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// Other services

Port 4391: American Printware IMServer Protocol

Port 4391 is assigned by IANA to apwi-imserver (American Printware IMServer Protocol).

4391

How apwi-imserver works

Other services
client:4391apwi-imserverresponse client:4391apwi-imserverresponse

The client sends a request to port 4391 and the apwi-imserver 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 4391 dangerous?

We have no specific security note for port 4391. 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 4391

ServicePortTransportDescriptionReferenceUpdated
apwi-imserver 4391 TCP American Printware IMServer Protocol - 2009-02-12
-4391-Reserved--

// source: IANA service name and port number registry

03 Check it

Which program uses port 4391, and is it reachable?

Linux

sudo ss -ltnup 'sport = :4391'      # process listening on 4391
sudo lsof -nP -i :4391

macOS

sudo lsof -nP -i :4391

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 4391 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 4391
netstat -ano | findstr :4391

From another machine

nmap -p 4391 -sV <host>
nc -zv <host> 4391

Block it in the firewall

sudo ufw deny 4391/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 4391" -Direction Inbound -LocalPort 4391 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 4391 used for?

Port 4391 is assigned by IANA to apwi-imserver (American Printware IMServer Protocol).

Is port 4391 TCP or UDP?

Port 4391 is registered for TCP.

Is it safe to open port 4391?

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 4391 is open?

On Linux run "ss -ltnup 'sport = :4391'", on macOS "lsof -nP -i :4391", and on Windows "netstat -ano | findstr :4391" to see the local process. From another machine, "nmap -p 4391 <host>" shows whether it is reachable.

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