[gPXE-devel] environment variable expansion in 'filename'?

[Miller, Shao]

I have been watching Joshua Oreman's presentation at
http://vimeo.com/11948216 and was reminded of something...

His use-cached setting helps a PXE -> gPXE to avoid a second DHCP
transaction, by re-using the parameters obtained by the initial PXE.  In
this instance, the filename option will uselessly remain the name of the
gPXE program itself.  This is why any scenario making use of the
use-cached setting will most likely have an embedded script (such as
gpxelinux.0 does, with Syslinux).  Thus use of use-cached seems to imply
a one-to-one relationship between chained-to gPXE binaries and the files
they will boot.

So if you'd rather have a single instance of the gPXE program, maintain
the ability to boot a DHCP-specified filename, and avoid a second DHCP
transaction, some additional data (another filename) must be provided.
Since this additional filename would be specific to gPXE, it might make
sense to implement as a gPXE encapsulated option, and might provide a
fine opportunity to allow for extended rules (such as allowing variable
expansion) to apply.

As another note, I don't think re-purposing DHCP option 209 would work,
after all, since PXELINUX itself would attempt to fetch using the
literal ${xxx} sequence inside any URIs, which would not work.  Only if
gPXE's file-fetching performed variable expansion might this work, but
that seems like a lot of work to consider for which URIs expansion
should be performed, etc.

- Shao Miller