Re: [edk2-devel] [PATCH v1] NetworkPkg/UefiPxeBcDxe: Fix PXE_BOOT_SERVERS usage in boot info parse flow

["Michael Brown" <mcb30@ipxe.org>]

On 20/08/2020 11:44, Maciej Rabeda wrote:
> @Michael
> I am now wondering whether bit 3 is actually relevant to server choice.
> 
> Bit 3:
> == 0 -> prompt user to choose a boot file. Which means to me: show 
> minimal menu with prompt (tag 10 - PXE_MENU_PROMPT) and options (tag 9 - 
> PXE_BOOT_MENU).
> == 1 -> do not prompt user. If boot file name is present (option 67), 
> download that boot file.
> 
> Bit 3 does not seem to specify/regulate which server to use.
> 
> Choice of server IP might look like:
> 
> if (option 43 is present, tag 6 is present, tag_6.bit_2 is set and tag 8 
> is present and valid)
>          take server IP from tag 8 (PXE_BOOT_SERVERS)
> 
> else if (option 66 is present)
>          take server IP from option 66 (TFTP server name)
> 
> else if (option 54 is present)
>          take server IP from option 54 (Server Identifier)
> 
> else
>          failure

RFC 2132 defines option 66 as a hostname (not an IP address): it is the 
equivalent of the non-option "sname" field.

RFC 2132 defines option 54 as the DHCP server identifier, which is 
unrelated to the TFTP server.

In the simple case (with no PXE menus involved), the TFTP server IP is 
provided by the non-option "siaddr" field.

If option 60 is set to "PXEClient" and option 43 tag 9 is present and 
option 43 tag 6 bit 3 is clear then this initiates a convoluted process 
in which the user is first presented with an interactive menu 
(constructed from the contents of option 43 tag 9) in order to select a 
"boot server type", after which a second convoluted process is performed 
to query the network using a protocol that is almost, but not quite, 
entirely unlike DHCP.  The TFTP server IP and boot filename are 
eventually taken from the selected response packet in this final 
almost-DHCP exchange.

Michael