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<chapter>
  <header>
    <copyright>
      <year>2020</year><year>2022</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
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          http://www.apache.org/licenses/LICENSE-2.0

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      See the License for the specific language governing permissions and
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    <title>Configuration in SSH</title>
    <prepared></prepared>
    <docno></docno>
    <approved></approved>
    <date></date>
    <rev></rev>
    <file>configurations.xml</file>
  </header>

  <section>
    <marker id="introduction"/>
    <title>Introduction</title>
    <p>The OTP SSH app can be configurated by a large amount of <i>Options</i>. This chapter will not go into
    details of what each of the options does. It will however describe and define different ways by which they
    could be entered.
    </p>
    <p>Options for hardening are described in the <seeguide marker="hardening">Hardening SSH</seeguide> chapter.
    How the options for algorithm configuration interact are described in the
    <seeguide marker="configure_algos">Configuring algorithms in SSH</seeguide> chapter.
    </p>
  </section>

  <section>
    <title>Options configuration</title>
    <p>There are from OTP-23.0 two main ways to set an option:
    </p>
    <list>
      <item>Like before, in the <c>Options</c> parameter in the Erlang code
      in a call to for example
      <seemfa marker="ssh#daemon/3">ssh:daemon/3</seemfa> or
      <seemfa marker="ssh#connect/3">ssh:connect/3</seemfa> or
      any of their variants. Example:
      <code>ssh:connect(22, [{user,"foo"}])</code>
      </item>

      <item>In <seefile marker="kernel:config">OTP Configuration Parameters</seefile>:
      <p/>
      <list>
      <item>In the erl command line:
      <pre>erl -ssh user \"foo\"</pre>
      </item>
      <item>In the <c>ssh.app</c> file, in the <c>env</c> part
      <code>
{application, ssh,
 [{description, "SSH-2 for Erlang/OTP"},
  {vsn, "4.9"},
  {modules, [ssh,
        ...
	     ssh_xfer]},
  {registered, []},
  {applications, [kernel, stdlib, crypto, public_key]},
  {env, [{user, "bar"]}, % &lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt;&lt; HERE
  {mod, {ssh_app, []}},
       ...       
</code>
      </item>
      <item>In a .config file:
      <pre>erl -config ex1</pre>
      where <c>ex1.config</c> contains:
      <code>[
{ssh, [{user, "foo"}]}
].
</code>
      </item>
      </list>
    <p>If the option is intended only for a server or for a client, it may be set in this way:
    </p>
    <code>[
{ssh, [{server_options,[{user, "foo"}]},
       {client_options,[{user, "bar"}]}
].
    </code>
    <p>A server (daemon) will use the user name <c>foo</c>, and a client will use the name <c>bar</c>.</p>
      </item>
    </list>
  </section>

  <section>
    <title>Precedens</title>
    <p>If an option is set in more than one way, what happens?</p>
    <p>There is an ordering, which is:
    </p>
    <list>
      <item>Level 0: Hard-coded default values in the source code</item>
      <item>Level 1: <seefile marker="kernel:config">OTP Configuration Parameters</seefile></item>
      <item>Level 2: Options in the <seefile marker="kernel:config">OTP Configuration Parameters</seefile>
      <c>server_options</c> or <c>client_options</c></item>
      <item>Level 3: Options in argument list to a function</item>
    </list>    
    <p>If the same option is set at two different levels, the one at the highest level is used.
    </p>
    <p>The only exception is the
    <seetype marker="ssh#modify_algorithms_common_option">modify_algorithms</seetype>
    common option. They are all applied in ascending level order on the set of algorithms. So a
    <c>modify_algorithms</c> on level zero is applied before one of level one and so on.
    </p>
    <p>If there is an
    <seetype marker="ssh#preferred_algorithms_common_option">preferred_algorithms</seetype>
    option on some level the whole set is replaced by that in that option and <em>all modify_algorithms
    are applied</em> in level ordering.
    </p>
    <p>The reason for applying all
    <seetype marker="ssh#modify_algorithms_common_option">modify_algorithms</seetype>
    in level order, is to enable the user to add an algorithm that has been removed from the
    default set without code changes, only by adding an option in a config file.
    This can be used to interoperate with legacy systems that still uses algorithms no longer
    considered secure enough to be supported by default.
    </p>
    
