Port 1723 (PPTP)
Port 1723 is the control channel of PPTP, an old VPN protocol. The tunnel data itself travels as GRE (IP protocol 47), which is why PPTP often breaks behind NAT even with 1723 open.
- Service
- PPTP
- Protocol
- TCP
- IANA registry
- Officially assigned: pptp (pptp), TCP/UDP
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 1723?
PPTP with MS-CHAP v2 should be considered broken: Microsoft’s advisory 2743314 says captured authentication traffic can be used to recover credentials. Replace it with WireGuard, OpenVPN or IKEv2 and close 1723.
Sources
“Port 1723 is already in use”: how to fix it
The error means another program is already listening on port 1723. 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 :1723taskkill /PID <PID> /FmacOS
lsof -nP -iTCP:1723 -sTCP:LISTENkill <PID>Linux
sudo ss -ltnp | grep :1723sudo lsof -i :1723sudo 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 1723 used for?
Port 1723 is the control channel of PPTP, an old VPN protocol. The tunnel data itself travels as GRE (IP protocol 47), which is why PPTP often breaks behind NAT even with 1723 open.
Is port 1723 TCP or UDP?
Port 1723 uses TCP. IANA reserves the number for both TCP and UDP, but in practice only TCP is used.
Is it safe to open port 1723?
PPTP with MS-CHAP v2 should be considered broken: Microsoft’s advisory 2743314 says captured authentication traffic can be used to recover credentials. Replace it with WireGuard, OpenVPN or IKEv2 and close 1723.
How do I free port 1723 if it is already in use?
Find the process: on Windows run netstat -ano | findstr :1723 and stop it with taskkill /PID <pid> /F; on macOS or Linux run lsof -i :1723 and stop it with kill <pid>.