add gre / gretap docs

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

docs/tunneling/gre.md

@@ -0,0 +1,55 @@
+
+# Generic Routing Encapsulation (GRE)
+
+*Note:* GRE tunnels are unencrypted.
+
+GRE tunnels on layer 3 of the ISO/OSI model.
+
+## Configuring with ip
+
+To create a tunnel, you can use the following command:
+```
+ip link add <interface> type gre remote <remote> local <local> ttl 255
+```
+Replace `<interface>` with the desired interface name, `<remote>` with the IPv4 of the peer and `<local>` with your own IPv4.
+
+If you want to establish the tunnel over IPv6 instead, you can replace the `mode gre` with `mode ip6gre`.
+
+To delete a tunnel, you can use the following command:
+```
+ip link del <interface>
+```
+
+To assign an IP address to the interface, you can use the following command:
+```
+ip link del <interface>
+```
+
+To assign an IP address to the interface, you can use the following command:
+```
+ip addr add dev <interface> <ll>
+```
+Replace `<ll>` with your own link-local address.
+
+### Automatic start with ifupdown
+
+If you want to start the GRE tunnel automatically, you can use an ifupdown template which is placed under `/etc/network/interfaces.d`:
+```
+auto <interface>
+iface <interface> inet6 manual
+    pre-up ip link add <interface> type gre remote <remote> local <local> ttl 255
+    up ip addr add dev <interface> <ll>
+    post-down ip link del <interface>
+```
+
+The interface can then be stopped and started accordingly:
+```
+ifup <interface>
+```
+```
+ifdown <interface>
+```
+
+## GRETAP
+
+GRETAP tunnels operate on layer 2 of the ISO/OSI model. To use a GRETAP tunnel, you only have to change the mode: For IPv4 `gretap` and for IPv6 `ip6gretap`.