    <section>
      <title>Algorithm configuration</title>
      <p>There is a <seeguide marker="configure_algos#introduction">separate chapter</seeguide> about how
      <seetype marker="ssh#preferred_algorithms_common_option">preferred_algorithms</seetype> and
      <seetype marker="ssh#modify_algorithms_common_option">modify_algorithms</seetype>
      co-operate. How the different configuration levels affect them, is described here in this section.
      </p>
      <section>
	<title>The ssh:start/0 function</title>
	<p>If the application SSH is <em>not</em> <seemfa marker="ssh#start/0">started</seemfa>, the command
	<seemfa marker="ssh#default_algorithms/0">ssh:default_algorithms/0</seemfa>
	delivers the list of default (hardcoded) algorithms with respect to the support in the current cryptolib.
	</p>
	<p>If the application SSH <em>is</em> <seemfa marker="ssh#start/0">started</seemfa>, the command
	<seemfa marker="ssh#default_algorithms/0">ssh:default_algorithms/0</seemfa>
	delvers the list of algorithms after application of level 0 and level 1 configurations.
	</p>
	<p>Here is an example. The config-file has the following contents:</p>
	<code>
$ cat ex2.config
[
 {ssh, [{preferred_algorithms, [{cipher, ['aes192-ctr']},
       			        {public_key, ['ssh-rsa']},
                                {kex, ['ecdh-sha2-nistp384']},
                                {mac, ['hmac-sha1']}]}]}
].
	</code>
	<p>Erlang is started with <c>ex2.config</c> as configuration and we check the default set of
	algorithms before starting ssh:</p>
	<code>
$ erl -config ex2
Erlang/OTP 23 [RELEASE CANDIDATE 1] [erts-10.6.4] [source-96a0823109] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [hipe]

Eshell V10.6.4  (abort with ^G)
1> ssh:default_algorithms().
[{kex,['ecdh-sha2-nistp384','ecdh-sha2-nistp521',
       'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
       'diffie-hellman-group16-sha512',
       'diffie-hellman-group18-sha512',
       'diffie-hellman-group14-sha256','curve25519-sha256',
       'curve25519-sha256@libssh.org','curve448-sha512',
       'diffie-hellman-group14-sha1',
       'diffie-hellman-group-exchange-sha1']},
 {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
              'ecdsa-sha2-nistp256','ssh-ed25519','ssh-ed448','ssh-rsa',
              'rsa-sha2-256','rsa-sha2-512','ssh-dss']},
 {cipher,[{client2server,['chacha20-poly1305@openssh.com',
                          'aes256-gcm@openssh.com','aes256-ctr','aes192-ctr',
                          'aes128-gcm@openssh.com','aes128-ctr','aes256-cbc',
                          'aes192-cbc','aes128-cbc','3des-cbc']},
          {server2client,['chacha20-poly1305@openssh.com',
                          'aes256-gcm@openssh.com','aes256-ctr','aes192-ctr',
                          'aes128-gcm@openssh.com','aes128-ctr','aes256-cbc',
                          'aes192-cbc','aes128-cbc','3des-cbc']}]},
 {mac,[{client2server,['hmac-sha2-256','hmac-sha2-512',
                       'hmac-sha1']},
       {server2client,['hmac-sha2-256','hmac-sha2-512',
                       'hmac-sha1']}]},
 {compression,[{client2server,[none,'zlib@openssh.com',zlib]},
               {server2client,[none,'zlib@openssh.com',zlib]}]}]
	</code>
	<p>Note that the algorithms in the file <c>ex2.config</c> is not yet applied. They will be
	when we start ssh:
	</p>
	<code>
2> ssh:start().
ok
3> ssh:default_algorithms().
[{kex,['ecdh-sha2-nistp384']},
 {public_key,['ssh-rsa']},
 {cipher,[{client2server,['aes192-ctr']},
          {server2client,['aes192-ctr']}]},
 {mac,[{client2server,['hmac-sha1']},
       {server2client,['hmac-sha1']}]},
 {compression,[{client2server,[none,'zlib@openssh.com',zlib]},
               {server2client,[none,'zlib@openssh.com',zlib]}]}]
4> 

	</code>
	<p>We see that the algorithm set is changed to the one in the  <c>ex2.config</c>. Since
	<c>compression</c> is not specified in the file, the hard-coded default is still used
	for that entry.
	</p>
      </section>

      <section>
	<title>Establishing a connection (ssh:connect et al) or starting a daemon (ssh:daemon)</title>
	<p>Both when the client establishes a connection with ssh:connect or other functions, or a
	daemon is started with ssh:daemon, the option lists in the function calls are also used.
	</p>
	<p>If a client is started (ssh:connect et al), the environment variable <c>client_options</c> is used.
	Similarly for a daemon the <c>server_options</c> variable is handled.
	</p>
	<p>If any
	<seetype marker="ssh#preferred_algorithms_common_option">preferred_algorithms</seetype> is
	present, the one with the highest level is used, that is, the <c>Option</c> list parameter has the highest
	priority. Then the
	<seetype marker="ssh#modify_algorithms_common_option">modify_algorithms</seetype>
	on all levels in order starting with level 0 are applied.
	
