[Greylist-users] idea: mis use temporary errors

rbr debian at ipconsult.nl
Sun May 27 13:54:29 PDT 2007

Following an idea. We have tried it for a while, and it works, but it is a
'solution' that is open for debate.

As you all know:
[1] smtp knows temporary 4xx errors and permanent 5xx errors.
[2] spammers worst enemy is time or delay, because with increasing time the
chance of being blacklisted on Spamcop, Sorbs, etc is also increased.

The second fact is one of the reasons why greylisting does work against

The idea I would like to drop is a combination of [1] and [2].

Quite often we get mail like this:

 In:  EHLO vsmtp105.tin.it
 Out: 250-ipconsult.nl
 In:  RCPT TO:<Karen532 at ipconsult.nl>
 Out: 550 <Karen532 at ipconsult.nl>: Recipient address rejected: User unknown
 in local recipient table

For reasons unknown to us, all kind of email addresses are 'tried'. Luckely
they have not found any working email address yet.
The trace as copied-and-paste above, is not coming from the usual
dsl/cable/ppp-hacked-windows-PC, but from decent mailservers. Every detail
like hostname, mx-record, etc, is correct.
So, apparently there are a lot of decent mailservers that are available for
spammers, for testing the existence of email addresses at our domains.

A small extra piece of info: we do run a perl-script that checks the
mail-logfile. Based on the mail-logfile, this perl-script does add
IP-addresses to the blacklist of the firewall. Handling abuse at a firewall
level is less expensive in terms of cpu then handling the abuse at
mail-level. And it gives us the oppertunity to do a 'drop' at iptable level,
which does result in timeouts for the offending mailhost. Something that
result in timeouts for a offending mailhost is always good [2].

The idea is as follows: we have changed permanent 5xx error to a self-defined
4xx temporary error. This is, off course, a violation of the RFCs. But the
result is kind of funny. Those temporary errors codes are pickup up by the
perl script, and the offending mailhost is placed in the blacklist at TCP/IP
level, with a drop rule. Because the offending mailhost did receive a 4xx
error, he keeps on trying for a couple of hours up to 2 days. Those retries
do not bother us, because it is handled quite effeciently by ip-tables.

The result is:

 In:  EHLO vsmtp105.tin.it
 Out: 250-ipconsult.nl
 In:  RCPT TO:<Karen532 at ipconsult.nl>
 Out: 412 <Karen532 at ipconsult.nl>: Recipient address rejected: User unknown
 in local recipient table
 shorewall drop vsmtp105.tin.it

Any comments on this?
Mis-use of temporary 4xx errors in combination with a drop-rule in the
firewall, with the objective that the offending mailhost is waisting time.

RbR-IPConsult BV


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