usr.sbin/dhcpd/dhcpd.8

.\"	$OpenBSD: dhcpd.8,v 1.29 2017/08/29 08:20:18 jmc Exp $
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.\" CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
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.\" This software has been written for the Internet Software Consortium
.\" by Ted Lemon <mellon@fugue.com> in cooperation with Vixie
.\" Enterprises.  To learn more about the Internet Software Consortium,
.\" see ``http://www.isc.org/''.  To learn more about Vixie
.\" Enterprises, see ``http://www.vix.com''.
.\"
.Dd $Mdocdate: August 29 2017 $
.Dt DHCPD 8
.Os
.Sh NAME
.Nm dhcpd
.Nd Dynamic Host Configuration Protocol (DHCP) server
.Sh SYNOPSIS
.Nm dhcpd
.Bk -words
.Op Fl dfn
.Op Fl A Ar abandoned_ip_table
.Op Fl C Ar changed_ip_table
.Op Fl c Ar config-file
.Op Fl L Ar leased_ip_table
.Op Fl l Ar lease-file
.Op Fl u Ns Op Ar bind_address
.Op Fl Y Ar synctarget
.Op Fl y Ar synclisten
.Op Ar if0 Op Ar ... ifN
.Ek
.Sh DESCRIPTION
.Nm
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.
.Pp
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
.Xr dhcpd.conf 5
file.
.Pp
On startup,
.Nm
reads the
.Pa 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,
.Nm
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,
.Nm
may delete the leased IP from certain
.Xr 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.
.Pp
In order to keep track of leases across system reboots and server restarts,
.Nm
keeps a list of leases it has assigned in the
.Xr dhcpd.leases 5
file.
Before
.Nm
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,
.Nm
will not forget about a lease that it has assigned.
On startup, after reading the
.Pa dhcpd.conf
file,
.Nm
reads the
.Pa dhcpd.leases
file to refresh its memory about what leases have been assigned.
.Pp
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.
.Pp
BOOTP clients may also be served in the old standard way, which is
simply to provide a declaration in the
.Pa dhcpd.conf
file for each BOOTP client, permanently assigning an address to each client.
.Pp
Whenever changes are made to the
.Pa dhcpd.conf
file,
.Nm
must be restarted.
Because the DHCP server database is not as lightweight as a BOOTP database,
.Nm
does not automatically restart itself when it sees a change to the
.Pa dhcpd.conf
file.
.Pp
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.
.Sh COMMAND LINE
The names of the network interfaces on which
.Nm
should listen for broadcasts may be specified on the command line.
This should be done on systems where
.Nm
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,
and the
.Fl u
option is not given,
.Nm
will identify all network interfaces which are up, eliminating non-broadcast
interfaces if possible, and listen for DHCP broadcasts on each interface.
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl A Ar abandoned_ip_table
When an address is abandoned for some reason, add it to the
.Xr pf 4
table named
.Ar abandoned_ip_table .
This can be used to defend against machines "camping" on an address
without obtaining a lease.
When an address is properly leased,
.Nm
will remove the address from this table.
.It Fl C Ar changed_ip_table
When an address is leased to a different hardware address, delete it from the
.Xr pf 4
table named
.Ar changed_ip_table .
This feature complements the overload table in a stateful
.Xr pf 4
rule.
If a host appears to be misbehaving, it can be quarantined by using the
overload feature.
When the address is leased to a different machine,
.Nm
can remove the address from the overload table, thus allowing a well-behaved
machine to reuse the address.
.It Fl c Ar config-file
Use an alternate configuration file,
.Ar config-file .
Because of the importance of using the same lease database at all times when
running
.Nm
in production, this option should be used
.Em only
for testing database files in a non-production environment.
.It Fl d
Do not daemonize.
If this option is specified,
.Nm
will run in the foreground and log to
.Em stderr .
.It Fl f
An alias for
.Fl d .
.It Fl L Ar leased_ip_table
When an address is leased
.Nm
will insert it into the
.Xr pf 4
table named
.Ar 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;
.Xr 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.
.It Fl l Ar lease-file
Use an alternate lease file,
.Ar lease-file .
Because of the importance of using the same lease database at all times when
running
.Nm
in production, this option should be used
.Em only
for testing lease files in a non-production environment.
.It Fl n
Only test configuration, do not run
.Nm .
.It Fl u Ns Op Ar bind_address
Use a UDP socket instead of BPF for receiving and sending packets.
Only
.Dv DHCPINFORM
messages can be handled on this socket;
other messages are discarded.
With this option,
.Nm
can answer
.Dv DHCPINFORM
from clients on non Ethernet interfaces
such as
.Xr tun 4
or
.Xr pppx 4 .
If
.Ar bind_address
is specified,
.Nm
will bind to that address; otherwise
the limited broadcast address (255.255.255.255) is used as the default.
.It Fl Y Ar synctarget
Add target
.Ar synctarget
to receive synchronisation messages.
.Ar synctarget
can be either an IPv4 address for unicast messages
or a network interface name followed optionally by a colon and a numeric TTL
value for multicast messages to the group 224.0.1.240.
If the multicast TTL is not specified, a default value of 1 is used.
This option can be specified multiple times.
See also
.Sx SYNCHRONISATION
below.
.It Fl y Ar synclisten
Listen on
.Ar synclisten
for incoming synchronisation messages.
