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// Network services

Port 204: AppleTalk Echo

Port 204 is assigned by IANA to at-echo (AppleTalk Echo).

204

How at-echo works

Network services
client:204at-echoresponse client:204at-echoresponse

The client sends a request to port 204 and the at-echo 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
Network services
01 Security

Is port 204 dangerous?

We have no specific security note for port 204. Its risk depends on the program actually listening on it. General advice for network services services:

  • Answer only the networks that need the service (no open resolvers, relays or reflectors).
  • UDP services can be abused for DDoS amplification: rate-limit or block them at the edge.
  • Disable legacy test services that have no use in production.
02 Registry

IANA registry entries for port 204

ServicePortTransportDescriptionReferenceUpdated
at-echo 204 TCP, UDP AppleTalk Echo - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 204, and is it reachable?

Linux

sudo ss -ltnup 'sport = :204'      # process listening on 204
sudo lsof -nP -i :204

macOS

sudo lsof -nP -i :204

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 204 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 204
netstat -ano | findstr :204

From another machine

nmap -p 204 -sV <host>
sudo nmap -sU -p 204 <host>
nc -zv <host> 204

Block it in the firewall

sudo ufw deny 204/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 204" -Direction Inbound -LocalPort 204 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 204 used for?

Port 204 is assigned by IANA to at-echo (AppleTalk Echo).

Is port 204 TCP or UDP?

Port 204 is registered for TCP and UDP.

Is it safe to open port 204?

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. Answer only the networks that need the service (no open resolvers, relays or reflectors).

How do I check if port 204 is open?

On Linux run "ss -ltnup 'sport = :204'", on macOS "lsof -nP -i :204", and on Windows "netstat -ano | findstr :204" to see the local process. From another machine, "nmap -p 204 <host>" shows whether it is reachable.

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