This document is about PowerDNS 4.X. If you have PowerDNS 3.X, please see the PowerDNS 3.X documentation

PowerDNS Security Advisory 2012-01: PowerDNS Authoritative Server can be caused to generate a traffic loop

Affected versions of the PowerDNS Authoritative Server can be made to respond to DNS responses, thus enabling an attacker to setup a packet loop between two PowerDNS servers, perpetually answering each other's answers. In some scenarios, a server could also be made to talk to itself, achieving the same effect.

If enough bouncing traffic is generated, this will overwhelm the server or network and disrupt service.

As a workaround, if upgrading to a non-affected version is not possible, several options are available. The issue is caused by the packet-cache, which can be disabled by setting 'cache-ttl=0', although this does incur a performance penalty. This can be partially addressed by raising the query-cache-ttl to a (far) higher value.

Alternatively, on Linux systems with a working iptables setup, 'responses' sent to the PowerDNS Authoritative Server 'question' address can be blocked by issuing:

      iptables -I INPUT -p udp --dst $AUTHIP --dport 53 \! -f -m u32 --u32 "0>>22&0x3C@8>>15&0x01=1" -j DROP

If this command is used on a router or firewall, substitute FORWARD for INPUT.

To solve this issue, we recommend upgrading to the latest packages available for your system. Tarballs and new static builds (32/64bit, RPM/DEB) of and 3.0.1 have been uploaded to our download site. Kees Monshouwer has provided updated CentOS/RHEL packages in his repository. Debian, Fedora and SuSE should have packages available shortly after this announcement.

For those running custom PowerDNS versions, just applying this patch may be easier:

--- pdns/   (revision 2326)
+++ pdns/   (working copy)
@@ -253,7 +253,9 @@
+    if(P->d.qr)
+      continue;
     S.ringAccount("queries", P->qdomain+"/"+P->qtype.getName());
     if(logDNSQueries) {

It should apply cleanly to 3.0 and with little trouble to several older releases, including 2.9.22 and 2.9.21.

This bug resurfaced because over time, the check for 'not responding to responses' moved to the wrong place, allowing certain responses to be processed anyhow.

We would like to thank Ray Morris of for bringing this issue to our attention and Aki Tuomi for helping us reproduce the problem.