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

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

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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