Arch Linux dhcpd won't start on boot - cannot find subnet declaration

Question

I want to have a DHCP server running on Arch Linux, but I'm having problems. I'm having an issue with getting dhcpd to start on boot. It will start ok when I manually run sudo systemctl start [email protected] when I've logged in.

I'm running the latest version of Arch Linux (4.7.0-1) and I've followed the Arch networking wiki to get this set up.

My setup

• dhcpcd is disabled

• I have used systemd-networkd to do the static ip (it is enabled), the config is (/etc/systemd/network/wired.network):

  [Match]
  Name=eth0
  
  [Network]
  Address=10.0.0.2/24
  Gateway=10.0.0.1
  

  • My /etc/dhcpd.conf is simple:

    option domain-name-servers 10.0.0.2;   
    option subnet-mask 255.255.255.0;   
    option routers 10.0.0.0.1;   
    subnet 10.0.0.0 netmask 255.255.255.0 {
        range 10.0.0.5 10.0.0.50;   
    }
    

• I created my systemd service file (/etc/systemd/system/[email protected]) and enabled it with systemctl enable [email protected]:

  [Unit]
  Description=IPv4 DHCP server on %I
  Wants=network.target
  After=network-pre.target
  Before=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
  

When I reboot I get an error in the start up that says (when i run journalctl -xe):

 No subnet declaration for eth0 (no IPv4 addresses).
 ** Ignoring requests on eth0....

Then when I run systemctl start [email protected] it all runs fine no problems.

I've done a lot of googling but found no solution to this issue. I've tried tweaking the files (dhcpd.conf, [email protected] and wired.network) to hard code interfaces, add extra DHCP declarations, alter Unit wants etc and more; but nothing seems to work.

Answer 1

Are you trying to restrict dhcpd to only eth0? If not, you may simply use the standard systemd service file dhcpd4.service.

If you are, maybe try changing

After=network-pre.target
Before=network.target

to

After=network.target

This is also specified in the standard dhcpd4.service. It seems that dhcpd needs an active interface - which is not guaranteed if it is started before network.target.

Answer 2

I found the answer to the problem! The problem is that the boot process is trying to start DHCP service whilst the network is starting up/being configured, and so DHCP can't find an working interface to bind to. This is why it always worked when I ran the start dhcpd service manually and didn't work on boot.

I found a clue to the answer from this post: dhcp on arch linux - rasp-pi. The user used a restart declaration in the unit file, which made the service retry every 6 seconds until the service was up. - This temporary solution worked for me too.

I had a dig around the systemd web site on the special targets page and found this link to running services after the network is up. From this page's recommendations I made the following adjustments:

• In the [email protected] unit file I removed the Before=..., changed After to After=network-online.target and changed Wants to Wants=network-online.target
• I also enabled the systemd-networkd-wait-online.serviceto start on boot.

It now works on boot! However, the recommendations page does say that enabling systemd-networkd-wait-online.service does slow down your boot time and can do by 90 seconds (if I've read it correctly). Though I've only noticed about 1 or 2 seconds increase in boot time in my case.