监控类型
UDP port monitors
Check UDP services such as DNS, NTP, SIP, STUN and QUIC with protocol-correct probes or your own payload, and learn what open, closed and filtered mean for UDP.
- API type
udp- You enter
- Hostname or IP
- Default port
- Required
- Runs at most every
- 1 minute
- Needs
- None
UDP has no handshake, so the only way to know that a service works is to send it something it answers. A UDP monitor sends a probe datagram, sends it once more after half the timeout (single UDP packets do get lost), and waits for an answer from the exact address it contacted.
| What comes back | Result |
|---|---|
| A reply (for protocol probes, a valid answer to the request) | Up: Port 53/udp open · … |
| An ICMP "port unreachable" message | Down: the host is up but nothing listens on the port |
| An ICMP "host unreachable" message | Down |
| Nothing at all | Down, or warning if you choose so under "When nothing answers": the port is open but the service ignored the probe, or a firewall dropped it |
The check runs at most once a minute.
| Probe | Usual port | What is sent and checked |
|---|---|---|
| DNS query | 53 | A query for the name you enter (type A), or for the root name servers when you leave it empty. The reply must match the query; an error such as NXDOMAIN for your name fails the check. |
| NTP time request | 123 | An NTPv4 client request; the reply shows the stratum and clock offset. |
| SIP OPTIONS | 5060 | The usual VoIP health check; the reply's status line and server are shown. |
| STUN binding request | 3478 | For WebRTC, STUN and TURN servers; the reply shows the mapped address. |
| QUIC version negotiation | 443 | A QUIC packet with a reserved version, which a QUIC server must answer with the versions it supports. This shows that a QUIC (HTTP/3) endpoint answers; it is not an HTTP/3 request. |
| Custom payload | any | Your own bytes, as text or hex. Any reply counts, and "Must contain" checks it. |
In Text format, the payload is sent as UTF-8 with these escapes: \n, \r, \t, \0, \\ and \xHH for any byte. In Hex bytes format, enter pairs of hex digits; spaces, colons and 0x prefixes are ignored, so ff ff ff ff 54 and 0xff:0xff both work. Payloads are limited to 1200 bytes; an empty payload sends an empty datagram.
Some UDP services never answer unauthenticated packets, by design. WireGuard is an example. For those, a UDP monitor can only report "no reply"; monitor the host with ping or a related TCP service instead, or set "When nothing answers" to Warning.
| Field | API option | Default | Notes |
|---|---|---|---|
| Hostname, domain or IP | host | Required | Public hostname or IP address |
| Port | port | Required | DNS 53, NTP 123, SIP 5060, STUN 3478, QUIC / HTTP/3 443. |
| Probe | udpPreset | Custom payload | One of: Custom payload (custom), DNS query (dns), NTP time request (ntp), SIP OPTIONS (VoIP) (sip), STUN binding request (WebRTC) (stun), QUIC version negotiation (HTTP/3) (quic) |
| Payload (custom probe) | udpPayload | None | Up to 1,000 characters. Text with \n, \r, \t and \xHH escapes, or hex bytes. Leave empty to send an empty datagram. |
| Payload format | payloadFormat | Text | One of: Text (text), Hex bytes (hex) |
| Name to look up (DNS probe) | dnsName | None | Up to 253 characters. Empty: asks for the root name servers, which any DNS server answers. |
| Must contain (optional) | expected | None | Up to 300 characters |
| Timeout (seconds) | timeout | 5 | 2 to 60 |
| Warn if slower than (ms) | maxMs | None | 0 to 60,000 |
| When nothing answers | noReply | Down | One of: Down (down), Warning (warn) |
| Check interval | interval | Your plan's minimum | Never more often than every minute |
| Message | What it means | What to do |
|---|---|---|
Port 123/udp closed (ICMP port unreachable) | Nothing listens on the port. | Start the service or check the port number. |
No reply from port 5060/udp (open or filtered) | No answer within the timeout. | Check firewalls (they usually drop UDP silently) and that the probe matches the service. |
Host unreachable (ICMP) | A router reported that the host cannot be reached. | Check the host and its network. |
DNS NXDOMAIN for example.com | The DNS server answered, but the name you asked for does not exist there. | Check the name, or that the server is authoritative or recursive for it. |
Reply does not contain "…" | A reply arrived but does not contain your expected text. | Compare with what the service actually returns. |
The free UDP port checker sends the same probes once from the same server and shows the reply.