if you are running the old version of the GNU/Linux like Debian Lenny and your USB LAN is of Kontron (Industrial Computer Source / ICS Advent) DM9601 Fast Ethernet Adapter which is not working then here is a quick solution.
1. Execute the command lsusb, if it display something like this :
Bus 001 Device 005: ID 0fe6:8101 Kontron (Industrial Computer Source / ICS Advent) DM9601 Fast Ethernet Adapter then go to step 2 , it means you are actually using the correct usblan for which this solution will work.
2. Do this to add backport repository for Debian Lenny.
3. Update you kernel to 2.6.32 from backport repository.
4.Enjoy after reboot !!