[Xymon] What does the 'dns' test do?

Michael T Pins mtpins at nndev.org
Sun Mar 10 23:15:07 CET 2019

John Horne writes:

>Using the 4.3.28 Terabithia RPMs, can someone tell me what the 'dns' test
>actually does?

It uses the C-ARES libraries to do a DNS lookup for hostname on hostname.
In other words, if you have a server named bob, it will do a DNS lookup for
"bob" on host bob.

>We actually negate the test ('!dns'), and run a client-side dns test ('dnsr')
>to check that resolution is working. As far as I remember the standard 'dns'
>test checks that the client DNS service can resolve something. This will always
>fail in our case as the client DNS service is always restricted to just the
>local host. (No querying allowed from an external server.)

>I'm trying to work out why most of our servers are taking less than a second -
>giving a 'Server unavailable' reply, but others are taking around 20 seconds
>for a 'Timeout' reply. I know why the timeout is 20 seconds, I'm just wondering
>why it's not giving a 'Server unavailable' reply.

What happens if you try to telnet to port 53 from your xymon server to the
various servers?

"Server unavailable" means the server in question is either refusing
connections on port 53, or it's responding with some sort of error

"Timeout" generally means it's listening on port 53, but doesn't actually
answer.  (Or something is sending port 53 packets to /dev/null, so xymon
never gets any response.)

To be precise - xymon will say "Server unavailable" if the C-ARES library
returns ARES_ECONNREFUSED, and it will say "Timeout" if the C-ARES library

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