Amended documentation with a better way to setup DNS-resolving LXD co…

…ntainers from the host machine.

Signed-off-by: Kristof Kovacs <[email protected]>

doc/howto/network_bridge_resolved.md

@@ -70,6 +70,60 @@ You must repeat the commands after each reboot and after LXD is restarted, or ma
 
 ## Make the `resolved` configuration persistent
 
+### Solution "A"
+
+You can automate the `systemd-resolved` DNS configuration, so that it is applied on system start and takes effect when LXD creates the network interface.
+
+To do so, create a `systemd` network file named `/etc/systemd/network/<network_bridge>.network` with the following content:
+
+```
+[Match]
+Name=<network_bridge>
+[Network]
+DNS=<dns_address>
+Domains=~<dns_domain>
+```
+
+So for example, `/etc/systemd/network/lxdbr0.network` with the following content (your IP, of course):
+
+```
+[Match]
+Name=lxdbr0
+[Network]
+DNS=10.167.146.1
+Domains=~lxd
+```
+
+If you have NOT freshly installed `lxd` (you've rebooted since install), then you only need to reload `systemd-resolved`:
+
+    systemctl restart systemd-resolved.service
+
+In case you **haven't rebooted yet since first initializing `lxd`**, then you need to either A) reboot the system, or B) 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 configuration got applied by running:
+
+    resolvectl status
+
+The output should contain a part similar to this (you can see the DNS server and the `.lxd` domain mentioned):
+
+```
+[...]
+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
+[...]
+```
+
+This solution "A" is better than solution "B" in the sense that it is more resilient, and gets reapplied if the `lxdbr0` network is re-created for some reason (LXD update+restart occasionally does this).
+
+### Solution "B"
+
 You can automate the `systemd-resolved` DNS configuration, so that it is applied on system start and takes effect when LXD creates the network interface.
 
 To do so, create a `systemd` unit file named `/etc/systemd/system/lxd-dns-<network_bridge>.service` with the following content: