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Page 180 SonicWALL Internet Security Appliance Administrator’s Guide
SonicWALL 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 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.
Selecting Enable NAT Traversal in the Global VPN Settings section of the Summary tab 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 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
AES (Advanced Encryption Standard) Support
AES is an encryption algorithm for securings sensitive but unclassified material by U.S. Government
agencies. It may become the official encryption standard for commercial transactions in the private
sector. As a symmetric algorithm (same key for encryption and decryption), it uses block encryption
of 128 bits in size, supporting key sizes of 128, 192, and 256.
AES support is only available on the PRO 230, PRO 330, and GX series. Support is limited to 128
and 256 bit keys for IKE Phase One tunnels, and 128 bit keys for Phase Two tunnels for both IKE
and Manual key.