openssh unix dev - Dec 2007 - $HOME/.ssh/config and host/hostname resolution how-to?

Hiya,

What is the easiest way of looking up a hostname to see if it exists,  
taking into account the .ssh/config file?

If i have an entry in my file

host host1
   hostname server.domain.name

then usual OS calls won't recognise the 'alias' host1.  So, i need
to
make a call via ssh somehow to check if the name resolves without  
actually initiating a full blown 'expensive' connection to it, such  
as using 'ssh host1 true' or something (which also relies on
'true'
being on the path, or always being installed in the same place if the  
path is provided - neither of which can be guaranteed).

I tried ssh-keyscan, but this ignores the config file (probably  
correctly), so what is the more appropriate way to perform the name  
validation?  I have tried 'ssh -n host1' which seems to work - would  
this be the best option?

I am using:

   OpenSSH_4.6p1 Debian-5build1, OpenSSL 0.9.8e 23 Feb 2007

on Etch in this instance, but the method will need to be portable.

Thanks

   Duncs

Duncan Ferguson wrote:
> Hiya,
> 
> What is the easiest way of looking up a hostname to see if it exists,  
> taking into account the .ssh/config file?
> 
> If i have an entry in my file
> 
> host host1
>    hostname server.domain.name
> 
> then usual OS calls won't recognise the 'alias' host1.  So, i
need to
> make a call via ssh somehow to check if the name resolves without  
> actually initiating a full blown 'expensive' connection to it, such
> as using 'ssh host1 true' or something (which also relies on
'true'
> being on the path, or always being installed in the same place if the  
> path is provided - neither of which can be guaranteed).
> 
> I tried ssh-keyscan, but this ignores the config file (probably  
> correctly), so what is the more appropriate way to perform the name  
> validation?  I have tried 'ssh -n host1' which seems to work -
would
> this be the best option?
I'm not quite sure what you're trying to achieve, could you give an
example?

Depending on what you want, there's a couple of options.

If you just want to return quickly if the host is down, you can set the 
ConnectTimeout to a lowish value.

If you want to do something more complicated, (eg fail if your 
dial-on-demand link is down or similar) you could use a ProxyCommand and 
implement whatever logic you want in a shell script (that finishes by 
exec'ing netcat).

-- 
Darren Tucker (dtucker at zip.com.au)
GPG key 8FF4FA69 / D9A3 86E9 7EEE AF4B B2D4  37C9 C982 80C7 8FF4 FA69
     Good judgement comes with experience. Unfortunately, the experience
usually comes from bad judgement.

On Dec 19, 2007 11:34 AM, Duncan Ferguson <duncan_j_ferguson at
yahoo.co.uk> wrote:
> What is the easiest way of looking up a hostname to see if it exists,
> taking into account the .ssh/config file?
It sounds to me like you want to use ssh to parse ~/.ssh/config for
you. To avoid any overhead, you could simply parse it yourself.

Hamish