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 of PowerDNS and Performance and Tuning.

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 this period is 3 weeks. This period starts at most a week in the past, and continues at least a week into the future. This interval jumps with one-week increments every Thursday.

The time period used is always calculated based on the moment of rollover. The inception timestamp is the most recent Thursday 00:00:00 UTC, which is exactly one week ago at the moment of rollover. The expiry timestamp is the Thursday 00:00:00 UTC two weeks later from the moment of rollover. Graphically, it looks like this:

RRSIG(1) Inception                                                    RRSIG(1) Expiry
|                                                                                   |
v                                                                                   v
|================================ RRSIG(1) validity ================================|
                            |================================ RRSIG(2) validity ================================|
                            ^                                                                                   ^
                            |                                                                                   |
                            RRSIG(2) Inception                                                    RRSIG(2) Expiry

                            |----- RRSIG(1) served -----|----- RRSIG(2) served -----|

|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
thu fri sat sun mon tue wed thu fri sat sun mon tue wed thu fri sat sun mon tue wed thu fri sat sun mon tue wed thu
                                                        ^
                                                        |
                                                        RRSIG roll-over(1 to 2)

At all times, only one RRSIG per signed RRset per ZSK is served when responding to clients.

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 in the backend.

Signed AXFR

An outgoing zone transfer from a signing master contains all information required for the receiving party to rectify the zone without knowing the keys, such as signed NSEC3 records for empty non-terminals. The zone is not required to be rectified on the master.

Signatures and Hashing is similar as described in Online Signing.

BIND-mode operation

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.

Note

This sqlite database is different from the database used for the regular SQLite 3 backend.

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

Hybrid BIND-mode operation

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. See the backend specific documentation on how to initialize the database and backend.

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');

The SQLite 3 backend probably complements BIND mode best, since it does not require a database server process.

Note

The sqlite3 database must be created using the normal schema for this backend. The database created with pdnsutil create-bind-db will not work in this backend.