Remote Bridging with IP Tunnels mini-HOWTO

Alexander Alekseev

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v1.0, May 12, 2002

Revision History
Revision 1.0 2002-05-12 Revised by: aa
first release

This document describes how to unite two separate ethernet LANs with an IP tunnel between them.

1. How does it work?

You can transparently bridge traffic between 2 ethernet LANs to unite them, if both of them are connected to Internet.

There is no way to do a "real" bridge, you can only bridge third level protocols, which linux knows how to route, but ethernet traffic with those protocols will seem bridged. You can make 2 ethernet bridges, to bridge IP and/or IPX traffic. You cannot transparently bridge any other third level protocols between distinct LANs. You should read the rest of this document to determine whether you can bridge any other protocol.

1.1. Bridging IP over ethernet traffic between 2 LANs.

If you have:

PC1   (  /24)--|
PC3   (  /24)--|
PC5   (  /24)--|--[ eth0 - bridge_1 - eth1 ( ]

PC253 (|              
                                         | (  /24) PC2
                                         | (  /24) PC4
[ ( eth1 - bridge_2 - eth0 ] --| (  /24) PC6

                                         | ( PC254

bridge_1 and bridge_2 are your Linux bridges and externally connected to the Internet interface eth1. So and can be any valid Internet addresses given to you by your ISP.

So, you should:

  1. Get two linux computers with kernels 2.2 or 2.4. Kernels should be compiled with PPP and Advanced Router. You also need the iproute2 package properly installed. Information on iproute2 can be found in of your kernel in the comments under Advanced Router. You also need the following utilities:

    You can also find them on

    Please, keep in mind that you need special patches for pppd and the kernel if you want to do MS Chap and MS Encryption (MPPE). Refer to the PoPTop manual for instructions on how to get and install these patches.

  2. Connect your routers to Internet, or establish any other communication between them with the exception of IP.

  3. Make a PPTP tunnel between them. There are example configurations in the PoPToP (server) and pptp (client) manuals.

  4. Now you should have two bridges and an IP tunnel between then, possibly encrypted (refer to the PPP manual). Let's configure bridging.

  5. Remember that the bridge is really a router, so we need to run the following commands on our bridges (this assumes bridge_1 and bridge_2 are IP addresses, assigned to each end of the PPTP tunnel between bridges):

         bridge_1$ip route add via bridge_2
         bridge_1$ip route add via bridge_2
         bridge_1$ip route add via bridge_2
         bridge_1$ip route add via bridge_2
         bridge_1$ip route add via bridge_2

    On the other side:

         bridge_2$ip route add via bridge_1
         bridge_2$ip route add via bridge_1
         bridge_2$ip route add via bridge_1
         bridge_2$ip route add via bridge_1

    This will tell each of bridges which hosts are on the other side. You can do the same with the old-style route command. It will look like:

         bridge_1$route add -host gw bridge_2
         bridge_1$route add -host gw bridge_2
         bridge_1$route add -host gw bridge_2
         bridge_1$route add -host gw bridge_2
         bridge_1$route add -host gw bridge_2

    On the other side:

         bridge_2$route add -host gw bridge_1
         bridge_2$route add -host gw bridge_1
         bridge_2$route add -host gw bridge_1
         bridge_2$route add -host gw bridge_1

    Please note once more that bridge_1 and bridge_2 are not IP addresses given by your ISP, but IP addresses which you assigned to each end of the PPTP tunnel.

  6. Now you have two bridges and each of them knows where to find a particular IP. But how do you tell those computers to send their traffic for the remote network to the local bridge? You need tarpd.

    tarpd is a very simple daemon, which replies to arp requests for certain IP addresses. You only need to run a tarpd on each bridge, and specify the list of IP addresses found on the remote end.

    For example, for those two bridges you should run:

         bridge_1$tarpd eth0  \

    On the other side:

         bridge_2$tarpd eth0  \

    You specify 128 remote pairs (IP/mask. Mask should be in order not to confuse tarpd!) on each bridge.

  7. Enjoy your bridges!

2. Copyright

Copyright © 2002 Alexander Alekseev

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

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