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// Other services

Port 6556: Checkmk Monitoring Agent

Port 6556 is assigned by IANA to checkmk-agent (Checkmk Monitoring Agent).

6556

How checkmk-agent works

Other services
client:6556checkmk-agentresponse client:6556checkmk-agentresponse

The client sends a request to port 6556 and the checkmk-agent service answers.

Transport
TCP
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 6556 dangerous?

We have no specific security note for port 6556. 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 6556

ServicePortTransportDescriptionReferenceUpdated
checkmk-agent 6556 TCP Checkmk Monitoring Agent - 2024-08-06
-6556-Reserved--

// source: IANA service name and port number registry

03 Check it

Which program uses port 6556, and is it reachable?

Linux

sudo ss -ltnup 'sport = :6556'      # process listening on 6556
sudo lsof -nP -i :6556

macOS

sudo lsof -nP -i :6556

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 6556 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 6556
netstat -ano | findstr :6556

From another machine

nmap -p 6556 -sV <host>
nc -zv <host> 6556

Block it in the firewall

sudo ufw deny 6556/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 6556" -Direction Inbound -LocalPort 6556 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 6556 used for?

Port 6556 is assigned by IANA to checkmk-agent (Checkmk Monitoring Agent).

Is port 6556 TCP or UDP?

Port 6556 is registered for TCP.

Is it safe to open port 6556?

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 6556 is open?

On Linux run "ss -ltnup 'sport = :6556'", on macOS "lsof -nP -i :6556", and on Windows "netstat -ano | findstr :6556" to see the local process. From another machine, "nmap -p 6556 <host>" shows whether it is reachable.

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