dhcpd

From ArchWiki

dhcpd is the Internet Systems Consortium DHCP Server. It is useful for instance on a machine acting as a router on a LAN.

Note: dhcpd (DHCP (server) daemon) is not the same as dhcpcd (DHCP client daemon).

Installation

Install the dhcp package.

Usage

dhcpd includes two unit files, dhcpd4.service and dhcpd6.service, which can be used to control the daemon. They start the daemon on all network interfaces for IPv4 and IPv6 respectively. See #Listening on only one interface for an alternative.

Configuration

Assign a static IPv4 address to the interface you want to use (in our examples we will use eth0). The specified subnet should not overlap with that of other interfaces.

# ip link set up dev eth0
# ip addr add 139.96.30.100/24 dev eth0 # arbitrary address
Tip: Usually, one of the next three subnets is used for private networks, which are specially reserved and will not conflict with any host in the Internet:
  • 192.168/16 (subnet 192.168.0.0, netmask 255.255.0.0)
  • 172.16/12 (subnet 172.16.0.0, netmask 255.240.0.0)
  • 10/8 (for large networks; subnet 10.0.0.0, netmask 255.0.0.0)
See also RFC 1918.

To have your static ip assigned at boot, see Network configuration#Static IP address.

The default dhcpd.conf contains many uncommented examples, so relocate it:

# cp /etc/dhcpd.conf /etc/dhcpd.conf.example

To just listen on the subnet 139.96.30.0/24, you can create a minimal configuration file that looks like the following:

/etc/dhcpd.conf
option domain-name-servers 8.8.8.8, 8.8.4.4;
option subnet-mask 255.255.255.0;
option routers 139.96.30.100;
subnet 139.96.30.0 netmask 255.255.255.0 {
  range 139.96.30.150 139.96.30.250;
}

Note that:

  • If eth0 is the only interface that is in the subnet 139.96.30.0/24, as is usually the case for most of the time, then dhcpd will only be listening on that interface. It will not listen on other interfaces. This is usually what you want anyway.
  • If you want dhcpd to listen on other interfaces, modify the configuration file and specify the respective subnets of those respective interfaces.

If you need to provide a fixed IP address for a single specific device, you can define host blocks:

/etc/dhcpd.conf
option domain-name-servers 8.8.8.8, 8.8.4.4;
option subnet-mask 255.255.255.0;
option routers 139.96.30.100;
subnet 139.96.30.0 netmask 255.255.255.0 {
  range 139.96.30.150 139.96.30.250;
}
host macbookpro {
  hardware ethernet 70:56:81:22:33:44;
  fixed-address 139.96.30.199;
}

domain-name-servers option contains addresses of DNS servers which are supplied to clients. In our example we are using Google's public DNS servers. If you know a local DNS server (for example, provided by your ISP), you should consider using it. If you have configured your own DNS on a local machine, then use its address in your subnet (e. g. 139.96.30.100 in our example).

subnet-mask and routers defines a subnet mask and a list of available routers on the subnet. In most cases for small networks you can use 255.255.255.0 as a mask and specify an IP address of the machine on which you are configuring DHCP server as a router.

subnet blocks defines options for separate subnets, which are mapped to the network interfaces on which dhcpd is running. In our example this is one subnet 139.96.30.0/24 for single interface eth0, for which we defined the range of available IP addresses. Addresses from this range will be assigned to the connecting clients.

Listening on only one interface

Tango-view-refresh-red.pngThis article or section is out of date.Tango-view-refresh-red.png

Reason: In 2022, this is not accurate. The man page suggests this behavior, but in practice, dhcpd only listens on interfaces whose subnets are declared in its configuration file. (Discuss in Talk:Dhcpd)

If your computer is already part of one or several networks, it could be a problem if your computer starts giving ip addresses to machines from the other networks. It can be done by either configuring dhcpd or starting it as a daemon with systemctl.

Configuring dhcpd

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: dhcpd does not listen on interfaces whose subnet are not declared in its configuration file (Discuss in Talk:Dhcpd)

In order to exclude an interface, you must create an empty declaration for the subnet that will be configured on that interface.

This is done by editing the configuration file (for example):

/etc/dhcpd.conf
# No DHCP service in DMZ network (192.168.2.0/24)
subnet 192.168.2.0 netmask 255.255.255.0 {
}

Service file

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: Does not work with systemd 251 (Discuss in Talk:Dhcpd)

The default service file provided by dhcpd does not specify an interface. Use a drop-in unit file for the dhcpd4.service as follows:

/etc/systemd/system/dhcpd4.service.d/override.conf
[Service]
ExecStart=
ExecStart=/usr/bin/dhcpd -4 -q -cf /etc/dhcpd.conf -pf /run/dhcpd4/dhcpd.pid %I

This allows using dhcpd4.service as a template unit, binding dhcpd to a particular interface; for example dhcpd4@eth0.service, where eth0 is the first enumerated Ethernet device.

Use for PXE

PXE Configuration is done with the following two options:

/etc/dhcpd.conf
next-server 192.168.0.2;
filename "/pxelinux.0";

This section can either be in an entire subnet or just in a host definition. next-server is the IP of the TFTP Server, and filename is the filename of the image to boot. For more information see PXE.

See also