Port 3128 (Squid proxy)
Port 3128 is the default listening port of the Squid caching proxy, so it became the usual port for HTTP proxies in general. In the IANA registry the number belongs to an unrelated service (ndl-aas).
- Service
- Squid proxy
- Protocol
- TCP
- IANA registry
- Assigned to a different service: ndl-aas (Active API Server Port), TCP/UDP. The use described here is not registered.
- Commonly used by
Check if a port is open on your IP
We only check the address you are visiting from, one TCP port at a time. You cannot enter someone else’s address: this is not a port scanner.
Is it safe to open port 3128?
A proxy on 3128 open to everyone is quickly found and abused. Limit who may use it with http_access rules or a password, and check that your proxy works with our proxy checker.
“Port 3128 is already in use”: how to fix it
The error means another program is already listening on port 3128. Find its process ID (PID), then stop that program or start yours on a different port.
Windows (Command Prompt or PowerShell as administrator)
netstat -ano | findstr :3128taskkill /PID <PID> /FmacOS
lsof -nP -iTCP:3128 -sTCP:LISTENkill <PID>Linux
sudo ss -ltnp | grep :3128sudo lsof -i :3128sudo kill <PID>Replace <PID> with the number from the last column (Windows) or the PID column (macOS, Linux).
Why a port can look closed when your server is running
Most home networks sit behind a router doing NAT: the router owns the public address, and your computer has a private one such as 192.168.1.10. A connection from the internet reaches the router and stops there unless you add a port forwarding rule to your computer.
Many mobile and some home providers go further and use carrier-grade NAT (CGNAT): several customers share one public address, and your router itself gets an address from 100.64.0.0/10. Then no rule on your side can open a port; ask the provider for a public IP or use a tunnel or VPN service.
Even with forwarding in place, the firewall on the computer (Windows Defender Firewall, ufw, firewalld) must allow the port, and the program must listen on all interfaces (0.0.0.0), not only on 127.0.0.1.
UDP cannot be checked this way: a UDP service usually stays silent to packets it does not understand, so “no answer” means nothing. This check uses TCP only.
Frequently asked questions
What is port 3128 used for?
Port 3128 is the default listening port of the Squid caching proxy, so it became the usual port for HTTP proxies in general. In the IANA registry the number belongs to an unrelated service (ndl-aas).
Is port 3128 TCP or UDP?
Port 3128 uses TCP. This use is not registered with IANA.
Is it safe to open port 3128?
A proxy on 3128 open to everyone is quickly found and abused. Limit who may use it with http_access rules or a password, and check that your proxy works with our proxy checker.
How do I free port 3128 if it is already in use?
Find the process: on Windows run netstat -ano | findstr :3128 and stop it with taskkill /PID <pid> /F; on macOS or Linux run lsof -i :3128 and stop it with kill <pid>.