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add gre / gretap docs
Signed-off-by: Marek Küthe <m.k@mk16.de>
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# Generic Routing Encapsulation (GRE)
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*Note:* GRE tunnels are unencrypted.
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GRE tunnels on layer 3 of the ISO/OSI model.
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## Configuring with ip
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To create a tunnel, you can use the following command:
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```
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ip link add <interface> type gre remote <remote> local <local> ttl 255
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```
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Replace `<interface>` with the desired interface name, `<remote>` with the IPv4 of the peer and `<local>` with your own IPv4.
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If you want to establish the tunnel over IPv6 instead, you can replace the `mode gre` with `mode ip6gre`.
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To delete a tunnel, you can use the following command:
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```
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ip link del <interface>
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```
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To assign an IP address to the interface, you can use the following command:
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```
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ip link del <interface>
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```
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To assign an IP address to the interface, you can use the following command:
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```
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ip addr add dev <interface> <ll>
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```
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Replace `<ll>` with your own link-local address.
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### Automatic start with ifupdown
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If you want to start the GRE tunnel automatically, you can use an ifupdown template which is placed under `/etc/network/interfaces.d`:
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```
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auto <interface>
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iface <interface> inet6 manual
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pre-up ip link add <interface> type gre remote <remote> local <local> ttl 255
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up ip addr add dev <interface> <ll>
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post-down ip link del <interface>
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```
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The interface can then be stopped and started accordingly:
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```
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ifup <interface>
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```
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```
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ifdown <interface>
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```
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## GRETAP
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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`.
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