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
       parameters 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
       subnet declaration.   For clients with statically assigned addresses,
       or for installations where only known clients will be served, each such
       client must have a host declaration.   If parameters are to be applied
       to a group of declarations which are not related strictly on a per-
       subnet basis, the group declaration can be used.

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

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

       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
       dynamically assigned, class declarations and conditional declarations
       may be used to group parameter assignments based on information the
       client sends.

       When a client is to be booted, its boot parameters are determined by
       consulting that client's host declaration (if any), and then consulting
       any class declarations matching the client, followed by the pool,
       subnet 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 zappo.test.isc.org {
           host-specific parameters...
         host beppo.test.isc.org {
           host-specific parameters...
         host harpo.test.isc.org {
           host-specific parameters...

                                      Figure 1

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

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

                                      Figure 2

       As you can see in Figure 2, you can specify host addresses in
       parameters 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 test.isc.org domain, so it might
       make sense for a group-specific parameter to override the domain name
       supplied to these hosts:

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

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

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

       You may have noticed that while some parameters start with the option
       keyword, some do not.   Parameters starting with the option keyword
       correspond to actual DHCP options, while parameters that do not start
       with the option keyword either control the behavior of the DHCP server
       (e.g., how long a lease dhcpd will give out), or specify client
       parameters 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
       terminals come in a variety of models, and you want to specify the boot
       files for each model.   One way to do this would be to have host
       declarations for each server and group them by model:

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

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

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

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

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

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

       The pool declaration can be used to specify a pool of addresses that
       will be treated differently than another pool of addresses, even on the
       same network segment or subnet.   For example, you may want to provide
       a large set of addresses that can be assigned to DHCP clients that are
       registered to your DHCP server, while providing a smaller set of
       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

       subnet netmask {
         option routers;

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

         # Known clients get this pool.
         pool {
           option domain-name-servers ns1.example.com, ns2.example.com;
           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.

       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
       incorrect 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
       immediately allocated to the client.   If the address is available for
       allocation 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
       particular 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 possible, and there is no way to configure this behavior with
       version 3 of the ISC DHCP server.

       The DHCP server checks IP addresses to see if they are in use before
       allocating them to clients.   It does this by sending an ICMP Echo
       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
       statements, and only when the lease is thought by the DHCP server to be
       free - i.e., the DHCP server or its failover peer has not listed the
       lease as in use.

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

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

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

       This version of the ISC DHCP server supports the DHCP failover protocol
       as documented in draft-ietf-dhc-failover-07.txt.   This is not a final
       protocol document, and we have not done interoperability testing with
       other vendors' implementations of this protocol, so you must not assume
       that this implementation conforms to the standard.  If you wish to use
       the failover protocol, make sure that both failover peers are running
       the same version of the ISC DHCP server.

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

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

       You can put the server into the PARTNER-DOWN state either by using the
       omshell (1) command or by stopping the server, editing the last peer
       state declaration in the lease file, and restarting the server.   If
       you use this last method, be sure to leave the date and time of the
       start of the state blank:

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

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

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

       The failover protocol defines a primary server role and a secondary
       server role.   There are some differences in how primaries and
       secondaries act, but most of the differences simply have to do with
       providing 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 configured as secondary, and it doesn't matter too much which
       one is which.

       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
       contact with its partner will have expired.

       While the failed server is recovering, its partner remains in the
       partner-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
       communicated with their partner, they both come up in this recovery
       state and follow the procedure we have just described.   In this case,
       no service will be provided to DHCP clients until MCLT has expired.

       In order to configure failover, you need to write a peer declaration
       that configures the failover protocol, and you need to write peer
       references 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
       segment.    You must not tell one server it's doing failover on a
       particular 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 declaration that utilizes failover would look like this:

       pool {
            failover peer "foo";
            deny dynamic bootp clients;
            pool specific parameters

       Dynamic BOOTP leases are not compatible with failover, and, as such,
       you need to disallow BOOTP in pools that you are using failover for.

       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" {
         address anthrax.rc.vix.com;
         port 647;
         peer address trantor.rc.vix.com;
         peer port 847;
         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
         identifier.  Because this value is used as an identifier, it may not
         be omitted.

       The peer address statement

         peer address address;

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

       The port statement

         port port-number;

         The port statement declares the TCP port on which the server should
         listen for connections from its failover peer.

       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. The
         port number declared in the peer port statement may be the same as
         the port number declared in the port statement.

