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

Serving authoritative DNSSEC data

PowerDNS contains support for DNSSEC, enabling the easy serving of DNSSEC secured data, with minimal administrative overhead.

In PowerDNS, DNS and signatures and keys are (usually) treated as separate entities. The domain & record storage is thus almost completely devoid of DNSSEC record types.

Instead, keying material is stored separately, allowing operators to focus on the already complicated task of keeping DNS data correct. In practice, DNSSEC related material is often stored within the same database, but within separate tables.

If a DNSSEC configuration is found for a domain, the PowerDNS daemon will provide key records, signatures and (hashed) denials of existence automatically.

As an example, securing an existing zone can be as simple as:

$ pdnsutil secure-zone powerdnssec.org

Alternatively, PowerDNS can serve pre-signed zones, without knowledge of private keys.

A brief introduction to DNSSEC

DNSSEC is a complicated subject, but it is not required to know all the ins and outs of this protocol to be able to use PowerDNS. In this section, we explain the core concepts that are needed to operate a PowerDNSSEC installation.

Zone material is enhanced with signatures using 'keys'. Such a signature (called an RRSIG) is a cryptographic guarantee that the data served is the original data. DNSSEC keys are asymmetric (RSA, DSA, ECSDA or GOST), the public part is published in DNS and is called a DNSKEY record, and is used for verification. The private part is used for signing and is never published.

To make sure that the internet knows that the key that is used for signing is the authentic key, confirmation can be gotten from the parent zone. This means that to become operational, a zone operator will have to publish a representation of the signing key to the parent zone, often a ccTLD or a gTLD. This representation is called a DS record, and is a shorter (hashed) version of the DNSKEY.

Once the parent zone has the DS, and the zone is signed with the DNSSEC key, we are done in theory.

However, for a variety of reasons, most DNSSEC operations run with another layer of keys. The so called 'Key Signing Key' is sent to the parent zone, and this Key Signing Key is used to sign a new set of keys called the Zone Signing Keys.

This setup allows us to change our keys without having to tell the zone operator about it.

A final challenge is how to DNSSEC sign the answer 'no such domain'. In the language of DNS, the way to say 'there is no such domain' (NXDOMAIN) or there is no such record type is to send an empty answer. Such empty answers are universal, and can't be signed.

In DNSSEC parlance we therefore sign a record that says 'there are no domains between A.powerdnssec.org and C.powerdnssec.org'. This securely tells the world that B.powerdnssec.org does not exist. This solution is called NSEC, and is simple but has downsides - it also tells the world exactly which records DO exist.

So alternatively, we can say that if a certain mathematical operation (an 'iterated salted hash') is performed on a question, that no valid answers exist that have as outcome of this operation an answer between two very large numbers. This leads to the same 'proof of non-existence'. This solution is called NSEC3.

A PowerDNS zone can either be operated in NSEC or in one of two NSEC3 modes ('inclusive' and 'narrow').

Profile, Supported Algorithms and Record Types

PowerDNS aims to serve unexciting, standards compliant, DNSSEC information. One goal is to have relevant parts of our output be identical or equivalent to important fellow-traveller software like NLNetLabs' NSD.

Particularly, if a PowerDNS secured zone is transferred via AXFR, it should be able to contain the same records as when that zone was signed using ldns-signzone using the same keys and settings.

PowerDNS supports serving pre-signed zones, as well as online ('live') signed operations. In the last case, Signature Rollover and Key Maintenance are fully managed by PowerDNS.

Supported Algorithms

Supported Algorithms (See the IANA website for more information):

For the DS records, these digest algorithms are supported:

This corresponds to:

In order to facilitate interoperability with existing technologies, PowerDNS keys can be imported and exported in industry standard formats.

Note: Actual supported algorithms depend on the crypto-libraries PowerDNS was compiled against. To check the supported DNSSEC algoritms in your build of PowerDNS, run pdnsutil list-algorithms.

DNSSEC Modes of Operation

Traditionally, DNSSEC signatures have been added to unsigned zones, and then this signed zone could be served by any DNSSEC capable authoritative server. PowerDNS supports this mode fully.

