How to integrate with `systemd-resolved`

Important
This guide applies to managed bridge networks only.

If the system that runs LXD uses systemd-resolved to perform DNS lookups, you should notify resolved of the domains that LXD can resolve. To do so, add the DNS servers and domains provided by a LXD network bridge to the resolved configuration.

Note
The dns.mode option must be set to managed or dynamic if you want to use this feature.

Depending on the configured dns.domain, you might need to disable DNSSEC in resolved to allow for DNS resolution. This can be done through the DNSSEC option in resolved.conf.

Configure resolved

To add a network bridge to the resolved configuration, specify the DNS addresses and domains for the respective bridge.

DNS address

You can use the IPv4 address, the IPv6 address or both. The address must be specified without the subnet netmask.

To retrieve the IPv4 address for the bridge, use the following command:

lxc network get <network_bridge> ipv4.address

To retrieve the IPv6 address for the bridge, use the following command:

lxc network get <network_bridge> ipv6.address

DNS domain

To retrieve the DNS domain name for the bridge, use the following command:

lxc network get <network_bridge> dns.domain

If this option is not set, the default domain name is lxd.

Use the following commands to configure resolved:

resolvectl dns <network_bridge> <dns_address>
resolvectl domain <network_bridge> ~<dns_domain>

Note
When configuring resolved with the DNS domain name, you should prefix the name with ~. The ~ tells resolved to use the respective name server to look up only this domain.

Depending on which shell you use, you might need to include the DNS domain in quotes to prevent the ~ from being expanded.

For example:
resolvectl dns lxdbr0 192.0.2.10
resolvectl domain lxdbr0 '~lxd'

Note
Alternatively, you can use the systemd-resolve command. This command has been deprecated in newer releases of systemd, but it is still provided for backwards compatibility.
systemd-resolve --interface <network_bridge> --set-domain ~<dns_domain> --set-dns <dns_address>

The resolved configuration persists as long as the bridge exists. You must repeat the commands after each reboot and after LXD is restarted, or make it persistent as described below.

Make the `resolved` configuration persistent

There are two approaches to automating systemd-resolved configuration to ensure that it persists when the LXD bridge network is re-created. Use only one of these approaches, described below.

The first approach is recommended because it is more resilient. It applies your desired configuration whenever your system is rebooted, and whenever the LXD bridge network is re-created outside of a system reboot. For example, updating and restarting LXD can occasionally cause its bridge network to be re-created.

If you are unable to use the recommended approach, the alternative approach can be used. The alternative approach applies your desired configuration only when your system is rebooted. If LXD re-creates its bridge network outside of a system reboot, you must reapply the configuration manually.

Recommended approach

Create a `systemd` network file

Get the network bridge address with the following command:

lxc network get lxdbr0 ipv4.address

Create a systemd network file named /etc/systemd/network/<network_bridge>.network with the following content:

[Match]
Name=<network_bridge>
[Network]
Address=<network_bridge_address>
DNS=<dns_address>
Domains=~<dns_domain>

Example file content for /etc/systemd/network/lxdbr0.network (insert your own DNS value):

[Match]
Name=lxdbr0
[Network]
Address=10.167.146.1/24
DNS=10.167.146.1
Domains=~lxd

Apply the updated configuration

If you have rebooted since you first installed LXD, you only need to reload systemd-resolved:

systemctl restart systemd-resolved.service

If you have not rebooted your system since you first installed LXD, you must either:

reboot the system, or
reload systemd-networkd (to reload the .network files) and restart lxd (to add the routing):

networkctl reload
snap restart lxd

You can test that the updated configuration was applied by running:

resolvectl status

The output should contain a section similar to the example shown below. You should see the configured DNS server and the ~lxd domain:

[...]
Link 4 (lxdbr0)
    Current Scopes: DNS
         Protocols: -DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
Current DNS Server: 10.167.146.1
       DNS Servers: 10.167.146.1
        DNS Domain: ~lxd
[...]

Alternative approach

Warning
This approach only automates applying your desired configuration when your system is rebooted. If LXD re-creates its bridge network outside of a system reboot, you must reapply the configuration manually with the following command:
systemctl restart lxd-dns-<bridge_network>.service
Example:
systemctl restart lxd-dns-lxdbr0.service

Create a systemd unit file named /etc/systemd/system/lxd-dns-<network_bridge>.service with the following content:

[Unit]
Description=LXD per-link DNS configuration for <network_bridge>
BindsTo=sys-subsystem-net-devices-<network_bridge>.device
After=sys-subsystem-net-devices-<network_bridge>.device

[Service]
Type=oneshot
ExecStart=/usr/bin/resolvectl dns <network_bridge> <dns_address>
ExecStart=/usr/bin/resolvectl domain <network_bridge> <dns_domain>
ExecStopPost=/usr/bin/resolvectl revert <network_bridge>
RemainAfterExit=yes

[Install]
WantedBy=sys-subsystem-net-devices-<network_bridge>.device

Replace <network_bridge> in the file name and content with the name of your bridge (for example, lxdbr0). Also replace <dns_address> and <dns_domain> as described in Configure resolved.

Then enable and start the service with the following commands:

sudo systemctl daemon-reload
sudo systemctl enable --now lxd-dns-<network_bridge>

If the respective bridge already exists (because LXD is already running), you can use the following command to check that the new service has started:

sudo systemctl status lxd-dns-<network_bridge>.service

You should see output similar to the following:

~$ sudo systemctl status lxd-dns-lxdbr0.service

● lxd-dns-lxdbr0.service - LXD per-link DNS configuration for lxdbr0
     Loaded: loaded (/etc/systemd/system/lxd-dns-lxdbr0.service; enabled; vendor preset: enabled)
     Active: inactive (dead) since Mon 2021-06-14 17:03:12 BST; 1min 2s ago
    Process: 9433 ExecStart=/usr/bin/resolvectl dns lxdbr0 n.n.n.n (code=exited, status=0/SUCCESS)
    Process: 9434 ExecStart=/usr/bin/resolvectl domain lxdbr0 ~lxd (code=exited, status=0/SUCCESS)
   Main PID: 9434 (code=exited, status=0/SUCCESS)

To check that resolved has applied the settings, use resolvectl status <network_bridge>:

~$ resolvectl status lxdbr0

Link 6 (lxdbr0)
      Current Scopes: DNS
DefaultRoute setting: no
       LLMNR setting: yes
MulticastDNS setting: no
  DNSOverTLS setting: no
      DNSSEC setting: no
    DNSSEC supported: no
  Current DNS Server: n.n.n.n
         DNS Servers: n.n.n.n
          DNS Domain: ~lxd

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How to integrate with systemd-resolved