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Re: [Xen-users] VM slow after being unused for a while

To: Itamar Reis Peixoto <itamar@xxxxxxxxxxxxxxxx>
Subject: Re: [Xen-users] VM slow after being unused for a while
From: Nico Kadel-Garcia <nkadel@xxxxxxxxx>
Date: Sat, 12 May 2007 09:15:45 +0100
Cc: Nasse Gris <nassegris@xxxxxxxxx>, xen-users@xxxxxxxxxxxxxxxxxxx
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Itamar Reis Peixoto wrote:
try to edit /etc/ssh/sshd_config
and change UseDNS to NO restart ssh server -------------------- Itamar Reis Peixoto
Unfortunately, that argument doesn't do what you think it does. And it confuses a lot of people!

Here's the situation at least up through OpenSSH 3.9p1.

OpenSSH, for logging purposes, does a reverse DNS on any contacting IP address. The UseDNS option says whether to verify that the reverse DNS matches a valid forward DNS for that host. But disabling UseDNS does *NOT, NOT, NOT* turn off the reverse DNS lookup! Any number of us have submitted patches for this over the years: I submitted some when I dealt with large remotely deployed networks. (When you manage thousands of machines deployed in data centers all over the world, you can be absolutely certain a lot of them will not have valid reverse DNS, or even have DNS working properly, and you need to be able to log in quickly in a crunch.)

The option you need is in your sshd init script. You need to use the additional options "-u 0", to set the namelength of the recorded DNS entry to 0 so that the reverse DNS isn't actually done. (Why the SSH authors think setting an arglength to 0 should cause undocumented behavior and not throw an error, instead of obeying the UseDNS option in the configuraton file more correctly, I leave to people who think the "chroot" option of OpenSSH actually means a chroot cage for SSH users to protect them from accessing the filesystem outside their home directory. It doesn.t.)

I like OpenSSH, I use it a lot, but I've disagreed volubly with the authors on a few points over the years. This is one of them.

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