The program was launched on Ubuntu 14.04, but refuses to work on Ubuntu 16.04, what should I do?

Question

ubuntu

In general, this is how I have a program that worked fine on Ubuntu 14.04, but the same program refuses to work on Ubuntu 16.04. This program is installed on the mirror server and is engaged in IP spoofing in packets. That is, there is a server, a user connects to it, and in outgoing packets the IP is changed to the user's ip and then sent to the game server, that is, a packet arrives at the game server with the client address, and not the address of the mirror server.
So the problem is that while Ubuntu 14.04 was installed on the mirror server, everything was fine, but with the installation of 16.04, packets do not reach the destination server. I suspect that the problem is in the library. But where to look?

Answer 1

In the program logs, of course (it is possible in the system /var/log/syslog), enable in the parameters, detailing to the maximum.

Answer 2

That is, packages enter the program, but do not exit it?
The first step is to recompile the program. If it doesn't help, turn on debugging for the program, if it exists, look in the logs - where does it send what it sends and whether it swears. And if it didn't help - look at the source code of the program - how and where and with what it sends packets.

Answer 3

If you installed 16.04 completely, then you simply forgot that in 14.04 you also configured iptables with some rules, or maybe even trite ipv4 packet forwarding was not included.