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.

process-no-validate

The default mode until PowerDNS Recursor 4.5.0.

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

The default mode since PowerDNS Recursor 4.5.0.

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.

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 responses will be be validated and queries will be answered with a SERVFAIL in case of bogus data, even if the client did not request validation by setting the AD or DO bit.

Note: the CD-bit is honored for process, log-fail and validate. This mean that even if validation fails, results are returned if the CD-bit is set by the client. For log-fail, failures will be logged too.

What, when?

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

	off	process-no-validate	process	log-fail	validate
Perform validation	No	No	Only on +AD or +DO from client	Always (logs result)	Always
SERVFAIL on bogus	No	No	Only on +AD or +DO and -CD from client	Only on +AD or +DO and -CD from client	If -CD from client
AD in response on authenticated data	Never	Never	Only on +AD or +DO from client	Only on +AD or +DO from client	Only on +AD or +DO from client
RRSIGs/NSECs in answer on +DO from client	No	Yes	Yes	Yes	Yes

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.

Current trust anchors can be queried from the recursor by sending a query for “trustanchor.server CH TXT”. This query will (if allow-trust-anchor-query is enabled) return a TXT record per trust-anchor in the format "DOMAIN KEYTAG [KEYTAG]...".

Trust Anchors

The PowerDNS Recursor ships with the DNSSEC Root key built-in.

Note: it has no support 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 addTA(). 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 example.com, use the following config in the Lua file:

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

For PowerDNS Recursor 4.1.x and below, use the addDS() function instead.

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

Reading trust anchors from files

New in version 4.2.0.

It is also possible to read the Trust Anchors from a BIND-style zonefile using the readTrustAnchorsFromFile() in the lua-config-file. Only the DS and DNSKEY records from this file are read. This file is (by default) re-read every 24 hours for updates. Debian and its derivatives ship the dns-root-data package that contains the DNSSEC root trust anchors in /usr/share/dns/root.key.

To only use the distribution-provided Trust Anchors, add the following to the lua-config-file:

clearTA() -- Remove built-in trust-anchors
readTrustAnchorsFromFile("/usr/share/dns/root.key") -- Use these keys

Note

When using readTrustAnchorsFromFile(), any runtime changes to Trust Anchors (see below) will be overwritten when the file is refreshed. To prevent this, set the interval parameter to 0. This will disable automatic reloading of the file.

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.

Current negative trust anchors can be queried from the recursor by sending a query for “negativetrustanchor.server CH TXT”. This query will (if allow-trust-anchor-query is enabled) return a TXT record per negative trust-anchor in the format "DOMAIN [REASON]".

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.