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