[adsl] ADSL Routers and multiple Port 25 forwarding

[Jp Wise]

Another couple of maybe options which would be a bit trial and error 
would be:

1) Put a router than can do routing based on port addresses in between 
the two adsl routers and the mail server.
1a) Port forward from one adsl model to port 25
1b) Port forward from the other adsl modem to port 26 (with 26 activated 
on server 2 aswell).  this gives you different port numbers for 
determining which router it should send it back to via the in between 
router.

2) Second NIC (or 2nd ip/subnet on existing nic) - maybe.
2a) routing tables may still be an issue with this. In the back of my 
mind the last time I properly played with routing the system will still 
go with the 'default default' even if it's via a different nic. But you 
might have an option to bind the mail server in SBS2003 to a local 
interface and see how it goes with the return routing.


I've used something similar to the first when I had a linux box with two 
links (adsl & cable). The cable was primary mx, the adsl secondary mx.  
Connections coming in on Cable:25 were forwarded to the mail server on 
port 25. Connections coming in on ADSL:25 were forwarded to the mail 
server on port 26.  When the return packets came back from the mail 
server if it was from mail:25 the iptables were configured to send it 
went out via cable. If it came from mail:26 it was sent out via 
adsl.     The mail server of course also needs to be enabled for 
connections on port 26 aswell.

Jp.

Mark Foster wrote:
>> Last night adsl1 fell over and no email was delivered (as expected)
>> I have tried forwarding both adsl routers to server1 but it is sbs2003 and
>> will not allow this it seems.
>>     
>
> This will probably be because its default gateway will point back to 
> adsl2's router, so any attempt to communicate with it through adsl1 will 
> have all return packets routed via adsl2 - and the remote end will not 
> have any idea what is going on. (Aka, no tcp sessions can be established).
>
>
>   
>> Can anyone suggest how i can forward port 25 from M1122 to XH1169 and then
>> onto server1 or if not possible I have a spare G604T that i can use in place
>> of the M1122.
>>     
>
> Can't really be done, as it'd require you to be able to deal with pinholes 
> on the internal interface of the XH1169 and then re-forward the traffic to 
> a different address, via the same interface.  This isn't exactly typical 
> behavior so I doubt the box has been configured to allow this.
>
> If you have two mail servers with one acting as secondary MX for the other 
> and vice versa, the only way to force traffic to one MTA in particular 
> would, from my POV, be to close Port 25 on the other and/or cause the 
> inbound systems to talk to the preferred mail server via its own MX record 
> / IP address.
>
> The alternative would be to change the default gateway on server1 to point 
> to the other DSL modem.  (Basically you can use only one route at a time. 
> To have different routes you would need different costs - one would be 
> preferred over the other - and the mailsystem would need to see the 
> preferred one actually fail / cease to be available, before it'll pick the 
> next lowest metric from its routing table and send traffic there.
>
> This is similar to corporate routers (say on frame relay) which have an 
> ISDN link as secondary; the ISDN Link is present but has a much higher 
> cost, so is not used unless the primary route (the frame link) dissapears 
> from the routing table - eg, the interface drops.
>
> Hope this helps. I dare say there are people on here more experienced at 
> some of this than I.
>
> Mark.
> _______________________________________________
> adsl mailing list
> adsl at lists.unixathome.org
> http://lists.unixathome.org/mailman/listinfo/adsl
>
>
>