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Port 399 (iso-tsap-c2)

IANA assigns port 399 to iso-tsap-c2: ISO Transport Class 2 Non-Control over TCP.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
iso-tsap-c2 TCP ISO Transport Class 2 Non-Control over TCP Yanick Pouffary -
iso-tsap-c2 UDP ISO Transport Class 2 Non-Control over UDP Yanick Pouffary -

Check what is listening on port 399

Linux: is anything listening on TCP 399?
sudo ss -tlnp 'sport = :399'
Linux: UDP 399
sudo ss -ulnp 'sport = :399'
Linux / macOS: which process owns the port
sudo lsof -nP -i :399
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':399 '
Windows (Command Prompt)
netstat -ano | findstr :399
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 399 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 399
From Windows
Test-NetConnection HOST -Port 399
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 399

Block port 399 in the firewall

ufw
sudo ufw deny 399/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 399 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 399 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 399' -Direction Inbound -Protocol TCP -LocalPort 399 -Action Block
ufw
sudo ufw deny 399/udp
iptables
sudo iptables -A INPUT -p udp --dport 399 -j DROP
nftables
sudo nft add rule inet filter input udp dport 399 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 399' -Direction Inbound -Protocol UDP -LocalPort 399 -Action Block

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