[ipxe-devel] DHCP/ProxyDHCP settings ordering bug

[Alex Williamson]

On Fri, 2013-06-07 at 13:28 +0100, Michael Brown wrote:
> On 06/06/13 23:37, Alex Williamson wrote:
> > When iPXE tries to boot in the above environment, it tries to load
> > tftp://10.0.0.1/pxelinux.0 as the boot file.  The next-server comes from
> > server1's DHCP packet and filename comes from server2's.  Clearly this
> > is wrong.
> >
> > From what I can gather, this is a result of neither packet having a
> > priority and the ordering of adding the settings from the DHCP packet
> > before the ProxyDHCP packet.  list_add_tail bites us.  DHCP packets
> > don't support reallocation, so I can't add an artificial priority to get
> > them in the correct order.  Registering the settings from the ProxyDHCP
> > packet before the DHCP packet does work, but it's ugly and prone to
> > being broken by changes to the list manipulation code in settings.
> > Suggestions?
> 
> You could work around it by adding an artifical priority to "net0".  The 
> DHCP settings will show up as "net0.dhcp" while the ProxyDHCP settings 
> will show up as just "proxydhcp" (outside the scope of "net0"); setting 
> "net0/priority" will therefore affect "net0.dhcp" but not "proxydhcp". 
> This is an ugly hack and I wouldn't suggest using it, but it can be done.

We'd actually want to de-prioritize net0 though, which I'm not sure we
can do since priority is a uint8_t.

> As for a proper fix: it's difficult to reconcile the conflicting 
> requirements of the PXE spec (which conflicts with itself) and iPXE's 
> settings model (which incorporates multiple sources of settings not 
> envisioned by the PXE spec).  The best solution I can think of off-hand 
> would be for fetch_next_server_and_filename() to ensure that both 
> next-server and filename come from the same settings block.
> 
> This could be done by using fetch_setting_origin() to determine the 
> settings block containing the filename and then using that as the 
> settings block when fetching next-server.  (As in your example, some 
> DHCP servers will provide a spurious next-server defaulting to the DHCP 
> server's own IP address; it therefore seems sensible to treat the 
> presence of a filename as indicating the intended presence of a 
> next-server+filename pair.)
> 
> Thoughts?

I like it.  It seems like a good idea in general that next-server should
come from the same settings block as filename.  The PXE spec doesn't
specify which to use if both dhcp and proxydhcp packets specify a
filename, so I guess it's just an implementation detail that iPXE would
use net0.dhcp in that case.  Thanks,

Alex