UDP

Sends a UDP datagram and waits for a sensible reply. UDP has no connection to establish, so unlike Port this has to speak a little of the protocol to know anything happened.

Use it when

You run a service that answers over UDP: a nameserver, an NTP server, a STUN/TURN server for video, or something of your own.

What to enter

The three named probes send a real request of that protocol and validate the shape of the answer. Raw sends exactly your bytes and, if you gave one, looks for your expected string in the reply.

Exactly when it's DOWN

• No valid reply after 3 attempts.
• An ICMP "port unreachable" comes back, which means nothing is listening.
• A reply arrives but does not match what the probe expects — for raw, your expected text is absent.

What it doesn't do

• UDP is unreliable by design, so a lost packet is indistinguishable from a dead service. That is why it retries three times; it is also why a low failure threshold produces false alarms here more than elsewhere.
• Many networks rate-limit or drop UDP from unfamiliar sources.
• Raw mode has no protocol knowledge: if you send bytes and expect nothing, any reply at all counts.

Example

ns1.example.com:53 with the DNS probe. For something custom: host collector.example.com:9000, probe Raw, payload hex:01ff, expected reply hex:02.

Alerts

Down, recovered, latency.