In addition, PowerDNS supports taking care of the signing itself, in which case PowerDNS operates differently from most tutorials and handbooks. This mode is easier however.

For relevant tradeoffs, please see Security and Performance.

Online Signing

In the simplest situation, there is a single "SQL" database that contains, in separate tables, all domain data, keying material and other DNSSEC related settings.

This database is then replicated to all PowerDNS instances, which all serve identical records, keys and signatures.

In this mode of operation, care should be taken that the database replication occurs over a secure network, or over an encrypted connection. This is because keying material, if intercepted, could be used to counterfeit DNSSEC data using the original keys.

Such a single replicated database requires no further attention beyond monitoring already required during non-DNSSEC operations.

Records, Keys, signatures, hashes within PowerDNS in online signing mode

Within PowerDNS live signing, keys are stored separately from the zone records. Zone data are only combined with signatures and keys when requests come in over the internet.

Each zone can have a number of keys associated with it, with varying key lengths. Typically 1 or at most 2 of these keys are employed as actual Zone Signing Keys (ZSKs). During normal operations, this means that only 1 ZSK is 'active', and the other is inactive.

Should it be desired to 'roll over' to a new key, both keys can temporarily be active (and used for signing), and after a while the old key can be inactivated. Subsequently it can be removed.

As elucidated above, there are several ways in which DNSSEC can deny the existence of a record, and this setting too is stored away from zone records, and lives with the DNSSEC keying material.

(Hashed) Denial of Existence

PowerDNS supports unhashed secure denial of existence using NSEC records. These are generated with the help of the (database) backend, which needs to be able to supply the 'previous' and 'next' records in canonical ordering.

The Generic SQL Backends have fields that allow them to supply these relative record names.

In addition, hashed secure denial of existence is supported using NSEC3 records, in two modes, one with help from the database, the other with the help of some additional calculations.

NSEC3 in 'broad' or 'inclusive' mode works with the aid of the backend, where the backend should be able to supply the previous and next domain names in hashed order.

NSEC3 in 'narrow' mode uses additional hashing calculations to provide hashed secure denial of existence 'on the fly', without further involving the database.

Signatures

In PowerDNS live signing mode, signatures, as served through RRSIG records, are calculated on the fly, and heavily cached. All CPU cores are used for the calculation.

RRSIGs have a validity period, in PowerDNS by default this period starts at most a week in the past, and continues at least a week into the future.

Precisely speaking, the time period used is always from the start of the previous Thursday until the Thursday two weeks later. This two-week interval jumps with one-week increments every Thursday.

Note: Why Thursday? POSIX-based operating systems count the time since GMT midnight January 1st of 1970, which was a Thursday. PowerDNS inception/expiration times are generated based on an integral number of weeks having passed since the start of the 'epoch'.

PowerDNS also serves the DNSKEY records in live-signing mode. Their TTL is derived from the SOA records minimum field. When using NSEC3, the TTL of the NSEC3PARAM record is also derived from that field.

Pre-signed records

In this mode, PowerDNS serves zones that already contain DNSSEC records. Such zones can either be slaved from a remote master, or can be signed using tools like OpenDNSSEC, ldns-signzone or dnssec-signzone.

Even in this mode, PowerDNS will synthesize NSEC(3) records itself because of its architecture. RRSIGs of these NSEC(3) will still need to be imported. See the Presigned migration guide.

Front-signing

As a special feature, PowerDNS can operate as a signing server which operates as a slave to an unsigned master.

In this way, if keying material is available for an unsigned zone that is retrieved from a master server, this keying material will be used when serving data from this zone.

As part of the zone retrieval, the equivalent of pdnsutil rectify-zone is run to make sure that all DNSSEC-related fields are set correctly.

Signatures and Hashing is similar as described above

BIND-mode operation

Starting with PowerDNS 3.1, the bindbackend can manage keys in an SQLite3 database without launching a separate gsqlite3 backend.

To use this mode, add bind-dnssec-db=/var/db/bind-dnssec-db.sqlite3 to pdns.conf, and run pdnsutil create-bind-db /var/db/bind-dnssec-db.sqlite3. Then, restart PowerDNS.

