bugtraq - Re: possible open relay hole in qmail-smtpd-auth patch

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Date: Wed, 16 Jul 2003 16:48:18 -0400
From: Valdis.Kletnieks@...edu
To: Uwe Ohse <uwe@...e.de>
Cc: smtpauth@...t.elysium.pl, qmail@...t.cr.yp.to, bugtraq@...urityfocus.com
Subject: Re: possible open relay hole in qmail-smtpd-auth patch

On Wed, 16 Jul 2003 11:54:18 -0000, Uwe Ohse <uwe@...e.de>  said:

> While i'm at it: Your qmail-1.03-jms1-antispam.patch not only violates
> the SMTP protocol (replying OK when the mail will definitively not reach
> the recipient

Actually, it is *quite* legal to reply a '250 OK' on something that will eventually
bounce.  If it weren't, you couldn't have an MX site that had no direct knowledge of
all possible valid userids (think about it - your main mail hub goes down, mail
starts piling up at your off-site backup MX - and THAT mail server has to say
'250 OK' without being able to tell for *SURE* that 'userid@...e.de' is or is not valid.

RFC2821, section 6.1 says:

6.1 Reliable Delivery and Replies by Email

   When the receiver-SMTP accepts a piece of mail (by sending a "250 OK"
   message in response to DATA), it is accepting responsibility for
   delivering or relaying the message.  It must take this responsibility
   seriously.  It MUST NOT lose the message for frivolous reasons, such
   as because the host later crashes or because of a predictable
   resource shortage.

   If there is a delivery failure after acceptance of a message, the
   receiver-SMTP MUST formulate and mail a notification message.  This
   notification MUST be sent using a null ("<>") reverse path in the
   envelope.  The recipient of this notification MUST be the address
   from the envelope return path (or the Return-Path: line).  However,
   if this address is null ("<>"), the receiver-SMTP MUST NOT send a
   notification.  Obviously, nothing in this section can or should
   prohibit local decisions (i.e., as part of the same system
   environment as the receiver-SMTP) to log or otherwise transmit
   information about null address events locally if that is desired.  If
   the address is an explicit source route, it MUST be stripped down to
   its final hop.

So it's perfectly valid to issue a 250 OK for a bad address, as long as
you bounce it if there's an issue later.

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