add docs for creating a dummy interface

Signed-off-by: Marek Küthe <m.k@mk16.de>

docs/routing/create-dummy-interface.md

@@ -0,0 +1,43 @@
+
+# Create a dummy interface
+
+For routes to be installed into the kernel, the source IP must be bound to an interface. Often the CRXN address is not on any interface. Therefore one creates a dummy interface. Alternatively, you can bind the CRXN IP address to the loopback interface. If the CRXN IP address is not bound to an interface, the `Netlink: Invalid argument` error can occur at bird.
+
+You can create a dummy interface with the following command:
+```
+ip link add crxn type dummy
+ip link set dev crxn up
+```
+Here `crxn` is the name of the interface.
+
+To bind the CRXN IP address to the interface you can use the following command:
+```
+ip addr add dev crxn <ip>/128
+```
+Replace `<ip>` with the CRXN IP address.
+
+To delete the dummy interface you can use the following command:
+```
+ip link del crxn
+```
+
+## Automatic start with ifupdown
+
+```
+auto crxn
+iface crxn inet6 manual
+    pre-up ip link add crxn type dummy
+    up ip addr add dev crxn <ip>/128
+    post-down ip link del crxn
+```
+
+Alternatively, you can use the following configuration:
+```
+auto crxn
+iface crxn inet6 static
+	address <ip>
+	netmask 128
+    pre-up ip link add crxn type dummy
+    post-down ip link del crxn
+```
+Here the assignment of the IP address is delegated to ifupdown.

docs/routing/index.md

@@ -3,3 +3,4 @@
 
 - [Forwarding](forwarding)
 - [Setting up Bird](bird)
+- [Create a dummy interface](create-dummy-interface)