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

Fri Aug 21 11:15:22 UTC 2020

@Michael
I stand corrected on option 66. I have confused myself with "next 
server" comment in EFI code.

Bugzilla 2876
----------------
The more I get into the problem with a fresh mind and a cup of tea, the 
more I realize that it is indeed a problem in reporters configuration.
Reporter stated:
- Failing scenario: tag 6 == 0x7
- Working scenario: tag 6 == 0x3

In the failing scenario, user sets bit 2 (so PXE_BOOT_SERVERS are being 
used) and PXE_BOOT_SERVERS contains ProxyDHCP IP instead of TFTP server.
It should work fine if ProxyDHCP and TFTP sits on the same machine (with 
the same IP).
Case closed, patch to be dropped, BZ to be rejected.

Philosophy / PXE Forced Mode
----------------
All I could find about 'PXE Forced Mode' is some non-spec 
tooling/documentation.
https://knowledge.broadcom.com/external/article?legacyId=HOWTO7071
https://knowledge.broadcom.com/external/article/180499/pxe-forced-mode-utility.html

PDF from the first link's page sets up tag 6 to value 11 (bit 3, 1, 0 
are set) and setting up tag 8 with some values.
Now I am sharing the shudder. Why is PXE_BOOT_SERVERS being used when 
tag 6, bit 3 is set?
All I can blame is PXE spec being very vague on the matter or some kind 
of ancient knowledge that I am not aware of.

Thanks,
Maciej.

On 21-Aug-20 11:11, Laszlo Ersek wrote:
> On 08/20/20 15:41, Michael Brown wrote:
>> 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.
> *shudder*
>
> I'll 100% defer to you and Maciej on this -- this is very complicated.
>
> To begin with, I'm not fully clear what the purpose of edk2 git commit
> ecec42044078 ("Update PXE driver to support PXE forced mode.",
> 2014-01-06) was.
>
> What on Earth is "PXE forced mode"?
>
> Siyuan, can you please explain?
>
> And then I don't know whether the bug report at
>
>    https://bugzilla.tianocore.org/show_bug.cgi?id=2876
>
> really has merit.
>
> In the words of the reporter, the presently discussed patch would still
> qualify as a "work-around", for making the PXE client ignore
> PXE_BOOT_SERVERS, via clearing option#43 tag#6 bit#2 in the DHCP server
> response. But IMO the more important question is whether it is valid for
> the DHCP server (config) at their site to (a) populate PXE_BOOT_SERVERS,
> (b) put (apparently!) the ProxyDHCP IP address in PXE_BOOT_SERVERS.
>
> Like, I'd like to be convinced that the server config at the reporter's
> site is not *invalid* in the first place. If it's invalid, then we
> shouldn't be complicating the edk2 client code with a workaround. Even
> if we adopted the workaround, the reporter would still have to
> *activate* it, by manually clearing the bit in question (see at the very
> end of <https://bugzilla.tianocore.org/show_bug.cgi?id=2876#c4>).
>
> For me one big difficulty is that the PXE config options are scattered
> about a forest of specs. Last time I spent more than an hour cursing and
> hunting for them.
>
> At Red Hat, over the last few years I've received an immense amount of
> bug reports related to PXEv4/PXEv6 booting with edk2. In almost every
> case, it was a bug in the reporter's server configuration. Yes,
> anecdotal evidence. It makes me very reluctant to change the edk2 code,
> especially that the reporter of TianoCore#2876 has seemingly stopped
> communications.
>
> Note how the bug report makes references to various attachments, such as
> RAR files and one "Serva32.exe", regarding a reproducer. But until now,
> with the latest comment being #9, those files have *not* been attached.
> So it's not like we can set up some virtual machines on a virtual
> network and fire up wireshark or tcpdump, to see the actual traffic.
>
> I'm happy to pull out of this review session, as I trust you Michael and
> Maciej to do the right here. I'm happy to offer some level of regression
> testing, if you got new patches. I'd also be OK to simply close
> TianoCore#2876 as INVALID (due to insufficient data).
>
> Thanks
> Laszlo
>

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