Access denied when mounting IPv4 NFS share on dual-stacked client

[Russell Phillips]

Here is my (censored) /etc/exports on my server (Running Ubuntu 12.04), which is currently dual-stacked:

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# Public Storage (write)
#ipv6
/export 2001:xxxx:xxxx:xxxx::/64(rw,async,no_subtree_check)
/export/home 2001:xxxx:xxxx:xxxx::/64(rw,async,no_subtree_check,nohide,no_root_squash)
/export/home/pub/unmirrored 2001:xxxx:xxxx:xxxx::/64(rw,async,no_subtree_check,nohide,no_root_squash)

# ipv4
/export 192.168.1.0/24(rw,async,no_subtree_check)
/export/home 192.168.1.0/24(rw,async,no_subtree_check,nohide,no_root_squash)
/export/home/pub/unmirrored 192.168.1.0/24(rw,async,no_subtree_check,nohide,no_root_squash)
-------------------

This is the mount command I issue from the client:
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sudo mount bender:/export/home/pub /mnt/pub -t nfs
-------------------

This config works as expected. However, I get the following error if I remove the IPv6 config lines from /etc/exports, and attempt to mount a share on a dual-stacked client running Quantal:

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mount.nfs: access denied by server while mounting bender:/export/home/pub
-------------------

If I then disable IPv6 at the client, it will then mount as expected.

Most importantly: This does not break in Ubuntu 12.04. A dual-stacked 12.04 client will successfully mount an NFS share which is only exported over IPv4.

In short, to replicate the error:
- Both client and server are dual-stacked
- Client is running Quantal
- /etc/exports on server is exporting NFS shares ONLY over an IPv4 subnet.