Re: [PATCH - nfs-utils] Fix fallback from tcp to udp

[NeilBrown <neilb@suse.de>]

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On Thu, 20 Feb 2014 15:37:02 -0500 "J. Bruce Fields" <bfields@fieldses.org>
wrote:

> On Thu, Feb 20, 2014 at 12:50:15PM -0500, Steve Dickson wrote:
> > 
> > 
> > On 02/17/2014 06:43 PM, NeilBrown wrote:
> > > 
> > > Protocol negotiation in mount.nfs does not correctly negotiate with a
> > > server which only support NFSv3 and UDP.
> > > 
> > > When mount.nfs attempts an NFSv4 mount and fails with ECONNREFUSED
> > > it does not fall back to NFSv3, as this is not recognised as a
> > > "does not support NFSv4" error.
> > > However ECONNREFUSED is a clear indication that the server doesn't
> > > support TCP, and ipso facto does not support NFSv4.
> > > So ECONNREFUSED should trigger a fallback from v4 to v2/3.
> > I'm also pretty this is the error returned when the server is 
> > down or more pointy when server is rebooting...
> 
> Probably worth checking that.

It is certainly possible that there is a window during boot when a server
will RST any SYN to port 2049.  The window may be very small, but it will
usually be there.

It is possible to configure a server to start listening before enabling the
interface, and so close the window completely.  But we certainly cannot
assume any server does this.

> 
> > Do we really want to fallback at this point?
> 
> >From a bz comment (#984901, not sure why it's private):
> 
> Any NFS server has to support either tcp or rpcbind.  But it's OK for a
> server to support only of those two.  So the only way to handle both
> cases while continuing to retry after ECONNREFUSED is to alternate
> between trying nfs4/tcp and rpcbind until you can connect to one or the
> other.
> 
> If it's the rpcbind call that succeeds first then I think we want to do
> one more try of nfs4/tcp just to make sure it didn't just come up,
> before falling back to v3.
> 
> The rpcbind call is done in userspace, if I understand right, so I think
> this is doable.  Looking at utils/mount/ I don't understand the mount
> process well enough to understand exactly how to do it.  Maybe
> everything but the final nfs_sys_mount needs to be moved out of
> nfs_do_mount_v3v2 into a new nfs_do_probe_v3v2 and nfs_autonegotiate
> should alternate between nfs_try_mount_v4 and nfs_do_probe_v3v2 as long
> as both return ECONNREFUSED, calling nfs_try_mount_v3v2 only if
> nfs_try_mount_v4 has failed after a succesful nfs_do_probe_v3v2?
> 
> Except the v3v2 mount logic seems to actually modify the mount_options,
> so probably that doesn't quite work.

I had come to much the same conclusion after reading Steve's mail:  when TCP
fails we need rpcbind to be sure what to do.
I suspect it should be fairly straight forward to implement (I'm less
pessimistic than you).  I'll have a go on Monday.

Thanks,
NeilBrown

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