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Port 666 (doom)

IANA assigns port 666 to doom: doom Id Software.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
doom TCP doom Id Software ddt -
doom UDP doom Id Software ddt -

Check what is listening on port 666

Linux: is anything listening on TCP 666?
sudo ss -tlnp 'sport = :666'
Linux: UDP 666
sudo ss -ulnp 'sport = :666'
Linux / macOS: which process owns the port
sudo lsof -nP -i :666
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':666 '
Windows (Command Prompt)
netstat -ano | findstr :666
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 666 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 666
From Windows
Test-NetConnection HOST -Port 666
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 666

Block port 666 in the firewall

ufw
sudo ufw deny 666/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 666 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 666 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 666' -Direction Inbound -Protocol TCP -LocalPort 666 -Action Block
ufw
sudo ufw deny 666/udp
iptables
sudo iptables -A INPUT -p udp --dport 666 -j DROP
nftables
sudo nft add rule inet filter input udp dport 666 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 666' -Direction Inbound -Protocol UDP -LocalPort 666 -Action Block

Related ports

664oob-ws-https, asf-secure-rmcp665sun-dr667disclose668mecomm

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.