How to not break nfsroot

Ferenc Wagner wferi at
Tue Apr 27 09:44:39 UTC 2010


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.


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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