CyberHunter
Local dataset, no network
// Other services

Port 1721: caicci

Port 1721 is assigned by IANA to caicci (caicci).

1721

How caicci works

Other services
client:1721caicciresponse client:1721caicciresponse

The client sends a request to port 1721 and the caicci 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 1721 dangerous?

We have no specific security note for port 1721. 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 1721

ServicePortTransportDescriptionReferenceUpdated
caicci 1721 TCP, UDP caicci - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 1721, and is it reachable?

Linux

sudo ss -ltnup 'sport = :1721'      # process listening on 1721
sudo lsof -nP -i :1721

macOS

sudo lsof -nP -i :1721

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 1721 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 1721
netstat -ano | findstr :1721

From another machine

nmap -p 1721 -sV <host>
sudo nmap -sU -p 1721 <host>
nc -zv <host> 1721

Block it in the firewall

sudo ufw deny 1721/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 1721" -Direction Inbound -LocalPort 1721 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 1721 used for?

Port 1721 is assigned by IANA to caicci (caicci).

Is port 1721 TCP or UDP?

Port 1721 is registered for TCP and UDP.

Is it safe to open port 1721?

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 1721 is open?

On Linux run "ss -ltnup 'sport = :1721'", on macOS "lsof -nP -i :1721", and on Windows "netstat -ano | findstr :1721" to see the local process. From another machine, "nmap -p 1721 <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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