HTTPfy

Port 504 (citadel)

IANA assigns port 504 to citadel.

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

IANA assignment

Service nameProtoDescriptionAssigneeReference
citadel TCP citadel Art Cancro -
citadel UDP citadel Art Cancro -

Check what is listening on port 504

Linux: is anything listening on TCP 504?
sudo ss -tlnp 'sport = :504'
Linux: UDP 504
sudo ss -ulnp 'sport = :504'
Linux / macOS: which process owns the port
sudo lsof -nP -i :504
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':504 '
Windows (Command Prompt)
netstat -ano | findstr :504
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 504 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 504
From Windows
Test-NetConnection HOST -Port 504
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 504

Block port 504 in the firewall

ufw
sudo ufw deny 504/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 504 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 504 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 504' -Direction Inbound -Protocol TCP -LocalPort 504 -Action Block
ufw
sudo ufw deny 504/udp
iptables
sudo iptables -A INPUT -p udp --dport 504 -j DROP
nftables
sudo nft add rule inet filter input udp dport 504 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 504' -Direction Inbound -Protocol UDP -LocalPort 504 -Action Block

Related ports

502Modbus/TCP503intrinsa505mailbox-lm506ohimsrv

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