have to run pppoeconf every time

Hello, my PC is a common one, with 8139 netcard, and without any special hardware.
I have only one PC, and connect with internet through adsl. directly, without router or hub.

I installed Knoppix5.1.1 as Debian in my hardware, and I found this is a good distro except one bore.
I have to configure my pppoe by typing "pppoeconf" in konsole every time after booting my knoppix, or internet cannot be connected. I noticed that one line of boot scrolling is "Failed to bring up eth0"

On my other partitions, I installed Simplymepis and Dreamlinux. With this systems, I needn't configure pppoe every time. Internet conncetion is always ready after booting.

what is the matter? What can i do with it?

Knoppix is well known to break after a hard disk install, and problems with networking may be the most common symptom. I have been seeing this for over for years now. You didn't say, but if it worked fine from CD then you have already demonstrated this. I urge you to try a distro like Debian that does not have this and other problems, but offers all of the software and features that Knoppix does.

While Knoppix has pppoe software to run your dsl connection without a router, I urge you to get one and operate safely (I'm assuming your dsl modem does not have one built in, you imply it does not). Even with only one computer you are much better off behind a NAT router, particularly if you ever run Windows.