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// Other services

Port 1851: ctcd

Port 1851 is assigned by IANA to ctcd (ctcd).

1851

How ctcd works

Other services
client:1851ctcdresponse client:1851ctcdresponse

The client sends a request to port 1851 and the ctcd 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
Registered (1024-49151)
Category
Other services
01 Security

Is port 1851 dangerous?

We have no specific security note for port 1851. 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 1851

ServicePortTransportDescriptionReferenceUpdated
ctcd 1851 TCP, UDP ctcd - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 1851, and is it reachable?

Linux

sudo ss -ltnup 'sport = :1851'      # process listening on 1851
sudo lsof -nP -i :1851

macOS

sudo lsof -nP -i :1851

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 1851 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 1851
netstat -ano | findstr :1851

From another machine

nmap -p 1851 -sV <host>
sudo nmap -sU -p 1851 <host>
nc -zv <host> 1851

Block it in the firewall

sudo ufw deny 1851/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 1851" -Direction Inbound -LocalPort 1851 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 1851 used for?

Port 1851 is assigned by IANA to ctcd (ctcd).

Is port 1851 TCP or UDP?

Port 1851 is registered for TCP and UDP.

Is it safe to open port 1851?

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 1851 is open?

On Linux run "ss -ltnup 'sport = :1851'", on macOS "lsof -nP -i :1851", and on Windows "netstat -ano | findstr :1851" to see the local process. From another machine, "nmap -p 1851 <host>" shows whether it is reachable.

05 Related
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