Scenario description

You might have an old existing VPN with some partner which to a gateway that is not under your control. That peer gateway used to be also a CheckPoint device but get’s exchanged for some other 3rd party vendor firewall with out prior notification. When the peer gateway gets changed, the key exchange seems to work, but connection’s fail and nothing seems to be showing up in SmartView tracker although connection logging is on.

Finding out what is happening and trying to fix it

After seeing no traffic in SmartView tracker, you log in to the gateway and might look at tcpdump to see if there is actually any traffic coming from the peer gateway. After seeing there is actual traffic coming you should try a few kernel level debug commands on VPN to get some info on whats happening. For example run “fw ctl zdebug -m VPN all” and you might see something like this in the output:

 [cpu_1];[fw4_0];fw_log_drop_conn: Packet <dir 1, 192.168.200.100:0 -> 172.32.200.100:0 IPP 50>, dropped by do_inbound, Reason: decryption failed

Well this still isn’t saying too much, but at least is giving a hint. Next lets take a look at the secure XL debug, maybe it gives some more info. To get the output prepare the debug first by using the following commands:

[Expert@HostName:0]# fw ctl debug 0
[Expert@HostName:0]# fw ctl debug -buf 32000
[Expert@HostName:0]# sim dbg -m vpn all
[Expert@HostName:0]# sim dbg -m mgr + vpn

Now start the debug:

fw ctl kdebug -T -f > /tmp/debug.txt

Let it run for a bit and close it with ctrl+c. Go through the log file, you might find something like this:

;11Jul2017 13:14:57.017362;[cpu_1];[SIM-xxxx]vpn_ipsec_decrypt: Packet does not match block size (xx xx xx);

;11Jul2017 13:14:57.017362;[cpu_1];[SIM-xxxx]vpn_decrypt: IPSEC decryption failed;

Well this is odd. You contact the peer gateway’s technical contact asking if have they changed anything lately and find out that they changed their firewalls..

Fixing the issue

To remedy the issue create a new gateway object that is of the “Interoperable Device” type and has all the same settings/parameters as the previous the peer gateway object that was with the “Check Point gateway” type. Change the peer GW object in the VPN community to the newly created “Interoperable device” and install policy. And the traffic starts flowing again.

As it seems Check Point is handling VPN tunnels differently between Check Point devices and Interoperable devices.