Nat Traversal Support - SonicWALL TELE3 SP Administrator's Manual

NAT Traversal Support

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 and addresses
the traffic problem 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 (207.126.101.100: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.
NAT Traversal support is transparent, but 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 tab:
• 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 doesn't support VPN NAT Traversal
SonicWALL VPN Page 109