[CentOS] /etc/gai.conf fails to prefer IPv4 over IPv6 for NFS

Fri Jun 8 17:42:42 UTC 2018
Steve Rikli <sr at genyosha.net>

Using CentOS 6.9 with IPv4 configured, and _not_ disabling IPv6 yet, we
want this NFS client system to prefer the IPv4 address of a dual-stack
remote NFS server, which has both A and AAAA records in DNS.

Otherwise we get a minutes long pause while automounter tries to mount
the IPv6 address -- I can see automounter's '/bin/mount' running, using
the AAAA record of the server, until that times-out and the IPv4 address
of the server mounts successfully.

Disabling IPv6 altogether works around the problem as expected, but is
a bigger hammer than we wanted to use.  :-)

I found posts from others in a similar situation, and proposed solutions
included modifying /etc/gai.conf to use:

   precedence ::ffff:0:0/96  100

I cp'd /usr/share/doc/glibc-common-2.12/gai.conf to /etc/ and tried that,
with several other permutations (e.g. uncommenting other blocks of "label"
and "precedence" settings) but none achieved the desired effect.

Am I misunderstanding how this is supposed to work?

|| Steve Rikli             ||| Well, we've stared at it... that oughta ||
|| Systems Administrator   ||| fix it!  Let's get outta here.          ||
|| Genyosha Networks       |||                                         ||
|| sr at genyosha.net         |||                   - Crow, MST3K         ||