       The max-response-delay statement

         max-response-delay seconds;

         The max-response-delay statement tells the DHCP server how many
         seconds 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
         connection 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 DHCP server how many
         BNDUPD messages it can send before it receives a BNDACK from the
         failover peer.   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
         communicating.   A value of something like 3600 is probably
         reasonable, but again bear in mind that we have no real operational
         experience with this.

       The split statement

         split index;

         The split statement specifies the split between the primary and
         secondary for the purposes of load balancing.   Whenever a client
         makes a DHCP request, the DHCP server runs a hash on the client
         identification.   If the hash comes out to less than the split value,
         the primary answers.   If it comes out to equal to or more than the
         split, the secondary answers.   The only meaningful value is 128, and
         can only be configured on the primary.

       The hba statement

         hba colon-separated-hex-list;

         The hba statement specifies the split between the primary and
         secondary 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

           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.  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.
         As such, the use of hba is deprecated.

       The load balance max seconds statement

         load balance max seconds seconds;

         This statement allows you to configure a cutoff after which load
         balancing 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.

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

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

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

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

       class "ras-clients" {

       In addition to classes, it is possible to declare subclasses.   A
       subclass is a class with the same name as a regular class, but with a
       specific 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
       constant value to use in matching the match expression for the class.
       When class matching is done, the server will evaluate the match
       expression 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

       Subclasses can be declared with or without scope.   In the above
       example, 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

       In the above example, if you had a single client that needed some
       configuration 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
       provide 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
       matching the circuit ID, a new one will be created and logged in the
       dhcpd.leases file, and the client will be classified in this new class.
       Once the client has been classified, it will be treated according to
       the rules of the class, including, in this case, being subject to the
       per-site limit of four leases.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

       The first point to understand about this style of DNS update is that
       unlike the ad-hoc style, the DHCP server does not necessarily always
       update both the A and the PTR records.   The FQDN option includes a
       flag which, when sent by the client, indicates that the client wishes
       to update its own A record.   In that case, the server can be
       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
       "radish.org" domain, whose hostname is "jschmoe".   The server is for
       the "example.org" domain.   The DHCP client indicates in the FQDN
       option that its FQDN is "jschmoe.radish.org.".   It also indicates that
       it wants to update its own A record.   The DHCP server therefore does
       not attempt to set up an A record for the client, but does set up a PTR
       record for the IP address that it assigns the client, pointing at
       jschmoe.radish.org.   Once the DHCP client has an IP address, it can
       update its own A record, assuming that the "radish.org" DNS server will
       allow it to do so.

       If the server is configured not to allow client updates, or if the
       client doesn't want to do its own update, the server will simply choose
       a name for the client from either the fqdn option (if present) or the
       hostname option (if present).  It will use its own domain name for the
       client, just as in the ad-hoc update scheme.  It will then update both
       the A and PTR record, using the name that it chose for the client.   If
       the client sends a fully-qualified domain name in the fqdn option, the
       server uses only the leftmost part of the domain name - in the example
       above, "jschmoe" instead of "jschmoe.radish.org".

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

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

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

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

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

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

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

       When you set your DNS server up to allow updates from the DHCP server,
       you may be exposing it to unauthorized updates.  To avoid this, you
       should use TSIG signatures - a method of cryptographically signing
       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

       The DNS server must be configured to allow updates for any zone that
       the DHCP server will be updating.  For example, let us say that clients
       in the sneedville.edu domain will be assigned addresses on the 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;
         secret pRP5FapFoJ95JEL06sv4PQ==;

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

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

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

       key DHCP_UPDATER {
         algorithm hmac-md5;
         secret pRP5FapFoJ95JEL06sv4PQ==;

       zone EXAMPLE.ORG. {
         key DHCP_UPDATER;

       zone 17.127.10.in-addr.arpa. {
         key DHCP_UPDATER;

       The primary statement specifies the IP address of the name server whose
       zone information is to be updated.

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

       You should choose your own secret key, of course.  The ISC BIND 8 and 9
       distributions come with a program for generating secret keys called
       dnssec-keygen.  The version that comes with BIND 9 is likely to produce
       a substantially more random key, so we recommend you use that one even
       if you are not using BIND 9 as your DNS server.  If you are using BIND
       9's dnssec-keygen, the above key would be created as follows:

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

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

            dnskeygen -H 128 -u -c -n DHCP_UPDATER

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

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

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

       You must create the /var/log/named-auth.info and /var/log/update-
       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.   Events are used to implement DNS updates, so you should
       not define your own event handlers if you are using the built-in DNS
       update mechanism.

