Port 1701: L2TP
Port 1701 is L2TP, the Layer 2 Tunneling Protocol used to create VPN tunnels. L2TP has no encryption of its own, so it is almost always paired with IPsec, which negotiates over ports 500 and 4500.
| Port number | 1701 |
|---|---|
| Service | Layer 2 Tunneling Protocol |
| Protocol | UDP |
| Category | Remote access |
| Default encryption | No |
How do I check whether port 1701 is open?
Port 1701 is UDP, which changes the answer: there is no handshake to complete, so nothing outside the host can prove the port is open. Asking the host itself is the reliable route.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | sudo nmap -sU -p 1701 example.com | UDP has no handshake, so there is nothing to complete. nmap infers the state from whether an ICMP port-unreachable comes back, needs root, and is slow and easy to misread. A silent port may be open, filtered, or simply not replying to a probe it does not recognise. |
| What is listening locally? | ss -ulnp | grep ':1701 ' | Shows the UDP socket bound to port 1701. This is the reliable way to answer the question, because you are asking the host itself rather than guessing from outside. |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 1701 used for?
Port 1701 is L2TP, the Layer 2 Tunneling Protocol used to create VPN tunnels. L2TP has no encryption of its own, so it is almost always paired with IPsec, which negotiates over ports 500 and 4500.
Is port 1701 TCP or UDP?
Port 1701 (L2TP) uses UDP.
Is port 1701 secure?
Port 1701 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 1701 on my firewall?
Only if you specifically need L2TP. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.