From http://sirlagz.net/2012/01/29/how-to-pxe-boot-systemrescuecd/, changed to use HTTP as that is faster than TFTP.
We need the ISO file that contains the files we will need to PXE boot SystemRescueCD. Once we have the ISO file, we need 4 files off the ISO file, which are:
Once we have these 4 files, we need to place them onto our PXE server, put the sysrcd.* on the webserver. I have used the directory /tftpboot/srcd for my files on the server.
We will need to edit the file that contains the configuration for the PXE server. If you have been following my PXE Server guide then the file is called default in the pxelinux.cfg directory. Using your favourite text editor, add the following lines to the configuration file:
serial 0 115200 default sr prompt 0 timeout 0 label sr kernel srcd/rescue64 append initrd=srcd/initram.igz netboot=http://18.104.22.168/srcd/sysrcd.dat console=ttyS0,115200n8
The last two lines will need customisation to your setup if you use a different kernel, or the IP address of your PXE server is different, as will the path for the kernel, initram and netboot parameters.
Once the configuration file is updated, you should now be able to boot SystemRescueCD over the network with little difficulty.