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[ScreenOS]Possible Issues with NAT-Traversal Draft-02

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Article ID: KB7813 KB Last Updated: 12 Aug 2013Version: 5.0
Summary:
Beginning with ScreenOS 5.1.0, NAT Traversal Draft-02, which uses UDP port 4500, is supported. This could cause an issue in some NAT-T environments.
Symptoms:
Environment:
  • Aggressive mode
  • NAT-T  / NAT Traversal
  • UDP port 4500
  • Dial-Up VPN
  • Remote Client / NSR / NetScreen-Remote Client

Symptoms & Errors:

  • Upgrading to ScreenOS 5.1.0 breaks aggressive mode VPNs with NAT-Traversal
Cause:

Solution:
In ScreenOS 5.1.0 and higher, when ScreenOS detects a NAT Traversal packet, it forces the IKE Negotiation packet to use UDP port 4500, instead of UDP port 500.  If the upstream router has an access list that blocks UDP port 4500, this breaks any NAT Traversal VPN from establishing.  The resolution is to add an access list policy on the upstream router to allow UDP port 4500 for NAT Traversal IKE packets.

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