DHCPD(8) | System Manager's Manual | DHCPD(8) |
dhcpd
— Dynamic
Host Configuration Protocol (DHCP) server
dhcpd |
[-dfn ]
[-A abandoned_ip_table]
[-C changed_ip_table]
[-c config-file]
[-L leased_ip_table]
[-l lease-file]
[-Y synctarget]
[-y synclisten]
[if0 [... ifN]] |
The Internet Software Consortium DHCP Server,
dhcpd
, implements the Dynamic Host Configuration
Protocol (DHCP) and the Internet Bootstrap Protocol (BOOTP). DHCP allows
hosts on a TCP/IP network to request and be assigned IP addresses, and also
to discover information about the network to which they are attached. BOOTP
provides similar functionality, with certain restrictions.
The DHCP protocol allows a host which is unknown to the network administrator to be automatically assigned a new IP address out of a pool of IP addresses for its network. In order for this to work, the network administrator allocates address pools in each subnet and enters them into the dhcpd.conf(5) file.
On startup, dhcpd
reads the
dhcpd.conf file and stores a list of available
addresses on each subnet in memory. When a client requests an address using
the DHCP protocol, dhcpd
allocates an address for
it. Each client is assigned a lease, which expires after an amount of time
chosen by the administrator (by default, one day). When a leased IP address
is assigned to a new hardware address, dhcpd
may
delete the leased IP from certain
pf(4) tables. Before leases
expire, the clients to which leases are assigned are expected to renew them
in order to continue to use the addresses. Once a lease has expired, the
client to which that lease was assigned is no longer permitted to use the
leased IP address.
In order to keep track of leases across system reboots and server
restarts, dhcpd
keeps a list of leases it has
assigned in the
dhcpd.leases(5) file.
Before dhcpd
grants a lease to a host, it records
the lease in this file and makes sure that the contents of the file are
flushed to disk. This ensures that even in the event of a system crash,
dhcpd
will not forget about a lease that it has
assigned. On startup, after reading the dhcpd.conf
file, dhcpd
reads the
dhcpd.leases file to refresh its memory about what
leases have been assigned.
BOOTP support is also provided by this server. Unlike DHCP, the BOOTP protocol does not provide a protocol for recovering dynamically-assigned addresses once they are no longer needed. It is still possible to dynamically assign addresses to BOOTP clients, but some administrative process for reclaiming addresses is required. By default, leases are granted to BOOTP clients in perpetuity, although the network administrator may set an earlier cutoff date or a shorter lease length for BOOTP leases if that makes sense.
BOOTP clients may also be served in the old standard way, which is simply to provide a declaration in the dhcpd.conf file for each BOOTP client, permanently assigning an address to each client.
Whenever changes are made to the
dhcpd.conf file, dhcpd
must
be restarted. Because the DHCP server database is not as lightweight as a
BOOTP database, dhcpd
does not automatically restart
itself when it sees a change to the dhcpd.conf
file.
DHCP traffic always bypasses IPsec. Otherwise there could be situations when a server has an IPsec SA for the client and sends replies over that, which a newly booted client would not be able to grasp.
The names of the network interfaces on which
dhcpd
should listen for broadcasts may be specified
on the command line. This should be done on systems where
dhcpd
is unable to identify non-broadcast
interfaces, but should not be required on other systems. If no interface
names are specified on the command line, dhcpd
will
identify all network interfaces which are up, eliminating non-broadcast
interfaces if possible, and listen for DHCP broadcasts on each
interface.
The options are as follows:
-A
abandoned_ip_tabledhcpd
will remove the address from this table.-C
changed_ip_tabledhcpd
can remove the address from the overload
table, thus allowing a well-behaved machine to reuse the address.-c
config-filedhcpd
in production, this option
should be used only for testing database files in a
non-production environment.-d
dhcpd
to log to
stderr. This can be useful for debugging, and also
at sites where a complete log of all dhcp activity must be kept, but
syslogd(8) is not reliable
or otherwise cannot be used. Normally, dhcpd
will
log all output using the
syslog(3) function with the
log facility set to LOG_DAEMON
.-f
dhcpd
as a foreground process, rather than
allowing it to run as a daemon in the background. This is useful when
running dhcpd
under a debugger, or when running it
out of inittab on System V systems.-L
leased_ip_tabledhcpd
will insert it
into the pf(4) table named
leased_ip_table. Addresses are removed from the
table when the lease expires. Combined with the table of abandoned
addresses, this can help enforce a requirement to use DHCP on a network,
or can place DHCP users in a different class of service. Users are
cautioned against placing much trust in Ethernet or IP addresses;
ifconfig(8) can be used
to trivially change the interface's address, and on a busy DHCP network,
IP addresses will likely be quickly recycled.-l
lease-filedhcpd
in production, this option should be used
only for testing lease files in a non-production
environment.-n
dhcpd
.-Y
synctarget-y
synclistenThe syntax of the dhcpd.conf(5) file is discussed separately. This section should be used as an overview of the configuration process, and the dhcpd.conf(5) documentation should be consulted for detailed reference information.
