On Wed, Nov 04, 2009 at 04:46:16PM +0000, Paul Tansom wrote:
The issue/problem is the NAT forwarding, I can't see a way to handle two streams of incoming SMTP mail with one sent to port 25 and one sent to a non-standard port. E.g. there's SMTP messages arriving at the 'outside' of the NAT router and the port forwarding and firewall are set up to route them to my desktop machine's port 25. There's no way I can see to split off mail for, say, test.isbd.net to a different port from that sent to chris.isbd.net.
** end quote [Chris G]
As you say, you won't be able to differentiate between mail domains to route them differently using a basic NAT router. What you can do is telnet in to the alternative port and run the SMTP conversation manually, something along the lines of:
[snip details, thanks]
Alternatively if you have a server doing the forwarding then you may have the ability to configure a different port when you configure the domain routing details. That will depend on your setup. I've not tried it, although I keep meaning to check whether the Exim 4 hubbed_host feature can manage it, preferably on a per domain basis.
The router doing NAT is a Draytek Vigor 2820N which can do lots of clever things, I'll have a dig around.
I do actually have two ADSL connections which the 2820N load balances (it has an ethernet WAN port as well as an ADSL connection), I can send mail to the other router as it has its own 'outside' address (in fact I'm already doing this). I suppose I could simply disconnect the other router from the 2820's ethernet WAN port (it'll fail over to using just one WAN) and then port map mail on the other router to a different port. It's all a bit messy though! :-)
It's all down to how much effort I want to put into checking it before I get daring and do the 'big switchover'.