Backup MXes

Wed Nov 16 19:13:41 PST 2005

Quoting David Wolfskill (david at catwhisker.org):

> Rather, just as a sewer line needs to flow into a line that does
> not have a smaller diameter, the anti-spam measures must merely not
> become stricter as the mail passes from one relay to another.

Yes, lower-numbered MXes must be no _more_ restrictive than higher ones.
Saying they should all have the same policy is just my Procrustean solution.

> [Note:  in case it's not clear, all of my anti-spam measures are
> performed during the SMTP conversation, by my MTA.  The choices are:
> accept; silently discard; reject (either permanently or temporarily).]

Ditto.

> That does, howevber, "assume" (ahem!) a "well-behaved" SMTP client.
> Then again, if the SMTP client is sufficiently ill-behaved as to lose
> mail in such a circumstance, whose problem is that?  Probably the
> administrator of the client; probably the user of the client.  But the
> administrator of the server?  Maybe, but if I were forced to decide one
> way or the other, I'd select "not" for that case.

As I said separately, I'd expect to have a substitute MX for my
one-and-only-MTA online within much less than four _hours_, never mind
four days -- and figure I can live with the few not-well-behaved senders
that reject immediately instead of resending within very brief time
periods.

> I have been known to implement a variation on that theme: Have a
> backup MX all right, properly advertised, but as long as the primary
> is functioning, have the backup MX not listen to 25/tcp at all, so
> spammers get "connection refused."
> 
> Now, if the objective were to capture spam, a variant might be to
> advertise a higher-numbered MX, and as long as the primary MX is
> working OK, accept the mail, but rather than deliver it as addressed,
> assume that it's spam....  After all, no legitimate SMTP client has
> any business sending mail to the higher-numbered MX unless the
> lower-numbered MX fails to respond.

I like both these ideas.  The latter sounds like a dandy setup for a
spamtrap host -- if you have the bandwidth and patience for such things.