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// Monitoring & logging

Port 391: SynOptics SNMP Relay Port

Port 391 is assigned by IANA to synotics-relay (SynOptics SNMP Relay Port).

391

How synotics-relay works

Monitoring & logging
monitoringdevicesynotics-relay monitoringdevicesynotics-relay

The monitoring server collects metrics, alerts or logs from devices over port 391.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
Monitoring & logging
01 Security

Is port 391 dangerous?

We have no specific security note for port 391. Its risk depends on the program actually listening on it. General advice for monitoring & logging services:

  • Restrict management and monitoring protocols to the monitoring network.
  • Replace community strings and default credentials.
  • Send logs over TLS.
02 Registry

IANA registry entries for port 391

ServicePortTransportDescriptionReferenceUpdated
synotics-relay 391 TCP, UDP SynOptics SNMP Relay Port - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 391, and is it reachable?

Linux

sudo ss -ltnup 'sport = :391'      # process listening on 391
sudo lsof -nP -i :391

macOS

sudo lsof -nP -i :391

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 391 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 391
netstat -ano | findstr :391

From another machine

nmap -p 391 -sV <host>
sudo nmap -sU -p 391 <host>
nc -zv <host> 391

Block it in the firewall

sudo ufw deny 391/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 391" -Direction Inbound -LocalPort 391 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 391 used for?

Port 391 is assigned by IANA to synotics-relay (SynOptics SNMP Relay Port).

Is port 391 TCP or UDP?

Port 391 is registered for TCP and UDP.

Is it safe to open port 391?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Restrict management and monitoring protocols to the monitoring network.

How do I check if port 391 is open?

On Linux run "ss -ltnup 'sport = :391'", on macOS "lsof -nP -i :391", and on Windows "netstat -ano | findstr :391" to see the local process. From another machine, "nmap -p 391 <host>" shows whether it is reachable.

05 Related
:161 SNMP udp · tcp · high risk :162 SNMP trap udp · tcp · moderate risk :9090 Prometheus / admin consoles tcp · udp · high risk :514 Syslog udp · tcp · moderate risk :6514 Syslog over TLS tcp · udp · dccp · low risk :5601 Kibana tcp · udp · high risk :412 Trap Convention Port tcp · udp :561 tcp · udp

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