EOIP

Kernel mode EOIP (Ethernet Over IP) tunnel compatible with MikroTik RouterOS

There are several projects doing the same job with userland utilities via tap interfaces and raw sockets. While a userland application is easier to install and maintain it lacks the performance and stability of an in-kernel module. Especially for the simple job of adding and stripping the EOIP tunneling headers. The userland tunneling application may be good for testing, research or concept proof projects but not suitable for production environments with high bandwidth requirements.

This project's goals are:

• to solve the performance issue with EOIP on Linux
• to make EOIP tunneling support a standard part of the Linux world

Install

This code was developed on a 3.2.44 linux kernel and tested on the next 3.2.x releases. Patches and out-of-tree builds are provided for 3.2/3.16/4.19 kernels. It should not be hard to adapt it to different Linux kernels.

• To patch a kernel tree:

cd path-to-kernel-source/linux-X.Y.Z
patch -p1 < path-to-eoip/kernel-patch/kernel-X.Y.Z-eoip-gre-demux.patch
patch -p1 < path-to-eoip/kernel-patch/kernel-X.Y.Z-eoip-buildconf.patch
patch -p1 < path-to-eoip/kernel-patch/kernel-X.Y.Z-eoip.patch

afterwards configure the kernel in the usual ways make (menu/x/...)config and do not forget to select IP: EOIP tunnels over IP located under Networking options from Networking support

EOIP tunnel depends on IP: GRE demultiplexer - if it not selected then EOIP tunnel is not shown at all

Unless the target is a limited embedded system, it is recommended to build EOIP and GRE demux as modules.

• To build the modules out of the kernel tree:

cd path-to-eoip/out-of-tree-X.Y.Z
make
make install

For this to work at least the running kernel's headers should be available.

On Debian/Ubuntu systems this build process will place the newly built modules in /lib/modules/x.x.x.x/misc. Note that there will be two versions of gre.ko (the GRE demux). At least on 3.2/3.16/4.19 it is safe to replace the original version with the modified one because it is backwards compatible.

The eoip.ko module cannot operate properly without the newly built version of GRE demux (gre.ko). If the original gre.ko is loaded then it should be removed and the newly built gre.ko loaded before loading eoip.ko.

• To build the userland management utility eoip:

cd path-to-eoip
make

Example how to build out of tree

This example may be used as a checklist for an out of tree build:

root@ubuntu-18_04:~/eoip# make
cc -Wall -Os -c libnetlink.c
cc -Wall -Os -o eoip eoipcr.c libnetlink.o
strip eoip
root@ubuntu-18_04:~/eoip# cd out-of-tree-4.15.x/
root@ubuntu-18_04:~/eoip/out-of-tree-4.15.x# make
patching file eoip.c
using 4.15.0 version of gre_demux.c
found running kernel version of gre.h
patching file gre.h
Hunk #1 succeeded at 24 (offset 15 lines).
patching file gre_demux.c
Hunk #3 succeeded at 137 (offset -9 lines).
Hunk #4 succeeded at 173 (offset -9 lines).
make[1]: Entering directory '/usr/src/linux-headers-4.15.0-106-generic'
  CC [M]  /root/eoip/out-of-tree-4.15.x/eoip.o
  CC [M]  /root/eoip/out-of-tree-4.15.x/gre.o
  Building modules, stage 2.
  MODPOST 2 modules
  CC      /root/eoip/out-of-tree-4.15.x/eoip.mod.o
  LD [M]  /root/eoip/out-of-tree-4.15.x/eoip.ko
  CC      /root/eoip/out-of-tree-4.15.x/gre.mod.o
  LD [M]  /root/eoip/out-of-tree-4.15.x/gre.ko
make[1]: Leaving directory '/usr/src/linux-headers-4.15.0-106-generic'
root@ubuntu-18_04:~/eoip/out-of-tree-4.15.x# insmod ./gre.ko
root@ubuntu-18_04:~/eoip/out-of-tree-4.15.x# insmod ./eoip.ko
root@ubuntu-18_04:~/eoip/out-of-tree-4.15.x# ../eoip add name eoip0 local 203.0.113.113 remote 198.51.100.100 tunnel-id 8421
root@ubuntu-18_04:~/eoip/out-of-tree-4.15.x# ip li set eoip0 up

