Adding a serial port to your Linksys WRT54G, WRT54GS, WRT54GL and probably a wide range more is really really easy. The only thing we need is a serial to ttl converter. I personally followed the guide right here: http://www.rwhitby.net/projects/wrt54gs – works perfectly, and very well laid out. This http://www.compsys1.com/workbench/On_top_of_the_Bench/Max233_Adapter/max233_adapter.html is where I purchased my MAX232 kit from.
This is the device in it’s entirety. Leave the “Ct/Rt” unconnected, and connect + (+5V), – (GND), Tx and Rx according to the guide. You can then use your favorite serial terminal client to connect using 11500,8N1. You’ll need a null modem cable (serial crossover) to connect. Now it’s been some time since I worked on the Linksys WRT range. The earlier versions allow for a full firmware flash, later versions have seriously crippled hardware and a proprietory OS (VxWorks), and as such as pretty useless to hobbyists. If soldering this tiny board is too much trouble, you can also get hold of a MAX2323CPE chip, which only needs 4 ceramic capacitors to do it’s job.