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Remote Bridging with IP Tunnels mini-HOWTO

Alexander Alekseev

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.

Table of Contents
1. How does it work?
    1.1. Bridging IP over ethernet traffic between 2 LANs.
    1.2. What about other protocols?
2. Copyright
    2.1. GNU Free Documentation License
    2.2. PREAMBLE
    2.13. How to use this License for your documents

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:
    ��+�pppd (PPP daemon) - [] ftp://
    ��+�PopTop (PPTP server) - [] http://
    ��+�PPTP (Linux PPTP Client, by C.S. Ananian) - [http://] http://
    ��+�tarpd (a trivial proxy arp daemon) - [
        utils/net/tarpd-1.6.tar.gz] htp://
    You can also find them on [] http://
    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!

1.2. What about other protocols?

Really, I can say nothing about other protocol routing. I never used them.
But I suppose if you are familiar with other protocols, it should not be too
difficult to bridge it this way.

2. Copyright

Copyright � 2002 Alexander Alekseev

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