check_udp error message

[Roger Bell_West]

On Fri, Sep 06, 2013 at 01:44:06PM +0200, Marco Borsani wrote:
>I already have read that wiki...

OK, I'll try to explain in a different way.

With a TCP-based service there's a standard handshake. You send a SYN,
you get a SYN+ACK, you send an ACK; if that happens, the TCP port is
open. (It may not be responding in a useful way, but it is at least
accepting connections.) So check_tcp doesn't need anything more than
an address and port number to be a useful generic monitor.

With UDP there is no handshaking except what's defined in the
application layer. If you send something that's not what the listening
server expects, the response you get could be an error or could be
nothing at all; it's entirely up to the listening server. The concept
of an "open UDP port" is not really very meaningful in itself. You can
check that you don't get an ICMP Port Unreachable when you send random
junk to the port (I assume that's what the old check_udp is doing;
I've never used it), but there are lots of reasons you might not get
that packet back, including firewall policies; it's no sort of
reliable test.

So your UDP service, of which you still refuse to give any detail, is
expecting something and will return something else. You need to send
it that something, and look for the something else in the response --
just as you send a URL and look for body text when you're using
check_http.

Roger

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