NAT-T support for IKE on UDP port 500

Cloudflare IPsec now supports the standard NAT traversal (NAT-T) flow, where IKE begins on UDP port 500 and switches to UDP port 4500 after NAT is detected.

Previously, devices behind NAT had to be configured to initiate IKE on UDP port 4500 directly. Devices that started on UDP port 500 could not complete the IKE handshake when NAT was in the path. This required custom configuration on devices such as VeloCloud SD-WAN edges, Cisco IOS-XE routers, and Juniper SRX firewalls, and was not possible on every platform.

What changed:

• Devices behind NAT can now initiate IKE on either UDP port 500 or UDP port 4500.
• Devices that start IKE on UDP port 500 and switch to UDP port 4500 after NAT detection now complete the handshake successfully.
• No configuration change is required on Cloudflare. The change is available for all IPsec tunnels on Cloudflare WAN and Magic Transit.

This change does not affect existing tunnels:

• Tunnels using UDP port 500 with no NAT detected continue to operate as before.
• Tunnels configured to start IKE on UDP port 4500 continue to operate as before.
• NAT detection logic is unchanged.

For configuration details, refer to GRE and IPsec tunnels.