Patches for BOOTP/DHCP code to support Windows Server DHCP

John Howie john at thehowies.com
Sun Jun 1 02:23:36 UTC 2014


Hi all,

I apologize for the cross posting of this email, but I believe it will be
of interest to people across all three groups. Please feel free to forward
to additional groups if you feel they would benefit.

I have seen a few posts on and off over the years about Windows Server
DHCP not working with FreeBSD. Specifically, the Windows Server DHCP would
not return the boot host name/IP address and the root path. The typical
response to ³why won¹t it work" has typically been that there is a flaw in
Windows Server DHCP code. I did a little digging and found that the
problem actually lies in code in FreeBSD.

Section 3.5 of RFC 2131 (the DHCP RFC) states that ³...Second, in its
initial DHCPDISCOVER or DHCPREQUEST message, a client may provide the
server with a list of specific parameters the client is interested inŠ²
and ³...The client can inform the server which configuration parameters
the client is interested in by including the 'parameter request list'
option.  The data portion of this option explicitly lists the options
requested by tag numberŠ². A DHCP Server is not required to return any
parameter that a client does not ask for. It appears that the ISC-DHCP
server, which is recommended by most, will return configured options
regardless of whether or not the client asks for them.

There are two places in the FreeBSD codebase that DHCP is used to boot the
system over a network. The first is in the boot loader, which uses code in
lib/libstand. The second is in the NFS code, in sys/nfs. The code is
sys/nfs is not always used if the boot loader sets up the environment for
the NFS code, either by passing parameters to the kernel (as PXEBOOT
appears to do), or information is configured in the boot loader
configuration files, e.g. /boot/loader.rc.

I have attached two unified diff files which I ask people to test, before
I submit them for inclusion into the codebase as fixes. The first,
bootp.c.diff fixes the code in lib/libstand/bootp.c to request the boot
host (option 12, aka TAG_HOSTNAME) and the NFS root path (option 17, aka
TAG_ROOTPATH). This fix has been tested with PXEBOOT on an amd64 box and
ubldr on an ARM/RaspberryPI system. The second, bootp_subr.c.diff, fixes
code in sys/nfs/bootp_subr.c to request the same options and also to fix
bugs that erroneously reported the IP address of the BOOTP/DHCP server.
The code assumed that the BOOTP/DHCP server was also the boot host. Please
send me all feedback directly.

The diff files work with 10.0-RELEASE through 10.0-RELEASE-p3, but will
likely work with 9.0 and also CURRENT and STABLE, including 11.0, as the
code is old code that does not appear to have changed in a  while. If you
want to try it on those systems please, please make sure you have backup
copies just in case.

If you do not have experience configuring Windows Server DHCP just drop me
an email, and I will send you a cheat sheet to get you up and running.

I am going to grab the latest ubldr code to see if I can get it to work
more like PXEBOOT, that appears to pass parameters to the kernel to avoid
the need for the NFS BOOTP/DHCP process. If you test on an ARM system with
ubldr in RELEASE you will see a lot of unnecessary network activity going
on, that we should be able to fix.

Regards,

John

john at thehowies.com (personal) | jhowie at email.arizona.edu (academic) |
j.howie at napier.ac.uk (academic) | jhowie at cloudsecurityalliance.org (work)



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