I. Background
BIND 9 is an implementation of the Domain Name System (DNS) protocols.
The named(8) daemon is an Internet Domain Name Server. DNS requests
contain a query id which is used to match a DNS request with the response
and to make it harder for anybody but the DNS server which received the
request to send a valid response.
II. Problem Description
The BIND DNS implementation does not randomize the UDP source port when
doing remote queries, and the query id alone does not provide adequate
randomization.
III. Impact
The lack of source port randomization reduces the amount of data the
attacker needs to guess in order to successfully execute a DNS cache
poisoning attack. This allows the attacker to influence or control
the results of DNS queries being returned to users from target systems.
IV. Workaround
Limiting the group of machines that can do recursive queries on the DNS
server will make it more difficult, but not impossible, for this
vulnerability to be exploited.
To limit the machines able to perform recursive queries, add an ACL in
named.conf and limit recursion like the following:
acl example-acl {
192.0.2.0/24;
};
options {
recursion yes;
allow-recursion { example-acl; };
};
V. Solution
Perform one of the following:
1) Upgrade your vulnerable system to 6-STABLE or 7-STABLE, or to the
RELENG_7_0 or RELENG_6_3 security branch dated after the correction
date.
2) To patch your present system:
The following patches have been verified to apply to FreeBSD 6.3 and
7.0 systems.
a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.
[FreeBSD 6.3]
# fetch http://security.FreeBSD.org/patches/SA-08:06/bind63.patch
# fetch http://security.FreeBSD.org/patches/SA-08:06/bind63.patch.asc
[FreeBSD 7.0]
# fetch http://security.FreeBSD.org/patches/SA-08:06/bind7.patch
# fetch http://security.FreeBSD.org/patches/SA-08:06/bind7.patch.asc
b) Execute the following commands as root:
# cd /usr/src
# patch
NOTE WELL: This update causes BIND to choose a new, random UDP port for
each new query; this may cause problems for some network configurations,
particularly if firewall(s) block incoming UDP packets on particular
ports. The avoid-v4-udp-ports and avoid-v6-udp-ports options should be
used to avoid selecting random port numbers within a blocked range.
NOTE WELL: If a port number is specified via the query-source or
query-source-v6 options to BIND, randomized port selection will not be
used. Consequently it is strongly recommended that these options not
be used to specify fixed port numbers.
The latest revision of this advisory is available at FreeBSD