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

       The include statement

        include "filename";

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

       The shared-network statement

        shared-network name {
          [ parameters ]
          [ declarations ]

       The shared-network statement is used to inform the DHCP server that
       some IP subnets actually share the same physical network.  Any subnets
       in a shared network should be declared within a shared-network
       statement.  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
       subnet.   Such addresses are specified using the range declaration.

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

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

       The 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

       The host statement

        host hostname {
          [ parameters ]
          [ declarations ]

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

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

       If client-specific boot parameters must change based on the network to
       which the client is attached, then multiple host declarations should be
       used.  The host declarations will only match a client if one of their
       fixed-address statements is viable on the subnet (or shared network)
       where the client is attached.  Conversely, for a host declaration to
       match a client being allocated a dynamic address, it must not have any
       fixed-address statements.  You may therefore need a mixture of host
       declarations for any given client...some having fixed-address
       statements, 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
       matching 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.

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

       The group statement

        group {
          [ parameters ]
          [ declarations ]

       The group statement is used simply to apply one or more parameters to a
       group of declarations.   It can be used to group hosts, shared
       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
       keywords 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
       control general server behavior with respect to clients based on scope.
       In a non-pool context, the ignore keyword can be used in place of the
       deny keyword to prevent logging of denied requests.

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

       The unknown-clients keyword

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

       The unknown-clients flag is used to tell dhcpd whether or not to
       dynamically 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.

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

       The booting keyword

        allow booting;
        deny booting;
        ignore booting;

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

       The duplicates keyword

        allow duplicates;
        deny duplicates;

       Host declarations can match client messages based on the DHCP Client
       Identifer 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.
       Unfortunately, a malicious or buggy client can, using DHCPDECLINE
       messages, completely exhaust the DHCP server's allocation pool.   The
       server will reclaim these leases, but while the client is running
       through the pool, it may cause serious thrashing in the DNS, and it
       will also cause the DHCP server to forget old DHCP client address

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

       The client-updates keyword

        allow client-updates;
        deny client-updates;

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

       The uses of the allow and deny keywords shown in the previous section
       work pretty much the same way whether the client is sending a
       DHCPDISCOVER 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 done.

       When a DHCPREQUEST message is processed, address allocation simply
       consists 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.

       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
         therefore may not receive responses from the DHCP server.   The DHCP
         server can be made to always broadcast its responses to clients by
         setting this flag to 'on' for the relevant scope; relevant scopes
         would be inside a conditional statement, as a parameter for a class,
         or as a parameter for a host declaration.   To avoid creating excess
         broadcast traffic on your network, we recommend that you restrict the
         use of this option to as few clients as possible.   For example, the
         Microsoft DHCP client is known not to have this problem, as are the
         OpenTransport and ISC DHCP clients.

       The always-reply-rfc1048 statement

         always-reply-rfc1048 flag;

         Some BOOTP clients expect RFC1048-style responses, but do not follow
         RFC1048 when sending their requests.   You can tell that a client is
         having this problem if it is not getting the options you have
         configured 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
         information 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
         DHCPNAK 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 time.

         Usually, writing authoritative; at the top level of the file should
         be sufficient.   However, if a DHCP server is to be set up so that it
         is aware of some networks for which it is authoritative and some
         networks 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

       The boot-unknown-clients statement

         boot-unknown-clients flag;

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

       The ddns-hostname statement

         ddns-hostname name;

         The name parameter should be the hostname that will be used in
         setting up the client's A and PTR records.   If no ddns-hostname is
         specified in scope, then the server will derive the hostname
         automatically, 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 ddns-rev-domainname statement

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

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

       The ddns-update-style parameter

         ddns-update-style style;

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

       The 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
         expiration time.

