CyberHunter
Local dataset, no network
// Other services

Port 5766: OpenMail NewMail Server

Port 5766 is assigned by IANA to openmailns (OpenMail NewMail Server).

5766

How openmailns works

Other services
client:5766openmailnsresponse client:5766openmailnsresponse

The client sends a request to port 5766 and the openmailns 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 5766 dangerous?

We have no specific security note for port 5766. 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 5766

ServicePortTransportDescriptionReferenceUpdated
openmailns 5766 TCP, UDP OpenMail NewMail Server - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 5766, and is it reachable?

Linux

sudo ss -ltnup 'sport = :5766'      # process listening on 5766
sudo lsof -nP -i :5766

macOS

sudo lsof -nP -i :5766

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 5766 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 5766
netstat -ano | findstr :5766

From another machine

nmap -p 5766 -sV <host>
sudo nmap -sU -p 5766 <host>
nc -zv <host> 5766

Block it in the firewall

sudo ufw deny 5766/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 5766" -Direction Inbound -LocalPort 5766 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 5766 used for?

Port 5766 is assigned by IANA to openmailns (OpenMail NewMail Server).

Is port 5766 TCP or UDP?

Port 5766 is registered for TCP and UDP.

Is it safe to open port 5766?

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 5766 is open?

On Linux run "ss -ltnup 'sport = :5766'", on macOS "lsof -nP -i :5766", and on Windows "netstat -ano | findstr :5766" to see the local process. From another machine, "nmap -p 5766 <host>" shows whether it is reachable.

05 Related
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