Re: Problem with multiple ADSL lines

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Hi Mohan,

Thanks a lot for your reply. I haven't tried it yet, but I have no doubts it will work.

However, if you have time, could you please explain to me why a request received from IP1 by host H1, having as default route IP2, can not be routed through IP1; I have a routing table saying that the route to IP1 is through the proper eth card.
I have used tcpdump to check the packets coming/in/out from the systems and it seems that there is a packet going from ADSL1 to H1, but nothing on the other way... it seems like H1 doesn't know what to do with it.

Could you please explain (or point me to URLs explaining that) why it will change if I set the modems to bridge mode? I still want to do NAT, will it change if I do it from the Linux router now?

I also do not understand what you imply with
"3. Source NAT rules use ISP1_IP and routing rule redirects it to link
connected to ISP2. Flip similarly for ISP1."
Could you be a bit more descriptive?

Forgive me for being a bit ignorant of all that; possibly I'm trying to understand too much without basic technical background on routing, etc.
One more time, thanks a lot for your help.

On 10/12/07, Mohan Sundaram <mohan.tux@xxxxxxxxx > wrote:
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 <>; while if someone hits
> http://myADSL2_IP I would redirect him to
> <>.
This is your ADSL router(s) config.
> If I have rules such as the following all works well:
>    ip rule add from <> table ADSL1_rules
>    ip rule add from <> 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.

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