CentOS - Jun 2018 - /etc/gai.conf fails to prefer IPv4 over IPv6 for NFS

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?

Cheers,
sr.
-- 
|| 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         ||

On 06/08/2018 10:42 AM, Steve Rikli wrote:
> 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
>
 From my reading of that file, you'd need to uncomment all of the 
default precedence lines, and modify the last one.? You couldn't use 
that one line, alone.? It's hard to tell if that's what you did. 
/etc/gai.conf would contain:

precedence? ::1/128?????? 50
precedence? ::/0????????? 40
precedence? 2002::/16???? 30
precedence ::/96????????? 20
precedence ::ffff:0:0/96? 100

...also of note is that gai.conf should only affect the getaddrinfo() 
API.? If the client is using an older API like gethostbyname(), it won't 
order DNS results correctly.

In article <669037eb-029c-eb3b-0c60-6a5121142bb8 at gmail.com>,
Gordon Messmer  <centos at centos.org> wrote:
>On 06/08/2018 10:42 AM, Steve Rikli wrote:
>
>> 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
>
> From my reading of that file, you'd need to uncomment all of the 
> default precedence lines, and modify the last one.?? You couldn't use 
> that one line, alone.?? It's hard to tell if that's what you did. 
> /etc/gai.conf would contain:
>
> precedence?? ::1/128???????????? 50
> precedence?? ::/0?????????????????? 40
> precedence?? 2002::/16???????? 30
> precedence ::/96?????????????????? 20
> precedence ::ffff:0:0/96?? 100
Yes, I tried that permutation too.  Some posts claimed you only need the
last "100" precendence line, but I tried it both ways, as well as
other
combinations, by following the file's comments.
> ...also of note is that gai.conf should only affect the getaddrinfo() 
> API.?? If the client is using an older API like gethostbyname(), it
won't
> order DNS results correctly.
I suspect you're right; how to tell what system call is used by CentOS 6.9
NFS automounter?

I've watched the automounter daemon in foreground debug mode, and it does
show the NFS server mount attempt with IPv6 first, but the actual API call
name isn't displayed there.

This seems the most likely explanation, I'd just like to know for certain
before I give up on gai.conf and restort to disabling IPv6 or other
workarounds (e.g. /etc/nfsmount.conf).

Thanks,
sr.
-- 
|| 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         ||