[Mimedefang] Revisit: Filtering on HELO

Philip Prindeville philipp_subx at redfish-solutions.com
Mon Mar 26 11:56:29 EDT 2007

David F. Skoll wrote:
> Philip Prindeville wrote:
>> Remember: the original point was that if a host is
>> multi-homed {A.A.A.A, B.B.B.B, C.C.C.C}, that it is
>> incorrect behavior to say "EHLO [B.B.B.B]" on a
>> socket that is bound to any local interface *other
>> than* B.B.B.B.
> Why is it incorrect?  A multihomed host can call itself any of its
> IP addresses.
> The only kind of HELO filtering I do on my server is to reject remote
> hosts that call themselves "www.roaringpenguin.com" or
> [], because I know they're lying.
> --
> David.

It's incorrect because the originating machine might
be hosting several logic, distinct domains, each with
its own IP address... which are *not* interchangeable.

The server (the one receiving the HELO) can't make
any valid assumptions about whether the client is truly
multihomed (and all addresses are equivalent), or if
it has a bunch of cloned interfaces, each with a unique
address and a separate instance of the MTA running
on each (and each domains' MX pointing to *just one*
IP address on that multihomed machine, not ALL of

Web farms, VMware, etc. all throw whatever assumptions
we had about multi-homing out the window.


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