smtp load balancer???

Carl Lewis carl at
Wed Dec 7 22:34:38 GMT 2005

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Chris Conn wrote:
>>> avhost      5       IN      A
>>> avhost        5       IN      A
>>> gives you a 5 second TTL on both records.  Remove one, reload DNS, 
>>> and 5 seconds later it never existed.
>> Been there, done that. My FreeBSD machines worked fine, my Macs worked 
>> fine, my clients with Windows 2003 server continued to cache the 
>> record. Two weeks ago I changed the IP on from 
>> to, yet I still have traffic. This is 
>> not supposed to be so, I have had plenty of people tell me is not so, 
>> yet I have traffic. When I call a client and have them go to their 
>> office server and run "ipconfig /flushdns" everything works again.
>> DAve
> Hmm, that's too bad.  My Windows DNS server does not behave like that.  
> My other UNIX servers are fine too, and to my knowledge I have not found 
> a MS client that would override the record TTL.
> It does not really matter since I do not rely on this method, however my 
> experience differs from yours it seems when it comes to DNS ttls.
I have to echo the, don't go with DNS for load balancing mail-servers.
There's a surprising amount of broken machines out there.
We moved to a load balancer solution a couple of years ago because of
that problem and the fact that some OS's didn't properly handle multiple
MX records.
I've been using a simple pair of linux boxes with keepalived (as a
"front-end" to IPVS) for load balancing. they have a spread of services
shared between them (SMTP/HTTP/HTTPS/Z39.50) and act as each others hot
spare. They live on the same DMZ VLAN but in different buildings.

The Load balancers handle a lot of traffic 24x7 and hardly break a 
sweat, it really does seem to be a fairly low overhead process.

For mail, the single IP on the load balancer's is distributed to
5 small PC's running sendmail/MailScanner etc. It's brilliant!
I can actually do maintenance/patching/upgrades etc during the
day by simply pulling out a box or two at a time with no need
to post outages or hope no one notices. In  the near future I'll be
able to add in another 2 or 3 boxes and seamlessly handle the forever
increasing flow of mail.


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