MirBSD manpage: dhcpd.conf(5)

DHCPD.CONF(5)                BSD Reference Manual                DHCPD.CONF(5)


     dhcpd.conf - dhcpd configuration file


     The dhcpd.conf file contains configuration information for dhcpd(8), the
     Internet Software 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(8). 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 say how to do something (e.g., how long a lease to
     offer), whether to do something (e.g., should dhcpd(8) 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 as-
     signed to clients, or to apply a group of parameters to a group of de-
     clarations. In any group of parameters and declarations, all parameters
     must be specified before any declarations which depend on those parame-
     ters may be specified.

     Declarations about network topology include the shared-network and the
     subnet declarations. If clients on a subnet are to be assigned addresses
     dynamically, a range declaration must appear within the subnet declara-
     tion. For clients with statically assigned addresses, or for installa-
     tions 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 de-
     clarations 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(8) 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 sub-
     net 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 declara-
     tions for these two networks may be enclosed in a shared-network declara-

     Some sites may have departments which have clients on more than one sub-
     net, 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,
     there is currently no way to group parameter assignments other than by
     network topology.

     When a client is to be booted, its boot parameters are determined by
     first consulting that client's host declaration (if any), then consulting
     the group declaration (if any) which enclosed that host declaration, then
     consulting the subnet declaration for the subnet on which the client is
     booting, then consulting the shared-network declaration (if any) contain-
     ing that subnet, and finally consulting the top-level parameters which
     may be specified outside of any declaration.

     When dhcpd(8) tries to find a host declaration for a client, it first
     looks for a host declaration which has a fixed-address parameter which
     matches the subnet or shared network on which the client is booting. If
     it doesn't find any such entry, it then tries to find an entry which has
     no fixed-address parameter. If no such entry is found, then dhcpd(8) acts
     as if there is no entry in the dhcpd.conf file for that client, even if
     there is an entry for that client on a different subnet or shared net-


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

     Example 1

           global parameters...

           shared-network ISC-BIGGIE {
             shared-network-specific 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...

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

     Example 2

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

     As you can see in Example 2, it's legal to specify host addresses in
     parameters as domain names rather than as numeric IP addresses. If a
     given hostname resolves to more than one IP address (for example, if that
     host has two ethernet interfaces), both addresses are supplied to the

     In Example 1, you can see that both the shared-network statement and the
     subnet statements can have parameters. Let us say that the shared network
     ISC-BIGGIE supports an entire department - perhaps the accounting depart-
     ment. If accounting has its own domain, then a shared-network-specific
     parameter might be:

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

     All subnet declarations appearing in the shared-network declaration would
     then have the domain-name option set to "accounting.isc.org" instead of
     just "isc.org".

     The most obvious reason for having subnet-specific parameters as shown in
     Example 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 in-
     stead 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 Example 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 behaviour of the DHCP server
     (e.g., how long a lease dhcpd(8) will give out), or specify client param-
     eters that are not optional in the DHCP protocol (for example, server-
     name and filename).

     In Example 1, each host had host-specific parameters. These could include
     such things as the hostname option, the name of a file to download (the
     filename parameter) and the address of the server from which to download
     the file (the next-server parameter). In general, any parameter can ap-
     pear 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 termi-
     nals 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 ncd5 { hardware ethernet 0:c0:c3:11:90:23; }
             host ncd6 { hardware ethernet 0:c0:c3:91:a7:8; }
             host ncd7 { hardware ethernet 0:c0:c3:cc:a:8f; }

     The shared-network statement

           shared-network name {

     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 ad-
     dresses 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 net-
     work 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

     The subnet statement

           subnet subnet-number netmask netmask {

     The subnet statement is used to provide dhcpd(8) 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 ad-
     dresses 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

     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 omitted.

     The host statement

           host hostname {

     There must be at least one host statement for every BOOTP client that is
     to be served. host statements may also be specified for DHCP clients,
     although this is not required unless booting is only enabled for known

     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 parameter, or more than one host statement may be

     If client-specific boot parameters must change based on the network to
     which the client is attached, then multiple host statements should be

     If a client is to be booted using a fixed address if it's possible, but
     should be allocated a dynamic address otherwise, then a host statement
     must be specified without a fixed-address clause. 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 ad-
     dress 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.

