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// Other services

Port 399: ISO Transport Class 2 Non-Control over TCP

Port 399 is assigned by IANA to iso-tsap-c2 (ISO Transport Class 2 Non-Control over TCP).

399

How iso-tsap-c2 works

Other services
client:399iso-tsap-c2response client:399iso-tsap-c2response

The client sends a request to port 399 and the iso-tsap-c2 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 399 dangerous?

We have no specific security note for port 399. 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 399

ServicePortTransportDescriptionReferenceUpdated
iso-tsap-c2 399 TCP, UDP ISO Transport Class 2 Non-Control over TCP - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 399, and is it reachable?

Linux

sudo ss -ltnup 'sport = :399'      # process listening on 399
sudo lsof -nP -i :399

macOS

sudo lsof -nP -i :399

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 399 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 399
netstat -ano | findstr :399

From another machine

nmap -p 399 -sV <host>
sudo nmap -sU -p 399 <host>
nc -zv <host> 399

Block it in the firewall

sudo ufw deny 399/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 399" -Direction Inbound -LocalPort 399 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 399 used for?

Port 399 is assigned by IANA to iso-tsap-c2 (ISO Transport Class 2 Non-Control over TCP).

Is port 399 TCP or UDP?

Port 399 is registered for TCP and UDP.

Is it safe to open port 399?

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 399 is open?

On Linux run "ss -ltnup 'sport = :399'", on macOS "lsof -nP -i :399", and on Windows "netstat -ano | findstr :399" to see the local process. From another machine, "nmap -p 399 <host>" shows whether it is reachable.

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