[ipxe-devel] [PATCH] [dhcp] Use random transaction ID to associate messages

[Amos Kong]

----- Original Message -----
> Whole Archive: http://marc.info/?l=kvm&m=131609166918121&w=2
> 
> ----- Original Message -----
> > On Thu, 2011-09-15 at 10:43 -0400, Amos Kong wrote:
> > > ----- Original Message -----
> > > > ----- Original Message -----
> > > > > On Thu, 15 Sep 2011 21:00:38 +0800, Amos Kong
> > > > > <akong at redhat.com>
> > > > > wrote:
> > > > >
> > > > > > + netdev->xid = random();
> > > > >
> > > > > This will not work for reboots. The decision that the
> > > > > hardware
> > > > > address
> > > > > is
> > > > > choosen was not accidental. Not sure if some DHCP server will
> > > > > count
> > > > > on
> > > > > the
> > > > > ID. (RFC 2131 "Retain DHCP client configuration across server
> > > > > reboots,
> > > > > and,
> > > > > whenever possible, a DHCP client should be assigned the same
> > > > > configuration
> > > > > parameters despite restarts of the DHCP mechanism"). If not
> > > > > so I
> > > > > am
> > > > > fine
> > > > > with the patch.
> > >
> > > Hi Hagen,
> > >
> > > rfc2131 clearly describes that we need a random xid,
> > > I don't think xid is a port of "DHCP client configuration",
> > > it only be used to associate messages and responses between
> > > client
> > > and server.
> > >
> > > I would post a patch to ipxe maillist later if it's ok.
> > 
> > rfc2131 only required that "A DHCP client MUST choose 'xid's in
> > such a
> > way as to minimize the chance of using an 'xid' identical to one
> > used
> > by another client.".
> > 
> > The 'random xid' suggestion is listed merely as an example.
> > 
> > The way I see it using a xid based on MAC instead of a random
> > number
> > is
> > safer since the odds for same MAC on the same network are pretty
> > slim
> > since it would cause problems on other layers in the network.
> 
> Users may boot up a QEMU guest without default mac address, it's easy
> to repeat.
> 
> Yaniv, what real problem do you touched? only not in accordance to
> the RFC?

Answer from Yaniv:
Indeed. I saw some people complaining on inability to get a DHCP address, and
from the network capture saw that. We have no idea why the failed most of the
times to get an IP address with PXE boot.