SSH not working over IPSec tunnel (Strongswan)

Here’s a step-by-step guide to help you troubleshoot and fix the issue of SSH not working over an IPSec tunnel using Strongswan:

1. Verify IPSec tunnel configuration:
   • Check the configuration files on both the client and server for Strongswan (e.g., /etc/ipsec.conf, /etc/ipsec.secrets). Ensure that the tunnel settings, including encryption algorithms, authentication methods, and IPSec policies, match on both ends.
   • Use the ipsec statusall command on both the client and server to verify that the IPSec tunnel is established and active. Check for any errors or discrepancies in the output.
2. Verify network connectivity:
   • Ping the server from the client and vice versa to confirm basic network connectivity over the IPSec tunnel. If ICMP traffic is blocked, you may need to adjust firewall settings or enable ICMP specifically for troubleshooting purposes.
3. Check firewall settings:
   • Ensure that the necessary ports for SSH (typically port 22) are allowed through the firewall on both the client and server. Check any firewall rules or security groups that could be blocking SSH traffic over the IPSec tunnel.
4. Review SSH configuration:
   • Check the SSH server configuration file (/etc/ssh/sshd_config) on the server. Ensure that SSH is listening on the correct network interface and port, and that any specific settings (e.g., AllowUsers, PermitRootLogin) are correctly configured.
   • Validate SSH client configuration on the client side, ensuring that the correct SSH key and user are specified.
5. Verify SSH key-based authentication:
   • Make sure the SSH public key for the SSH user is correctly added to the server’s ~/.ssh/authorized_keys file. Double-check the permissions of the authorized_keys file and the .ssh directory (typically 700 for .ssh and 600 for authorized_keys).
6. Check Strongswan logs:
   • Examine the Strongswan logs for any error messages or warnings that might indicate issues with the IPSec tunnel or traffic. The logs are usually located in /var/log/ directory and can vary based on your system configuration. Look for any specific entries related to the IPSec negotiation or traffic flow.
7. Validate IPSec policies and transforms:
   • Ensure that the IPSec policies and transforms (encryption, authentication, key exchange) match on both the client and server. Check the configuration files and compare the settings. Use the ip xfrm policy and ip xfrm state commands to inspect the IPSec policies and state on both ends.
8. Test with other services:
   • To isolate the issue, try testing other services (e.g., HTTP, FTP) over the IPSec tunnel. This can help determine if the problem is specific to SSH or affects all network traffic.

If you’re unable to resolve the issue using these steps, consider consulting the Strongswan documentation, seeking assistance from the Strongswan community forums, or reaching out to a network administrator or expert with experience in IPSec configurations.