I spent some time trying to figuring this out. I found my core solution, but iam not satisfied, or more likely i dont fully understand the result, which buggs me.
My setting: I have wall plug(port) with attached internal VLAN on which multicast IPTV runs. When i plug this into set-top box, all works fine.
In the middle, iam plugging small lab pc, on which i can capture real time logs from the set-top box for debugging purposes. The issue is -
At first the box did not play anything. Nor archive based streams, nor life streams. I solved this half-way. Using pimd deamon for mcast traffic forwarding, i was able to start playing archive (stored) streams. But real-time does not work. The archive-storage servers and real-time streaming servers are two different devices.
Whats bugging me is WHY and WHAT does the simple machine debian-8 lab PC in the middle doing, that it actually prohibits set-top box, to work like its directly plugged into the wall. I understood the need of forwarding mcast traffic, even tho i was told that pimd is not optimal since its overkill and may cause trouble if i plug the PC to wifi for example (some possibility of joining of somebody on the same wifi and such as its forwarding mcast everywhere). I was not given better solution and thats why iam here.
The PC in the middle is without firewall, no rules, simply put, id like to image it and set it as just simple passive device, which can capture the logs based on the IP on set-top box and does not interfere with communication between box and outer VLAN.
Some solutions hinting me towards setting bridge on lab-pc eth0 and eth1. That did not work well, or i mismanaged the setting.
Other solution is to take passive device like hub which sends traffic everywhere, and simply put everything next to each other and listen from the lab-pc. But i hate it since iam completely sure there must be some elegant SW based solution. - mabye other then to find out all the IPs i need to forward between these two ports, so the set-top box does not feel hes not directly plugged. That sounds more like brute force.
I lack happy-to help or explain people around me and i cant let it go even tho i already got what i needed (logs from archive-streams behavior)
Ill keep looking and asking too :)
Setting of lab-pc:
eth0 - outer world/VLAN - gets IP via dhcp
eth1 - set-top box plugged in - got static IP
set-top box - acquire its own IP on which i can listen to in the logs.