CyberHunter
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// Other services

Port 646: LDP

Port 646 is assigned by IANA to ldp (LDP).

646

How ldp works

Other services
client:646ldpresponse client:646ldpresponse

The client sends a request to port 646 and the ldp 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 646 dangerous?

We have no specific security note for port 646. 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 646

ServicePortTransportDescriptionReferenceUpdated
ldp 646 TCP, UDP LDP - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 646, and is it reachable?

Linux

sudo ss -ltnup 'sport = :646'      # process listening on 646
sudo lsof -nP -i :646

macOS

sudo lsof -nP -i :646

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 646 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 646
netstat -ano | findstr :646

From another machine

nmap -p 646 -sV <host>
sudo nmap -sU -p 646 <host>
nc -zv <host> 646

Block it in the firewall

sudo ufw deny 646/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 646" -Direction Inbound -LocalPort 646 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 646 used for?

Port 646 is assigned by IANA to ldp (LDP).

Is port 646 TCP or UDP?

Port 646 is registered for TCP and UDP.

Is it safe to open port 646?

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 646 is open?

On Linux run "ss -ltnup 'sport = :646'", on macOS "lsof -nP -i :646", and on Windows "netstat -ano | findstr :646" to see the local process. From another machine, "nmap -p 646 <host>" shows whether it is reachable.

05 Related
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