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// Other services

Port 588: CAL

Port 588 is assigned by IANA to cal (CAL).

588

How cal works

Other services
client:588calresponse client:588calresponse

The client sends a request to port 588 and the cal 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 588 dangerous?

We have no specific security note for port 588. 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 588

ServicePortTransportDescriptionReferenceUpdated
cal 588 TCP, UDP CAL - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 588, and is it reachable?

Linux

sudo ss -ltnup 'sport = :588'      # process listening on 588
sudo lsof -nP -i :588

macOS

sudo lsof -nP -i :588

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 588 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 588
netstat -ano | findstr :588

From another machine

nmap -p 588 -sV <host>
sudo nmap -sU -p 588 <host>
nc -zv <host> 588

Block it in the firewall

sudo ufw deny 588/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 588" -Direction Inbound -LocalPort 588 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 588 used for?

Port 588 is assigned by IANA to cal (CAL).

Is port 588 TCP or UDP?

Port 588 is registered for TCP and UDP.

Is it safe to open port 588?

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 588 is open?

On Linux run "ss -ltnup 'sport = :588'", on macOS "lsof -nP -i :588", and on Windows "netstat -ano | findstr :588" to see the local process. From another machine, "nmap -p 588 <host>" shows whether it is reachable.

05 Related
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