After this, you can use pdnsutil secure-zone and all other pdnsutil commands on your BIND zones without trouble.

Hybrid BIND-mode operation

Warning: This mode is only supported in 3.0, 3.0.1 and 3.4.0 and up! In 3.1 to 3.3.1, the bindbackend always did its own key storage. In 3.4.0 and up hybrid bind mode operation is optional and enabled with the bindbackend hybrid config option.

PowerDNS can also operate based on 'BIND'-style zone & configuration files. This 'bindbackend' has full knowledge of DNSSEC, but has no native way of storing keying material.

However, since PowerDNS supports operation with multiple simultaneous backends, this is not a problem.

In hybrid mode, keying material and zone records are stored in different backends. This allows for 'bindbackend' operation in full DNSSEC mode.

To benefit from this mode, include at least one database-based backend in the 'launch' statement. The Generic SQLite backend (gsqlite3) probably complements BIND mode best, since it does not require a database server process.

Warning: For now, it is necessary to execute a manual SQL 'insert' into the domains table of the backend hosting the keying material. This is needed to generate a zone-id for the relevant domain. Sample SQL statement:

insert into domains (name, type) values ('powerdnssec.org', 'NATIVE');

pdnsutil

pdnsutil (previously called pdnssec) is a powerful command that is the operator-friendly gateway into PowerDNS configuration. Behind the scenes, pdnsutil manipulates a PowerDNS backend database, which also means that for many databases, pdnsutil can be run remotely, and can configure key material on different servers.

For a list of available commands, see the manpage.

DNSSEC Defaults

Since version 4.0, when securing a zone using pdnsutil secure-zone, a single ECDSA (algorithm 13, ECDSAP256SHA256) key is generated that is used as ZSK. Before 4.0, 3 RSA (algorithm 8) keys were generated, one as the KSK and two ZSKs. As all keys are online in the database, it made no sense to have this split-key setup.

The default negative answer strategy is NSEC.

Note: not all registrars support algorithm 13.

Migration

This chapter discusses various migration strategies, from existing PowerDNS setups, from existing unsigned installations and finally from previous non-PowerDNS DNSSEC deployments.

From an existing PowerDNS installation

To migrate an existing database-backed PowerDNS installation, ensure you are running at least PowerDNS 3.3.3 and preferably 3.4 or newer.

If you run an older version of PowerDNS, please upgrade to 3.4 and apply all the changes in database schemas as shown in the upgrade documentation.

Warning: Once the relevant backend-dnssec switch has been set, stricter rules apply for filling out the database! The short version is: run pdnsutil rectify-all-zones, even those not secured with DNSSEC! For more information, see the DNSSEC documentation for Generic SQL backends.

To deliver a correctly signed zone with the DNSSEC defaults, invoke:

pdnsutil secure-zone ZONE

To view the DS records for this zone (to transfer to the parent zone), run

pdnsutil show-zone ZONE

For a more traditional setup with a KSK and a ZSK, use the following sequence of commands:

pdnsutil add-zone-key ZONE ksk 2048 active rsasha256
pdnsutil add-zone-key ZONE zsk 1024 active rsasha256
pdnsutil add-zone-key ZONE zsk 1024 inactive rsasha256

This will add a 2048-bit RSA Key Signing Key and two 1024-bit RSA Zone Signing Keys. One of the ZSKs is inactive and can be rolled to if needed.

From existing non-DNSSEC non-PowerDNS setups

It is recommended to migrate to PowerDNS before securing your zones. After that, see the instructions above.

From existing DNSSEC non-PowerDNS setups, pre-signed

Industry standard signed zones can be served natively by PowerDNS, without changes. In such cases, signing happens externally to PowerDNS, possibly via OpenDNSSEC, ldns-sign or dnssec-sign.

PowerDNS needs to know if a zone should receive DNSSEC processing. To configure, run pdnsutil set-presigned ZONE.

If you import presigned zones into your database, please do not import the NSEC or NSEC3 records. PowerDNS will synthesize these itself. Putting them in the database might cause duplicate records in responses. zone2sql filters NSEC and NSEC3 automatically.

