diskless+pxe notes

[Danny Braniss]

> Hi,
> i finally decided to try and use pxeboot to replace the etherboot
> method I was using so far for diskless setups.
> 
> The goal is to fully share the server's root and /usr directories,
> as documented in diskless(8). I'd like to share the following
> notes, hopefully to go in the manpage.
> 
> 	cheers
> 	luigi
> 
Hi,
	With a slightly modified libstand/bootp.c - a PR was sent
way back, but you can check
	ftp://ftp.cs.huji.ac.il/users/danny/freebsd/diskless-boot
you can control the diskless boot options.
by comenting out kernel= in
	/boot/defaults/loader.conf
you can set in the dhcpd.conf.
since most of the tags received via dhcp are placed in kenv,
the crucial options are there!

BTW, we use diskless servers/workstations for 90% of our hosts, the
exception being:
 - the dhcp/tftp server
 - a 'lagged' server - the router/server get confused :-)
 - our mail servers, there is a bug somewhere, where some critical
   network resources get deadlocked.
 - our developement servers.

the / of the diskless is almost identical to the server, but for
many reasons, I like to keep it appart. The trick to overcome the
read-only problem, is using unionfs for /etc:
in rc.initdiskless:

if [ -e /conf/union ]; then
    kldload unionfs
    mount_md 4096 /.etc
    mount_unionfs -o transparent /.etc /etc
fi

the /conf is nfs mounted  from a central site, the location is passed
via dhcp:

confpath=`kenv conf-path`
if [ -n "$confpath" ] ; then
    if [ "`expr $confpath : '\(.*\):'`" ] ; then
        echo Mounting $confpath on /conf
        mount_nfs $confpath /conf
        chkerr $? "mount_nfs $confpath /conf"
        to_umount="${to_umount} $confpath"
    fi
fi


the actual rc.conf is configured like this:
eval `kenv | sed -n 's/^rc\.//p'`
rm -f /etc/rc.conf /etc/rc.conf.local
for fc in $conf0 $conf1 $conf2 $conf3 $conf4 $conf5 $conf6 $conf7 $conf8 
$conf9 rc.conf.$hostname
do
    ho=`expr $fc : '\(.*\):'`
    fl=`expr $fc : '.*/\(.*\)'`
    if [ "${ho}" != "" ]; then
        mp=`expr $fc : '\(.*\)/.*'`
        mount_nfs $mp /mnt > /dev/null 2>&1
        if [ -f /mnt/$fl ]; then
            echo "# from $fc /mnt/$fl" >> /etc/rc.conf
            cat /mnt/$fl >> /etc/rc.conf
        fi
        umount /mnt > /dev/null 2>&1
    elif [ -e /conf/$fc ] ; then
        echo "# from /conf/$fc" >> /etc/rc.conf
        cat /conf/$fc >> /etc/rc.conf
    fi
done

> -------------- root path configuration -----------------
> 
>     There seems to be a well known problem in pxeloader, see
>     kern/106493 , where pxeloader defaults to using a root path of
>     /pxeroot when offered "/" .  The patch suggested in
> 
> 	http://www.freebsd.org/cgi/query-pr.cgi?pr=106493
> 
>     is trivial and judging from it I believe this is addressing a
>     true bug and not a feature.  Fortunately there is a workaround
>     (suggested in the PR) which is using "//" as a root path.
> 
> ------------- sharing /boot with the server ---------------
> 
>     I believe it is quite useful to share the whole root
>     partition between the server and the diskless client.
>     This would require at a minimum some conditional code
>     in loader.conf (or loader.rc, etc) so that at least you
>     point to different kernels.
> 
>     A minimalistic approach can be adding this line to /boot/loader.conf
> 
> 	bootfile="kernel\\${loaddev};kernel"
> 
>     The variable $loaddev contains the name of the load device,
>     which is "pxe0" in the case of pxeboot, and disk* in other
>     cases when loading from the local disk.
> 	If you make sure that there is no 'kernel.disk*' on the
>     directory, and instead there is a kernel.pxe0 in the same
>     directory, then the diskless machines and the server will boot
>     from the proper file.
> 
>     Unfortunately i don't know how to implement a conditional
>     in /boot/loader.conf -- otherwise one could do much nicer things
>     such as differentiate which modules to load and so on.
> 
> --------------- pxeloader bug in 7.x ---------------------------
> Also worth mentioning is an annoying bug in pxeloader as compiled
> on 7.x, see http://www.freebsd.org/cgi/query-pr.cgi?pr=118222
> i.e. the pxeloader in 7.x fails to proceed and prints a message
> "can't figure out which disk we are booting from".
> 
> The workaround is using a pxeloader from FreeBSD6 works.
> I guess this is a compiler-related problem (given that 6.x uses gcc 3.4
> as a compiler, while 7.x uses gcc 4.2).
> 
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