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// Other services

Port 504: citadel

Port 504 is assigned by IANA to citadel (citadel).

504

How citadel works

Other services
client:504citadelresponse client:504citadelresponse

The client sends a request to port 504 and the citadel 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
Well-known (0-1023)
Category
Other services
01 Security

Is port 504 dangerous?

We have no specific security note for port 504. 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 504

ServicePortTransportDescriptionReferenceUpdated
citadel 504 TCP, UDP citadel - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 504, and is it reachable?

Linux

sudo ss -ltnup 'sport = :504'      # process listening on 504
sudo lsof -nP -i :504

macOS

sudo lsof -nP -i :504

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 504 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 504
netstat -ano | findstr :504

From another machine

nmap -p 504 -sV <host>
sudo nmap -sU -p 504 <host>
nc -zv <host> 504

Block it in the firewall

sudo ufw deny 504/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 504" -Direction Inbound -LocalPort 504 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 504 used for?

Port 504 is assigned by IANA to citadel (citadel).

Is port 504 TCP or UDP?

Port 504 is registered for TCP and UDP.

Is it safe to open port 504?

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 504 is open?

On Linux run "ss -ltnup 'sport = :504'", on macOS "lsof -nP -i :504", and on Windows "netstat -ano | findstr :504" to see the local process. From another machine, "nmap -p 504 <host>" shows whether it is reachable.

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