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// Other services

Port 3900: Unidata UDT OS

Port 3900 is assigned by IANA to udt_os (Unidata UDT OS), udt-os (Unidata UDT OS IANA assigned this well-formed service name as a replacement for "udt_os".).

3900

How udt_os works

Other services
client:3900udt_osresponse client:3900udt_osresponse

The client sends a request to port 3900 and the udt_os service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned2 registered services
Range
Registered (1024-49151)
Category
Other services
01 Security

Is port 3900 dangerous?

We have no specific security note for port 3900. 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 3900

ServicePortTransportDescriptionReferenceUpdated
udt_os 3900 TCP, UDP Unidata UDT OS - -
udt-os 3900 TCP, UDP Unidata UDT OS IANA assigned this well-formed service name as a replacement for "udt_os". - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 3900, and is it reachable?

Linux

sudo ss -ltnup 'sport = :3900'      # process listening on 3900
sudo lsof -nP -i :3900

macOS

sudo lsof -nP -i :3900

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 3900 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 3900
netstat -ano | findstr :3900

From another machine

nmap -p 3900 -sV <host>
sudo nmap -sU -p 3900 <host>
nc -zv <host> 3900

Block it in the firewall

sudo ufw deny 3900/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 3900" -Direction Inbound -LocalPort 3900 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 3900 used for?

Port 3900 is assigned by IANA to udt_os (Unidata UDT OS), udt-os (Unidata UDT OS IANA assigned this well-formed service name as a replacement for "udt_os".).

Is port 3900 TCP or UDP?

Port 3900 is registered for TCP and UDP.

Is it safe to open port 3900?

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 3900 is open?

On Linux run "ss -ltnup 'sport = :3900'", on macOS "lsof -nP -i :3900", and on Windows "netstat -ano | findstr :3900" to see the local process. From another machine, "nmap -p 3900 <host>" shows whether it is reachable.

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