Adds a "how to" for routed nic devices on VMs

doc/howto/instances_routed_nic_vm.md

@@ -0,0 +1,58 @@
+(instances-routed-nic-vm)=
+# How to add a routed NIC device to a virtual machine
+
+When adding a {ref}`routed NIC device <nic-routed>` to an instance, you must configure the instance to use the link-local gateway IPs as default routes.
+For containers, this is configured for you automatically.
+For virtual machines, the gateways must be configured manually or via a mechanism like `cloud-init`.
+
+To configure the gateways with `cloud-init`, firstly initialize an instance:
+
+    lxc init ubuntu:22.04 jammy --vm
+
+Then add the routed NIC device:
+
+    lxc config device add jammy eth0 nic nictype=routed parent=my-parent-network ipv4.address=192.0.2.2 ipv6.address=2001:db8::2
+
+In this command, `my-parent-network` is your parent network, and the IPv4 and IPv6 addresses are within the subnet of the parent.
+
+Next we will create a `netplan` configuration file `netplan.yaml` to use with the instance:
+
+    cat <<EOF > netplan.yaml
+    network:
+      version: 2
+      ethernets:
+        enp5s0:
+          routes:
+          - to: default
+            via: 169.254.0.1
+          - to: default
+            via: fe80::1
+          link-local:
+          - ipv4
+          - ipv6
+          addresses:
+          - 192.0.2.2/32
+          - 2001:db8::2/128
+    EOF
+
+This `netplan` configuration adds the {ref}`static link-local next-hop addresses <nic-routed>` (`169.254.0.1` and `fe80::1`) that are required.
+Additionally, we enable link-local addressing for IPv4 and IPv6, and add the addresses we configured in our routed NIC device.
+For more information on `netplan`, see [their documentation](https://netplan.readthedocs.io/en/latest/).
+
+```{note}
+This `netplan` configuration does not include a name server.
+To enable DNS within the instance, you must set a valid DNS IP address.
+If there is a `lxdbr0` network on the host, the name server can be set to that IP instead.
+```
+
+Finally, we add this `netplan` configuration to the instance using the `cloud-init.network-config` configuration key:
+
+    lxc config set jammy cloud-init.network-config "$(cat netplan.yaml)"
+
+```{note}
+Before you start your instance, make sure that you have {ref}`configured the parent network <nic-routed>` to enable proxy ARP/NDP.
+```
+
+You can then start your instance with:
+
+    lxc start jammy

doc/instances.md

@@ -9,6 +9,7 @@ The following how-to guides cover common operations related to instances:
 
 Access files </howto/instances_access_files.md>
 Access the console </howto/instances_console.md>
+Add a routed NIC to a VM </howto/instances_routed_nic_vm.md>
 Back up instances </howto/instances_backup.md>
 Configure instances </howto/instances_configure.md>
 Create instances </howto/instances_create.md>
@@ -42,6 +43,7 @@ Use cloud-init </cloud-init>
 Run commands </instance-exec.md>
 Access the console </howto/instances_console.md>
 Access files </howto/instances_access_files.md>
+Add a routed NIC to a VM </howto/instances_routed_nic_vm.md>
 Troubleshoot errors </howto/instances_troubleshoot.md>
 /explanation/instance_config.md
 Container environment </container-environment>

doc/reference/devices_nic.md

@@ -381,7 +381,7 @@ IP addresses, gateways and routes
       169.254.0.1
       fe80::1
 
-  For VMs, the gateways must be configured manually or via a mechanism like `cloud-init`.
+  For VMs, the gateways must be configured manually or via a mechanism like `cloud-init` (see the {ref}`how to guide <instances-routed-nic-vm>`).
 
   ```{note}
   If your container image is configured to perform DHCP on the interface, it will likely remove the automatically added configuration.
@@ -400,11 +400,8 @@ Multiple IP addresses
 Parent interface
 : This NIC can operate with and without a `parent` network interface set.
 
-  With the `parent` network interface set, proxy ARP/NDP entries of the instance's IPs are added to the parent interface, which allows the instance to join the parent interface's network at layer 2.
-
-DNS
-: The name servers must be configured inside the instance, because they are not set automatically.
-  To do this, set the following `sysctls`:
+: With the `parent` network interface set, proxy ARP/NDP entries of the instance's IPs are added to the parent interface, which allows the instance to join the parent interface's network at layer 2.
+: To enable this, the following network configuration must be applied on the host via `sysctl`:
 
    - When using IPv4 addresses: