[syslinux] Getting HTTP path-prefix to work with syslinux.efi

[Alexander Perlis]

>> Doesn't work. Apparently the Dell UEFI PXE firmware doesn't know HTTP.
>
> There's a lot of variation. Do you have a shell option
> in your boot selections?

None of Dell OptiPlex 990 (firmware A19), 9010 (firmware A22), nor 9020 
(firmware A16) seem have a built-in EFI shell option, but in all three 
cases I had success running the external EFI shell from 
github.com/tianocore/edk2/tree/master/EdkShellBinPkg (naming it 
/efi/boot/bootx64.efi on a FAT USB stick).


> I'm guessing a lack of TCPv4Sb (an alias for the
> EFI_TCP4_SERVICE_BINDING_PROTOCOL ).

You're onto something. All three systems do report TCPv4Sb identically 
in their "guid" output,

TCPv4Sb : 00720665-67EB-4A99-BAF7-D3C33A1C7CC9

yet in the "dh -p Net" output, only the OptiPlex 9020 lists TCPv4Sb. 
That is also the only one of the three systems for which syslinux.efi is 
indeed able to load HTTP URLs.

What does it mean for TCPv4Sb to be listed under "guid" yet not under 
"dh -p Net"? Curiously, on the OptiPlex 990, the "dh -p Net" output is 
bizarrely essentially empty:


Dell OptiPlex 990 A19 (EFI 2.0 Revision 4.632 American Megatrends):

Shell> dh -p Net
Handle dump by protocol 'Net'



Dell OptiPlex 9010 A22 (EFI 2.31 Revision 4.653 American Megatrends):

Shell> dh -p Net
Handle dump by protocol 'Net'

  326: DevPath (..9,0x0)/MAC(xxxxxxxxxxxx,0x0))Net MNPSb ARPSb DHCPv4Sb 
IPv4Sb IPv4Config UDPv4Sb MTFTPv4Sb HiiConfAccess

  341: DevPath (..HCP,0.0.0.0,0.0.0.0,0.0.0.0))Load Net Pxebc

  349: DevPath (..00:0000:0000:0000:0000:0000))Load Net Pxebc



Dell OptiPlex 9020 A16 (EFI 2.31 Revision 4.653 American Megatrends):

Shell> dh -p Net
Handle dump by protocol 'Net'

  335: DevPath (..9,0x0)/MAC(xxxxxxxxxxxx,0x0))Net MNPSb ARPSb DHCPv4Sb 
TCPv4Sb IPv4Sb IPv4Config UDPv4Sb MTFTPv4Sb HiiConfAccess

  344: DevPath (..00:0000:0000:0000:0000:0000))Load Net Pxebc

  359: DevPath (..HCP,0.0.0.0,0.0.0.0,0.0.0.0))Load Net Pxebc



Thanks,
Alex