dhcpd
needs to know the subnet numbers and
netmasks of all subnets for which it will be providing service. In
addition, in order to dynamically allocate addresses, it must be assigned
one or more ranges of addresses on each subnet which it can in turn assign
to client hosts as they boot. Thus, a very simple configuration providing
DHCP support might look like this:
subnet 239.252.197.0 netmask 255.255.255.0 { range 239.252.197.10 239.252.197.250; }
Multiple address ranges may be specified like this:
subnet 239.252.197.0 netmask 255.255.255.0 { range 239.252.197.10 239.252.197.107; range 239.252.197.113 239.252.197.250; }
If a subnet will only be provided with BOOTP service and no dynamic address assignment, the range clause can be left out entirely, but the subnet statement must appear.
For example, in an office environment where systems are added from time to time and removed from time to time, but move relatively infrequently, it might make sense to allow lease times of a month or more. In a final test environment on a manufacturing floor, it may make more sense to assign a maximum lease length of 30 minutes - enough time to go through a simple test procedure on a network appliance before packaging it up for delivery.
It is possible to specify two lease lengths: the default length that will be assigned if a client doesn't ask for any particular lease length, and a maximum lease length. These are specified as clauses to the subnet command:
subnet 239.252.197.0 netmask 255.255.255.0 { range 239.252.197.10 239.252.197.107; default-lease-time 600; max-lease-time 7200; }
This particular subnet declaration specifies a default lease time of 600 seconds (ten minutes), and a maximum lease time of 7200 seconds (two hours). Other common values would be 86400 (one day), 604800 (one week) and 2592000 (30 days).
Each subnet need not have the same lease - in the case of an office environment and a manufacturing environment served by the same DHCP server, it might make sense to have widely disparate values for default and maximum lease times on each subnet.
host haagen { hardware ethernet 08:00:2b:4c:59:23; fixed-address 239.252.197.9; filename "haagen.boot"; }
These options can be specified on a per-subnet basis, and, for BOOTP clients, also on a per-client basis. In the event that a BOOTP client declaration specifies options that are also specified in its subnet declaration, the options specified in the client declaration take precedence. A reasonably complete DHCP configuration might look something like this:
subnet 239.252.197.0 netmask 255.255.255.0 { range 239.252.197.10 239.252.197.250; default-lease-time 600; max-lease-time 7200; option subnet-mask 255.255.255.0; option broadcast-address 239.252.197.255; option routers 239.252.197.1; option domain-name-servers 239.252.197.2, 239.252.197.3; option domain-name "isc.org"; }
A BOOTP host on that subnet that needs to be in a different domain and use a different name server might be declared as follows:
host haagen { hardware ethernet 08:00:2b:4c:59:23; fixed-address 239.252.197.9; filename "haagen.boot"; option domain-name-servers 192.5.5.1; option domain-name "vix.com"; }
A more complete description of the dhcpd.conf file syntax is provided in dhcpd.conf(5).
dhcpd
supports realtime synchronisation of
the lease allocations to a number of dhcpd
daemons
running on multiple machines, using the -Y
and
-y
options.
The following example will accept incoming multicast and unicast synchronisation messages, and send outgoing multicast messages through the network interface em0:
# /usr/sbin/dhcpd -y em0 -Y em0
The second example will increase the multicast TTL to a value of 2, add the unicast targets foo.somewhere.org and bar.somewhere.org, and accept incoming unicast messages sent to example.somewhere.org only.
# /usr/sbin/dhcpd -y example.somewhere.org -Y em0:2 \ -Y foo.somewhere.org -Y bar.somewhere.org
If the file /var/db/dhcpd.key exists,
dhcpd
will calculate the message-digest fingerprint
(checksum) for the file and use it as a shared key to authenticate the
synchronisation messages. The file itself can contain any data. For example,
to create a secure random key:
# dd if=/dev/arandom of=/var/db/dhcpd.key bs=2048 count=1
The file needs to be copied to all hosts sending or receiving synchronisation messages.
All hosts using synchronisation must use the same configuration in the /etc/dhcpd.conf file.
pf(4), dhcpd.conf(5), dhcpd.leases(5), dhclient(8), dhcrelay(8), pxeboot(8)
R. Droms, Interoperation Between DHCP and BOOTP, RFC 1534, October 1993.
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.
T. Lemon and S. Cheshire, Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4), RFC 3396, November 2002.
T. Lemon, S. Cheshire, and B. Volz, The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4, RFC 3442, December 2002.
dhcpd
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>.
We realize that it would be nice if one could send a
SIGHUP
to the server and have it reload the
database. This is not technically impossible, but it would require a great
deal of work, our resources are extremely limited, and they can be better
spent elsewhere. So please don't complain about this on the mailing list
unless you're prepared to fund a project to implement this feature, or
prepared to do it yourself.
January 3, 2014 | OpenBSD-5.5 |