sign

sign adds DNSSEC records to zone files.

Description

The sign plugin is used to sign (see RFC 6781) zones. In this process DNSSEC resource records are added. The signatures that sign the resource records sets have an expiration date, this means the signing process must be repeated before this expiration data is reached. Otherwise the zone’s data will go BAD (RFC 4035, Section 5.5). The sign plugin takes care of this. Sign works, but has a couple of limitations, see the “Bugs” section.

Only NSEC is supported, sign does not support NSEC3.

Sign works in conjunction with the file and auto plugins; this plugin signs the zones files, auto and file serve the zones data.

For this plugin to work at least one Common Signing Key, (see coredns-keygen(1)) is needed. This key (or keys) will be used to sign the entire zone. Sign does not support the ZSK/KSK split, nor will it do key or algorithm rollovers - it just signs.

Sign will:

  • (Re)-sign the zone with the CSK(s) when:

    • the last time it was signed is more than a 6 days ago. Each zone will have some jitter applied to the inception date.

    • the signature only has 14 days left before expiring.

    Both these dates are only checked on the SOA’s signature(s).

  • Create signatures that have an inception of -3 hours (minus a jitter between 0 and 18 hours) and a expiration of +32 days for every given DNSKEY.

  • Add or replace all apex CDS/CDNSKEY records with the ones derived from the given keys. For each key two CDS are created one with SHA1 and another with SHA256.

  • Update the SOA’s serial number to the Unix epoch of when the signing happens. This will overwrite any previous serial number.

Thus there are two ways that dictate when a zone is signed. Normally every 6 days (plus jitter) it will be resigned. If for some reason we fail this check, the 14 days before expiring kicks in.

Keys are named (following BIND9): K<name>+<alg>+<id>.key and K<name>+<alg>+<id>.private. The keys must not be included in your zone; they will be added by sign. These keys can be generated with coredns-keygen or BIND9’s dnssec-keygen. You don’t have to adhere to this naming scheme, but then you need to name your keys explicitly, see the keys file directive.

A generated zone is written out in a file named db.<name>.signed in the directory named by the directory directive (which defaults to /var/lib/coredns).

Syntax

sign DBFILE [ZONES...] {
    key file|directory KEY...|DIR...
    directory DIR
}
  • DBFILE the zone database file to read and parse. If the path is relative, the path from the root plugin will be prepended to it.
  • ZONES zones it should be sign for. If empty, the zones from the configuration block are used.
  • key specifies the key(s) (there can be multiple) to sign the zone. If file is used the KEY’s filenames are used as is. If directory is used, sign will look in DIR for K<name>+<alg>+<id> files. Any metadata in these files (Activate, Publish, etc.) is ignored. These keys must also be Key Signing Keys (KSK).
  • directory specifies the DIR where CoreDNS should save zones that have been signed. If not given this defaults to /var/lib/coredns. The zones are saved under the name db.<name>.signed. If the path is relative the path from the root plugin will be prepended to it.

Keys can be generated with coredns-keygen, to create one for use in the sign plugin, use: coredns-keygen example.org or dnssec-keygen -a ECDSAP256SHA256 -f KSK example.org.

Examples

Sign the example.org zone contained in the file db.example.org and write the result to ./db.example.org.signed to let the file plugin pick it up and serve it. The keys used are read from /etc/coredns/keys/Kexample.org.key and /etc/coredns/keys/Kexample.org.private.

example.org {
    file db.example.org.signed

    sign db.example.org {
        key file /etc/coredns/keys/Kexample.org
        directory .
    }
}

Running this leads to the following log output (note the timers in this example have been set to shorter intervals).

[WARNING] plugin/file: Failed to open "open /tmp/db.example.org.signed: no such file or directory": trying again in 1m0s
[INFO] plugin/sign: Signing "example.org." because open /tmp/db.example.org.signed: no such file or directory
[INFO] plugin/sign: Successfully signed zone "example.org." in "/tmp/db.example.org.signed" with key tags "59725" and 1564766865 SOA serial, elapsed 9.357933ms, next: 2019-08-02T22:27:45.270Z
[INFO] plugin/file: Successfully reloaded zone "example.org." in "/tmp/db.example.org.signed" with serial 1564766865

Or use a single zone file for multiple zones, note that the ZONES are repeated for both plugins. Also note this outputs multiple signed output files. Here we use the default output directory /var/lib/coredns.

. {
    file /var/lib/coredns/db.example.org.signed example.org
    file /var/lib/coredns/db.example.net.signed example.net
    sign db.example.org example.org example.net {
        key directory /etc/coredns/keys
    }
}

This is the same configuration, but the zones are put in the server block, but note that you still need to specify what file is served for what zone in the file plugin:

example.org example.net {
    file var/lib/coredns/db.example.org.signed example.org
    file var/lib/coredns/db.example.net.signed example.net
    sign db.example.org {
        key directory /etc/coredns/keys
    }
}

Be careful to fully list the origins you want to sign, if you don’t:

example.org example.net {
    sign plugin/sign/testdata/db.example.org miek.org {
        key file /etc/coredns/keys/Kexample.org
    }
}

This will lead to db.example.org be signed twice, as this entire section is parsed twice because you have specified the origins example.org and example.net in the server block.

Forcibly resigning a zone can be accomplished by removing the signed zone file (CoreDNS will keep on serving it from memory), and sending SIGUSR1 to the process to make it reload and resign the zone file.

Also See

The DNSSEC RFCs: RFC 4033, RFC 4034 and RFC 4035. And the BCP on DNSSEC, RFC 6781. Further more the manual pages coredns-keygen(1) and dnssec-keygen(8). And the file plugin’s documentation.

Coredns-keygen can be found at https://github.com/coredns/coredns-utils in the coredns-keygen directory.

Bugs

keys directory is not implemented. Glue records are currently signed, and no DS records are added for child zones.