[ale] weirdness with slow ssh connections (IPv6 related?)

[Jeff Lightner]

My first thought was the /etc/resolv.conf on the host where you're
coming from then I read your comment about -4.  Using fully qualified
domain prevents going through the list of search domains in resolv.conf
so if you many of those it might make a difference (and of course IP
ignores it altogether).

On reviewing man page for resolv.conf just now I see it also has option
inet6:

"inet6  sets RES_USE_INET6 in _res.options.  This has the  effect
of trying a AAAA query before an A query inside the gethostbyname()
function, and of mapping  IPv4  responses  in IPv6  ''tunnelled form''
if no AAAA records are found but an A record set exists."

Is it possible someone put that in your resolv.conf?   For that matter
does your DNS server have AAAA records and A records? 

While you're at it you might want to verify the resolv.conf doesn't have
unreachable name servers listed.

-----Original Message-----
From: ale-bounces at ale.org [mailto:ale-bounces at ale.org] On Behalf Of
Steven A. DuChene
Sent: Wednesday, October 28, 2009 9:16 PM
To: ale at ale.org
Subject: [ale] weirdness with slow ssh connections (IPv6 related?)

I am working on a project with 50 - 70 servers in the AT&T data center
on University. They have either RHEL5U3 or CentOS5U3 on the systems.
I have DNS configured with the normal bind 9 packages that come with
CentOS5U3. My problem is when using ssh to get between the systems
if i use the short name for a system it takes a long time to get to
a shell prompt on the remote system. If I use the fully qualified
domain name or the IP address of I use a "-4" with the short name
I get to a shell prompt immediately. So to illustrate:

root at adm #  ssh new75

takes a LONG time to get shell prompt.

Any of the following get me a shell prompt almost immediately:

root at adm #  ssh -4 new75

root at adm #  ssh new75.wholedomain.here

root at adm #  ssh 10.4.5.6   (IP address of new75 system)

Since the "-4" option seems to have a positive effect I have gone
to each system and made sure the IPv6 kernel modules are not loaded
and the network file in /etc/sysconfig/ has the following line in
it:

NETWORKING_IPV6=no

but that made little or no difference to the problem. The lines
I added to the /etc/modprobe.conf file look like this:

alias net-pf-10 off
alias ipv6 off

I believe it some strange interaction problem between IPv6 and DNS
but I do not have a clear plan on what to try next.

Any suggestions?
--
Steven DuChene


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