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Unable to Create Multiple Proposals with Differing Diffie-Hellman Groups in IKE Phase 1 or Phase 2

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Article ID: KB6698 KB Last Updated: 06 Aug 2010Version: 4.0
Summary:
NS-Remote is configured with one Phase 1 and one Phase 2 proposal.  On the Juniper Firewall, Phase 1 is configured with multiple proposals.  The proposals include DH1 and DH2.  For example, pre-g1-des-sha and pre-g2-des-sha.  The VPN is failing to negotiate.
Symptoms:
Environment:
  • Diffie-Hellman (DH)
  • IKE Phase 1 or Phase 2
  • Custom Proposal
  • Differing DH groups

Symptoms & Errors:

  • Error Message:  Mismatched DH group in the Phase 2 Proposals. The DH group must be identical in all proposals of a VPN Configuration. Please try again. 

  • VPN fails to negotiate past Phase 1.  On the Firewall, the debug ike detail shows:

    ## 2006-12-04 20:56:13 : IKE<172.19.51.188> mismatched group for phase 1 aggressive mode.
    ## 2006-12-04 20:56:13 : IKE<172.19.51.188> cannot convert proposals from peer (IKE Gate)

Configuration Example:

  1. Create custom p1 proposal pre-g5-3des-md5
  2. Create custom p2 proposal g5-esp-3des-md5
  3. Create Ike gateway, select two proposals from the drop menus: pre-g2-3des-md5 and pre-g5-3des-md5
  4. Create an AutokeyIKE VPN, using PFS, and selecting from the drop menus: g2-esp-3des-md5 and g5-esp-3des-md5, after clicking Return and OK you will received the following error message:
"Mismatched DH group in the Phase 2 Proposals. The DH group must be identical in all proposals of a VPN Configuration. Please try again."
Solution:
When configuring either Phase 1 or Phase 2, the same DH group must be selected:

  • Due to protocol limitations, multiple proposals with differing Diffie-Hellman groups in IKE Phase 2 is not supported.   The error message is generated as expected.

  • When configuring multiple proposals for Phase 1, you have to use the same Diffie-Helman group in all proposals.  Using different Diffie-Helman groups will result in a failure in the IKE negotiation.

    For example, assume the peer is configured for pre-g2-des-sha.  The following configuration will fail:
    pre-g1-des-sha
    pre-g2-des-sha


    However, the following configuration will succeed:
    pre-g2-des-md5
    pre-g2-des-sha
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