how do I handle ssh response with expect Post: 302221525

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Top Forums Shell Programming and Scripting how do I handle ssh response with expect Post 302221525 by buffoonix on Monday 4th of August 2008 04:12:17 PM

08-04-2008

Registered User

Hi,

I haven't read your Expect script because it's been far too long since I last wrote anything in Tcl.
Anyway, I think there is no real need for any sophisticated Expect prompting logic here
since the warning you encounter from your SSH client about an unknown host identity
can be easily circumvented.
If your SSH client connects to a remote SSH server whose host identity it cannot verify,
either because it is the first connect to this host, or maybe the remote host's SSH server was started with different host keys meanwhile (maybe its admin updated SSH and neglected restoring its host key) it will warn you as long as StrictHostKeyChecking isn't set to "no" (per default it is set to "ask", see man ssh_config).
If it is the first connect and you have verified that the presented fingerprint of the remote host key is correct (or you trust it anyway) you simply need to confirm this warning with yes.
Your SSH client will then create a file $HOME/.ssh/known_hosts (if it hasn't existed yet)
and append the public host key offered from the remote SSH server to it.
From then on it will never again ask you as long as the host key on the remote server or the entry in your local known_hosts file for that host will not change.
In that respect it even wouldn't help if you provided an extra yes response in your Expect prompt logic.
However, there are even other ways how you can connect if you don't care for strict host key checking at all (which maybe isn't advisable in a potentially hostile environment)
You could run the SSH command with the following options:

Code:

$ ssh -q -o BatchMode=yes -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null -l remote_login remote_host "command opts args..."

This will (even if the host key changed, or there is a real man-in-the-middle attack!) don't care about the validity of the host key's fingerprint and automatically "add" any offered host key to the bit bucket /dev/null.
The quiet option -q will suppress any warning text of this action,
and BatchMode will not prompt for any passwords or passphrases.
So you should run this command with distributed RSA keys which have either no passphrase attached to them, or have started an ssh-agent a priori which had added the necessary RSA key for this connection.
Please, consult man ssh and man ssh_config for details.

buffoonix

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LEARN ABOUT LINUX

ssh-keysign

SSH-KEYSIGN(8)						    BSD System Manager's Manual 					    SSH-KEYSIGN(8)

NAME

     ssh-keysign -- ssh helper program for host-based authentication

SYNOPSIS

     ssh-keysign

DESCRIPTION

     ssh-keysign is used by ssh(1) to access the local host keys and generate the digital signature required during host-based authentication with
     SSH protocol version 2.

     ssh-keysign is disabled by default and can only be enabled in the global client configuration file /etc/ssh/ssh_config by setting
     EnableSSHKeysign to ``yes''.

     ssh-keysign is not intended to be invoked by the user, but from ssh(1).  See ssh(1) and sshd(8) for more information about host-based authen-
     tication.

FILES

     /etc/ssh/ssh_config
	     Controls whether ssh-keysign is enabled.

     /etc/ssh/ssh_host_dsa_key
     /etc/ssh/ssh_host_ecdsa_key
     /etc/ssh/ssh_host_rsa_key
	     These files contain the private parts of the host keys used to generate the digital signature.  They should be owned by root, read-
	     able only by root, and not accessible to others.  Since they are readable only by root, ssh-keysign must be set-uid root if host-
	     based authentication is used.

     /etc/ssh/ssh_host_dsa_key-cert.pub
     /etc/ssh/ssh_host_ecdsa_key-cert.pub
     /etc/ssh/ssh_host_rsa_key-cert.pub
	     If these files exist they are assumed to contain public certificate information corresponding with the private keys above.

SEE ALSO

     ssh(1), ssh-keygen(1), ssh_config(5), sshd(8)

HISTORY

     ssh-keysign first appeared in OpenBSD 3.2.

AUTHORS

     Markus Friedl <markus@openbsd.org>

BSD
								  August 31, 2010							       BSD