Port 561MONITOR Protocol
Port 561 is assigned to the MONITOR protocol, an older diagnostic and monitoring tool used primarily on UDP to facilitate remote system management and status querying. While it is not widely used today, it historically enabled system administrators to access network device performance data, troubleshoot issues, and observe system health remotely..
- transport
- udp
- in transit
- cleartext
- assignment
- official
- risk
- 5/10
- lookups
- 7,059
single transport
payload readable on path
registered with iana
caution
rank 725 of 993 · top 73%
Technical Details
what runs on :561Overview:
Port 561 is officially designated for the MONITOR protocol, which is utilized primarily over the UDP transport layer. Its main purpose is system and network monitoring through the exchange of probe and status packets. The protocol typically facilitates administrators with performance metrics, device statuses, and basic remote management functionality across distributed systems.
Protocol Functionality:
Because MONITOR uses UDP, it favors fast, connectionless communication suitable for lightweight status updates but without delivery guarantees. It commonly sends small packets that query various system resources such as CPU load, memory usage, interface statistics, or basic health indicators. These data points assist in performance trending, outage detection, and proactive maintenance planning.
Modern Relevance:
Given the evolution of more sophisticated network monitoring solutions—like SNMP (Simple Network Management Protocol) or dedicated telemetry systems—the MONITOR protocol is now largely obsolete. However, some legacy systems may still generate traffic on this port, and awareness is important for comprehensive network management and auditing.
Security Information
exposure of :561risk score
5/ 10caution
worth attention. how exposed you are depends on configuration — don't leave it reachable from the internet without a reason.
network services averages 3.9 across 604 ports — this one sits 1.1 above.
in transit
cleartext
credentials and payloads are readable by anyone on path
assignment
official
registered with iana for this service — scanners fingerprint it by number
reachable over
udp
udp is connectionless — source addresses are trivially spoofed and it is a reflection favourite
security overview
Common Vulnerabilities:
- Unauthorized information disclosure due to lack of authentication and encryption, allowing attackers to eavesdrop on monitoring data.
- Packet spoofing or injection, since UDP is connectionless and susceptible to forged packets potentially disrupting monitoring operations.
- Reflection/amplification attacks where open services respond to spoofed requests, which can contribute to denial-of-service (DoS) abuse.
Mitigations:
- Filter inbound and outbound traffic on UDP port 561 to restrict access to trusted monitoring stations.
- Deploy network segmentation and firewall rules to isolate legacy monitoring traffic from the broader network.
- Replace or augment MONITOR with more secure protocols like SNMPv3, which provides encryption and authentication.
- Regularly audit open ports and services, disabling or decommissioning unnecessary legacy protocols to reduce attack surface.
Related Ports
the 8 most looked-up other ports in network services — 604 ports carry that label.
| port | service | risk | |||
|---|---|---|---|---|---|
| :9080 | Groove RPC | TCPUDP | Web Services | caution | 70.8k |
| :6543 | Jetnet | UDP | Network Services | caution | 65.3k |
| :5938 | TeamViewer | TCPUDP | Remote Access | caution | 65.0k |
| :8291 | Winbox MikroTik Admin | TCP | Security | caution | 62.9k |
| :3233 | WhiskerControl Protocol | TCPUDP | Network Services | caution | 61.9k |
| :3268 | Microsoft Global Catalog (GC) | TCPUDP | Security | caution | 54.6k |
| :135 | Microsoft EPMAP | TCPUDP | Security | caution | 51.9k |
| :3128 | Tatsoft Default HTTP Proxy | TCP | Web Services | caution | 46.8k |
risk mix of the 8 listed
- caution100%
0 of 8 encrypted