Port 1701 (L2TP)

Layer 2 Tunneling Protocol, usually L2TP/IPsec on older VPN setups.

Registered (user) port, 1024-49151 UDP VPN Expose only to trusted sources

Port 1701 is normally used over UDP; the checker tests TCP only, so a "filtered" result does not mean the UDP service is down.

What port 1701 is used for

Layer 2 Tunneling Protocol, usually L2TP/IPsec on older VPN setups.

Expose only to trusted sources. L2TP itself is unencrypted; only use it inside IPsec. Modern setups should prefer IKEv2, WireGuard or OpenVPN.

Service nameProtoDescriptionAssigneeReference

l2f TCP l2f - -

l2tp TCP l2tp Andy Valencia -

l2f UDP l2f - -

l2tp UDP l2tp Andy Valencia -

Linux: UDP 1701

sudo ss -ulnp 'sport = :1701'

Linux / macOS: which process owns the port

sudo lsof -nP -i :1701

Windows (Command Prompt)

netstat -ano | findstr :1701

Windows (PowerShell)

Get-NetUDPEndpoint -LocalPort 1701

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 1701

Replace 203.0.113.10 with the address or network you trust. With ufw, make sure the default incoming policy is deny.

ufw (one trusted source)

sudo ufw allow from 203.0.113.10 to any port 1701 proto udp

iptables

sudo iptables -A INPUT -p udp --dport 1701 -s 203.0.113.10 -j ACCEPT
sudo iptables -A INPUT -p udp --dport 1701 -j DROP

nftables

sudo nft add rule inet filter input ip saddr 203.0.113.10 udp dport 1701 accept
sudo nft add rule inet filter input udp dport 1701 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'UDP 1701 trusted' -Direction Inbound -Protocol UDP -LocalPort 1701 -RemoteAddress 203.0.113.10 -Action Allow

All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.