CyberHunter
Local dataset, no network
// Other services

Port 521: ripng

Port 521 is assigned by IANA to ripng (ripng).

521

How ripng works

Other services
client:521ripngresponse client:521ripngresponse

The client sends a request to port 521 and the ripng 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 521 dangerous?

We have no specific security note for port 521. 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 521

ServicePortTransportDescriptionReferenceUpdated
ripng 521 TCP, UDP ripng - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 521, and is it reachable?

Linux

sudo ss -ltnup 'sport = :521'      # process listening on 521
sudo lsof -nP -i :521

macOS

sudo lsof -nP -i :521

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 521 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 521
netstat -ano | findstr :521

From another machine

nmap -p 521 -sV <host>
sudo nmap -sU -p 521 <host>
nc -zv <host> 521

Block it in the firewall

sudo ufw deny 521/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 521" -Direction Inbound -LocalPort 521 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 521 used for?

Port 521 is assigned by IANA to ripng (ripng).

Is port 521 TCP or UDP?

Port 521 is registered for TCP and UDP.

Is it safe to open port 521?

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 521 is open?

On Linux run "ss -ltnup 'sport = :521'", on macOS "lsof -nP -i :521", and on Windows "netstat -ano | findstr :521" to see the local process. From another machine, "nmap -p 521 <host>" shows whether it is reachable.

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