I have an adsl modem acting as a bridge to my router my setup was working well for more than year now ,
but then the internet went slow . I set the modem back to PPPoE the problem was with the DNS server of the ISP so using ifup i set the dns t 8.8.8.8 (google) the connection is back ,
but when i set the modem back as a bridge slow internet
does the router know that my dns is 8.8.8.8 or i should set the dns on the router ?:X
I can’t see an option to set the dns on the router ( i just enter my username and password from the isp to use PPPoE mode )
by the way i called the isp support they kept telling me to restart , i told them i did but i never restarted my pc . I’m sure if i tell them i’m running linux they will blame the os ! 
Your router does not know anything about your DNS server. No need. When the resolver on your system needs the DNS server it will open a connection to 8.8.8.8:53 (in your case). For the router this is the same as opening a connection to 130.57.4.15:80 for getting a page from these Forums.
And yes, I fear that when you tell them they will blame Linux instead of theirt own ignorance. (BTW my ISP won’t do that, but I am a lucky one).
Thanks for explaining how i’m using the dns server
i got it working the dns is set on my pc , their dns is so slow !