Before you can do anything with Net::SSH, you need to require the net/ssh module:

require 'net/ssh'

Once you have required the net/ssh module, you can begin an SSH session by calling Net::SSH.start. This may be used in one of two ways. If called without a block, it will return a reference to the new session as an instance of a Net::SSH::Session. Used this way, you must explicitly close the session when you are finished with it.

session = Net::SSH.start( 'host', 'user', 'passwd' )
...
session.close

The other approach involves attaching a block to the start method. When used this way, the new session is passed to the block, and the session is automatically closed when the block exits.

Net::SSH.start( 'host', 'user', 'passwd' ) do |session|
  ...
end

If you need to specify a different port on the host to connect to (the default is 22), you can specify it immediately after the host parameter, like so:

Net::SSH.start( 'host', 1234, 'user', 'passwd' ) do |session|
  ...
end

Using Keyword Arguments

Some people prefer using keyword arguments for functions with more than a couple of parameters. The start method supports this approach as well, although the host parameter is always positional and always comes first.

Net::SSH.start( 'host',
                :password=>'passwd', 
                :port=>1234,
                :username=>'user',
       ... ) do |session|
  ...
end

(More about the “...” stuff, later.)

Failed Authentication

If the username and/or password given to start are incorrect, authentication will fail. If authentication fails, a Net::SSH::AuthenticationFailed exception will be raised.

Just as with the OpenSSH version of the ssh utilities, Net::SSH supports authentication using public/private keys.

I don’t know what public/private keys are… Explain, please?

Public key/private key encryption is just one way of hiding information from prying eyes. The idea is that you have two tokens: a public key, and a private key. The private key is yours alone—you never let anyone else see it. The public key, on the other hand, is distributable. You give it to anyone that you want to be able to communicate with you securely.

The remote party uses your public key to encrypt information. Anything encrypted with your public key may only be decrypted with the corresponding private key, and since you have the only copy of that, you can rest easily knowing that no one can easily intercept your communications!

Net::SSH allows you to define a private key, which it will then attempt to use during authentication with the remote server. If the remote server has a copy of the corresponding public key, you will be able to log into that remote server without having to specify a password. Not only is this convenient, but for Ruby scripts, it is much more secure, since you don’t have to hard-code your password in your script.

Setting up public/private keys

Net::SSH, by default, will use the private keys that you have set up for use with ssh. These keys are called “id_dsa” and “id_rsa”, and are located under your home directory, either in a ”.ssh” subdirectory, or a ”.ssh2” subdirectory.

The “id_dsa” key is the preferred key (since it uses the stronger DSA encryption), but both DSA and RSA are supported.

To create these keys, you can use the “ssh-keygen” utility from OpenSSH. Alternatively, if you have the Net::SSH::Utilities package installed, you can use the “rb-keygen” utility (which is a pure-Ruby implementation of most of the functionality of ssh-keygen).

ssh-keygen -t dsa

(If you would rather use an RSA key, replace “dsa” with “rsa” in the command given above.)

Accept all the defaults when prompted. You will also be asked for a passphrase. This passphrase is an additional level of protection, which prevents anyone from being able to use your private key without knowing the passphrase. Unfortunately, it also means that you have to enter the passphrase every time you use your key. It is up to you what price you want to pay for security, but if you can leave the passphrase blank. In this case, anyone that has a copy of your private key can use it, but it’s a little more convenient to deal with.

Once you create your keys, you then need to set up your account on each remote server so that it knows about your public key. To do this, log into the remote server and edit (or create) the file (in your home directory) ”.ssh/authorized_keys”. Just copy the contents of your public key (in your local machine’s home directory, called ”.ssh/id_dsa.pub” or ”.ssh/id_rsa.pub”) into the “authorized_keys” file on a line of its own. Then save the file and logout. Everything should now be set up.

(Note: if you have an SSH client installed, it will typically have its own key generation utility. You can use that instead, if you prefer.)

Connecting using public/private keys

Public/private keys are always tried before the explicit password authentication, even if you provide a password. Thus, if you only want to use public/private key authentication, simply remove the password from the argument list. If you can successfully obtain a session handle, then your keys are set up correctly!

Net::SSH.start( 'host', 'user' ) do |session|
  ...
end

Furthermore, if your USER environment variable is set to the username that you want to log into the remote machine as, you can even leave the username parameter off:

Net::SSH.start( 'host' ) do |session|
  ...
end

Using keys with passphrases

When you use a private key that was created with a passphrase, you will be prompted to enter the passphrase when the key is loaded. This may make such a key inappropriate for use in automated environments, but it is certainly more secure than the use of unprotected private keys.

Using an SSH agent

Most SSH clients come with what is called an agent. This is a program that is continually running, and which keeps track of all of a user’s keys. When an SSH client needs to perform an operation using one of the user’s keys, it requests the operation via the agent, rather than performing the operation itself directly with a key.

The benefit of this is what is known as single sign-on. If any of your keys have a passphrase, this allows you to enter the passphrase once (when the key is loaded by the agent), and then any SSH program you use will never prompt you for that passphrase again.

Net::SSH includes support for interfacing with an SSH agent. This includes support for the PuTTY agent on Windows systems.

On Unixish systems, you allow your Net::SSH programs to interface with a running agent by making sure that the SSH_AGENT_SOCK environment variable is set to the location of the Unix domain socket that the agent is listening to. Also, make sure you have added all of your keys to the agent (typically by running the ssh-add utility.

On Windows, the pageant process will be detected automatically, if it is running.

A future version of Net::SSH may include it’s own agent implementation as well, to make using an agent on a variety of platforms simpler.

Agent forwarding is available, and may be requested with the :forward_agent option to Net::SSH.start. Agents are not forwarded by default.

There are various additional options that you can specify when connecting. These options allow you to specify such things as the cipher algorithm to use, whether or not the data stream will be compressed, or explicit paths to the private keys to use.

Options are specified as a hash in the last parameter to the start method. If using the keyword parameters version of the start method, the options hash is whatever is left after processing the :username, :password, and :port options.

The complete list of available options, and their valid values, is given in the following table.

For example, the following code snippet will connect to the given remote host, and requests that the ssh-rsa host key type be used, with the blowfish-cbc cipher algorithm, and requests that the given private key file be used. Also, the data stream will be compressed.

require 'net/ssh'
require 'logger'

Net::SSH.start(
  'host', 'user',
  :host_key => "ssh-rsa",
  :encryption => "blowfish-cbc",
  :keys => [ "/tmp/temporary-key" ],
  :compression => "zlib"
) do |session|
  ...
end

Alternatively, you can use Net::SSH::Session to start your SSH sessions. The Net::SSH.start interface described above is simply a convenience for creating a new Session object explicitly.

require 'net/ssh'

Net::SSH::Session.new(
  'host', 'username', 'password',
  :compression => "zlib"
) do |session|
  ...
end

Note that Net::SSH::Session#new accepts the same parameters as Net::SSH.start, and may also be called without a block.