     The group statement

           group {

     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 behaviour of
     dhcpd(8) to various sorts of requests.

     The unknown-clients keyword

           allow unknown-clients;
           deny unknown-clients;

     The unknown-clients flag is used to tell dhcpd(8) whether or not to
     dynamically assign addresses to unknown clients. Dynamic address assign-
     ment to unknown clients is allowed by default.

     The bootp keyword

           allow bootp;
           deny bootp;

     The bootp flag is used to tell dhcpd(8) whether or not to respond to
     bootp queries. Bootp queries are allowed by default.

     The booting keyword

           allow booting;
           deny booting;

     The booting flag is used to tell dhcpd(8) 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 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

     The max-lease-time statement

           max-lease-time time;

     time should be the maximum length in seconds that will be assigned to a
     lease if the client requesting the lease asks for a specific expiration

     The hardware statement

           hardware hardware-type hardware-address;

     In order for a BOOTP client to be recognized, its network hardware ad-
     dress 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 an 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 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 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 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 state-
     ment) is to be loaded. server-name should be a numeric IP address or a
     domain name. If no next-server parameter applies to a given client, the
     DHCP server's IP address is used.

     The fixed-address statement

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

     The fixed-address statement is used to assign one or more fixed IP ad-
     dresses 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 as-
     signed the address which corresponds to the network on which it is boot-
     ing. If none of the addresses in the fixed-address statement are on the
     network on which the client is booting, that client will not match the
     host declaration containing that fixed-address statement. Each address
     should be either an IP address or a domain name which resolves to one or
     more IP addresses.

     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(8) assigns infinite leases to all BOOTP
     clients. However, it may make sense in some situations to set a cutoff
     date for all BOOTP leases - for example, the end of a school term, or the
     time at night when a facility is closed and all machines are required to
     be powered off.

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

           W 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 ex-
     pressed 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

     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 ex-
     treme caution.

     The get-lease-hostnames statement

           get-lease-hostnames flag;

     The get-lease-hostnames statement is used to tell dhcpd(8) whether or not
     to look up the domain name corresponding to the IP address of each ad-
     dress in the lease pool and use that address for the DHCP hostname op-
     tion. 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 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 exam-

           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.

     The authoritative statement


           not authoritative;

     The DHCP server will normally assume that the configuration information
     about a given network segment is known to be correct and is authorita-
     tive. So if a client requests an IP address on a given network segment
     that the server knows is not valid for that segment, the server will
     respond with a DHCPNAK message, causing the client to forget its IP ad-
     dress and try to get a new one.

     If a DHCP server is being configured by somebody who is not the network
     administrator and who therefore does not wish to assert this level of au-
     thority, then the statement "not authoritative" should be written in the
     appropriate scope in the configuration file.

     Usually, writing not 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 au-
     thoritative for some subnets within a shared network, but not authorita-
     tive for others, nor is it meaningful to specify that the server is au-
     thoritative for some host declarations and not others.

     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.

     If use-lease-addr-for-default-route is enabled and an option routers
     statement are both in scope, the routers option will be preferred. The
     rationale for this is that in situations where you want to use this
     feature, you probably want it enabled for a whole bunch of Windows 95
     machines, and you want to override it for a few other machines. Unfor-
     tunately, if the opposite happens to be true for your site, you are prob-
     ably better off not trying to use this flag.

     The always-reply-rfc1048 statement

           always-reply-rfc1048 flag;

     Some BOOTP clients expect RFC 1048-style responses, but do not follow RFC
     1048 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 RFC 1048 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 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 reach-
     able 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 inter-
     face 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 pur-
     pose 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.

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


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

     RFC 2132, RFC 2131.


     dhcpd(8) was written by Ted Lemon <mellon@vix.com> under a contract with
     Vixie Labs.

     The current implementation was reworked by Henning Brauer

MirBSD #10-current             January 1, 1995                               9

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