Check DNS Authoritative

An Icinga/Nagios plugin that checks all authoritative nameservers for a zone and reports on discrepancies between them.

It checks for:

• servers that are unresponsive, lame (REFUSED, SERVFAIL, an upward referral, or some other non-answer), or that answer without the AA bit set (answering from cache, forwarding, etc.)
• a matching NS set between the parent's delegation and the zone apex
• matching apex NS sets across all of the zone's servers
• matching serial numbers (with optional warning/critical thresholds on the spread)
• the presence of CDS/CDNSKEY records (optional)

Every address (IPv6 and IPv4) of every nameserver is checked individually, so a nameserver whose address family instances disagree, or which is only partially reachable, is detected. The set of servers checked is the delegation NS set from the parent, plus any names given with --ns, plus any additional names discovered in the apex NS sets returned by the servers themselves.

Behaviour

Output

The plugin reports OK, WARNING, or CRITICAL depending on supplied arguments and the current status of the zone being checked.

• unresponsive, lame, or non-authoritative servers: CRITICAL
• apex NS set does not match the delegation (plus any --ns names): WARNING
• apex NS sets disagree between servers: CRITICAL
• serial numbers disagree: CRITICAL by default; see the serial threshold options below
• CDS/CDNSKEY present: OK unless --cds warning or --cds critical is given

Problem output names the offending servers (with their addresses) to give an operator a starting point for troubleshooting. Where servers disagree with each other, each group of servers is reported with the answer it gave:

ZONE CRITICAL - serials disagree (spread 2): 2026070802 [ns1.example.com. (192.0.2.1), ns2.example.com. (192.0.2.2)]; 2026070800 [ns3.example.com. (192.0.2.3)]

Counts of each problem type are emitted as performance data for trending.

Failures obtaining supporting information (e.g. inability to retrieve the NS set from the parent zone) result in an UNKNOWN return value and an explanatory message. For example:

ZONE UNKNOWN - No answer from ns1.example.com., ns2.example.com. querying parent NS set for example.org.

Serial number thresholds

Serial spread is the largest distance between any two servers' serial numbers, computed with RFC 1982 serial arithmetic. By default any spread at all is CRITICAL. A threshold you set is the behaviour you get:

• --serial-warning N alone: WARNING above a spread of N, never CRITICAL
• --serial-critical N alone: CRITICAL above a spread of N
• both: the usual two-tier thresholds

Usage

check_dns_auth -z ZONE [options]

Releasing

Build and verify the source distribution and wheel before uploading:

python -m build
twine check dist/*

Upload to TestPyPI first if you want a dry run:

twine upload --repository testpypi dist/*

Upload the final release to PyPI:

twine upload dist/*

Acknowledgements

This is initially inspired by Duane Wessels's perl plugin check_zone_auth.