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[ScreenOS] What is NAT-T draft 2 and how does the Firewall detect it?



Article ID: KB8119 KB Last Updated: 21 Dec 2012Version: 7.0
This article provides information about NAT-T draft 2 and how the firewall detects it.

  • NAT-T Draft 2

  • UDP 4500

What is NAT-T draft 2?

Due to IKE/IPSec-aware devices attempting to process IKE and IPSec packets, the NAT-T draft 2 was implemented. NAT-T draft 2 changes the UDP IKE SRC and DST port numbers from 500 to 4500. This UDP header, using port 4500, is found between the outer IP and the ESP or AH header, thus changing the IPSec protocol field from 50 (ESP) or 51 (AH) to 17 (for UDP).

The IKE packet would appear as:

[IP Header] [UDP Header] [ISAKMP Header] [Payload]

The [ UDP Header] , after NAT-T is detected, is broken out as:

[Src port 4500] [Dst port 4500] [Length] [Checksum] [Non-esp marker (0000) ] [Payload]

The non-ESP marker is 4 bytes of zero (0000), is added to the UDP segment to distinguish an encapsulated ISAKMP packet from an encapsulated ESP packet which does not have a marker.

How does the Juniper Firewall detect the difference between the draft versions of NAT-T?

The Firewall sends two MD-5 hashes in the Vendor ID payload during the first two exchanges of Phase 1 negotiations. One hash for "draft 0" the other for "draft 2"

 MD-5 hash of "draft-ietf-ipsec-nat-t-ike-00" 4485152d 18b6bbcd 0be8a846 9579ddcc
 MD-5 hash of "draft-ietf-ipsec-nat-t-ike-02"  90cb8091 3ebb696e 086381b5 ec427b1f

Both VPN peers must send and receive at least one of these in order for NAT-T setup to continue. If hashes for both drafts are sent and received, the Firewall will use NAT-T draft 2 implementation.

Screen OS 5.1 and later supports NAT-T based on draft-ietf-ipsec-nat-t-ike-02.txt and draft-ietf-ipsec-udp-encaps-02.txt, as well as version 0 of these drafts.

Note: For more information on NAT Traversal, please refer to the Concepts and Examples ScreenOS Reference Guide.

Volume 5: Virtual Private Networks
Chapter 7 -- Advanced Virtual Private Network Features

ScreenOS 5.4

ScreenOS 6.2

ScreenOS 6.3

or KB4741 - NAT Traversal Overview

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