[PATCH v2 1/3] net: Get pxe config file from dhcp option 209

[Heinrich Schuchardt]

Am 9. November 2023 14:04:40 GMT-07:00 schrieb Tom Rini <trini at konsulko.com>:
>On Wed, Nov 08, 2023 at 12:24:24PM +0000, Peter Robinson wrote:
>> > > > > > > Allow dhcp server pass pxe config file full path by using option 209
>> > > > > > >
>> > > > > > > Signed-off-by: Sean Edmond <seanedmond at microsoft.com>
>> > > > > > > ---
>> > > > > > >   cmd/Kconfig |  4 ++++
>> > > > > > >   cmd/pxe.c   | 10 ++++++++++
>> > > > > > >   net/bootp.c | 21 +++++++++++++++++++++
>> > > > > > >   3 files changed, 35 insertions(+)
>> > > > > > >
>> > > > > > > diff --git a/cmd/Kconfig b/cmd/Kconfig
>> > > > > > > index 5bc0a92d57..adbb1a6187 100644
>> > > > > > > --- a/cmd/Kconfig
>> > > > > > > +++ b/cmd/Kconfig
>> > > > > > > @@ -1826,6 +1826,10 @@ config BOOTP_PXE_CLIENTARCH
>> > > > > > >       default 0x15 if ARM
>> > > > > > >       default 0x0 if X86
>> > > > > > >
>> > > > > > > +config BOOTP_PXE_DHCP_OPTION
>> > > > > > > +    bool "Request & store 'pxe_configfile' from BOOTP/DHCP server"
>> > > > > > > +    depends on BOOTP_PXE
>> > > > > >
>> > > > > > Why should this be disabled by default?
>> > > > > >
>> > > > > > Do we really want a separate config variable?
>> > > > > >
>> > > > > I expect most won't use this option to get the file path (they'll use
>> > > > > the default paths as per the PXE specification).  It makes more sense
>> > > > > for me to keep it optional, like many of the other options?
>> > > >
>> > > > RFC 5701 seems to require this option. Hence we should make it default
>> > > > yes. Boards that have a build size issue can opt out.
>> > >
>> > > The PXELINUX specification
>> > > (https://wiki.syslinux.org/wiki/index.php?title=PXELINUX) doesn't state that
>> > > option 209 is required. In the abense of option 209, PXELINUX will try the
>> > > following default configuration files (this example is provided on the wiki
>> > > link):
>> > >  /mybootdir/pxelinux.cfg/b8945908-d6a6-41a9-611d-74a6ab80b83d
>> > >  /mybootdir/pxelinux.cfg/01-88-99-aa-bb-cc-dd
>> > >  /mybootdir/pxelinux.cfg/C0A8025B
>> > >  /mybootdir/pxelinux.cfg/C0A8025
>> > >  /mybootdir/pxelinux.cfg/C0A802
>> > >  /mybootdir/pxelinux.cfg/C0A80
>> > >  /mybootdir/pxelinux.cfg/C0A8
>> > >  /mybootdir/pxelinux.cfg/C0A
>> > >  /mybootdir/pxelinux.cfg/C0
>> > >  /mybootdir/pxelinux.cfg/C
>> > >  /mybootdir/pxelinux.cfg/default
>> > >
>> > > If 209 is requested/provided it tries the provided "config file" before the
>> > > default paths.  RFC 5071 should be seen as an extension for PXELINUX (not a
>> > > requirement).  RFC 5071 only states "The Config File Option MUST be supplied
>> > > by the DHCP server if it appears on the Parameter Request List".
>> >
>> > We also want to be careful about overall size growth on common options,
>> > even if someone can opt-out. Those are usually last-ditch stop-gaps and
>> > it's better to make sure we really need functionality X/Y/Z by default,
>> > for everyone. Especially with all of the work going on to make HTTP(s)
>> > based network installs a viable option, how much more do we want to
>> > change the PXE case, for everyone. I'm personally somewhat looking for
>> > the use case to be well defined for a "this must be default". Network
>> > provisioning is a thing, yes, but for whom/what, and in turn how broadly
>> > do we need this to be in the vast majority of our boards (since it would
>> > come in via BOOT*DEFAULTS or DISTRO_DEFAULTS).
>> 
>> If PXE is being enabled I think it's useful in that it means the PXE
>> client can be largely stateless because it can all be provided by
>> central netowork configs as opposed to either hard wiring it into
>> firmware or someone having to manually set them. This is likely also
>> useful from the PoV less reasons to have a shell if it can be dealt
>> with in code.
>> 
>> From the PoV of defaults, with or without HTTP boot, I suspect it
>> probably makes sense to review whether PXE as a whole is enabled by
>> default. That said a number of silicon vendors are actually actively
>> removing PXE from their reference FW in favour of HTTP Boot due to
>> security and a number of other reasons (easier for firewalls,
>> caching/CDN, edge use cases outside of the data centre).
>
>I guess I don't understand how frequent a use case PXE boot is, on the
>types of platforms U-Boot is usually found on. Clearly, some people and
>usecases use it. But I also think part of why it's default enabled
>within the "distro" frameworks is because initially we didn't have
>"parse extlinux.conf" entirely split from "do pxeboot" as the conf file
>parsing started there. I guess just something to evaluate down the road,
>and perhaps make it a regular thing to evaluate defaults and provide
>some notice when we might disable features.
>

For security reason it is preferable to disable PXE boot if it is not really needed. So default no would make sense.

Best regards

Heinrich