backend_connect source address and config_servername

[Bron Gondwana]

Hi,

I'm working on setting up replication for our (FastMail.FM) new servers.

One issue I've run in to is that our machines are set up as follows:

10.*: imap<$n>.internal
66.111.4.*: imap<$n>.messagingengine.com

Now we can argue all we like about whether the IMAP servers should have
an external network connection at all (it's pretty heavily firewalled,
but for various hysterical reasons I can't cut their direct outside
connection just yet.

So - I'd very much prefer config_servername to contain
imap<$n>.messagingengine.com since that's what the world sees on the
connection string - but when I do that, backend_connect binds the
_source_ of its TCP connections to the external IP address, despite
being asked to make a connection to another 10.* address.

I guess I have two questions:

1) Is there a good reason for this behaviour, binding to a specific
   interface/IP address on all outbound connections?

2) If so, would it make sense to have a separate configuration item
   (config_backend_ip ?) that could override the default behaviour?

Regards,

Bron.

-- 
  Bron Gondwana
  brong at fastmail.fm