Userland management utility

eoip - tunnel management utility

Important notes

• Normally EoIP tunnels in MikroTiks work well by only specifying the remote IP address. This code requires to configure both ends in a symmetrical way - each end of the tunnel should have the local IP address configured and equal to the remote IP address configured on the other end.
• This code does not support the keepalive option; configure the tunnel on MikroTik's end with !keepalive.
• It is a good idea to use IP fragmentation and to set MTU on both ends to 1500; using clamp-tcp-mss is pointless in this case. For performance it would be best if the transport network's MTU is 42+tunnel MTU (42 bytes is the EoIP protocol overhead) but obviously that is not the case over the Internet.
• The EoIP protocol is connection-less and requires both ends to be able to reach the other end. In case only one end has a public IP, the other end may establish a private network by using another protocol that works over NAT (e.g. PPTP, L2TP, etc.) and run EoIP on top of the newly established private network.
• Security warning: EoIP is a simple encapsulation and does implement any transport security.

Usage

• to create new eoip tunnel interface:

    eoip add tunnel-id <tunnel-id> [name <if-name>]
             [local <src-address>] [remote <dst-address>]
             [link <ifindex>] [ttl <ttl>] [tos <tos>]

• to change existing eoip tunnel interface:

    eoip change name <if-name> tunnel-id <tunnel-id>
                [local <src-address>] [remote <dst-address>]
                [link <ifindex>] [ttl <ttl>] [tos <tos>]

• to list existing eoip tunnels:

    eoip list

Example how to configure

To configure an Ethernet tunnel between a MikroTik with IP 198.51.100.11 and a Linux box with IP 203.0.113.50 with tunnel id 1234:

On MikroTik:

/interface eoip add clamp-tcp-mss=no !keepalive local-address=198.51.100.11 remote-address=203.0.113.50 tunnel-id=1234 mtu=1500 name=eoip1234

On Linux:

eoip add name eoip1234 local 203.0.113.50 remote 198.51.100.11 tunnel-id 1234

Roadmap

• make a patch for iproute2 to include eoip support

• work towards making this code good enough for inclusion in official kernel/iproute2 releases

  The inclusion of a reverse-engineered proprietary protocol which violates GRE standards is not going to happen in the official Linux kernel. Creating a patch for iproute2 is pointless as it would require patching and replacing one more component that in turn would make supporting a Linux system with kernel mode EoIP even harder. Thus using the included simple tool would be easier and preferred.

  The problem in making a stand-alone EoIP kernel module is that it requires replacing gre_demux (gre.ko). Linux kernel does not support overloading IP protocol handlers and EoIP does not fit anywhere in the standard gre_demux logic. A relatively sane solution might be to include the GRE demultiplexing logic in the EoIP kernel module itself, to provide a gre alias and to blacklist the original gre.ko. In this way GRE and PPTP would still be able to coexist with EoIP.

• make a DKMS package

• implement the keepalive option and probably make it the default

Development process

This code was developed based on information gathered from sniffed datagrams and information from similar projects without involving any reverse engineering of code from the closed source commercial product.

The protocol is not documented and although it looks like there are no deviations in the header format this cannot be guaranteed in all environments or for future releases of the commercial product.

Protocol spec

After the IP header (which can be fragmented, MTU 1500 is usually used for tunnels) a GRE-like datagram follows. Note that RFC 1701 is mentioned in MikroTik's docs but there is nothing in common between the standard and the actual protocol used.

Header format (taken from https://github.com/katuma/eoip):

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       GRE FLAGS 0x20 0x01     |      Protocol Type  0x6400    | = MAGIC "\x20\x01\x64\x00"
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|   Encapsulated frame length   |           Tunnel ID           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ethernet frame...                                             |

Strangely enough the frame length is kept into network byte order while tunnel ID is in little endian byte order.

Bugs

This code was tested and works without problems on quite a few different 32/64bit x86 systems.

No testing was done on non-x86 and big endian hardware.

There is no guarantee that there are no bugs left. Patches are welcome.

License

All code and code modifications in this project are released under the GPL licence. Look at the COPYING file.