You need to send a message that the listening daemon will understand. The 
check_snmp plugin speaks the SNMP language so it knows how to construct a 
packet that snmpd will understand. I imagine an OID is a requirement for 
check_snmp, as well as community string or security credentials, those are 
all part of the "send". If you want to use check_udp for this, then you 
have to construct that packet yourself.

On the flip-side, you should know the expected response to a known query. 
If you make an SNMP request for . you should expect a 
string from the "syscontact" line from your snmpd.conf in response.

Just like you would use check_http rather than check_tcp for an Apache 
service, it's probably best to use a custom program like check_dns rather 
than check_udp to check a DNS service. In the case of SNMP, I prefer to 
use or write scripts that are specific to what I'm polling rather than 
using a generic request/response program. For instance, a program like could be alot more robust than running "check_snmp -o 
." alone.


On Thu, 5 Sep 2013, Marco Borsani wrote:

> Date: Thu, 5 Sep 2013 09:06:38 +0200
> From: Marco Borsani <>
> Reply-To: Nagios Users List <>
> To: 'Andreas Ericsson' <>,
>     'Nagios Users List' <>
> Subject: [Nagios-users] R: check_udp error message
> Ok, but what can I "send" and "receive" to/from the host?
> I also tried old version of check_udp (that does not need any string), but I
> read :
> No data was recieved from host!
> No response from host on port XX
> Or simply:
> No response from host on port XX
> I see another strange situation.
> On port 161 I can use check_snmp (work correctly), but if I use check_udp on
> port 161 it does not work at all !
> How can be possible ?
> Regards
> -----Messaggio originale-----
> Da: Andreas Ericsson []
> Inviato: marted? 3 settembre 2013 15:54
> A: Nagios Users List
> Cc: Marco Borsani
> Oggetto: Re: [Nagios-users] check_udp error message
> On 2013-09-03 12:33, Marco Borsani wrote:
>> Hi all
>> I need to control many UDP ports.
>> I run the command:
>> ./check_udp ?H <IP_ADDRESS> ?p 88
>> I receive following error message (with state UNKNOWN) :
>> With UDP checks, a send/expect string must be specified
>> Can anyone help me to solve it?
>> Those parameters should be optional..
> UDP is a connection-less protocol. The "expect" string could indeed be
> optional, and we could just expect to get something at all back when we send
> something, but without sending anything we won't even touch the network, so
> the remote host has no idea that we're trying to talk to it and we won't
> know if the port is up.
> That's why "send", at least, is not optional.

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