CyberHunter
Local dataset, no network
// Other services

Port 4990: BusySync Calendar Synch. Protocol

Port 4990 is assigned by IANA to busycal (BusySync Calendar Synch. Protocol).

4990

How busycal works

Other services
client:4990busycalresponse client:4990busycalresponse

The client sends a request to port 4990 and the busycal 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 4990 dangerous?

We have no specific security note for port 4990. 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 4990

ServicePortTransportDescriptionReferenceUpdated
busycal 4990 TCP, UDP BusySync Calendar Synch. Protocol - 2008-01-07

// source: IANA service name and port number registry

03 Check it

Which program uses port 4990, and is it reachable?

Linux

sudo ss -ltnup 'sport = :4990'      # process listening on 4990
sudo lsof -nP -i :4990

macOS

sudo lsof -nP -i :4990

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 4990 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 4990
netstat -ano | findstr :4990

From another machine

nmap -p 4990 -sV <host>
sudo nmap -sU -p 4990 <host>
nc -zv <host> 4990

Block it in the firewall

sudo ufw deny 4990/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 4990" -Direction Inbound -LocalPort 4990 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 4990 used for?

Port 4990 is assigned by IANA to busycal (BusySync Calendar Synch. Protocol).

Is port 4990 TCP or UDP?

Port 4990 is registered for TCP and UDP.

Is it safe to open port 4990?

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 4990 is open?

On Linux run "ss -ltnup 'sport = :4990'", on macOS "lsof -nP -i :4990", and on Windows "netstat -ano | findstr :4990" to see the local process. From another machine, "nmap -p 4990 <host>" shows whether it is reachable.

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