I have substituted my Speedport W 921V with an RouterBOARD 750GL as I needed to mask the DNS. The Speedport has an integrated VoIP modem, which is connected to my ISDN phone system. Since I can’t connect the WAN port of the Speedport to RouterBOARD using DHCP - I don’t know what’s wrong with those guys - I was thinking of faking an ISP and providing a PPPoE server, hence this is the only way to establish an internet connection using the Speedport.
So I configured port 5 of my RouterBOARD and set up a PPPoE server. It is possible to connect to it from my Linux and it forwards the DNS well. But when I try to connect from the Speedport I get an error message:
Verbindung fehlgeschlagen (R56-0002 - Es ist ein technischer Fehler aufgetreten).
Connection failed (R56-0002 - technical malfunction).
Okay, there is not a lot information in it 
.
In the log I am afraid is no further information, so I get an DoS attack message, but since the router, speedport, is not directly connected to the internet and should be behind the RouterBOARD firewall.
DoS(Denial of Service) Angriff IP Spoofing wurde entdeckt. (FW101)
DoS(Denial of Service) attack IP Spoofing recognized. (FW101)
My first question is if my plan is possible and if so where is my mistake?
Thanks for your help.
Best wishes