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// Other services

Port 3999: Norman distributes scanning service

Port 3999 is assigned by IANA to nvcnet (Norman distributes scanning service).

3999

How nvcnet works

Other services
client:3999nvcnetresponse client:3999nvcnetresponse

The client sends a request to port 3999 and the nvcnet 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 3999 dangerous?

We have no specific security note for port 3999. 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 3999

ServicePortTransportDescriptionReferenceUpdated
nvcnet 3999 TCP, UDP Norman distributes scanning service - 2007-11-06

// source: IANA service name and port number registry

03 Check it

Which program uses port 3999, and is it reachable?

Linux

sudo ss -ltnup 'sport = :3999'      # process listening on 3999
sudo lsof -nP -i :3999

macOS

sudo lsof -nP -i :3999

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 3999 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 3999
netstat -ano | findstr :3999

From another machine

nmap -p 3999 -sV <host>
sudo nmap -sU -p 3999 <host>
nc -zv <host> 3999

Block it in the firewall

sudo ufw deny 3999/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 3999" -Direction Inbound -LocalPort 3999 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 3999 used for?

Port 3999 is assigned by IANA to nvcnet (Norman distributes scanning service).

Is port 3999 TCP or UDP?

Port 3999 is registered for TCP and UDP.

Is it safe to open port 3999?

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 3999 is open?

On Linux run "ss -ltnup 'sport = :3999'", on macOS "lsof -nP -i :3999", and on Windows "netstat -ano | findstr :3999" to see the local process. From another machine, "nmap -p 3999 <host>" shows whether it is reachable.

05 Related
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