configuring-dns.md

DNS Configuration

By default, DNS resolution on CoreOS is handled through /etc/resolv.conf, which is a symlink to /run/systemd/resolve/resolv.conf. This file is managed by systemd-resolved. Normally, systemd-resolved gets DNS IP addresses from systemd-networkd, either via DHCP or static configuration. DNS IP addresses can also be set via systemd-resolved's resolved.conf. See Network configuration with networkd for more information on systemd-networkd.

Using a local DNS cache

systemd-resolved includes a caching DNS resolver. To use it for DNS resolution and caching, you must enable it via nsswitch.conf by adding resolv to the hosts section.

Here is an example cloud-config snippet to do that:

#cloud-config
write_files:
  - path: /etc/nsswitch.conf
    permissions: 0644
    owner: root
    content: |
      # /etc/nsswitch.conf:

      passwd:      files usrfiles
      shadow:      files usrfiles
      group:       files usrfiles

      hosts:       files usrfiles resolv dns
      networks:    files usrfiles dns

      services:    files usrfiles
      protocols:   files usrfiles
      rpc:         files usrfiles

      ethers:      files
      netmasks:    files
      netgroup:    files
      bootparams:  files
      automount:   files
      aliases:     files

Only nss-aware applications can take advantage of the systemd-resolved cache. Notably, this means that statically linked Go programs and programs running within Docker/rkt will use /etc/resolv.conf only, and will not use the systemd-resolve cache.