all of its	logging	on the
	 specified  log	 facility  once	the dhcpd.conf file has	been read.  By
	 default the DHCP server logs to the daemon  facility.	 Possible  log
	 facilities  include  auth,  authpriv,	cron,  daemon, ftp, kern, lpr,
	 mail, mark, news, ntp,	 security,  syslog,  user,  uucp,  and	local0
	 through  local7.   Not	 all  of these facilities are available	on all
	 systems, and there may	be other facilities available  on  other  sys-

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

	      local7.debug /var/log/dhcpd.log

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

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

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

	 log-threshold-high percentage;

	 log-threshold-low percentage;

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

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

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

	     "Threshold	  logging   disabled  for  shared  subnet  of  ranges:

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

       The max-lease-time statement

	 max-lease-time	time;

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

       The min-lease-time statement

	 min-lease-time	time;

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

       The min-secs statement

	 min-secs seconds;

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

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

       The next-server statement

	 next-server server-name;

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

       The omapi-port statement

	 omapi-port port;

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

       The one-lease-per-client	statement

	 one-lease-per-client flag;

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

       The pid-file-name statement

	 pid-file-name name;

	 Name should be	the name of the	DHCP server's process ID  file.	  This
	 is  the file in which the DHCP	server's process ID is stored when the
	 server	starts.	 By  default,  this  is	 RUNDIR/   Like  the
	 lease-file-name  statement,  this  statement must appear in the outer
	 scope of the configuration file.  It has no effect if	overridden  by
	 the -pf flag or the PATH_DHCPD_PID environment	variable.

	 The dhcpv6-pid-file-name statement

	    dhcpv6-pid-file-name name;

	    Name  is the name of the pid file to use if	and only if the	server
	    is running in DHCPv6 mode.	By default, this is  DBDIR/
	    This statement, like pid-file-name,	must appear in the outer scope
	    of the configuration file.	It has no effect if overridden by  the
	    -pf	  flag	 or  the  PATH_DHCPD6_PID  environment	variable.   If
	    dhcpv6-pid-file-name is not	specified, but pid-file-name  is,  the
	    latter value will be used.

	 The ping-check	statement

	    ping-check flag;

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

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

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

	 The ping-timeout statement

	    ping-timeout seconds;

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

	 The preferred-lifetime	statement

	    preferred-lifetime seconds;

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

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

	 The prefix-length-mode	statement

	    prefix-length-mode mode;

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

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

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

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

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

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

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

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

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

	 The remote-port statement

	    remote-port	port;

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

	 The server-identifier statement

	    server-identifier hostname;

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

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

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

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

	 The server-id-check statement

	    server-id-check flag;

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

	    Unless  overridden	by use of the server-identifier	statement, the
	    value the server uses as its id will be the	first IP address asso-
	    ciated  with  the  physical	network	interface on which the request

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

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

	    Care should	be taken before	enabling this option.

	 The server-duid statement

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

	    server-duid	EN enterprise-number enterprise-identifier ;

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

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

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

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

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

	    The	default	server-duid type is LLT.

	 The server-name statement

	    server-name	name ;

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

	 The dhcpv6-set-tee-times statement

	    dhcpv6-set-tee-times flag;

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

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

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

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

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

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

	 The site-option-space statement

	    site-option-space name ;

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

	 The stash-agent-options statement

	    stash-agent-options	flag;

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

	 The update-conflict-detection statement

	    update-conflict-detection flag;

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

	 The update-optimization statement

	    update-optimization	flag;

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

	 The update-static-leases statement

	    update-static-leases flag;

	    The	update-static-leases flag, if enabled, causes the DHCP	server
	    to	do  DNS	 updates  for  clients even if those clients are being
	    assigned their IP address using a fixed-address statement  -  that
	    is,	the client is being given a static assignment.	It is not rec-
	    ommended 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 signifi-
	    cant performance impact in environments that place	heavy  demands
	    on the DHCP	server.

	 The use-host-decl-names statement

	    use-host-decl-names	flag;

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

		group {
		  use-host-decl-names on;

		  host joe {
		    hardware ethernet 08:00:2b:4c:29:32;

	    is equivalent to

		  host joe {
		    hardware ethernet 08:00:2b:4c:29:32;
		    option host-name "joe";

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

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

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

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

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

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

	 The vendor-option-space statement

	    vendor-option-space	string;

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

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

	 my-parameter =	expression ;

       For example:

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

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

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

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

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

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

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

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

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

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

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



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