Hi,

As suggested I tested the modem at bridge mode. There was no problem with port 12000. However, I noticed a few (minor) things:
- I received a new IP address.
- The "line speed" dropped 20% at the first test. At the second test it was back to normal. [Only Registered Users Can See LinksClick Here To Register]
- The "internet light" at the modem is off.
- I could no longer access the web configurator of the modem. I had to reset the modem to the factory settings to switch back to "Router mode".

Again, I forwarded port 12000 to the router and added the above mentioned rule to the firewall:

Packet direction: WAN to LAN
Source IP: any
Destination: <IP address router>
Service: Any(TCP)
Action: Permit

I think adding this rule is more or less the same as bridging all traffic to the router. Correct me if I'm wrong. FYI, there's one more line in the firewall rules:

Packet direction: WAN to LAN
Source IP: any
Destination: <xxx.x.x.x - xxx.x.x.x>
Service: Multicast (IGMP:0)
Action: Permit

This is the #1 line. The line I added is #2. I don't know what #1 is for, but it came with the factory settings after the reset and it was there before.

As some of you mentioned already there's a risk in opening ports. I think all (TCP-) traffic through port 12000 ends up at my Dreambox. I guess that goes for all Dreamboxes, not just mine. Maybe it's unnecessary but it raises concerns with me. Was the risk of opening ports ever discussed? (OK, I could use the search-button).

Since the current configuration is working fine I will keep it as it is, at least for now.

Thanks to all of you.