Pfsense blocks packets with ip header field id set to 0

[hong10]

Hi all

I'm currently having following problem:
We have two networks which are connected via IPSEC VPN (both sides use pfsense).
Everything works as expected, except one application.

The application (running on win2003) tries to get data from a cash box. They use a UDP
based service, port 6.

When I investigated the issue, i noticed that the packets from the server gets through the
VPN. The cash box also answers (I can see the incoming packets on the lan interface
of pfsense) but are not forwarded to the VPN tunnel.

Wenn ich checked the packets in detail, I noticed that the UDP packets from the cash box
set the ID field in the IP header to 0, which is very unusual.

So here is the question:
Is pfsense checking the ID header and drops the packet? and if yes, is there a way to disable
that?

I know that pfsense can replace id fields with random values, however it does not have any
effects here.

Any hints?

Thank you