Tag: VPN

CheckPoint to Amazon AWS VPN connection issue

When trying to create a VPN tunnel between a CheckPoint firewall and Amazon managed VPN service I happened upon a unpleasant surprise.

Namely when using stronger crypto methods than defined by default in the guides by CheckPoint or Amazon you will run in to an issue, that the CheckPoint device will start dropping traffic after Phase2 key exchanges for a ~5 minute time period. To be more exact the traffic from Amazon to the hosts/networks behind the CheckPoint GW will start failing and connections started from behind the CheckPoint device will continue working as before. Namely Amazon VPN service refreshes it’s keys 5 minutes before the lifetime set in the VPN properties and CheckPoint close to 30 seconds. It actually wouldn’t be a problem if Amazon would use the same parameters as were used to initially establish the tunnel, but it doesn’t. It will actually use DH group 2 to initiate key exchange after which the CheckPoint device will start dropping the traffic coming in from the Amazon service with the following error:

encryption failure: Packet was decrypted with methods which are different from the methods according to the security policy - Gateway and Peer use different DH groups

After talking to both CheckPoint and Amazon support, I can say that the only thing you can do to remedy this is actually setting the DH group to 2  for PFS.

Although Amazon in its documentation(here) states it supports a bunch of different DH groups, and yet it defaults to DH group 2 when initiating the connection it self. To be honest, to me it seems a bit strange that the AWS VPN actually mirrors the encryption/integrity settings of the previous negotiation, but doesn’t remember the PFS settings and defaults to DH group 2. When talking to support services the only thing that AWS support suggested was to force the CheckPoint device to exchange keys before the AWS service does. Unfortunately you cannot do that according to Check Point support services, as there is no such setting available and that timer is around 30s+- some random number of seconds prior to the end of the life time set in the VPN properties.




Enabling DPD on VPN instead of tunnel_test on R77 gateway

To keep VPN tunnels alive Check Point uses by default it’s proprietary tunnel_test protocol. In order to get it working with 3rd party vendors it isn’t enough to have the partner device set as an “Interoperable device” and set the tunnel keep alive method on your gateway object as DPD. You also need to set the peer gateway’s tunnel keep alive method as DPD, because by default it is still set to tunnel_test.

To change the keep alive methods you need to do the following as described on Check Point’s website here:

  1. In GuiDBedit, go to Network Objects > network_objects > <gateway> > VPN > tunnel_keepalive_method.
  2. For the Value, select a permanent tunnel mode.
  3. Save.
  4. Install policy on the gateways.

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CheckPoint VPN with failed connection not showing up in SmartView Tracker

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, -> 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.

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