Tunnel does not establish

First check the service status at Status > Services. If the IPsec service is stopped, double check that it is enabled at

VPN > IPsec. Also, if using mobile clients, ensure that on the Mobile clients tab, the enable box is also checked.

If the service is running, check the firewall logs (Status > System Logs, Firewall tab) to see if the connection is being blocked, and if so, add a rule to allow the blocked traffic. Rules are normally added automatically for IPsec, but that feature can be disabled.

The single most common cause of failed IPsec tunnel connections is a configuration mismatch. Often it is something small, such as a DH group set to 1 on side A and 2 on side B, or perhaps a subnet mask of /24 on one side and /32  on the other. Some routers (Linksys, for one) also like to hide certain options behind “Advanced” buttons or make assumptions. A lot of trial and error may be involved, and a lot of log reading, but ensuring that both sides match precisely will help the most.

Depending on the Internet connections on either end of the tunnel, it is also possible that a router involved on one side or the other does not properly handle IPsec traffic. This is a larger concern with mobile clients, and networks where NAT is involved outside of the actual IPsec endpoints. The problems are generally with the ESP protocol and problems with it being blocked or mishandled along the way. NAT Traversal (NAT- T) encapsulates ESP in UDP port 4500 traffic to work around these issues.