How to not break nfsroot

Glenn Satchell glenn.satchell at
Wed Apr 28 00:35:13 UTC 2010

Hi Feri

I don't have any direct experience with nfsroot/dbcp but I think that 
you will need to make some changes to dhclient-script. This is a fairly 
generic script designed to implement the actions for each OS/platform. 
eg the commands for linux vary from, say, BSD.

There is a man page for dhclient-script which strongly suggests not 
modifying the original. There is a mechanism where you can override 
actions using /etc/dhclient-enter-hooks.

In /etc/dhclient-enter-hooks you could do your own test for PREINIT and 
skip the ifconfig 0 up, then exit, eg something like this (note I 
haven't tested this!)

if [ x$reason = xPREINIT ]; then
   # don't change anything
   exit_with_hooks 0

This way the corresponding code in dhclient-script does not get a chance 
to be run.

On the dhcp server side you probably want to be sure that steps 2 and 3 
deliver the same IP address to the client.

HTH and happy to have some more discussin along these lines.


On 04/27/10 19:44, Ferenc Wagner wrote:
> Hi,
> I'd much appreciate your thoughts about this.  If I didn't succeed in
> making the issue clear, I can try again.  If I brought it up on the
> wrong forum, please direct me to a better one.
> Regards,
> Feri.
> Ferenc Wagner<wferi at>  writes:
>> When booting an NFS rooted system, one generally has to go through three
>> DHCP round trips:
>>   1. PXE BIOS does DHCP to setup networking for downloading the kernel
>>      and the initramfs image,
>>   2. initramfs does DHCP to setup networking for mounting the NFS root,
>>   3. the booting system does DHCP to set host and domain name and DNS
>>      resolvers.
>> But 3 is inherently problematic, because a cold-starting dhclient brings
>> down its network interface, and NFS root can't tolerate that, the system
>> deadlocks (cf.
>> Now, it's technically possible to transfer the DHCP reply packet from
>> the PXE phase down to dhclient.  It's also possible to create a lease
>> file for dhclient by parsing this packet.  But, to my knowledge, there's
>> no way to coax dhclient into skipping the PREINIT phase (that's where
>> dhclient-script unconditionally downs the network interface), even
>> though it's willing to skip discovery if it finds a valid lease.
>> Also, while it may be possible to skip any interaction with the DHCP
>> server if the valid lease (or the saved reply packet) contains all the
>> options the client config needs, it should still be acted upon, as
>> resolv.conf (as an example) should be set up.  If some information is
>> missing, a DHCPINFORM would probably be the most appropriate choice.
>> Now it seems to me that the nfsroot scenario can't be handled without
>> tailoring dhclient-script, but I'd like to hear your opinion about the
>> best handling of this whole problem domain.

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