The Linksys GPL source code at ftp://ftp.linksys.com/opensourcecode/wrp400/

The source code does not build on Ubuntu, but does build on CentOS 5.

It builds a somewhat-bootable "cybertan_rom_bin" image, but that built image is not the same as the image in the firmware upgrade binary.

It also builds the u-boot binary, including the secret console-locking code.

What seems to be missing is means to build the "cybertan_half_bin" image, and the tool to turn those images into a flashable binary (with the "WRP400 FiRmWaRe" header).

From examination of the source code, it seems that the WRP400 is just a rebranded version of the CyberTan WGV414 (http://www.cybertan.com.tw/Products/WGV414.htm).

The u-boot configuration in the source code is "cywgv414dls".