The format for
.Ar synclisten
is the same as for
.Ar synctarget ,
above.
This option can be specified only once.
See also
.Sx SYNCHRONISATION
below.
.El
.Sh CONFIGURATION
The syntax of the
.Xr dhcpd.conf 5
file is discussed separately.
This section should be used as an overview of the configuration process,
and the
.Xr dhcpd.conf 5
documentation should be consulted for detailed reference information.
.Bl -tag -width 3n
.It Subnets
.Nm
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:
.Bd -literal -offset indent
subnet 239.252.197.0 netmask 255.255.255.0 {
  range 239.252.197.10 239.252.197.250;
}
.Ed
.Pp
Multiple address ranges may be specified like this:
.Bd -literal -offset indent
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;
}
.Ed
.Pp
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.
.It Lease Lengths
DHCP leases can be assigned almost any length from zero seconds to infinity.
What lease length makes sense for any given subnet, or for any given
installation, will vary depending on the kinds of hosts being served.
.Pp
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.
.Pp
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:
.Bd -literal -offset indent
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;
}
.Ed
.Pp
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).
.Pp
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.
.It BOOTP Support
Each BOOTP client must be explicitly declared in the
.Xr dhcpd.conf 5
file.
A very basic client declaration will specify the client network interface's
hardware address and the IP address to assign to that client.
If the client needs to be able to load a boot file from the server,
that file's name must be specified.
A simple BOOTP client declaration might look like this:
.Bd -literal -offset indent
host haagen {
  hardware ethernet 08:00:2b:4c:59:23;
  fixed-address 239.252.197.9;
  filename "haagen.boot";
}
.Ed
.It Options
DHCP (and also BOOTP with Vendor Extensions) provides a mechanism
whereby the server can provide the client with information about how
to configure its network interface (e.g., subnet mask), and also how
the client can access various network services (e.g., DNS, IP routers,
and so on).
.Pp
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:
.Bd -literal -offset indent
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";
}
.Ed
.Pp
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:
.Bd -literal -offset indent
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";
}
.Ed
.El
.Pp
A more complete description of the
.Pa dhcpd.conf
file syntax is provided in
.Xr dhcpd.conf 5 .
.Sh SYNCHRONISATION
.Nm
supports realtime synchronisation of the lease allocations to
a number of
.Nm
daemons running on multiple machines,
using the
.Fl Y
and
.Fl y
options.
.Pp
The following example will accept incoming multicast and unicast
synchronisation messages, and send outgoing multicast messages through
the network interface
.Ar em0 :
.Bd -literal -offset indent
# /usr/sbin/dhcpd -y em0 -Y em0
.Ed
.Pp
The second example will increase the multicast TTL to a value of 2,
add the unicast targets
.Ar foo.somewhere.org
and
.Ar bar.somewhere.org ,
and accept incoming unicast messages sent to
.Ar example.somewhere.org
only.
.Bd -literal -offset indent
# /usr/sbin/dhcpd -y example.somewhere.org -Y em0:2 \e
	-Y foo.somewhere.org -Y bar.somewhere.org
.Ed
.Pp
If the file
.Pa /var/db/dhcpd.key
exists,
.Nm
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:
.Bd -literal -offset indent
# dd if=/dev/random of=/var/db/dhcpd.key bs=2048 count=1
.Ed
.Pp
The file needs to be copied to all hosts
sending or receiving synchronisation messages.
.Pp
All hosts using synchronisation must use the same configuration in the
.Pa /etc/dhcpd.conf
file.
.Sh FILES
.Bl -tag -width "/var/db/dhcpd.leases~  " -compact
.It Pa /etc/dhcpd.conf
DHCPD configuration file.
.It Pa /var/db/dhcpd.leases
DHCPD lease file.
.El
.Sh SEE ALSO
.Xr pf 4 ,
.Xr dhcpd.conf 5 ,
.Xr dhcpd.leases 5 ,
.Xr dhclient 8 ,
.Xr dhcrelay 8 ,
.Xr pxeboot 8
.Sh STANDARDS
.Rs
.%A R. Droms
.%D October 1993
.%R RFC 1534
.%T Interoperation Between DHCP and BOOTP
.Re
.Pp
.Rs
.%A R. Droms
.%D March 1997
.%R RFC 2131
.%T Dynamic Host Configuration Protocol
.Re
.Pp
.Rs
.%A S. Alexander
.%A R. Droms
.%D March 1997
.%R RFC 2132
.%T DHCP Options and BOOTP Vendor Extensions
.Re
.Pp
.Rs
.%A T. Lemon
.%A S. Cheshire
.%D November 2002
.%R RFC 3396
.%T Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)
.Re
.Pp
.Rs
.%A T. Lemon
.%A S. Cheshire
.%A B. Volz
.%D December 2002
.%R RFC 3442
.%T The Classless Static Route Option for Dynamic Host Configuration Protocol (DHCP) version 4
.Re
.Sh AUTHORS
.An -nosplit
.Nm
is based on software from the Internet Software Consortium,
written by
.An Ted Lemon Aq Mt mellon@vix.com
under a contract with Vixie Labs.
The current implementation was reworked for
.Ox
by
.An Henning Brauer Aq Mt henning@openbsd.org .
.Sh BUGS
We realize that it would be nice if one could send a
.Dv 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.