       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 and ddns-update-style is set to
         interim.   Forward updates are enabled by default.   If this
         statement is used to disable forward updates, the DHCP server will
         never attempt to 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 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 assignes 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 YYYY/MM/DD HH:MM:SS

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

       The dynamic-bootp-lease-length statement

         dynamic-bootp-lease-length length;

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

       The filename statement

         filename "filename";

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

       The fixed-address declaration

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

         The fixed-address declaration is used to assign one or more fixed IP
         addresses to a client.  It should only appear in a host declaration.
         If more than one address is supplied, then when the client boots, it
         will be assigned the address that corresponds to the network on which
         it is booting.  If none of the addresses in the fixed-address
         statement 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 get-lease-hostnames statement

         get-lease-hostnames flag;

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

       The hardware statement

         hardware hardware-type hardware-address;

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

       The lease-file-name statement

         lease-file-name name;

         Name should be the name of the DHCP server's lease file.   By
         default, this is /var/lib/dhcpd/dhcpd.leases.   This statement must
         appear in the outer scope of the configuration file - if it appears
         in some other scope, it will have no effect.

       The 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
         networks...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
         compile-time for all others.  You can be sure if your server is
         compiled with USE_SOCKETS if you see lines of this format at startup:

          Listening on Socket/eth0

         Note also that since this bind()s all DHCP sockets to the 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

         In addition to setting this value, you may need to modify your
         syslog.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 how to change the default log facility.  When this
         parameter is used, the DHCP server prints its startup message a
         second time after parsing the configuration file, so that the log
         will be as complete as possible.

       The max-lease-time statement

         max-lease-time time;

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

       The min-lease-time statement

         min-lease-time time;

         Time should be the minimum length in seconds that will be assigned to
         a lease.

       The min-secs statement

         min-secs seconds;

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

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

       The next-server statement

         next-server server-name;

         The next-server statement is used to specify the host address of the
         server from which the initial boot file (specified in the filename
         statement) is to be loaded.   Server-name should be a numeric IP
         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
         DHCPREQUEST, it has forgotten any lease not mentioned in the
         DHCPREQUEST - i.e., the client has only a single network interface
         and it does not remember leases it's holding on networks to which it
         is not currently attached.   Neither of these assumptions are
         guaranteed or provable, so we urge caution in the use of this

       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 /var/run/dhcpd.pid.   Like the
         lease-file-name statement, this statement must appear in the outer
         scope of the configuration file.

       The ping-check statement

         ping-check flag;

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

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

       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 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 configuration 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 configuratoin value should generally not be used.

       The server-identifier statement

         server-identifier hostname;

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

       The server-name statement

         server-name name ;

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

       The site-option-space statement

         site-option-space name ;

         The site-option-space statement can be used to determine from what
         option space site-local options will be taken.   This can be used in
         much the same way as the vendor-option-space statement.  Site-local
         options in DHCP are those options whose numeric codes are greater
         than 128.   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 allocated on an ad
         hoc basis, it is quite possible that one vendor's DHCP client might
         use the same option code that another vendor's client uses, for
         different purposes.   The site-option-space option can be used to
         assign a different set of site-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 RENEWING state.   This
         works around a problem with relay agent information 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-optimization statement

         update-optimization flag;

         If the update-optimization parameter is false for a given client, the
         server will attempt a DNS update for that client each time the client
         renews its lease, rather than only attempting an update when it
         appears to be necessary.   This will allow the DNS to heal from
         database inconsistencies more easily, but the cost is that the DHCP
         server must do many more DNS updates.   We recommend leaving this
         option enabled, which is the default.  This option only affects the
         behavior of the interim DNS update scheme, and has no effect on the
         ad-hoc DNS update scheme.   If this parameter is not specified, or is
         true, the DHCP server will only update when the client information
         changes, the client gets a different lease, or the client's lease

       The update-static-leases statement

         update-static-leases flag;

         The update-static-leases flag, if enabled, causes the DHCP server to
         do DNS updates for clients even if those clients are being assigned
         their IP address using a fixed-address statement - that is, the
         client is being given a static assignment.   This can only work with
         the interim DNS update scheme.   It is not recommended because the
         DHCP server has no way to tell that the update has been done, and
         therefore will not delete the record when it is not in use.   Also,
         the server must attempt the update each time the client renews its
         lease, which could have a significant 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 provided for
         the host declaration will be supplied to the client as its hostname.
         So, for example,

             group {
               use-host-decl-names on;

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

         is equivalent to

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

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

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

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

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

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

       The vendor-option-space statement

         vendor-option-space string;

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

       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
       evaluation to an option, define the option as follows:

         my-parameter = expression ;

       For example:

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

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

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

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

       dhcpd.conf(5) was written by Ted Lemon under a contract with Vixie
       Labs.   Funding for this project was provided by Internet Systems
       Consortium.  Information about Internet Systems Consortium can be found
       at http://www.isc.org.