SSH includes the ability to authenticate users using public keys. Instead of authenticating the user with a password, the SSH server on the remote machine will verify a challenge signed by the user's private key against its copy of the user's public key. To have a full programming control of the machine with Perl and automatic ssh-authentication you can do it this way:

• Install GRID::Machine. It will help you controlling SSH from Perl. You will have Perl RPC at hand.

• Generate a public key use the ssh-keygen utility. For example:

    local.machine$ ssh-keygen -t rsa -N ''
  

  The option -t selects the type of key you want to generate. There are three types of keys: rsa1, rsa and dsa. The -N option is followed by the passphrase. The -N '' setting indicates that no pasphrase will be used. This is useful when used with key restrictions or when dealing with cron jobs, batch commands and automatic processing which is the context in which this module was designed. If still you don't like to have a private key without passphrase, provide a passphrase and use ssh-agent to avoid the inconvenience of typing the passphrase each time. ssh-agent is a program you run once per login sesion and load your keys into. From that moment on, any ssh client will contact ssh-agent and no more passphrase typing will be needed.

  By default, your identification will be saved in a file /home/user/.ssh/id_rsa. Your public key will be saved in /home/user/.ssh/id_rsa.pub.

• Once you have generated a key pair, you must install the public key on the remote machine. To do it, append the public component of the key in

             /home/user/.ssh/id_rsa.pub
  

  to file

             /home/user/.ssh/authorized_keys
  

  on the remote machine. If the ssh-copy-id script is available, you can do it using:

    local.machine$ ssh-copy-id -i ~/.ssh/id_rsa.pub user@remote.machine
  

  Alternatively you can write the following command:

    $ ssh remote.machine "umask 077; cat >> .ssh/authorized_keys" < /home/user/.ssh/id_rsa.pub
  
  

  The umask command is needed since the SSH server will refuse to read a /home/user/.ssh/authorized_keys files which have loose permissions.

• Edit your local SSH configuration file /home/user/.ssh/config (see man ssh_config in UNIX) and create a new section for automatic connections to that host. Here follows an example:

   ...
  
   # A new section inside the config file: 
   # it will be used when writing a command like: 
   #                     $ ssh gridyum
  
   Host gridyum
  
   # My username in the remote machine
   user my_login_in_the_remote_machine
  
   # The actual name of the machine: by default the one provided in the
   # command line
   Hostname real.machine.name
  
   # The port to use: by default 22
   Port 2048
  
   # The identitiy pair to use. By default ~/.ssh/id_rsa and ~/.ssh/id_dsa
   IdentityFile /home/user/.ssh/yumid
  
   # Useful to detect a broken network
   BatchMode yes
  
   # Useful when the home directory is shared across machines,
   # to avoid warnings about changed host keys when connecting
   # to local host
   NoHostAuthenticationForLocalhost yes
  

   # Another section ...
   Host another.remote.machine an.alias.for.this.machine
   user mylogin_there
  
   ...
  

  This way you don't have to specify your login name on the remote machine even if it differs from your login name in the local machine, you don't have to specify the port if it isn't 22, etc. This is the recommended way to work with GRID::Machine. Avoid cluttering the constructor new.

• Once the public key is installed on the server you should be able to authenticate using your private key

    $ ssh remote.machine
    Linux remote.machine 2.6.15-1-686-smp #2 SMP Mon Mar 6 15:34:50 UTC 2006 i686
    Last login: Sat Jul  7 13:34:00 2007 from local.machine
    user@remote.machine:~$
  
  

  You can also automatically execute commands in the remote server:

    local.machine$ ssh remote.machine uname -a
    Linux remote.machine 2.6.15-1-686-smp #2 SMP Mon Mar 6 15:34:50 UTC 2006 i686 GNU/Linux
  

Hope it helps

Casiano