[LARTC] Problem with multiple ADSL lines

[Mohan Sundaram]

Kostas Flokos wrote:
> Hi all,
> I have managed to setup a Fedora 7 box with 3 ethernet cards and two 
> ADSL modem/routers from different suppliers as LARTC recommends. I am 
> able to direct traffic for specific internal IPs either to one or the 
> other ADSL line. However, I am faced with two problems I am struggling 
> for the solution:
>  
> 1. I have opened a few ports on the ADSL router/firewalls to talk to 
> internal hosts; say when someone hits http://myADSL1_IP I would redirect 
> him to 192.168.0.10 <http://192.168.0.10>; while if someone hits 
> http://myADSL2_IP I would redirect him to 192.168.0.20 
> <http://192.168.0.20>. 
This is your ADSL router(s) config.
> If I have rules such as the following all works well:
>    ip rule add from 192.168.0.10 <http://192.168.0.10> table ADSL1_rules
>    ip rule add from 192.168.0.20 <http://192.168.0.20> table ADSL2_rules
These are rules on your linux machine to send traffic back on the same 
link they came from.
> Unfortunately, if I want to do the reverse it does not work. I can't 
> have a host prefer one ADSL line, but still receive traffic from the 
> other ADSL line. 
This can be done by switching the rule table above. Obviously NAT is 
being used. However, this will break. You cannot initiate a connection 
to one IP and match the reply or ack for that connection from another 
IP. TCP/IP basics. Assymetric routing works when no address translations 
take place.
> It may sound weird, but I only want to have one host 
> reply to any of the two IPs, either from ISP1 or ISP2. With the current 
> configuration I can't. It works ok the default ISP of the host, but 
> can't make it to work for the other.
One way of doing this would be:
1. Set the routers to bridge mode.
2. Assign the ISP IPs to the NICs.
3. Source NAT rules use ISP1_IP and routing rule redirects it to link 
connected to ISP2. Flip similarly for ISP1.

Let us hope your ISP2 does not filter out traffic having source outside 
his subnet when it arrives from within.

Mohan