Layer 7 Load Balancer

Les Mikesell lesmikesell at
Mon Nov 19 19:52:42 UTC 2007

Luc MAIGNAN wrote:

>>> My problem : I have two mails servers and I want to distribute 
>>> incoming mails to a server depending of the domain (e.g. 
>>> somebody at  to server1 and somebody at to server2)

>> Uh, why?  Is there some reason DNS won't do what you need?  With 2 
>> separate domains, just point your MX record to the correct server.
>> Or am I missing something obvious here?

 > No, because the MX of the two domains points to the same IP (services
 > are dispatched behind this IP)

Nothing at the IP level can distinguish inbound connections for the 
different destinations and they are all going to be connecting to port 
25.  However, you can configure sendmail at that IP address (or wherever 
you direct port 25) to accept for both domains and relay to the other 
one at an address specified in mailertable.   The problem you'll have 
with this is that the first machine won't know the users on the other 
domain and will relay everything.  If you get a lot of spam to 
non-existent users, then the real server will reject them and the relay 
will have to construct and try to deliver a bounce, clogging up your 
queues.  To avoid that you might use virtusertable to list all the valid 
recipients at the other domain and a default rejection if they don't 
change too much, or something like MimeDefang which can check the real 
host for the recipient via smpt before accepting at all.

   Les Mikesell
    lesmikesell at

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