DNS Configuration

By default, DNS resolution on Flatcar Container Linux 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 resolve to the hosts section.

Here is an example Butane Config snippet to do that:

variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/nsswitch.conf
      overwrite: true
      mode: 0644
      contents:
        inline: |
          # /etc/nsswitch.conf:

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

          hosts:       files usrfiles resolve 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.

To use systemd-resolved as the default DNS resolver for all applications on the host, switch to the systemd-resolved provided stub-resolv.conf:

variant: flatcar
version: 1.0.0
storage:
 links:
   - path: /etc/resolv.conf
     overwrite: true
     target: /run/systemd/resolve/stub-resolv.conf

This is known to interfere with Kubernetes in certain situations.

Special considerations regarding Multicast DNS (mDNS)

In some network environments, the reserved top level domain .local is used to identify internal devices.

This is in contradiction with RFC 6762 but could be difficult to change across an entire environment.

Indeed, you will get SERVFAIL response from resolved when trying to resolve names ending with .local.

A simple workaround is to add local or yourcompany.local as a search domain in your network configuration. Here is an example with butane configuration:

variant: flatcar
version: 1.0.0
storage:
  files:
    - path: /etc/systemd/network/00-eth0.network
      contents:
        inline: |
          [Match]
          Name=eth0

          [Network]
          DNS=1.2.3.4
          Address=10.0.0.101/24
          Gateway=10.0.0.1
          Domains=yourcompany.local