From DD-WRT Wiki

How to backup your CFE

So you're afraid you will ever need to use a backup of your CFE in case you brick your router sooner or later.

If you own a Linksys WRT54G/GL/GS and your CFE is already screwed, you could try the Skynet repairkit to make yourself a new CFE.

There are two methods to backup the CFE, as follows:

by telnet/SSH

just enter the following command for all, except micro builds:

dd if=/dev/mtd/0 of=/tmp/cfe.bin

Then get the files with e.g. WinSCP from the /tmp directory.

On micro builds it's little more tricky:

cat /dev/mtd/0 > /tmp/cfe.jpg

Then point your browser to http://routerip/user/cfe.jpg and save as cfe.bin

You could do the same for mtd/1 (kernel, firmware), mtd/2 (jffs part if any) and mtd/3 (nvram)

by JTAG

this assumes you already have a JTAG cable ready to use on your router, use the following command:

wrt54g -backup:cfe