I have a small office network (VMs on XEN hypervisor): Domain Controller, Windows Server, Centos 7 server - joined the domain (say, C1); and another Centos 7 server not on the domain (say, C2). Also, half-dozen workstations - all on the domain as well.
I changed the router a few days ago, after which I am getting a weird problem: domain-based computers can't access domain-based Linux box (C1). Only C1. If I port forward router to C1 - external computers can access it from the internet without a problem. Also, I can access Windows boxes and non-domain Linux (C2) without problems.
C2 (non-domain Linux box) can access C1 for a few minutes after C1 reboot, and then it can't (maybe it takes a few minutes for C1 to connect to domain... don't have an explanation).
The behavior doesn't change if I turn off firewall. I can get on C1 from outside, and also from Xen control panel (XenCenter). Everything looks fine on the box itself - but maybe I am not checking something crucial
The router is cheap consumer model - not a whole lot to screw up. But it is the only thing that changed (or was touched) recently.