Older versions of DD-WRT didn't provide a firmware file for the WL-300g, but only for the WL-500gx. So I had to patch it for the WL-300g to accept.
The router ID was found quick: In the
0x356fc4, there's the string
57 4c 35 30 30 67 78
WL500gx. After changing it to
WL300g (with a space after the "g" to fill up the digit), the router didn't reject the
firmware directly, but initiated the update, then rebooted quickly after that and showed the old firmware again.
According to the DD-WRT manual for the WL-500g, the router should have a recovery mode which you can enable by holding the Reset button while powering on the device. But seems this mode wasn't there in my router.
So there has to be some sort of checksum which tells the router whether the firmware file is corrupt or not. In the DD-WRT forums I read somewhere, that there are only some informational bytes different between a normal DD-WRT binary firmware and the ASUS TRX format - maybe the ID-string at the end.
Therefore I compared the two files
dd-wrt.v23_generic.bin and voilà: Besides the few bytes at the end of
the file, there were 4 additional bytes different at position
0x08. This HAD to be the checksum. The remaining question was, from
which bytes it was calculated. Since the seemingly compressed data began at
0x28, I tried from there to the end and used
Hex-Workshop to calculate different checksums, but nothing matched what was in these 4 bytes. The next
try was from
0x10 but also no match. Then I tried starting directly after the checksum in the header, from
0x0c and bang:
The CRC32 checksum matched that in the header.
So I calculated the CRC32 of my modified file and changed the header - and now I could upload the DD-WRT file using the normal firmware upgrade of the original firmware. After installing v23, you can easily update to v24 using the DD-WRT web interface.