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// Other services

Port 1921: NoAdmin

Port 1921 is assigned by IANA to noadmin (NoAdmin).

1921

How noadmin works

Other services
client:1921noadminresponse client:1921noadminresponse

The client sends a request to port 1921 and the noadmin 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 1921 dangerous?

We have no specific security note for port 1921. 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 1921

ServicePortTransportDescriptionReferenceUpdated
noadmin 1921 TCP, UDP NoAdmin - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 1921, and is it reachable?

Linux

sudo ss -ltnup 'sport = :1921'      # process listening on 1921
sudo lsof -nP -i :1921

macOS

sudo lsof -nP -i :1921

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 1921 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 1921
netstat -ano | findstr :1921

From another machine

nmap -p 1921 -sV <host>
sudo nmap -sU -p 1921 <host>
nc -zv <host> 1921

Block it in the firewall

sudo ufw deny 1921/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 1921" -Direction Inbound -LocalPort 1921 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 1921 used for?

Port 1921 is assigned by IANA to noadmin (NoAdmin).

Is port 1921 TCP or UDP?

Port 1921 is registered for TCP and UDP.

Is it safe to open port 1921?

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 1921 is open?

On Linux run "ss -ltnup 'sport = :1921'", on macOS "lsof -nP -i :1921", and on Windows "netstat -ano | findstr :1921" to see the local process. From another machine, "nmap -p 1921 <host>" shows whether it is reachable.

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