I'm trying to reverse engineer a binary running on a zyxel router. My first approach would have been to change the password within the .bin firmware itself, but as seen here, I'm having some trouble extracting part of the files. I hooked the router up using a serial cable and have managed to interupt the booting process. Considering it's running Linux, is there a way to start the router up in some type of single user mode which will imediately fall back to /bin/sh and not require login? 
Reversing a router, single user mode
firmware;networking