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

Recursion with the Authoritative Server

From 2.9.5 onwards, PowerDNS offers both authoritative nameserving capabilities and a recursive nameserver component. These two halves are normally separate but many users insist on combining both recursion and authoritative service on one IP address. This can be likened to running Apache and Squid both on port 80.

However, many sites want to do this anyhow and some with good reason. For example, a setup like this allows the creation of fake domains which only exist for local users. Such domains often don't end on ".com" or ".org" but on ".intern" or ".name-of-isp".

PowerDNS can cooperate with either its own recursor or any other you have available to deliver recursive service on its port.

By specifying the recursor option in the configuration file, questions requiring recursive treatment will be handed over to the IP address specified. An example configuration might be recursor=203.0.113.7, which designates 203.0.113.7 as the nameserver to handle recursive queries.

Warning: Using recursor is NOT RECOMMENDED as it comes with many potentially nasty surprises. For more info, you can read Dan Bernstein's article on this topic.

Take care not to point recursor to the PowerDNS Authoritative Server itself, which leads to a very tight packet loop!

By specifying allow-recursion, recursion can be restricted to netmasks specified. The default is to allow recursion from everywhere. Example: allow-recursion=203.0.113.0/24, 198.51.100.0/26, 192.0.2.4, ::1.

Details

Questions carry a number of flags. One of these is called 'Recursion Desired'. If PowerDNS is configured to allow recursion, AND such a flag is seen, AND the IP address of the client is allowed to recurse via PowerDNS, then the packet may be handed to the recursing backend.

If a Recursion Desired packet arrives and PowerDNS is configured to allow recursion, but not to the IP address of the client, resolution will proceed as if the RD flag were unset and the answer will indicate that recursion was not available.

It is also possible to use a resolver living on a different port. To do so, specify a recursor like this: recursor=192.0.2.1:5300.

Reminder: according to RFC3986 for IPv6, the notation is to encode the IPv6 IP number in square brackets like this: recursor=[::1]:5300, as they explain in section 3.2.2: Host:

A host identified by an Internet Protocol literal address, version 6 [RFC3513] or later, is distinguished by enclosing the IP literal within square brackets ("[" and "]"). This is the only place where square bracket characters are allowed in the URI syntax. In anticipation of future, as-yet-undefined IP literal address formats, an implementation may use an optional version flag to indicate such a format explicitly rather than rely on heuristic determination.

So, be careful! The authoritative pdns service won't communicate with pdns-recursor if you write wrongly the IPv6 IP number in the recursor line of pdns.conf. Therefore, ~~recursor=::1:5300~~ won't work because of the missing required square brackets ("[" and "]") enclosing the IP literal. Please respect IPv6 notation.

If the backend does not answer a question within a large amount of time, this is logged as 'Recursive query for remote 198.51.100.15 with internal id 0 was not answered by backend within timeout, reusing id'. This may happen when using 'BIND' as a recursor as it is prone to drop queries which it can't answer immediately.

To make sure that the local authoritative database overrides recursive information, PowerDNS first tries to answer a question from its own database. If that succeeds, the answer packet is sent back immediately without involving the recursor in any way. This means that for questions for which there is no answer, PowerDNS will consult the recursor for an recursive query, even if PowerDNS is authoritative for a domain! This will only cause problems if you 'fake' domains which don't really exist. This also means that if you delegate a subzone to another set or authoritative servers, when a request comes in for that sub-zone, PowerDNS will respond with a delegation response (as that is the answer from the authoritative perspective) and will not involve the recursor.

If you want to create such fake domains or override existing domains, please set the allow-recursion-override feature (available from 2.9.14 until 2.9.22.6).

Some packets, like those asking for MX records which are needed for SMTP transport of email, can be subject to 'additional processing'. This means that a recursing nameserver is obliged to try to add A records (IP addresses) for any of the mail servers mentioned in the packet, should it have these addresses available.

If PowerDNS encounters records needing such processing and finds that it does not have the data in its authoritative database, it will send an opportunistic quick query to the recursing component to see if it perhaps has such data. This question is worded such that the recursing nameserver should return immediately such as not to block the authoritative nameserver.

This marks a change from pre-2.9.5 behaviour where a packet was handed wholesale to the recursor in case it needed additional processing which could not proceed from the authoritative database.