VPN NAT Traversal is an Internet Draft proposed to IETF (Internet Engineering Task Force) to overcome problems faced when IPSec traffic is intended to pass through a NAT device. NAT Traversal addresses the issue of UDP (User Datagram Protocol) encapsulation by wrapping an IPSec packet inside a UDP packet when a NAT or NAPT (Network Address Port Translator) device is detected between peers.
Encapsulation of the IPSec packet requires decapsulation of the IPSec packet. Since ESP-protected packets are exchanged between IKE peers using one of three methods, gateway to gateway, client to gateway, and client to client, the IKE peers must support the same method of UDP encapsulation. IKE peers exchange a known value to determine if they both support NAT Traversal. If the IKE peers agree, IKE probes or discovery payloads are used to determine if a NAT or NAPT device is present. Only if a NAT or NAPT device is detected is UDP encapsulation is used for IPSec packets.
NAT/NAT Traversal devices use dynamic mappings where a private IP address and source port (192.168.168.168:X) are temporarily bound to a shared public IP address and an unused port (220.127.116.11:Y). This binding is dissolved after a period of inactivity (minutes or seconds), enabling pool reuse.
IPSec VPNs protect traffic exchanged between authenticated endpoints, but authenticated endpoints cannot be dynamically re-mapped mid-session for NAT traversal to work. Therefore, to preserve a dynamic NAT binding for the life of an IPSec session, a 1-byte UDP is designated as a NAT Traversal keepalive and acts as a heartbeat sent by the VPN device behind the NAT or NAPT device. The keepalive is silently discarded by the IPSec peer.
Selecting Enable NAT Traversal in the Global VPN Settings section of the Summary page allows VPN tunnels to support this protocol, and log messages are generated by the SonicWALL when a IPSec Security Gateway is detected behind a NAT/NAPT device. The following log messages are found on the View Log page:
Peer IPSec Gateway behind a NAT/NAPT device
Local IPSec Security Gateway behind a NAT/NAPT device
No NAT/NAPT device detected between IPSec Security
Peer IPSec Security Gateway doesnt support VPN NAT Traversal