CyberHunter
Local dataset, no network
// Other services

Port 5991: NUXSL

Port 5991 is assigned by IANA to nuxsl (NUXSL).

5991

How nuxsl works

Other services
client:5991nuxslresponse client:5991nuxslresponse

The client sends a request to port 5991 and the nuxsl service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Registered (1024-49151)
Category
Other services
01 Security

Is port 5991 dangerous?

We have no specific security note for port 5991. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 5991

ServicePortTransportDescriptionReferenceUpdated
nuxsl 5991 TCP, UDP NUXSL - 2002-03

// source: IANA service name and port number registry

03 Check it

Which program uses port 5991, and is it reachable?

Linux

sudo ss -ltnup 'sport = :5991'      # process listening on 5991
sudo lsof -nP -i :5991

macOS

sudo lsof -nP -i :5991

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 5991 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 5991
netstat -ano | findstr :5991

From another machine

nmap -p 5991 -sV <host>
sudo nmap -sU -p 5991 <host>
nc -zv <host> 5991

Block it in the firewall

sudo ufw deny 5991/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 5991" -Direction Inbound -LocalPort 5991 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 5991 used for?

Port 5991 is assigned by IANA to nuxsl (NUXSL).

Is port 5991 TCP or UDP?

Port 5991 is registered for TCP and UDP.

Is it safe to open port 5991?

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. Close the port if nothing on the host needs it.

How do I check if port 5991 is open?

On Linux run "ss -ltnup 'sport = :5991'", on macOS "lsof -nP -i :5991", and on Windows "netstat -ano | findstr :5991" to see the local process. From another machine, "nmap -p 5991 <host>" shows whether it is reachable.

05 Related
:4444 Metasploit default / krb524 tcp · udp · high risk :31337 Back Orifice / elite tcp · udp · high risk :1 TCP Port Service Multiplexer tcp · udp :5 Remote Job Entry tcp · udp :9 Discard tcp · udp · sctp · dccp :11 Active Users tcp · udp :17 Quote of the Day tcp · udp :18 Message Send Protocol (historic) tcp · udp

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