	</p>
	<p>We continue the example above by connecting to a server and modifying the <c>kex</c>
	algorithm set. We remove the only one (<c>'ecdh-sha2-nistp384'</c>) and add
	<c>'curve25519-sha256@libssh.org'</c>
	by appending it to the now empty list:</p>
	<code>
4> {ok,C} = ssh:connect(loopback, 22,
                        [{modify_algorithms,
                                 [{rm,
                                     [ {kex,['ecdh-sha2-nistp384']} ]
				  },
                                  {append,
			             [ {kex,['curve25519-sha256@libssh.org']} ]
				  }
				 ]
	                 }
                        ]).
{ok,&gt;0.118.0>}
	</code>
	<p>We check which algorithms are negotiated by the client and the server, and note that
	the (only) <c>kex</c> algorithm <c>'curve25519-sha256@libssh.org'</c> was selected:
	</p>
	<code>
5> ssh:connection_info(C, algorithms).
{algorithms,[{kex,'curve25519-sha256@libssh.org'},
             {hkey,'ssh-rsa'},
             {send_mac,'hmac-sha1'},
             {recv_mac,'hmac-sha1'},
             {encrypt,'aes192-ctr'},
             {decrypt,'aes192-ctr'},
             {compress,none},
             {decompress,none},
             {send_ext_info,false},
             {recv_ext_info,true}]}
	</code>
      </section>

      <section>
	<title>Example of modify_algorithms handling</title>
	<p>We will now check if the
	<seetype marker="ssh#modify_algorithms_common_option">modify_algorithms</seetype>
	on a lower level is applied to a
	<seetype marker="ssh#preferred_algorithms_common_option">preferred_algorithms</seetype>
	on a higher level. We will do that by enabling the <c>ssh-dss</c>
	algorithm that is supported, but not in the default set.
	</p>
	<p>The config file <c>ex3.config</c> has the contents:
	</p>
	<code>
[
 {ssh, [{modify_algorithms,
         [ {prepend, [{public_key, ['ssh-dss']}]} ]
        }]}
].
	</code>
	<p>A newly started erlang shell shows that no <c>'ssh-dss'</c> is present in the
	<c>public_key</c> entry:</p>
	<code>
1> proplists:get_value(public_key, ssh:default_algorithms()).
['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
 'ecdsa-sha2-nistp256','ssh-ed25519','ssh-ed448',
 'rsa-sha2-256','rsa-sha2-512','ssh-rsa']
2> 
	</code>
	<p>A call to <c>ssh:connect/3</c> removes all algorithms but one of each type:
	</p>
	<code>
2> ssh:start().
ok
3> {ok,C} = ssh:connect(loopback, 22,
                        [{preferred_algorithms,
                         [{public_key, ['ecdsa-sha2-nistp256']},
			  {kex, ['ecdh-sha2-nistp256']},
		          {cipher, ['chacha20-poly1305@openssh.com']},
			  {mac, ['hmac-sha2-256']},
			  {compression, [none]}
			  ]}
			 ]).
{ok,&lt;0.101.0>}
4> ssh:connection_info(C,algorithms).
{algorithms,[{kex,'ecdh-sha2-nistp256'},
             {hkey,'ssh-dss'},
             {send_mac,'chacha20-poly1305@openssh.com'},
             {recv_mac,'chacha20-poly1305@openssh.com'},
             {encrypt,'chacha20-poly1305@openssh.com'},
             {decrypt,'chacha20-poly1305@openssh.com'},
             {compress,none},
             {decompress,none},
             {send_ext_info,false},
             {recv_ext_info,true}]}
5>
	</code>
	<p>But <c>'ssh-dss'</c> is selected although the call inserted <em>only</em>
	<c>'ecdsa-sha2-nistp256'</c> as acceptable.
	</p>
	<p>This example showed that we could augment the set of algorithms with a
	config-file without the need to change the actual call.
	</p>
	<p>For demonstration purposes we used <c>prepend</c> instead of <c>append</c>.
	This forces the negotiation to select <c>ssh-dss</c> since the the full list
	of public key algorithms was
	<c>['ssh-dss','ecdsa-sha2-nistp256']</c>.
	Normally it is safer to append a non-default algorithm.
	</p>
      </section>
    </section>
  </section>
</chapter>
