How to set up access to a local web server?

Question

network hardware

Good afternoon, I've been close to killing which thread of the file in the system for 3 hours already :D.
There is a local network. There is access to it from the outside.
The network itself M is a router, A is machine 1, B is machine 2 is connected via Wi-Fi.
If on machine B I raise a simple web server built into php7, and on the router I redirect port 80 to port 80 of machine B , then everything is OK, everything opens and works from the outside.
But if I raise the same server on machine A that is running on debian, by the way, iptables is empty. There is no access to it from the outside, although the machine pings through the router, the server opens on the local network by ip.
By carB is chromeos and in iptables I registered access to port 80 there. I thought that I should do the same on machine A , despite the fact that the system is bare and there are no rules and everything should be allowed anyway, but nevertheless I registered it and the result is still no connection. I tried to specify other ports and redirected to them, but there is no access to machine A.
I sit and think that there is such a thing on machine B that is not on A . Maybe it's in the route? but if that were the problem then there would be no LAN access to A .
I'm not asking for a solution, I'm asking you to point out where to look and what to play with. Thanks in advance.

Answer 1

I found what the problem is, it turns out that the firewall on the router cut the packets. registered the rule for a local web server and received access. Now I look at iptables on the router and look for a rule for the wifi interface in order to understand which rule passed to the wifi machine. Thanks to all!