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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libssh2_channel_direct_tcpip_ex

libssh2_channel_direct_tcpip_ex(3)				  libssh2 manual				libssh2_channel_direct_tcpip_ex(3)

NAME

       libssh2_channel_direct_tcpip_ex - Tunnel a TCP connection through an SSH session

SYNOPSIS

       #include <libssh2.h>

       LIBSSH2_CHANNEL * libssh2_channel_direct_tcpip_ex(LIBSSH2_SESSION *session, const char *host, int port, const char *shost, int sport);

       LIBSSH2_CHANNEL * libssh2_channel_direct_tcpip(LIBSSH2_SESSION *session, const char *host, int port);

DESCRIPTION

       session - Session instance as returned by libssh2_session_init_ex(3)

       host - Third party host to connect to using the SSH host as a proxy.

       port - Port on third party host to connect to.

       shost - Host to tell the SSH server the connection originated on.

       sport - Port to tell the SSH server the connection originated from.

       Tunnel  a TCP/IP connection through the SSH transport via the remote host to a third party. Communication from the client to the SSH server
       remains encrypted, communication from the server to the 3rd party host travels in cleartext.

RETURN VALUE

       Pointer to a newly allocated LIBSSH2_CHANNEL instance, or NULL on errors.

ERRORS

       LIBSSH2_ERROR_ALLOC -  An internal memory allocation call failed.

SEE ALSO

       libssh2_session_init_ex(3)

libssh2 0.15							    1 Jun 2007					libssh2_channel_direct_tcpip_ex(3)