Port 391 SYNOTICS-RELAY
Port 391 is the IANA-assigned synotics-relay port for SynOptics SNMP Relay Port (TCP / UDP).
What is port 391 used for?
Port 391 is assigned to synotics-relay and uses TCP / UDP. The IANA registry describes the service as “SynOptics SNMP Relay Port”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach synotics-relay connects to port 391. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 391 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Browse all Monitoring and metrics ports
Source: IANA Service Name and Transport Protocol Port Number Registry.
Check and block port 391
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :391'
sudo ss -ulpn 'sport = :391'Check on Windows
Get-NetTCPConnection -LocalPort 391 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 391 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 391/tcp
sudo ufw deny 391/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 391 -j DROP
sudo iptables -A INPUT -p udp --dport 391 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 391 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Where port 391 is usually used
Most traffic on port 391 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Scrapers such as Prometheus pulling metrics from port 391 on a schedule.
- Dashboards and alerting rules querying the same endpoint.
- Incident reviews comparing metrics before and after a deployment.
Fetch metrics
Metrics endpoints usually return plain text or JSON. Check the port locally before exposing it to a monitoring network.
curl -s http://127.0.0.1:391/metrics | headCommon problems with port 391
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 391. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :391' or sudo lsof -i :391. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 391 at a glance
| Port number | 391 |
|---|---|
| Service name | synotics-relay |
| Transport protocol | TCP / UDP |
| Category | Monitoring and metrics |
| Range | Well-known port (0–1023) |
| Risk level | Standard/Low |
| Registered assignee | Not listed |
| Registry description | SynOptics SNMP Relay Port |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 391 used for?
Port 391 (SYNOTICS-RELAY) uses TCP / UDP. SynOptics SNMP Relay Port.
How do I check if port 391 is open?
On Linux run `sudo ss -tlpn 'sport = :391'` and `sudo ss -ulpn 'sport = :391'`. On Windows run `Get-NetTCPConnection -LocalPort 391 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 391 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 391; empty output usually means nothing is bound to it.
How do I block port 391?
With UFW run `sudo ufw deny 391/tcp` and `sudo ufw deny 391/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 391 -j DROP` and `sudo iptables -A INPUT -p udp --dport 391 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 391 safe to expose?
This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Nevertheless, keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.
Related ports
- Port 390 · uisUIS
- Port 392 · synotics-brokerSynOptics Port Broker Port
- Port 193 · srmpSpider Remote Monitoring Protocol
- Port 514 · shellcmd like exec, but automatic authentication is performed as for login…
- Port 192 · osu-nmsOSU Network Monitoring System
- Port 601 · syslog-connReliable Syslog Service
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy