DNSSEC in the PowerDNS Recursor

As of 4.0.0, the PowerDNS Recursor has support for DNSSEC processing and experimental support for DNSSEC validation.

DNSSEC settings

The PowerDNS Recursor has 5 different levels of DNSSEC processing, which can be set with the dnssec setting in the recursor.conf. In order from least to most processing, these are:

off

In this mode, no DNSSEC processing takes place. The PowerDNS Recursor will not set the DNSSEC OK (DO) bit in the outgoing queries and will ignore the DO and AD bits in queries. In this mode, the behaviour is equal to the PowerDNS Recursor 3.X.

process-no-validate

The default mode. In this mode the Recursor acts as a "security aware, non-validating" nameserver, meaning it will set the DO-bit on outgoing queries and will provide DNSSEC related RRsets (NSEC, RRSIG) to clients that ask for them (by means of a DO-bit in the query), except for zones provided through the auth-zones setting. It will not do any validation in this mode, not even when requested by the client.

process

When dnssec is set to process the behaviour is similar to process-no-validate. However, the recursor will try to validate the data if at least one of the DO or AD bits is set in the query; in that case, it will set the AD-bit in the response when the data is validated successfully, or send SERVFAIL when the validation comes up bogus.

Note: in 4.0.0, only the AD-bit was considered when determining whether to validate. This lead to interoperability issues with older client software. From 4.0.1-onward, the DO-bit is also taken into account when determining whether to validate.

log-fail

In this mode, the recursor will attempt to validate all data it retrieves from authoritative servers, regardless of the client's DNSSEC desires, and will log the validation result. This mode can be used to determine the extra load and amount of possibly bogus answers before turning on full-blown validation. Responses to client queries are the same as with process.

validate

The highest mode of DNSSEC processing. In this mode, all queries will be be validated and will be answered with a SERVFAIL in case of bogus data, regardless of the client's request.

What, when?

The descriptions above are a bit terse, here's a table describing different scenarios with regards to the dnssec mode.

Note: the dig tool sets the AD-bit in the query. This might lead to unexpected query results when testing. Set +noad on the dig commandline when this is the case.

Trust Anchor Management

In the PowerDNS Recursor, both positive and negative trust anchors can be configured during startup (from a persistent configuration file) and at runtime (which is volatile). However, all trust anchors are configurable.

Trust Anchors

The PowerDNS Recursor ships with the DNSSEC Root key built-in. Note: is has no support yet for RFC 5011 key rollover and does not persist a changed root trust anchor to disk.

Configuring DNSSEC key material must be done in the lua-config-file, using addDS. This function takes 2 arguments, the node in the DNS-tree and the data of the corresponding DS record. To e.g. add a trust anchor for the root and powerdns.com, use the following config in the Lua file:

addDS('.', "63149 13 1 a59da3f5c1b97fcd5fa2b3b2b0ac91d38a60d33a") -- This is not an ICANN root
addDS('powerdns.com', "44030 8 2 D4C3D5552B8679FAEEBC317E5F048B614B2E5F607DC57F1553182D49 AB2179F7")

Now (re)start the recursor to load these trust anchors.

Runtime Configuration of Trust Anchors

To change or add trust anchors at runtime, use the rec_control tool. These runtime settings are not saved to disk. To make them permanent, they should be added to the lua-config-file as described above.

Adding a trust anchor is done with the add-ta command:

$ rec_control add-ta domain.example 63149 13 1 a59da3f5c1b97fcd5fa2b3b2b0ac91d38a60d33a
Added Trust Anchor for domain.example. with data 63149 13 1 a59da3f5c1b97fcd5fa2b3b2b0ac91d38a60d33a

To view the currently configured trust anchors, run get-tas:

$ rec_control get-tas
Configured Trust Anchors:
.       63149 13 1 a59da3f5c1b97fcd5fa2b3b2b0ac91d38a60d33a
net.    2574 13 1 a5c5acb889a7ba9b5aa5bef2b0ac9fe1565ddaab

To remove a trust anchor, run clear-ta:

$ rec_control clear-ta domain.example
Removed Trust Anchor for subdomain.example

Note: The root trust anchor cannot be removed in this manner.

Negative Trust Anchors

Negative trust anchors (defined in RFC 7646 can be used to temporarily disable DNSSEC validation for a part of the DNS-tree. This can be done when e.g. a TLD or high-traffic zone goes bogus. Note that it is good practice to verify that this is indeed the case and not because of malicious actions.

To configure a negative trust anchor, use the addNTA() function in the lua-config-file and restart the recursor. This function requires the name of the zone and an optional reason:

addNTA('example.', "Someone messed up the delegation")
addNTA('powerdns.com') -- No reason given

Runtime Configuration of Negative Trust Anchors

The rec_control command can be used to manage the negative trust anchors of a running instance. These runtime settings are lost when restarting the recursor, more permanent NTAs should be added to the lua-config-file with addNTA().

Adding a negative trust anchor is done with the add-nta command (that optionally accepts a reason):

$ rec_control add-nta domain.example botched keyroll
Added Negative Trust Anchor for domain.example. with reason 'botched keyroll'

To view the currently configured negative trust anchors, run get-ntas:

$ rec_control get-ntas
Configured Negative Trust Anchors:
subdomain.example.      Operator failed key-roll
otherdomain.example.    DS in parent, no DNSKEY in zone

To remove negative trust anchor(s), run clear-nta:

$ rec_control clear-nta subdomain.example
Removed Negative Trust Anchors for subdomain.example

clear-nta accepts multiple domain-names and accepts '*' (beware the shell quoting) to remove all negative trust anchors.