Warning Right now, you will also need to configure NSEC(3) settings for pre-signed zones using pdnsutil set-nsec3. Default is NSEC, in which case no further configuration is necessary.

From existing DNSSEC non-PowerDNS setups, live signing

The pdnsutil tool features the option to import zone keys in the industry standard private key format, version 1.2. To import an existing KSK, use

pdnsutil import-zone-key ZONE FILENAME ksk

replace 'ksk' by 'zsk' for a Zone Signing Key.

If all keys are imported using this tool, a zone will serve mostly identical records to before, with the important change that the RRSIG inception dates will be different.

Note: Within PowerDNS, the 'algorithm' for RSASHA1 keys is modulated based on the NSEC3 setting. So if an algorithm=7 key is imported in a zone with no configured NSEC3, it will appear as algorithm 5!

DNSSEC advice & precautions

DNSSEC is a major change in the way DNS works. Furthermore, there is a bewildering array of settings that can be configured.

It is well possible to configure DNSSEC in such a way that your domain will not operate reliably, or even, at all. We advise operators to stick to the keying defaults of pdnsutil secure-zone.

Note: GOST may be more widely available in Russia, because it might be mandatory to implement this regional standard there.

It is possible to operate a zone with different keying algorithms simultaneously, but it has also been observed that this is not reliable.

Depending on your master/slave setup, you may need to tinker with the SOA-EDIT metadata on your master. This is described in the operational instructions below.

Packet sizes, fragments, TCP/IP service

DNSSEC answers contain (bulky) keying material and signatures, and are therefore a lot larger than regular DNS answers. Normal DNS responses almost always fit in the 'magical' 512 byte limit previously imposed on DNS.

In order to support DNSSEC, operators must make sure that their network allows for:

If any of the conditions outlined above is not met, DNSSEC service will suffer or be completely unavailable.

In addition, the larger your DNS answers, the more critical the above becomes. It is therefore advised not to provision too many keys, or keys that are unnecessarily large.

Operational instructions

Several How to's describe operational practices with DNSSEC:

Below, frequently used commands are described:

Publishing a DS

To publish a DS to a parent zone, utilize pdnsutil show-zone and take the DS from its output, and transfer it securely to your parent zone.

Going insecure

pdnsutil disable-dnssec ZONE

Warning: Going insecure with a zone that has a DS record in the parent zone will make the zone BOGUS. Make sure the parent zone removes the DS record before going insecure.

Setting the NSEC modes and parameters

As stated earlier, PowerDNS uses NSEC by default. If you want to use NSEC3 instead, issue:

pdnsutil set-nsec3 ZONE [PARAMETERS]

e.g.

pdnsutil set-nsec3 example.net '1 0 1 ab'

The quoted part is the content of the NSEC3PARAM records, as defined in RFC 5155 , in order:

To convert a zone from NSEC3 to NSEC operations, run:

pdnsutil unset-nsec3 ZONE

Warning: Don't change from NSEC to NSEC3 (or the other way around) for zones with algorithm 5 (RSASHA1), 6 (DSA-NSEC3-SHA1) or 7 (RSASHA1-NSEC3-SHA1).

SOA-EDIT: ensure signature freshness on slaves

As RRSIGs can expire, slave servers need to know when to re-transfer the zone. In most implementations (BIND, NSD), this is done by re-signing the full zone outside of the nameserver, increasing the SOA serial and serving the new zone on the master.

With PowerDNS in Live-signing mode, the SOA serial is not increased by default when the RRSIG dates are rolled.

For zones that use native replication PowerDNS will serve valid RRSIGs on all servers.

For master zones (where replication happens by means of AXFR), PowerDNS slaves will automatically re-transfer the zone when it notices the RRSIGs have changed, even when the SOA serial is not increased. This ensures the zone never serves old signatures.

If your DNS setup uses non-PowerDNS slaves, the slaves need to know when the signatures have been updated. This can be accomplished by setting the SOA-EDIT metadata for DNSSEC signed zones. This value controls how the value of the SOA serial is modified by PowerDNS.

