Little help w/ greylisting bypass for SMTP AUTH-ed sessions

Vernon Schryver vjs@calcite.rhyolite.com
Sun Dec 3 22:45:47 UTC 2006


> From: Pawel Osiczko 

> I was wondering if you could help me out w/ greylist bypass for authenticated
> sessions in sendmail.

Depending on the situation, there may be something better than fully
whitelisting clients.  People trying to use typical MUAs such as Netscape
that use SMTP as a mail submission protocol but do not understand any
4yz rejections, such as for too many recipients, have problems.
If possible, they should be whitelisted by IP address with lines like

submit  ip  10.2.3.0/24

That tells dccm or dccifd that the SMTP clients must not be greylisted
or be given 4yz rejections for conflicts in per-user DCC whiteclnt files
among targets, but does not otherwise whitelist them.  This way dccm
can still detect and reject spam, such as a trojan trying to send spam
through your smarthost.

Of course, clients that send legitimate bulk mail need to be fully
whitelisted.



>                       I generated sendmail.cf with hackmc -AROT. With dcc
> up and running and with sendmail authenticating against saslauthd, I specify
> option MTA-first to attempt to whitelist authenticated sessions. 'Cept it does
> not work. After TLS-ed AUTH PLAIN succeeds, the message is embargoed leaving
> client all hot, bothered, and confused. Here is what my sendmail sees:


> Nov 29 21:51:59 foo sm-mta[24096]: kAU4plca024096: <-- AUTH PLAIN AHBhYmxvAGxpVkYhMEQ=
> Nov 29 21:51:59 foo sm-mta[24096]: kAU4plca024096: --- 235 2.0.0 OK Authenticated

Is TRUST_AUTH_MECH set in the .mc file?
Does it include "PLAIN" or whatever?
When a message gets through, does the Received: header say that TLS
authentication worked with "verify=OK"?

As you can see from the changes that hackmc makes to sendmail.cf,
dccm simply obeys the sendmail ${dcc_notspam} macro that should be set
with this sendmail.cf line:

   R<$={TrustAuthMech}> $* $: $(macro {dcc_notspam} $@ authenticated $) <> $2

The ${dcc_notspam} mechanism works for me with STARTTLS self-signed PKI
certs.  However, there may be something wrong with SMTP-AUTH, because
Sam Leffler has reported being unable to make authentication whitelist
senders.  I do not have a test-bed for SMTP-AUTH.


The problem of users getting all hot and bothered by greylisting often
indicates that they know less than they think they do about email.  They
choose to not understand that a 4yz rejection will not delay their
incoming mail by more than 15 or 30 minutes if the sending SMTP client
is reasonable, and that the delay will happen only once.  For the most
part, such hot and bothered lusers are reacting to delays that are
already finished when they see them.  The hot luser problem is so common
that years ago an ISP suggested and I added a control that disables
per-user logging of greylisting.  That control is enabled by adding the
following line to /var/dcc/whiteclnt

option greylist-log-off


Vernon Schryver    vjs@rhyolite.com



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