I stumbled upon this same problem. I boot netboot's linux+initrd.gz from a harddisk through grub (fb=false modules=ppp-udeb DEBCONF_PRIORITY=low) but it seems impossible to establish a PPPoE connection.
$ zcat initrd.gz | cpio -t | grep -i ppp 32799 blocks -- To UNSUBSCRIBE, email to debian-bugs-dist-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org
