Re: [PATCH] mount.nfs: prefer IPv4 addresses over IPv6 (try #2)

[Jeff Layton <jlayton@redhat.com>]

On Wed, 20 Jan 2010 10:36:36 -0500
Chuck Lever <chuck.lever@oracle.com> wrote:

> 
> On Jan 20, 2010, at 8:29 AM, Jeff Layton wrote:
> 
> > On Tue, 19 Jan 2010 10:43:34 -0500
> > Chuck Lever <chuck.lever@oracle.com> wrote:
> >
> >>
> >> On Jan 19, 2010, at 8:27 AM, Jeff Layton wrote:
> >>
> >>> We're poised to enable IPv6 in nfs-utils in distros. There is a
> >>> potential problem however. mount.nfs will prefer IPv6 addrs.
> >>>
> >>> If someone has a working IPv4 server today that has an IPv6 address,
> >>> then clients may start trying to mount over that address. If the
> >>> server
> >>> doesn't support NFS serving over IPv6 (and virtually no linux  
> >>> servers
> >>> currently do), then the mount will start failing.
> >>>
> >>> Avoid this problem by making the mount code prefer IPv4 addresses
> >>> when they are available and an address family isn't specified.
> >>>
> >>> This is the second attempt at this patch. This moves the changes
> >>> into nfs_validate_options. Chuck also mentioned parameterizing this
> >>> behavior too. This patch doesn't include that, as I wasn't exactly
> >>> clear on what he had in mind.
> >>>
> >
> > After playing around with this some more, I'm coming to the conclusion
> > that my first patch for this (the one that modified nfs_lookup) is
> > really the best one.
> >
> > As Chuck pointed out, we need to have the address resolution behave
> > consistently, so I'd need to have other places that currently call
> > nfs_lookup do something similar.
> >
> > There are 4 callers of nfs_lookup currently:
> >
> > nfs_gethostbyname
> > nfs_umount_do_umnt
> > nfs_fix_mounthost_option
> > nfs_validate_options
> >
> > ...all of these with the exception of nfs_gethostbyname will need to  
> > do
> > this "sorting". nfs_gethostbyname passes in AF_INET for the family, so
> > it's guaranteed to return a IPv4 address or nothing anyway.
> >
> > I've tried a more-or-less exhaustive combination of proto=/mountproto=
> > options (and lack thereof) and I think with the original patch I  
> > posted
> > it works as expected.
> >
> > On IRC, Chuck mentioned replacing nfs_lookup with direct calls to
> > getaddrinfo, but that's a larger change and I don't really think it's
> > necessary.
> >
> > Chuck also suggested that we should have mount.nfs never attempt to  
> > use
> > an IPv6 address unless someone explicitly adds proto=tcp6|udp6. I'm  
> > not
> > a fan of that solution. I think if a hostname resolves to only an IPv6
> > address it ought to "just work" and mount via that address without
> > needing extra options.
> >
> > For discussion, the original patch follows:
> 
> For the record, we looked at Solaris behavior yesterday.  With bi- 
> family servers, its mount command tries IPv6 first, but appears smart  
> enough to fall back to IPv4.  One thing we haven't tried is to see how  
> difficult it would be to fix the real problem by adding proper  
> protocol family negotiation to our own mount command.  This patch is  
> predicated on the idea that would be hard to implement, which hasn't  
> been demonstrated.
> 
> I'm worried about putting this preference behavior in released code,  
> and then changing it yet again later.  I don't know how much time we  
> have to fix this, but if we have a few days, I could try implementing  
> real protocol family negotiation.
> 
> If you go forward with this solution, it should be made clear in the  
> documenting comments (and in the patch description) that this is a  
> temporary workaround.  If we intend to further correct this behavior  
> down the road, we should avoid building on the new behavior, even by  
> accident.
> 

Changing behavior is a valid concern and something we generally like to
avoid. OTOH, if someone doesn't specify proto=/mountproto= options
explicitly, then I think it's fair game to change the address
preference if we think it's reasonable and necessary.

I agree that doing proper negotiation would be ideal. I'll note that
with a 2.6.33-ish kernel, I quick -ECONNREFUSED errors when attempting
a NFSv4 mount to a server that doesn't support NFS over IPv6, so it
seems doable.

If you think you can get proper negotiation done within a few days,
then I say go for it and I'll hold off on asking Steve to take this
patch. If it turns out to be more difficult, we can always revisit this
patch.

-- 
Jeff Layton <jlayton@redhat.com>