CyberHunter
Local dataset, no network
// Other services

Port 3551: Apcupsd Information Port

Port 3551 is assigned by IANA to apcupsd (Apcupsd Information Port).

3551

How apcupsd works

Other services
client:3551apcupsdresponse client:3551apcupsdresponse

The client sends a request to port 3551 and the apcupsd 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 3551 dangerous?

We have no specific security note for port 3551. 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 3551

ServicePortTransportDescriptionReferenceUpdated
apcupsd 3551 TCP, UDP Apcupsd Information Port - 2002-07

// source: IANA service name and port number registry

03 Check it

Which program uses port 3551, and is it reachable?

Linux

sudo ss -ltnup 'sport = :3551'      # process listening on 3551
sudo lsof -nP -i :3551

macOS

sudo lsof -nP -i :3551

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 3551 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 3551
netstat -ano | findstr :3551

From another machine

nmap -p 3551 -sV <host>
sudo nmap -sU -p 3551 <host>
nc -zv <host> 3551

Block it in the firewall

sudo ufw deny 3551/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 3551" -Direction Inbound -LocalPort 3551 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 3551 used for?

Port 3551 is assigned by IANA to apcupsd (Apcupsd Information Port).

Is port 3551 TCP or UDP?

Port 3551 is registered for TCP and UDP.

Is it safe to open port 3551?

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 3551 is open?

On Linux run "ss -ltnup 'sport = :3551'", on macOS "lsof -nP -i :3551", and on Windows "netstat -ano | findstr :3551" to see the local process. From another machine, "nmap -p 3551 <host>" shows whether it is reachable.

05 Related
:4444 Metasploit default / krb524 tcp · udp · high risk :31337 Back Orifice / elite tcp · udp · high risk :1 TCP Port Service Multiplexer tcp · udp :5 Remote Job Entry tcp · udp :9 Discard tcp · udp · sctp · dccp :11 Active Users tcp · udp :17 Quote of the Day tcp · udp :18 Message Send Protocol (historic) tcp · udp

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