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f7fe2b319e
For a UEFI HTTP boot, we set the current working URI based on the
loaded image device path. The autoexec.ipxe script will be fetched
from the same directory as the iPXE binary itself.
For a BIOS or UEFI PXE boot, we do not explicitly set a current
working URI, but rely on the fact that registering the cached DHCP
settings block will cause the TFTP code to set the current working URI
to "tftp://${next-server}/". The autoexec.ipxe script will therefore
be fetched from the default directory (which is most probably the root
directory) of the TFTP server.
When using a UEFI shim, the shim will always fetch iPXE from the same
directory as the shim itself. This leads to a somewhat unintuitive
requirement for a UEFI PXE boot: the shim and iPXE must be placed in
the same directory, but the corresponding autoexec.ipxe script must be
placed in the root directory.
As with the loaded image device path for a UEFI HTTP boot, the
existence of a cached DHCP packet gives us a way to construct the URI
of our own binary. We can therefore choose to use this to set the
current working URI, so that the autoexec.ipxe script may be placed in
the same directory as the iPXE binary itself. This is the least
surprising location, and avoids the need for lengthy explanations in
documentation.
Choose to set the current working URI at the point that the cached
DHCP packet is recorded, rather than the point at which it is applied
and registered as a settings block. This avoids some awkward corner
cases (such as failing to find a matching network device for the
DHCPACK), and naturally ensures that we retrieve the next-server
address and filename from the same DHCP packet. We rely on the order
in which cached DHCP packets are recorded to impose a priority
ordering: later packets (e.g. PxeBSACK) will override earlier ones.
To avoid breaking existing setups that do place the autoexec.ipxe
script in the root directory, we modify the fetching logic to first
attempt to retrieve autoexec.ipxe from the current working URI, then
from the root directory of that URI.
As with commit a69afd7 ("[tftp] Use TFTP server URI only if no other
working URI is set"), this is technically a breaking change in
behaviour, but the new behaviour is almost certainly less surprising
than the existing behaviour. Scripts that rely on the current working
URI being set to the root of the TFTP server can use absolute URIs
(i.e. add an initial slash): this is more explicit and will work on
iPXE builds both before and after this change.
Signed-off-by: Michael Brown <mcb30@ipxe.org>