Technical Tip: Ensuring Compatibility in IPsec VPN Configuration

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Description	This article describes common configuration issues when configuring a Remote Access VPN with IPSec.
Scope	Dial-up VPN, FortiClient VPN.
Solution	It is crucial to ensure that both Phase 1 and Phase 2 settings align correctly between peers. Mismatches in encryption, authentication, or Diffie-Hellman (DH) groups can prevent the tunnel from establishing successfully. This article outlines potential issues in IPsec VPN settings and best practices to optimize security and compatibility. FortiClient configuration example: Common Issues in IPsec VPN Configuration: Mismatched Encryption Between Phase 1 and Phase 2:  Issue: Phase 1 is configured with DES and 3DES, while Phase 2 is set to AES128 and AES256. Impact: If the peer does not support both DES/3DES and AES, the tunnel will fail to be established. Recommendation: Configure AES128 or AES256 consistently in both Phase 1 and Phase 2 to maintain security and compatibility. Weak Encryption and Authentication in Phase 1: Issue: DES and 3DES are outdated and vulnerable to cryptographic attacks. MD5 is also weak due to susceptibility to hash collision attacks. Impact: Many modern devices reject these algorithms due to security concerns. Recommendation: Use AES128 or AES256 instead of DES/3DES. Replace MD5 with SHA256 or SHA512 for authentication to enhance cryptographic strength. DH Group Mismatch: Issue: Phase 1 allows DH Groups 14 and 20, while Phase 2 uses only DH Group 14. Impact: If the peers cannot agree on a common DH group, the VPN tunnel may not be established. Recommendation: Ensure that both Phase 1 and Phase 2 share at least one DH group. Using DH Group 14 or higher (e.g., 19 or 20) is recommended for stronger key exchanges. Related article: Troubleshooting Tip: Troubleshooting IPsec Site-to-Site Tunnel Connectivity