dhcpd

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

Installation

Install the dhcp package, available in the official repositories.

Configuration

Assign a static IPv4 address to the interface you want to use (in our examples we will use eth0). The first 3 bytes of this address cannot be exactly the same as those of another interface.

# ip link set up dev eth0
# ip addr add 139.96.30.100/24 dev eth0 # arbitrary address
Tip: Usually, the one of next three subnets is used for private networks, which are specially reserved and won't 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:

# mv /etc/dhcpd.conf /etc/dhcpd.conf.example

The minimal configuration file may look like:

/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;
}

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 servers (for example, provided by your ISP), you should use it. If you've 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're 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.

After configuring, start the dhcpd daemon with dhcpd4.service using systemctl. Optionally, enable it to start automatically on boot.

Now, to any computer you connect to the eth0 will be assigned an IPv4 address (from 139.96.30.150 to 139.96.30.250 in this example).

Tips and Tricks

Listening on only one interface

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

In order to exclude an interface, you must create an empty declartion 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

There is no service files provided by default to use dhcpd only on one interface so you need to create one:

 /etc/systemd/system/dhcpd4@.service
[Unit]
Description=IPv4 DHCP server on %I
Wants=network.target
After=network.target

[Service]
Type=forking
PIDFile=/run/dhcpd4.pid
ExecStart=/usr/bin/dhcpd -4 -q -pf /run/dhcpd4.pid %I
KillSignal=SIGINT

[Install]
WantedBy=multi-user.target

Now you can start dhcpd as a daemon which only listen to a specific interface, for example eth0.

# systemctl start dhcpd4@eth0.service

See also