Setting DNS for Docker daemon on OS with systemd

Question

The default DNS for Docker (e.g. 8.8.8.8) is blocked where I work, so I want to change the default. I've been able to do this using

$ docker daemon --dns <mydnsaddress>

but I want to do this using a systemd drop-in instead, since the official Docker docs recommend this way. I've made a /etc/systemd/system/docker.service.d/dns.conf file, and used things like this:

[Service]
DNS=<mydnsaddress>

But I just have no idea what the variable name is supposed to be. How do I set this? More importantly, is there a page that documents all config variables that can be used in systemd drop-ins for Docker?

(btw, this is Docker 1.9 on Ubuntu 15.10, although I don't suspect any bugs)

Answer 1

All .conf files in /etc/systemd/system/docker.service.d overrule the settings from the /usr/lib/systemd/system/docker.service file, which is almost what you tried.

Instead of putting a DNS=.. line in, you need to copy the ExecStart= part from the /usr/lib/systemd/system/docker.service file to dns.conf (or mydocker.conf). Add --dns $ip after the daemon part of the ExecStart. E.g.:

[Service]
ExecStart=
ExecStart=/usr/bin/docker daemon --dns 192.168.1.1 -H fd://

Where the 192.168.1.1 is the ip of the dns server.

Now restart docker via systemctl and docker should now restart with your own dns. (Checkable via systemctl status docker.service | grep dns).

Note that the empty ExecStart= is required, as systemctl only will overrule the ExecStart if it is cleared first.

Also note that a systemctl daemon-reload is needed after editing files in /etc/systemd/system/.

Last remark is that on some systems docker.service is not located in /usr/lib/systemd/system/, but in /lib/systemd/system/.