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// Other services

Port 11: Active Users

Port 11 is assigned by IANA to systat (Active Users).

11

How systat works

Other services
client:11systatresponse client:11systatresponse

The client sends a request to port 11 and the systat 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 11 dangerous?

We have no specific security note for port 11. 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 11

ServicePortTransportDescriptionReferenceUpdated
systat 11 TCP, UDP Active Users - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 11, and is it reachable?

Linux

sudo ss -ltnup 'sport = :11'      # process listening on 11
sudo lsof -nP -i :11

macOS

sudo lsof -nP -i :11

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 11 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 11
netstat -ano | findstr :11

From another machine

nmap -p 11 -sV <host>
sudo nmap -sU -p 11 <host>
nc -zv <host> 11

Block it in the firewall

sudo ufw deny 11/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 11" -Direction Inbound -LocalPort 11 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 11 used for?

Port 11 is assigned by IANA to systat (Active Users).

Is port 11 TCP or UDP?

Port 11 is registered for TCP and UDP.

Is it safe to open port 11?

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 11 is open?

On Linux run "ss -ltnup 'sport = :11'", on macOS "lsof -nP -i :11", and on Windows "netstat -ano | findstr :11" to see the local process. From another machine, "nmap -p 11 <host>" shows whether it is reachable.

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