[Xymon] What does the 'dns' test do?
john.horne at plymouth.ac.uk
Mon Mar 11 00:08:25 CET 2019
On Sun, 2019-03-10 at 16:15 -0600, Michael T Pins wrote:
> 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.
Okay, but what if FQDN names are not used, and the 'testip' option is set?
So for example: 10.1.2.3 bob # testip !dns
Does it do something like a reverse lookup - e.g. 'dig -x 10.1.2.3 @10.1.2.3'?
Or does it do a lookup like 'dig bob @10.1.2.3'?
> 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
> returns ARES_ETIMEOUT.
Yes, I think this is where the problem lies. Servers giving a 'Server
unavailable' reply are actually connected to, but the local (client) firewall
rejects them trying to connect on UDP port 53 (for DNS).
For the other servers, the network does not even allow a connection. So the DNS
request just times out. The request doesn't even get to the server to be
answered or rejected.
I assume the 'dns' test uses UDP port 53? The timeout servers should have had
the UDP port opened up to them from the Xymon server. I'm wondering if either
the TCP port has been opened on our network or if the Xymon test is using TCP
instead of UDP.
John Horne | Senior Operations Analyst | Technology and Information Services
University of Plymouth | Drake Circus | Plymouth | Devon | PL4 8AA | UK
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