Note: The SOA serial in the datastore will be untouched, SOA-EDIT is applied to DNS answers with the SOA record.

The default-soa-edit or default-soa-edit-signed configuration options can instead be set to ensure SOA-EDIT is set for every zone.

Possible SOA-EDIT values

The 'inception' refers to the time the RRSIGs got updated in live-signing mode. This happens every week (see Signatures). The inception time does not depend on local timezone, but some modes below will use localtime for representation.

INCREMENT-WEEKS

Increments the serial with the number of weeks since the UNIX epoch. This should work in every setup; but the result won't look like YYYYMMDDSS anymore.

For example: a serial of 12345678 will become 12348079 on Wednesday 13th of January 2016 (2401 weeks after the epoch).

INCEPTION-EPOCH

Sets the new SOA serial number to the maximum of the old SOA serial number, and age in seconds of the last inception. This requires your backend zone to use the number of seconds since the UNIX epoch as SOA serial. The result is still the age in seconds of the last change to the zone, either by operator changes to the zone or the 'addition' of new RRSIGs.

As an example, a serial of 12345678 becomes 1452124800 on Wednesday 13th of January 2016.

INCEPTION-INCREMENT

Uses YYYYMMDDSS format for SOA serial numbers. If the SOA serial from the backend is within two days after inception, it gets incremented by two (the backend should keep SS below 98). Otherwise it uses the maximum of the backend SOA serial number and inception time in YYYYMMDD01 format. This requires your backend zone to use YYYYMMDDSS as SOA serial format. Uses localtime to find the day for inception time.

This changes a serial of 2015120810 to 2016010701 on Wednesday 13th of January 2016.

Sets the SOA serial to the last inception time in YYYYMMDD01 format. Uses localtime to find the day for inception time.

Warning: The SOA serial will only change on inception day, so changes to the zone will get visible on slaves only on the following inception day.

Sets the SOA serial to the number of weeks since the epoch, which is the last inception time in weeks.

Warning: Same problem as INCEPTION.

EPOCH

Sets the SOA serial to the number of seconds since the epoch.

Warning: Don't combine this with AXFR - the slaves would keep refreshing all the time. If you need fast updates, sync the backend databases directly with incremental updates (or use the same database server on the slaves)

NONE

Ignore default-soa-edit and/or default-soa-edit-signed setings.

PKCS#11 support

Note: This feature is experimental, and not ready for production. Use at your own risk! Note: As of version 4.0, slot IDs are deprecated, and you are expected to use slot label instead

To enable it, compile PowerDNS Authoritative Server using --enable-experimental-pkcs11 flag on configure. This requires you to have p11-kit libraries and headers.

You can also log on to the tokens after starting server, in this case you need to edit your PKCS#11 cryptokey record and remove PIN or set it empty. PIN is required for assigning keys to zone.

Using with SoftHSM

To test this feature, a software HSM can be used. It is not recommended to use this in production.

Instructions on how to setup SoftHSM to work with the feature after compilation on ubuntu/debian (tested with Ubuntu 12 and 14).

Using CryptAS

Instructions on how to use CryptAS Athena IDProtect Key USB Token V2J Smart Card token on Ubuntu 14.

Secure transfers

From 3.3.1 and up, PowerDNS support secure DNSSEC transfers as described in draft-koch-dnsop-dnssec-operator-change. If the direct-dnskey option is enabled the foreign DNSKEY records stored in the database are added to the keyset and signed with the KSK. Without the direct-dnskey option DNSKEY records in the database are silently ignored.

Security

During typical PowerDNS operation, the private part of the signing keys are 'online', which can be compared to operating an HTTPS server, where the private key is available on the webserver for cryptographic purposes.

In some settings, having such (private) keying material available online is considered undesirable. In this case, consider running in pre-signed mode.

Performance

DNSSEC has a performance impact, mostly measured in terms of additional memory used for the signature caches. In addition, on startup or AXFR-serving, a lot of signing needs to happen.

Most best practices are documented in RFC 6781.

Thanks to, acknowledgements

PowerDNS DNSSEC has been made possible by the help & contributions of many people. We would like to thank: