配置Site-2-Site VPN隧道后出现报错，寻求有效解决方案

Hey there, sorry to hear your Site-to-Site VPN tunnel is giving you grief even after trying standard fixes—let’s walk through targeted troubleshooting steps to get this resolved.

1. Double-Check Core Configuration Match #

The most common culprit is mismatched parameters between the two endpoints. Verify these line by line:

• IPsec Policy Alignment: Ensure both sides use identical encryption algorithms (e.g., AES-256 vs AES-128), hash functions (SHA-256 vs SHA-1), IKE version (IKEv1 vs IKEv2), and DH group settings. Even one mismatch will block tunnel negotiation.
• Pre-Shared Key/Cert Validity: Confirm the pre-shared key is exactly the same (case-sensitive, no extra spaces) or that both endpoints trust each other’s SSL certificates (no expired certs, correct CA chains).
• Subnet Route Configuration: Make sure each device has a static route pointing the remote subnet to the VPN tunnel interface. For example, if your local subnet is 192.168.1.0/24 and the remote is 10.0.0.0/24, both sides need this route defined.

2. Firewall & NAT Traversal Checks #

IPsec traffic is often blocked by firewalls or mangled by NAT:

• Open Required Ports/Protocols: Ensure firewalls on both ends (and any intermediate network devices) allow:
  • UDP port 500 (IKE initial negotiation)
  • UDP port 4500 (NAT-Traversal, if either endpoint is behind NAT)
  • IP protocol 50 (ESP) and IP protocol 51 (AH)
• NAT Exemption Setup: If either endpoint is behind a NAT device (like a home router), configure "NAT exemption" to exclude IPsec traffic from being translated. This ensures the tunnel packets retain their original source/destination IPs for negotiation.
• NAT-T Enablement: Turn on NAT-Traversal on both VPN endpoints—this is critical when one or both sides are behind NAT.

3. Dig Into Device Logs #

Logs are your best friend here—they’ll tell you exactly why the tunnel is failing:

• Look for IKE negotiation errors like:
  • NO_PROPOSAL_CHOSEN: Directly means your IPsec policy parameters don’t match between endpoints. Go back to step 1 and align all settings.
  • AUTH_FAILED: Indicates invalid pre-shared key or certificate authentication failure. Double-check your credentials/certs.
  • INVALID_ID_INFORMATION: Mismatched ID types (e.g., one side uses IP address as ID, the other uses FQDN).
• For Linux-based VPNs (like strongSwan), run ipsec statusall to get real-time tunnel status and error details. For network appliances, check the system logs under the VPN or security section.

4. Rule Out Compatibility & Version Bugs #

• IKE Version Compatibility: Some older devices only support IKEv1, while newer ones default to IKEv2. If you’re mixing devices, force both to use the same IKE version.
• Firmware/Software Updates: Outdated VPN device firmware or software often has known IPsec bugs. Check the vendor’s support site for the latest stable version and update both endpoints (after backing up configurations).

5. Test & Validate Incrementally #

After adjusting any settings:

• Restart the VPN service on both endpoints (or reboot the devices if needed) to apply changes.
• Manually trigger a tunnel connection if your device requires it.
• Use diagnostic tools like ping from a local host to a remote host across the tunnel, or traceroute (Linux/macOS) / tracert (Windows) to verify traffic is routing through the tunnel.

内容的提问来源于stack exchange，提问作者Salim Ibrohimi