autofs reverts to mount via ipv4 for multi-homed ipv6 server

Hello everybody,

I would like to report the following behaviour which is IMO a bug.

1. Working scenario without autofs:
NFS client with ipv4 and static GUA (global unique address) and ULA (unique local address) ipv6 addresses. NFS server also with ipv4 and static GUA and ULA. DNS contains complete and consistent (forward/reverse lookup) information.

So considering ipv6 both server and client are multi-homed which is a legal configuration
with ipv6. Without autofs a simple mount -tnfs4 -o proto=tcp6 ... on the client works as expected, the correct source and destination addresses are chosen according to RFC 6724/3484 (removing/adding server ULA/GUA AAAA record in DNS to test).

2. Working scenario with autofs:
DNS AAAA entry for the server is either the GUA or the ULA but not both. The server is not multi-homed. In this case mounting via autofs uses ipv6 and the correct IP addresses (ULA or GUA) are selected for the mount according to the RFC.

3. Non-working scenario with autofs:
DNS contains ULA and GUA (two AAAA entries) for server, the server is multi-homed. However, due to RFC 6724/3484 address selection rules are clearly defined. Attempting to mount the same export as in 2. in this situation reverts to ipv4 instead of using ipv6 with source and destination addresses according to the RFC. No other configuration changes on server or client.

After reading https://bugs.launchpad.net/ubuntu/+source/autofs/+bug/1101779 I tried to recompile an autofs package with --with-libtirpc. Unfortunately this recompiled package fails with

automount[20444]: open_mount:247: parse(sun): cannot open mount module nfs (/usr/lib/x86_64-linux-gnu/autofs/mount_nfs.so: undefined symbol: clnt_dg_create)
automount[20444]: lookup(file): failed to open parse context

If someone has a solution for this and thinks --with-libtirpc would help or yield additional information I can test again.

I cannot test the scenario mentioned in 1101779 at the moment in a production environment by removing the servers ipv4 address.

What you expected to happen: Mount via ipv6 in scenario 3 as in scenario 2.
What happened instead: Mount via ipv4 in scenario 3.

Best Regards

Christof

# lsb_release -rd
Description: Ubuntu 14.04.4 LTS
Release: 14.04

# apt-cache policy autofs
autofs:
  Installed: 5.0.7-3ubuntu3.2
  Candidate: 5.0.7-3ubuntu3.2
  Version table:
 *** 5.0.7-3ubuntu3.2 0
        500 http://de.archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     5.0.7-3ubuntu3 0
        500 http://de.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages