Port / UDP
1701L2TP
Layer 2 Tunneling Protocol carries PPP frames across a network; in practice it is always paired with IPsec for confidentiality.
If it is internet-facing: Port 1701 is legitimate in the right place, but exposed carelessly it widens the attack surface noticeably.
Exposure
Internet-facing
Transport
Cleartext
Risk if public
Elevated
Exposure verdict
Port 1701 normally answers from the internet, but the transport is not confidential by default. Enforce the encrypted variant before the traffic leaves your network.
Traffic crosses the network readable and modifiable. Anything sensitive on this port needs a tunnel, a VPN, or a different protocol.
Registered with IANA on request, but bindable by any unprivileged user. Registration records intent — it does not reserve the port, so collisions between products are common.
Grouped with the other network services ports so a rule written for one can be checked against its neighbours.
What actually goes wrong
1 note- L2TP on its own provides no encryption — the protection comes from the IPsec transform underneath it.
Recommended firewall rule
Allow it, and harden the service
This port is expected to answer from anywhere, so the firewall is not where its security comes from.
ufw
sudo ufw allow 1701/udpiptables
sudo iptables -A INPUT -p udp --dport 1701 -j ACCEPTCheck what is really there
Listening locally
sudo ss -ulpn 'sport = :1701'The bind address is the answer that matters: 127.0.0.1 is local-only, 0.0.0.0 and :: mean every interface.
Listening (macOS)
sudo lsof -nP -iUDP:1701macOS ships lsof rather than ss; this names the owning process and user.
From outside
nmap -sUV -Pn -p 1701 <host>A version probe confirms whether L2TP is actually what answers, rather than trusting the number. Only scan hosts you are authorised to test.
Reachability
nc -vzu <host> 1701The quick yes/no when you only need to know whether a path exists through the firewall.