Red Hat Bugzilla – Bug 146732
CAN-2005-0337 open relay bug in postfix ipv6 patch
Last modified: 2007-11-30 17:07:16 EST
*** This bug has been split off bug 146731 ***
------- Original comment by Josh Bressers (Security Response Team) on 2005.01.31
Affects: Postfix with IPv6 patch on Linux
Credits: Peer Heinlein
Postfix is an MTA written by Wietse Venema with security in mind. The
code has a great security record.
Dean Strik provides an IPv6 patch for Postfix releases.
To read IPv6 addresses and netmasks on Linux, the patch uses
In some cases, the /proc/net/if_inet6 file is not available. The most
common reason being that Postfix runs chrooted without /proc mounted in
the chroot. A programming error in the IPv6 patch could result in
Postfix relaying emails to destinations that have IPv6 addresses for
their MX hosts.
If /proc/net/if_inet6 is not available, so Postfix does configure any
IPv6 addresses, the permit_mx_backup code erroneously returns success
for relay permissions to any IPv6 host.
The Postfix IPv6 patch documentation (IPV6_README) does note that
- It is not currently supported to use Postfix network daemons
(such as smtp and smtpd) chrooted on Linux systems without
mounting the proc filesystem under /var/spool/postfix/proc
This is because the proc filesystem is required on Linux to
obtain the system's IPv6 address information.
So the configuration where /proci is NOT available to Postfix is not
The problem is specific to the Linux operating system in unsupported
Postfix does not come chrooted by default. The IPv6 patch does not
change this behaviour. Packagers/distributors however may have changed
The permit_mx_backup setting is not used by default and must be
specificially configured by the Postfix administrator.
If Postfix cannot read the /proc/net/if_inet6 file and permit_mx_backup
is used, then Postfix will wrongly relay mail only to sites that have
IPv6 addresses (AAAA RRs in DNS) configured for at least one MX host.
Because of these four points, the impact of the relaying bug is very
Since the problem is in the IPv6 patch to Postfix, the version numbers
used here are those of the IPv6 patch. An administrator can query the
patch version number by issuing the command
Patch versions up to and including 1.25 are vulnerable to this problem.
Associated Postfix versions include Postfix 2.1.x and 2.0.x.
Postfix 2.2 snapshots with IPv6 included in the base Postfix
(2.2-20050111-nonprod and up) are NOT vulnerable to the problem.
IPv6 and TLS+IPv6 patch 1.26 provide a correction of the problem.
Peer Heinlein reported the problem to the IPv6 patch author.
Several workarounds exist for the problem:
a) Do not run the Postfix smtpd program chrooted.
This can be achieved by editing master.cf and putting an 'n' in
the chroot column of the smtpd line;
b) Make the /proc filesystem available under the chroot.
Upgrade the Postfix IPv6 patch to version 1.26 or higher. The patches
are available from
An advisory has been issued which should help the problem
described in this bug report. This report is therefore being
closed with a resolution of ERRATA. For more information
on the solution and/or where to find the updated files,
please follow the link below. You may reopen this bug report
if the solution does not work for you.