[LARTC] Combining 2 DSL Links Using Linux Gateway

[LARTC] Combining 2 DSL Links Using Linux Gateway

[K N]

Hello All,

I am new to the field of linux networking and routing and therefore seek
your assistance. Please excuse me if my queries appear to be trivial. I
felt this would be the best forum to learn things.

I have two DSL connections in my office which terminate into my LAN through
separate DSL routers. Each of the DSL routers have been assigned an
external IP address and an internal IP address (belonging to the same
subnet as my LAN). At present we manually divide the internet traffic
between the routers by selecting one of the two gateways. However, I want
to set up a linux box as a gateway which would do load balancing and act as
an equalcost multipath router as shown in the figure below. This gateway
will also act as a firewall.

		                           
                                    --------------
                                    |            |
                                    |            |
	------------                |            |
	|          |                |            |
--------| DSL      |----------------| LINUX BOX  |
EXT IP1	| ROUTER 1 | INT IP1   eth1 |            |
	|          |                | EQUAL COST |
	------------                |     &      |
                                    |  MULTIPATH |eth0
                                    |            |----------LAN
	                            |  GATEWAY   |INT IP0
	------------                |            |
	|          |                |    AND     |
--------| DSL      |----------------|            |
EXT IP2	| ROUTER 2 | INT IP2   eth2 |  FIREWALL  |
	|          |                |            |
	------------                |            |
                                    |            | 
                                    ---------------

EXT IP1 and EXT IP2 are the two public IP addresses alloted by the DSL
provider while INT IP1 and INT IP2 are the respective internal IP
addresses. The new gateway should automatically divide the traffic between
the two DSL links and in case on of the links is down it should shift the
entire traffic to the other one. When the link is restored, it should again
start distributing traffic between the two links. I also intend to have an
IPChains or IPTables based firewall on this gateway. 



Please let me know if the above arrangement is technically feasible. If
yes, how should I go about implementing it. Please suggest suitable
references and web links. Since at present, the DSL links terminate into
the local LAN switch the two routers have been assigned internal IP
addresses belonging to my network. How do I go about assigning interface
addresses to interfaces eth1 and eth2 (which should be INT IP1 and INT
IP2), since the routers are programmed by the DSL provider. In the new
arrangement, can I give them the same addresses as earlier?

Thanking you in anticipation,

Kamath


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