NAME
dhcpd.conf
—
DHCP server configuration
file
DESCRIPTION
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 220.177.244.7).
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(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 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 may 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, 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) containing 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 network.
EXAMPLES
A typical dhcpd.conf
file will look
something like this:
Example 1
global parameters... shared-network ISC-BIGGIE { shared-network-specific parameters ... subnet 204.254.239.0 netmask 255.255.255.224 { subnet-specific parameters ... range 204.254.239.10 204.254.239.30; } subnet 204.254.239.32 netmask 255.255.255.224 { subnet-specific parameters ... range 204.254.239.42 204.254.239.62; } } subnet 204.254.239.64 netmask 255.255.255.224 { subnet-specific parameters ... range 204.254.239.74 204.254.239.94; } 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 organization), 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 hostnames rather than as numeric IP addresses.
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 department. 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
204.254.239.1;
Note that the address here is specified numerically. This is not required - if you have a different hostname for each interface on your router, it's perfectly legitimate to use the hostname for that interface instead of the numeric address. However, in many cases there may be only one hostname 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 parameters 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 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 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; } }
REFERENCE: DECLARATIONS
The shared-network
statement informs the
DHCP server that some IP subnets actually share the same physical
network:
shared-network
name {
[parameters]
[declarations]
}
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 hostname (although it will never be used as such), or it may be any arbitrary name, enclosed in quotes.
The subnet
statement provides
dhcpd(8) with enough information to tell whether or not an IP address
is on that subnet:
subnet
subnet-numbernetmask
netmask { [parameters] [declarations] }
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 and netmask should be specified as numeric IP addresses. 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 gives the lowest and
highest IP addresses in a range:
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. 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.
There must be at least one host
statement
for every BOOTP client that is to be served:
host
hostname {
[parameters]
[declarations]
}
host
statements may also be specified for
DHCP clients, although this is not required unless booting is only enabled
for known hosts.
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 specified.
If client-specific boot parameters must change based on the
network to which the client is attached, then multiple
host
statements should be used.
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 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.
The group
statement is used simply to
apply one or more parameters to a group of declarations:
group
{
[parameters]
[declarations]
}
It can be used to group hosts, shared networks, subnets, or even other groups.
REFERENCE: ALLOW and DENY
The allow
and deny
statements can be used to control the behaviour of
dhcpd(8) to various sorts of requests.
The unknown-clients flag tells dhcpd(8) whether or not to dynamically assign addresses to unknown clients:
allow unknown-clients; deny unknown-clients;
Dynamic address assignment to unknown clients is allowed by default.
The bootp flag tells dhcpd(8) whether or not to respond to bootp queries:
allow bootp; deny bootp;
Bootp queries are allowed by default.
The booting flag tells dhcpd(8) whether or not to respond to queries from a particular client:
allow booting; deny booting;
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.
REFERENCE: PARAMETERS
The default-lease-time
statement specifies
the time in seconds that will be assigned to a lease
if the client requesting the lease does not ask for a specific expiration
time:
default-lease-time
timeThe max-lease-time
statement specifies the
maximum time in seconds that will be assigned to a
lease if the client requesting the lease asks for a specific expiration
time:
max-lease-time
timeThe hardware
statement allows a BOOTP
client to be recognized in a host
statement:
hardware
hardware-type hardware-addresshardware-type must be the name of a hardware
interface type. Currently, the ethernet
,
token-ring
and fddi
physical
interface types are recognized, although support for DHCP-over-IPsec virtual
interface type ipsec-tunnel
is provided. The
hardware-address should be a set of colon-separated
hexadecimal octets (0-ff) or a hostname that can be looked up in
ethers(5) when the configuration is read. The
hardware
statement may also be used for DHCP
clients.
The filename
statement can be used to
specify the name of the initial boot file which is to be loaded by a
client:
filename
"filename"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 can be used to
inform the client of the name of the server from which it is booting:
server-name
"name"name should be the name that will be provided to the client.
The next-server
statement specifies the
host address of the server from which the initial boot file (specified in
the filename
statement) is to be loaded:
next-server
server-nameserver-name should be a numeric IP address
or a hostname. If no next-server
parameter applies
to a given client, the DHCP server's IP address is used.
The fixed-address
statement assigns one or
more fixed IP addresses to a client:
fixed-address
address [, address ...]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 which corresponds to the network on
which it is booting. 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 hostname
which resolves to one or more IP addresses.
Clients with fixed addresses are not assigned DHCP leases, and may
therefore not be used with the -ACL
table options of
dhcpd(8).
The dynamic-bootp-lease-cutoff
statement
sets the ending time for all leases assigned dynamically to BOOTP
clients:
dynamic-bootp-lease-cutoff
dateBecause 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 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
sets the length of leases dynamically assigned to BOOTP clients:
dynamic-bootp-lease-length
lengthAt 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 get-lease-hostnames
statement tells
dhcpd(8) whether or not to look up the hostname corresponding to the
IP address of each address in the lease pool and use that address for the
DHCP hostname
option:
get-lease-hostnames
flagIf 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.
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:
use-host-decl-names
flagSo, 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.
The authoritative
statement:
authoritative
not authoritative
The DHCP server will normally assume that the configuration information about a given network segment is known to be correct and is authoritative. 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 address 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 authority, 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 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 others.
The use-lease-addr-for-default-route
statement:
use-lease-addr-for-default-route
flagIf 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. Unfortunately, if the opposite happens to be true for your site,
you are probably better off not trying to use this flag.
The always-reply-rfc1048
statement:
always-reply-rfc1048
flagSome 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 can be
used to define the value that is sent in the DHCP Server Identifier option
for a given scope:
server-identifier
hostnameThe 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 echo-client-id
statement controls RFC
6842 compliant behavior and has a default value of true:
echo-client-id
flagSome devices, especially old printers, require
echo-client-id
to be false.
When echo-client-id
is true
dhcpd(8) copies option dhcp-client-identifier (code 61) from DHCP
DISCOVER or REQUEST messages into the DHCP ACK or NAK reply sent to the
client.
REFERENCE: OPTION STATEMENTS
DHCP option statements are documented in the dhcp-options(5) manual page.
SEE ALSO
STANDARDS
R. Droms, Dynamic Host Configuration Protocol, RFC 2131, March 1997.
S. Alexander and R. Droms, DHCP Options and BOOTP Vendor Extensions, RFC 2132, March 1997.
M. Patrick, DHCP Relay Agent Information Option, RFC 3046, January 2001.
B. Patel, B. Aboba, S. Kelly, and V. Gupta, Dynamic Host Configuration Protocol (DHCPv4) Configuration of IPsec Tunnel Mode, RFC 3456, January 2003.
AUTHORS
dhcpd(8) was written by Ted Lemon <mellon@vix.com> under a contract with Vixie Labs.
The current implementation was reworked by Henning Brauer <henning@openbsd.org>.