etherip-kmod

Out-of-tree Linux kernel module for RFC3378 EtherIP over IPv6 endpoints.

The module registers a rtnetlink link kind named etherip6. A companion etherip6ctl helper is included because stock iproute2 does not know how to encode this module's private tunnel attributes.

Build

On Arch Linux, install headers matching the kernel you will load into:

sudo pacman -S linux-headers
make

If you need to build against a specific headers tree:

make KDIR=/usr/lib/modules/$(uname -r)/build

Load

sudo insmod etherip6.ko

Create a Tunnel

sudo ./etherip6ctl add eip0 local 2001:db8:1::1 remote 2001:db8:1::2
sudo ip link set eip0 up
sudo ip addr add 192.0.2.1/30 dev eip0

For link-local IPv6 endpoints, specify the egress interface:

sudo ./etherip6ctl add eip0 local fe80::1 remote fe80::2 link eth0

Delete the device with:

sudo ./etherip6ctl del eip0

Fragmentation

The transmit path permits local IPv6 fragmentation for encapsulated EtherIP packets that exceed the outer path MTU. This lets a larger inner Ethernet frame be split into multiple outer IPv6 fragments by the local host.

The device default MTU is 1500, but the module allows an overlay MTU up to 9000. For example, with a 1500-byte underlay and a 9000-byte overlay:

sudo ip link set eip0 mtu 9000

An inner 9000-byte payload is carried as a 9014-byte Ethernet frame, plus the 2-byte EtherIP header and 40-byte outer IPv6 header. The local IPv6 stack fragments that outer packet before transmission, and the peer must reassemble the IPv6 fragments before EtherIP decapsulation.

You can still set a smaller tunnel MTU when you want to avoid IPv6 fragments:

sudo ip link set eip0 mtu 1444

For a 1500-byte outer link, 1444 leaves room for the 40-byte IPv6 header, 2-byte EtherIP header, and 14-byte Ethernet header carried inside EtherIP.

Wire Format

The outer IPv6 packet uses Next Header 97. The EtherIP header is the RFC3378 two-byte version/reserved field with version 3, followed by the complete Ethernet frame.