UDP availability check
This check test the ability of the DNS to respond to queries over UDP for the intended domain.
The overwhelming majority of all DNS queries uses UDP as transport. This check monitor three key components:
- that the domain is active and available on the DNS server,
- that the DNS server allow queries over UDP, and
- that the queries isn't dropped (timed out).
A DNS server can serve thousands of domains. A wrongly configured domain is usually rejected by the DNS software at load time while correctly configured domains are loaded. This check verify that the specified zone is loaded and active.
From an availability standpoint, this check is vital and have a high effect on the availability metric. A failure to load a domain on one server could have serious effects on both user experience and availability ratings.
Event severities and messages
WARNING
The domain {domain name} accept UDP queries. Encountered 2-3 timeouts in 5 attempts.
The DNS is able to respond to queries over UDP for the specified domain, but encountered two or three timeouts/packet drops in 5 attempts.
The name server has the correct zone loaded but the timeouts/packet drops could be an issue.
ERROR
The domain {domain name} is virtually reachable over UDP. Encountered 4 timeouts in 5 attempts.
The DNS is barely able to respond to queries over UDP for the specified domain due to an 80% packet drop rate.
The name server has the correct zone loaded but the amount of timeouts/packet drops seriously impact the availability of the nameserver.
The domain {domain name} is unreachable.
All attempts to query the domain on the authoritative nameserver failed. The effect on the overall availability for the domain will drop considerably. As a result no other tests on this server will be performed.
UNKNOWN
Unable to perform check due to input data failure.
When this message appear usually one of the queries leading up to the final query have received an empty or garbled query response. Please report this through the normal support channel.
Corrupt or malformed response from nameserver.
The check received a query response with unreadable data fields. Please report this through the normal support channel.
Solutions, tips & tricks
Domain load failures are typically due to either syntax or typing errors inside the zone file or errors made in the name servers configuration file. The best place to start any troubleshooting is to check for errors and warnings in the log files. The DNS software can usually be configured to log messages to either separate log files or directly to the system log.
Check the log files for inconsistencies, error and warnings to determine if your zone file is loaded or what caused the failure. Correct the mistake and reload the service.
Timeouts and packet drops can cause the resolution time for a query to take in excess of 30 seconds. This is a disaster for interactive sessions and could very well mean lost customers or even worse. Check the result from the other locations to determine if the timeouts can be isolated to one monitor location or if it is more wide-spread. If all monitor locations show similar results chance are that the timeout issues are caused in the vicinity of your name server.
Keep in mind that packet drops and timeouts can come and go due to variations in network load.