DHCP daemon on Raspberry Pi

DHCP daemon on Raspberry Pi

19 Jun 2018 · by · in , , , ,

DHCP stands for “Dynamic Host Configuration Protocol”.    The DHCP daemon is the process which assigns IP addresses to computers when they join a network, and gives them other important information about the network, including DNS server addresses. A local network used for a wireless Access Point usually has a DHCP server associated with it. It’s important to note that this is a DHCP server for the local network, but the AP may also act as a DHCP client to get it’s own network configuration information from the upstream link. These are two different things.

There are several packages that you can use to run a DHCP server. I chose the ISC DHCP server package (isc-dhcp-server), but I later learned that one can also use the dnsmasq package as a DHCP server.1 Choose one or the other.

Here is how I set up and configured dhcpd on Raspbian Stretch, using the isc-dhcp-server. This page was originally written in 2018, but I recently updated it in December 2020.

  1. Install: Install the package:
    sudo apt-get install isc-dhcp-server
    When you install this package it is configured to run automatically at boot time (
  2. Configure: There are two files to edit or check:
    1. Move the existing file /etc/dhcp/dhcpd.conf out of the way so you can replace it with your own:
      sudo mv /etc/dhcp/dhcpd.conf /etc/dhcp/dhcpd.conf.ORIG
      (You should keep it because it contains good documentation and examples).
      Then create and edit a new file /etc/dhcp/dhcpd.conf containing:2

      #                                                                               
# Configuration file for DHCP server on Rasberry Pi                             
#                                                                               
ddns-update-style none;
option domain-name "wifipi.local";
option domain-name-servers 8.8.8.8, 1.1.1.1, 192.168.1.1;
default-lease-time 3600;
max-lease-time 86400;
authoritative;
log-facility local7;

# Configure service for local network 192.168.47.0 (the wireless AP)                    
subnet 192.168.47.0  netmask 255.255.255.0 {
    range 192.168.47.50  192.168.47.250;
    option routers 192.168.47.1;
}
##
    2. Also edit the file /etc/default/isc-dhcp-server to add the line: 
      INTERFACESv4="wlan0"

      Change the name of the interface to the local network if you are using something else.

  3. Add a Delay: While I could start the service “by hand” once the Pi had booted, I found that the ISC DHCP server would sometimes not start at boot time, even though it is configured at installation to do so. When that happened I found a complaint in the log file (viewed with `grep dhcp /var/log/syslog`) like this: 
    dhcpd[345]: Not configured to listen on any interfaces!

    The problem appears to be that the server is brought up at the same time the interface is being configured, and sometimes the interface is not ready yet. A simple solution3 is to add a slight delay to the init script that brings up the DHCP server. I edited the file /etc/init.d/isc-dhcp-server and found the line (in the start_daemon() function) which actually starts the daemon. I then added a sleep of a few seconds (at least 4 seemed to be needed) right before it. The code should look something like

            sleep 4
        start-stop-daemon --start --quiet --pidfile $PIDFILE \
                --exec /usr/sbin/dhcpd -- $VERSION -q -cf $CONF $INTERFACES
        sleep 2

    As you can see, there is already a sleep of 2 seconds right afterwards to let the daemon get started.

    A more elegant solution would be to create a systemd service file for this daemon. If that’s not been done in a newer release of the Raspberry Pi OS (I will check at some point) then I may do that and report the result. Another solution, of course, is to use dnsmasq instead.

Notes and References

  1. See Setting up a Raspberry Pi as a routed wireless access point at https://www.raspberrypi.org/documentation/configuration/wireless/access-point-routed.md ↩
  2. “How to: Make a Raspberry Pi Powered Wifi Repeater” by Dryfire117, https://pastebin.com/A4jUp2Nq ↩
  3. Found on StackExchange, of course: https://askubuntu.com/questions/58032/dhcp-server-doesnt-start-at-boot-because-of-wrong-startup-order ↩
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