[Xymon] c-ares

[henrik at hswn.dk]

Bit of a late reply, but I am slowly digging through my mail archive 
...

Den 2013-11-01 21:43, John Thurston skrev:
>> It's funny you bring this up right now. I am currently struggling 
>> with
>> my pre-production xymon instance. The only way I seem able to get it 
>> to
>> behave is to start xymonnet with --no-ares. When using c-ares, I
>> routinely get hundreds of test failures due to inability to resolve 
>> names.
>
> I've been working on this today and have found the undocumented
> option to xymonnet "--maxdnsqueue". With this, I can limit the number
> of queries xymonnet sends to name servers in each batch. When I set 
> it
> to =10, I get solid results. When I set it to =25 (or higher), I get
> hundreds of name resolution failures.
>
> When I use --no-ares, it also works fine.
>
> My original question remains, "Why is c-ares the default rather than
> the system resolver?" But more important is the practical question . 
> .

Xymon uses the c-ares resolver because your system resolver library can 
only handle one lookup at a time, and may take a very long time to 
timeout if a DNS server is not responding.

A consequence of this is that Xymon can quite easily flood a normal DNS 
server with queries, resulting in DNS lookups failing. I suspect that is 
what happens in your case. The simplest solution for this (and I would 
recoomend this for anyone running a Xymon network probe) is to install a 
local caching DNS server, which forwards all requests to your normal DNS 
servers. Such a setup is completely transparent to how Xymon works, but 
it moves the load to a dedicated DNS server running on your Xymon box. 
Since it will cache all of the responses after the initial request, it 
will be much more reliable and fast.

So you can choose:
a) Run xymonnet with --no-ares and live with the fact that xymonnet may 
take a very long time to run, resulting in purple tests and status 
messages not being up-to-date; or
b) install a local caching BIND server where you put the normal DNS 
servers in the "forwarders" section.


Regards,
Henrik