NAME

SYNOPSIS

DESCRIPTION

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.

COMMAND LINE

The options are as follows:

-A abandoned_ip_table

When an address is abandoned for some reason, add it to the pf(4) table named 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, dhcpd will remove the address from this table.

-C changed_ip_table

When an address is leased to a different hardware address, delete it from the pf(4) table named changed_ip_table. This feature complements the overload table in a stateful 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, dhcpd can remove the address from the overload table, thus allowing a well-behaved machine to reuse the address.

-c config-file

Use an alternate configuration file, config-file. Because of the importance of using the same lease database at all times when running dhcpd in production, this option should be used only for testing database files in a non-production environment.

-d

Do not daemonize. If this option is specified, dhcpd will run in the foreground and log to stderr.

-f

An alias for -d.

-L leased_ip_table

When an address is leased dhcpd 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-file

Use an alternate lease file, lease-file. Because of the importance of using the same lease database at all times when running dhcpd in production, this option should be used only for testing lease files in a non-production environment.

-n

Only test configuration, do not run dhcpd.

-u[bind_address]

Use a UDP socket instead of BPF for receiving and sending packets. Only DHCPINFORM messages can be handled on this socket; other messages are discarded. With this option, dhcpd can answer DHCPINFORM from clients on non Ethernet interfaces such as tun(4) or pppx(4). If bind_address is specified, dhcpd will bind to that address; otherwise the limited broadcast address (255.255.255.255) is used as the default.

-Y synctarget

Add target synctarget to receive synchronisation messages. 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 SYNCHRONISATION below.

-y synclisten

Listen on synclisten for incoming synchronisation messages. The format for synclisten is the same as for synctarget, above. This option can be specified only once. See also SYNCHRONISATION below.

CONFIGURATION

Subnets

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.

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.

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.

BOOTP Support

Each BOOTP client must be explicitly declared in the 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:

host haagen {
  hardware ethernet 08:00:2b:4c:59:23;
  fixed-address 239.252.197.9;
  filename "haagen.boot";
}
    

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).

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).

SYNCHRONISATION

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/random 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 /usr/local/etc/dhcpd.conf file.

FILES

/usr/local/etc/dhcpd.conf

DHCPD configuration file.

/var/db/dhcpd.leases

DHCPD lease file.

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.

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.

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