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// Other services

Port 3521: Telequip Labs MC3SS

Port 3521 is assigned by IANA to mc3ss (Telequip Labs MC3SS).

3521

How mc3ss works

Other services
client:3521mc3ssresponse client:3521mc3ssresponse

The client sends a request to port 3521 and the mc3ss 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 3521 dangerous?

We have no specific security note for port 3521. 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 3521

ServicePortTransportDescriptionReferenceUpdated
mc3ss 3521 TCP, UDP Telequip Labs MC3SS - 2002-06

// source: IANA service name and port number registry

03 Check it

Which program uses port 3521, and is it reachable?

Linux

sudo ss -ltnup 'sport = :3521'      # process listening on 3521
sudo lsof -nP -i :3521

macOS

sudo lsof -nP -i :3521

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 3521 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 3521
netstat -ano | findstr :3521

From another machine

nmap -p 3521 -sV <host>
sudo nmap -sU -p 3521 <host>
nc -zv <host> 3521

Block it in the firewall

sudo ufw deny 3521/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 3521" -Direction Inbound -LocalPort 3521 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 3521 used for?

Port 3521 is assigned by IANA to mc3ss (Telequip Labs MC3SS).

Is port 3521 TCP or UDP?

Port 3521 is registered for TCP and UDP.

Is it safe to open port 3521?

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 3521 is open?

On Linux run "ss -ltnup 'sport = :3521'", on macOS "lsof -nP -i :3521", and on Windows "netstat -ano | findstr :3521" to see the local process. From another machine, "nmap -p 3521 <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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