CyberHunter
Local dataset, no network
// Other services

Port 21800: TVNC Pro Multiplexing

Port 21800 is assigned by IANA to tvpm (TVNC Pro Multiplexing).

21800

How tvpm works

Other services
client:21800tvpmresponse client:21800tvpmresponse

The client sends a request to port 21800 and the tvpm 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 21800 dangerous?

We have no specific security note for port 21800. 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 21800

ServicePortTransportDescriptionReferenceUpdated
tvpm 21800 TCP, UDP TVNC Pro Multiplexing - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 21800, and is it reachable?

Linux

sudo ss -ltnup 'sport = :21800'      # process listening on 21800
sudo lsof -nP -i :21800

macOS

sudo lsof -nP -i :21800

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 21800 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 21800
netstat -ano | findstr :21800

From another machine

nmap -p 21800 -sV <host>
sudo nmap -sU -p 21800 <host>
nc -zv <host> 21800

Block it in the firewall

sudo ufw deny 21800/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 21800" -Direction Inbound -LocalPort 21800 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 21800 used for?

Port 21800 is assigned by IANA to tvpm (TVNC Pro Multiplexing).

Is port 21800 TCP or UDP?

Port 21800 is registered for TCP and UDP.

Is it safe to open port 21800?

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 21800 is open?

On Linux run "ss -ltnup 'sport = :21800'", on macOS "lsof -nP -i :21800", and on Windows "netstat -ano | findstr :21800" to see the local process. From another machine, "nmap -p 21800 <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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