link to ipsec debugging in redis playbook

service/donate.md

@@ -406,7 +406,8 @@ the tunnel:
     ipsec up civicrm::crm-int-01
 
 If still unsuccessful, check the output from that command, or logs from
-strongswan.
+strongSwan. See also the [IPsec documentation](howto/ipsec) for more
+troubleshooting tricks.
 
 If the tunnel is up, you can check that you can reach the service from the
 frontend server: