CyberHunter
Local dataset, no network
// Other services

Port 991: Netnews Administration System

Port 991 is assigned by IANA to nas (Netnews Administration System).

991

How nas works

Other services
client:991nasresponse client:991nasresponse

The client sends a request to port 991 and the nas 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
Well-known (0-1023)
Category
Other services
01 Security

Is port 991 dangerous?

We have no specific security note for port 991. 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 991

ServicePortTransportDescriptionReferenceUpdated
nas 991 TCP, UDP Netnews Administration System RFC 4707 2017-02-10

// source: IANA service name and port number registry

03 Check it

Which program uses port 991, and is it reachable?

Linux

sudo ss -ltnup 'sport = :991'      # process listening on 991
sudo lsof -nP -i :991

macOS

sudo lsof -nP -i :991

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 991 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 991
netstat -ano | findstr :991

From another machine

nmap -p 991 -sV <host>
sudo nmap -sU -p 991 <host>
nc -zv <host> 991

Block it in the firewall

sudo ufw deny 991/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 991" -Direction Inbound -LocalPort 991 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 991 used for?

Port 991 is assigned by IANA to nas (Netnews Administration System).

Is port 991 TCP or UDP?

Port 991 is registered for TCP and UDP.

Is it safe to open port 991?

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 991 is open?

On Linux run "ss -ltnup 'sport = :991'", on macOS "lsof -nP -i :991", and on Windows "netstat -ano | findstr :991" to see the local process. From another machine, "nmap -p 991 <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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