Security of the PowerDNS Recursor

For Security Advisories, see the dedicated page.

PowerDNS Security Policy

If you have a security problem to report, please email us at both security@powerdns.com and ahu@ds9a.nl. Please do not mail security issues to public lists, nor file a ticket, unless we do not get back to you in a timely manner. We fully credit reporters of security issues, and respond quickly, but please allow us a reasonable timeframe to coordinate a response.

We remind PowerDNS users that under the terms of the GNU General Public License, PowerDNS comes with ABSOLUTELY NO WARRANTY. This license is included in this documentation.

As of the 9th of September 2016, no actual security problems with PowerDNS Authoritative Server 3.4.10, Recursor 3.6.3, Recursor 3.7.2, or later are known about. This page will be updated with all bugs which are deemed to be security problems, or could conceivably lead to those. Any such notifications will also be sent to all PowerDNS mailing lists.

HackerOne

Security issues can also be reported on our HackerOne page and might fetch a bounty. Do note that only the PowerDNS software is in scope for the HackerOne program, not our websites or other infrastructure.

Disclosure Policy

  • Let us know as soon as possible upon discovery of a potential security issue, and we’ll make every effort to quickly resolve the issue.
  • Provide us a reasonable amount of time to resolve the issue before any disclosure to the public or a third-party.
  • We will always credit researchers in our Security Advisories.

Anti-spoofing

The PowerDNS Recursor uses a fresh UDP source port for each outgoing query, making spoofing around 64000 times harder. This raises the bar from ‘easily doable given some time’ to ‘very hard’. Under some circumstances, ‘some time’ has been measured at 2 seconds. This technique was first used by dnscache by Dan J. Bernstein and is standardized in RFC 5452

In addition, PowerDNS detects when it is being sent too many unexpected answers, and mistrusts a proper answer if found within a clutch of unexpected ones.

This behaviour can be tuned using the spoof-nearmiss-max.

Throttling

PowerDNS implements a very simple but effective nameserver. Care has been taken not to overload remote servers in case of overly active clients.

This is implemented using the ‘throttle’. This accounts all recent traffic and prevents queries that have been sent out recently from going out again.

There are three levels of throttling.

  • If a remote server indicates that it is lame for a zone, the exact question won’t be repeated in the next 60 seconds.
  • After 4 ServFail responses in 60 seconds, the query gets throttled too.
  • 5 timeouts in 20 seconds also lead to query suppression.

Security Polling

PowerDNS products can poll the security status of their respective versions. This polling, naturally, happens over DNS. If the result is that a given version has a security problem, the software will report this at level ‘Error’ during startup, and repeatedly during operations.

By default, security polling happens on the domain ‘secpoll.powerdns.com’, but this can be changed with the security-poll-suffix. If this setting is made empty, no polling will take place. Organizations wanting to host their own security zones can do so by changing this setting to a domain name under their control.

To make this easier, the zone used to host secpoll.powerdns.com is available here.

To enable distributors of PowerDNS to signal that they have backported versions, the PACKAGEVERSION compilation-time macro can be used to set a distributor suffix.

Details

PowerDNS software sadly sometimes has critical security bugs. Even though we send out notifications of these via all channels available, we find that not everybody actually find out about our security releases.

To solve this, PowerDNS software will start polling for security notifications, and log these periodically. Secondly, the security status of the software will be reported using the built-in metrics. This allows operators to poll for the PowerDNS security status and alert on it.

In the implementation of this idea, we have taken the unique role of operating system distributors into account. Specifically, we can deal with backported security fixes.

Finally, this feature can be disabled, or operators can have the automated queries point at their own status service.

Implementation

PowerDNS software periodically tries to resolve ‘auth-x.y.z.security-status.secpoll.powerdns.com|TXT’ or ‘recursor-x.y.z.security-status.secpoll.powerdns.com’.

The data returned is in one of the following forms:

  • NXDOMAIN or resolution failure -> 0
  • “1 Ok” -> 1
  • “2 Upgrade recommended for security reasons, see …” -> 2
  • “3 Upgrade mandatory for security reasons, see …” -> 3

In cases 2 or 3, periodic logging commences. The metric security-status is set to 2 or 3 respectively. If at a later date, resolution fails, the security-status is not reset to 1. It could be lowered however if we discover the security status is less urgent than we thought.

If resolution fails, and the previous security-status was 1, the new security-status becomes 0 (‘no data’). If the security-status was higher than 1, it will remain that way, and not get set to 0.

In this way, security-status of 0 really means ‘no data’, and can not mask a known problem.

Distributions

Distributions frequently backport security fixes to the PowerDNS versions they ship. This might lead to a version number that is known to us to be insecure to be secure in reality.

To solve this issue, PowerDNS can be compiled with a distribution setting which will move the security polls from: ‘auth-x.y.z.security-status.secpoll.powerdns.com’ to ‘auth-x.y.z-n.debian.security-status.secpoll.powerdns.com.

Note two things, one, there is a separate namespace for debian, and secondly, we use the package version of this release. This allows us to know that 4.0.1-1 (say) is insecure, but that 4.0.1-2 is not.

Configuration Details

The configuration setting security-poll-suffix is by default set to ‘secpoll.powerdns.com’. If empty, nothing is polled. This can be moved to ‘secpoll.yourorganization.com’.

If compiled with PACKAGEVERSION=3.1.6-abcde.debian, queries will be sent to “auth-3.1.6-abcde.debian.security-status.security-poll-suffix”.

Delegation

If a distribution wants to host its own file with version information, we can delegate dist.security-status.secpoll.powerdns.com to their nameservers directly.