UDP availability check
Prerequisites:
None.
Package:
Availability.
Executed on:
All nameserver IP addresses.
Check interval:
Every 5 minutes.
Impact on Availability:
High.
Impact on Integrity:
None.
Purpose
The ability to respond to DNS queries via UDP is critical for any DNS server. This check tests every authoritative DNS server for this capability.
The vast majority of DNS queries use UDP for transport. This check monitors two key components:
- that the domain is active and available on the DNS server,
- that the DNS server allows queries over UDP.
A DNS server can serve thousands of domains. A misconfigured domain will usually be rejected by the DNS software at load time, while correctly configured domains will be loaded. This check verifies that the specified zone is loaded and active. Failure to load a domain on a single DNS server could have serious implications for both user experience and availability ratings.
From an availability standpoint, this check is absolutely critical and it has a high impact on the availability metric.
Event severities and messages
INFO
UDP connection successful.
The DNS server has the zone loaded and responds to queries over UDP.
WARNING
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 unreachable 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 number of timeouts/packet drops is seriously affecting the availability of the name server.
The domain {domain name} is not accessible.
All attempts to query the domain from the authoritative name server have failed. The impact on the overall availability for the domain will be significantly reduced. As a result, no further tests will be performed on this server.
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, errors 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 a query to take over 30 seconds to resolve. This is a disaster for interactive sessions and could very well mean lost customers or worse. Check the results from the other locations to see if the timeouts can be isolated to one monitor location or if they are more widespread. If all the monitoring locations show similar results, it is likely that the timeout problems are being 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.