[adsl] ADSL Routers and multiple Port 25 forwarding

[Mark Foster]

> 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.