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Thread: Help me to Protect qmail - Part III

Message: 9506623
FROM: Dave Sill
DATE: 09/04/2002 12:40:34
SUBJECT: RE: Help me to Protect qmail - Part III


Alberto J. Azevedo <<EMAIL: PROTECTED>> wrote:

>The problem this time is this: another guy, (thats two now) send to
>the list this web page and my jaw is on the floor.

Matthias Andree is a well-known qmail-hater/Postfix-lover. However,
this "Qmail bugs" page is probably pretty convincing to those who
don't know the facts, so, for the record, I think we should address
his comments. I'll take a stab at it...anyone else who wants to can
chime in, of course.

1.1. qmail-smtpd memory exhaustion attacks (unfixed for almost five

  DJB and Venema take different approaches to resource limitation. DJB
  prefers to use general resource limitation tools like
  softlimit. Venema prefers to build resource limitation into each and
  every program that uses resources. Therefore, DJB doesn't consider
  this a bug, but Venema and Venemoids like Andree do. The fact is
  that an LWQ-style installation using softlimit on the qmail-smtpd
  processes is not vulnerable to these attacks.

1.2. qmail-smtpd bounce flood

  qmail-smtpd can't bounce messages to invalid recipients during the
  SMTP session, it has to accept everything and return a bounce
  if/when delivery fails. Determining the validity of a local
  recipient during the SMTP session would require qmail-smtpd to
  duplicate the functionality of qmail-local *and* to have
  qmail-local's access to home directories. Even then, in many cases,
  qmail-smtpd would be unable to determine conclusively whether an
  address is valid or not--consider the case where
  ~alias/.qmail-default exists or a virtual domain has a

  Besides being hard to do, validating recipients during the SMTP
  session makes it trivial for spammers to determine which addresses
  are deliverable. A spammer can quickly run through a dictionary of
  possible recipients and the MTA will obligingly validate them. The
  standard fix to this problem, tarpitting, is kludgy and adds
  unnecessary complexity to the SMTP daemon.

  Yes, return addresses can be forged, but that's a weakness inherent
  in SMTP.

2.1. Bounce message contents are not crashproof

  Yes, if the system crashes at the wrong moment a bounce message
  could be lost. Big deal. Many MTAs don't even preserve the entire
  original message in a bounce.

3.1. Deliberate RFC-1652 "8BITMIME" violation

  Yes, qmail always sends 8-bit data. Yes, this is theoretically a
  problem. In reality, it's not a problem. Go ahead and search the
  archives if you don't believe me.

3.2. RFC-2821 (SMTP) violation (Mail Routing)

  The RFC requires an MTA to try at least two MX's if a message is
  undeliverable to the first MX. qmail only tries alternative MXs when
  the first MX is unreachable. If the first MX accepts the connection
  and refuses the message, qmail doesn't try to deliver via a backup
  MX. This is a good thing, in practice, because primary MXs are
  almost always authorative for a domain. The behavior the RFC
  requires could (and does) result in spam being delivered through
  backup MXs.

3.3. Potential for Violation of RFC-2045 (MIME) and Lack of RFC-1894
(Delivery Status Notifications) support

  DSN is a huge, complex mechanism for what should be a simple
  process. qmail implements a simple bounce mechanism, trading
  conformity for simplicity, reliability, and security.

4.1. Local resource hogging

  Andree's benchmarks were designed to make qmail look bad, and they
  did. qmail does do lots of synchronous I/O--that's necessary to
  provide the reliability that qmail guarantees. But independent
  benchmarks show that qmail is faster than Postfix, Sendmail, and
  Exim. See: http://www.kyoto.wide.ad.jp/mta/eval1/eindex.html

4.2. Qmail is not softupdates aware

  Softupdates improves filesystem performance at the cost of data
  reliability. Softupdates guarantees a consistent filesystem, but it
  doesn't guarantee that everything written to disk is actually
  written to disk. qmail requires a filesystem that doesn't lie about
  whether data was actually written to disk.

4.3. Bandwidth hogging

  Yes, qmail "hogs" bandwidth in order to simplify the delivery
  process (and make qmail itself smaller, more reliable, and more
  secure) and deliver mail faster. As any solution to a complex
  engineering problem, qmail makes trade-offs and compromises. No MTA
  can simultaneously minimize bandwidth and maximize delivery
  rate. DJB chose to maximize delivery rate.

4.4. No client-side ESMTP PIPELINING

  Correct. qmail could possibly benefit from client-side ESMTP
  PIPELINING, though it's not clear that the benefits outweigh the

4.5 Delayed bounces (also waste of bandwidth and spam-friendly)

  See 1.2.

4.6. Parallel delivery of qmail-remote sets back

  One person has reported a problem. As Andree says, further research
  needs to be done in this area before any conclusion can be drawn,
  but the lack of problem reports on the qmail list indicates that
  this is a rare and/or minor problem.

4.7 Mail sent from "anonymous", particularly by daemons.

  This behavior is clearly intentional, and the workaround is simple
  and well-documented.

5.1. Security guarantee

  $500 might be ridiculously small, but many people pored over Knuth's
  TeX code for a chance at his $1 bug bounties. Many people would like
  nothing better than to find a security bug in qmail just because
  they dislike DJB.

  Venema didn't win the prize because (1) he didn't find a bug in
  qmail, and (2) DoS issues were explicitly excluded.

6.1. Copy bounces to postmaster

  So qmail doesn't have a "copy bounces to postmaster" feature. So
  what? Not only have I never wanted such a feature, I don't remember
  ever seeing anyone ask for it. (It could, of course, be implemented
  easily in qmail without patching.)



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