DSL uses a protocol called PPPoE, Point to Point Protocol over Ethernet, similar in some cases to dial-up (PPP). There is some support for it under Linux.
Red Hat supports it out of the box. However, I found it, in version 7.3, a bit quirky. (It would stop working sometimes for me. I dunno if this was just for me.) I hear that Mandrake 9 has support for it too. I have DSL and, before getting a smal switch/router to do the dial-in for me, I used a third party software: http://www.roaringpenguin.com/pppoe/
Also, the 2.4.X kernel (since version 2.4.9 I believe) supports kernel-mode.
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