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Saurabh Nanda
Asked: 2019-02-13 22:30:36 +0800 CST

Why is "systemd-resolved" required?

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I am in the process of upgrading my production server from Ubuntu 14.04 to 18.04 and noticed that there is something called systemd-resolved in 18.04 which run a local DNS cache of sorts. [1]

Why is this required? What is wrong with simply putting nameserver 1.1.1.1 or nameserver 8.8.8.8 in /etc/resolv.conf and calling it a day? Is it safe to disable systemd-resolved and revert to the simpler configuration?

[1] I noticed only because it is randomly crashing and causing all hostname lookups to fail, effectively crippling my app - https://askubuntu.com/questions/1117842/systemd-resolved-crashing-with-failed-to-set-up-mount-namespacing-invalid-argu

systemd

1 Answers

  1. The logic of it is that Ubuntu has a built in DNS cache, which it checks first when trying to resolve anything. In this version, by default, NetworkManager is not updating /etc/resolv.conf instead it’s calling application called resolvconf.

    This approach:

    service resolvconf disable-updates
update-rc.d resolvconf disable
service resolvconf stop

    May not make it through a reboot.

    To get rid of the problem, we can try harder:

    systemctl disable systemd-resolved.service
systemctl stop systemd-resolved

    add the following lines to the [main] section of NetworkManager.conf:

    dns=default

    Then restart net manager: 

    service network-manager restart

    That should make the system use the classic resolv.conf .

    An alternate dirty trick is to makes the service un-startable until you "unmask" it: 

    systemctl mask systemd-resolved

    The unsafe part of this may be that disabling it might break name resolution in some VPN scenarios. So if you want to use VPNs, make sure to do proper testing of their functionality after getting rid of systemd-resolved.

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