CyberHunter
Local dataset, no network
// Other services

Port 551: cybercash

Port 551 is assigned by IANA to cybercash (cybercash).

551

How cybercash works

Other services
client:551cybercashresponse client:551cybercashresponse

The client sends a request to port 551 and the cybercash 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 551 dangerous?

We have no specific security note for port 551. 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 551

ServicePortTransportDescriptionReferenceUpdated
cybercash 551 TCP, UDP cybercash RFC 1898 -

// source: IANA service name and port number registry

03 Check it

Which program uses port 551, and is it reachable?

Linux

sudo ss -ltnup 'sport = :551'      # process listening on 551
sudo lsof -nP -i :551

macOS

sudo lsof -nP -i :551

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 551 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 551
netstat -ano | findstr :551

From another machine

nmap -p 551 -sV <host>
sudo nmap -sU -p 551 <host>
nc -zv <host> 551

Block it in the firewall

sudo ufw deny 551/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 551" -Direction Inbound -LocalPort 551 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 551 used for?

Port 551 is assigned by IANA to cybercash (cybercash).

Is port 551 TCP or UDP?

Port 551 is registered for TCP and UDP.

Is it safe to open port 551?

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 551 is open?

On Linux run "ss -ltnup 'sport = :551'", on macOS "lsof -nP -i :551", and on Windows "netstat -ano | findstr :551" to see the local process. From another machine, "nmap -p 551 <host>" shows whether it is reachable.

05 Related
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