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Port 3999 (nvcnet)

IANA assigns port 3999 to nvcnet: Norman distributes scanning service.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
nvcnet TCP Norman distributes scanning service Kristian A Bognaes -
nvcnet UDP Norman distributes scanning service Kristian A Bognaes -

Registry dates for this port: registered 2007-11-06.

Check what is listening on port 3999

Linux: is anything listening on TCP 3999?
sudo ss -tlnp 'sport = :3999'
Linux: UDP 3999
sudo ss -ulnp 'sport = :3999'
Linux / macOS: which process owns the port
sudo lsof -nP -i :3999
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':3999 '
Windows (Command Prompt)
netstat -ano | findstr :3999
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 3999 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 3999
From Windows
Test-NetConnection HOST -Port 3999
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 3999

Block port 3999 in the firewall

ufw
sudo ufw deny 3999/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 3999 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 3999 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 3999' -Direction Inbound -Protocol TCP -LocalPort 3999 -Action Block
ufw
sudo ufw deny 3999/udp
iptables
sudo iptables -A INPUT -p udp --dport 3999 -j DROP
nftables
sudo nft add rule inet filter input udp dport 3999 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 3999' -Direction Inbound -Protocol UDP -LocalPort 3999 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.