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Nagios, 2nd Edition by Wolfgang Barth

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6.7.2 Monitoring UDP ports

It is not so simple to monitor UDP ports, since there is no standard connection setup, such as the three-way-handshake for TCP, in the course of which a connection is opened, but data is not yet transferred. For a stateless protocol such as UDP there is no regulated sequence for sent and received packets. The server can reply to a UDP packet sent by the client with a UDP packet, but it is not obliged to do this.

If you find an unoccupied port, the requested host normally sends back an ICMP port unreachable message, which evaluates the plugin. If there is no reply, there are two possibilities: either the service on the target port is not reacting to the request, or a firewall is filtering out network traffic (either the ...

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