Defining the BOOTFILE.

BOOTFILE=bootimage.bin

A string that typically defines the boot file which is fetched from a TFTP server by the client.

The string is passed on to the client unchanged, the DHCP Server does not interpret it. Which name the client can make sense of therefore depends entirely on the client. A plain file name is what a PXE ROM expects, and it is resolved against the root directory of the TFTP server. Clients that are able to load their boot file over HTTP, such as iPXE or UEFI HTTP boot, accept a complete URL instead:

BOOTFILE=http://192.168.0.2/autoexec.ipxe

A regular PXE ROM cannot do this, so a URL should only be sent to clients that are known to support it. The usual way to make sure of that is a scope section that is selected by IPSCOPE_n, see the PXE boot FAQ entry for a complete example.

The boot file name is limited to 128 characters.