linux-kernel - RE: [-next] Regression: ssh log in slowdown

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1725C2C3@AcuExch.aculab.com>
Date:	Fri, 13 Jun 2014 10:05:42 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Geert Uytterhoeven' <geert@...ux-m68k.org>
CC:	Tom Herbert <therbert@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux-Next <linux-next@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [-next] Regression: ssh log in slowdown

From: Geert Uytterhoeven
> Hi David,
> 
> On Fri, Jun 13, 2014 at 10:49 AM, David Laight <David.Laight@...lab.com> wrote:
> > From: Of Geert Uytterhoeven
> > ...
> >> Note that I'm also using NFS root, which doesn't seem to be affected.
> >> I can happily run "ls -R /" on the serial console during the 10 s delay in ssh.
> >
> > Are you sure that the delay during ssh login isn't just
> > a reverse DNS timeout?
> 
> Indeed, the ssh server sends a reverse DNS request twice, with 5s in between:
... 
> Interestingly, I don't see the forward DNS request after that, which
> does happen in the good case.

The forwards request is probably just copying some strange code from 'rshd'
that tried to verify the reverse lookup by doing a forwards lookup on the
result.
That in itself used to cause us grief.
The RDNS would (correctly) generate host.bar.baz.co.uk, since the 'domain'
in etc/resolv.conf was bar.baz.co.uk the forwards lookup first tried
host.bar.baz.co.uk.bar.baz.co.uk then host.bar.baz.co.uk.baz.co.uk
one of which always timed out :-(
(When the 'search' command was added we could avoid the request that
timed out.)

	David