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  <title>SSH - Advanced Usage &ndash; LandChad.net</title>
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  <header>
    <h1>SSH - Advanced Usage</h1>
  </header>
  <nav></nav>
  <main>
    <h2>Introduction</h2>
    <p>
      This page is dedicated to advanced SSH usage examples. We will discuss the following concepts:
      <ul>
        <li>config files (for client)</li>
        <li>tunneling</li>
        <li>jumping</li>
      </ul>
    </p>
    <h2>Config files</h2>
    <p>
      Config files allow you to specify certain rules for all or chosen hosts. The file has a really simple structure.
      It is divided into sections which begin with the <code>Host</code> keyword. Sections are read one by one and <b>the first
      matching section takes precedence over the remaining sections</b> - you write more specific sections at the top and the more general
      sections below.
    </p>
    <h3>Why even bother?</h3>
    <p>
      You might say that SSH client doesn't need any special configuration - you just type the user@host and that's it.
      Well, what happens when you manage multiple servers? Maybe you want to use a different pair of keys for each servers?
      Maybe the server uses a port other than the default 22 to avoid automated bots trying to log in?
    </p>
    <p>
      That's where config files come in handy!
    </p>
    <h3>Example scenario</h3>
    <p>
      Let's assume that you manage 3 servers, with the following access info:
      <ol>
        <li>very.long.hostname.example1.com
          <ul>
            <li>user: admin</li>
            <li>port: 22</li>
            <li>key name: id_rsa</li>
          </ul>
        </li>
        <li>example2.com
          <ul>
            <li>user: billthemaster</li>
            <li>port: 2222</li>
            <li>key name: example2_ecdsa</li>
          </ul>
        </li>
        <li>192.168.133.7
          <ul>
            <li>user: management</li>
            <li>port: 22</li>
            <li>key name: id_rsa</li>
          </ul>
        </li>
      </ol>
    </p>
    <p>
      You got tired having to always specify the identity file location with the <code>-i</code> option and the port with <code>-p</code> option for example2.com.
      Don't even mention <code>admin@very.long.hostname.example1.com</code>!
    </p>
    <p>In the given example, the config file could look like this:
<code><pre>Host server1
  HostName very.long.hostname.example1.com
  User admin
  IdentityFile ~/.ssh/id_rsa

Host server2
  HostName example2.com
  Port 2222
  User billthemaster
  IdentityFile ~/.ssh/example2_ecdsa

Host server3
  HostName 192.168.133.7
  User management
  IdentityFile ~/.ssh/id_rsa

Host *
  IdentityFile /path/to/some/other/key
</pre></code>
    </p>
    <p>You can see here usage of <code>Host *</code>. Options specified in this section will affect all other hosts.</p>
    <h3>But where do I put this file?</h3>
    <p>
      SSH looks for the options in the following order:
      <ol>
        <li>command line arguments</li>
        <li><code>~/.ssh/config</code></li>
        <li><code>/etc/ssh/ssh_config</code></li>
      </ol>
    </p>
    <p>You can also specify a custom path with the <code>-F</code> argument, for example:
    <code><pre>ssh -F ~/Documents/projects/someproject/config/ssh production</pre></code>
    </p>
    <p>
      ...or discard any config file:
      <code><pre>ssh -F /dev/null username@hostname</pre></code>
    </p>
    <p>There's more to ssh config files, but I direct you to <code>man ssh_config</code> for more information</p>
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