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authorHans Nilsson <[email protected]>2017-08-28 15:32:08 +0200
committerHans Nilsson <[email protected]>2017-09-11 12:11:42 +0200
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parentb3cb129a5518d0018340eea088acd409b616836a (diff)
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ssh: Document the option modify_algorithms and a User's Guide chapter
Diffstat (limited to 'lib/ssh/doc/src')
-rw-r--r--lib/ssh/doc/src/Makefile3
-rw-r--r--lib/ssh/doc/src/configure_algos.xml457
-rw-r--r--lib/ssh/doc/src/ssh.xml80
-rw-r--r--lib/ssh/doc/src/ssh_app.xml4
-rw-r--r--lib/ssh/doc/src/usersguide.xml1
5 files changed, 541 insertions, 4 deletions
diff --git a/lib/ssh/doc/src/Makefile b/lib/ssh/doc/src/Makefile
index a759854da4..adbda5a030 100644
--- a/lib/ssh/doc/src/Makefile
+++ b/lib/ssh/doc/src/Makefile
@@ -53,7 +53,8 @@ XML_PART_FILES = part_notes.xml \
XML_CHAPTER_FILES = notes.xml \
introduction.xml \
ssh_protocol.xml \
- using_ssh.xml
+ using_ssh.xml \
+ configure_algos.xml
BOOK_FILES = book.xml
diff --git a/lib/ssh/doc/src/configure_algos.xml b/lib/ssh/doc/src/configure_algos.xml
new file mode 100644
index 0000000000..ebd128c097
--- /dev/null
+++ b/lib/ssh/doc/src/configure_algos.xml
@@ -0,0 +1,457 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<!DOCTYPE chapter SYSTEM "chapter.dtd">
+
+<chapter>
+ <header>
+ <copyright>
+ <year>2017</year>
+ <year>2017</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.
+ You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+ Unless required by applicable law or agreed to in writing, software
+ distributed under the License is distributed on an "AS IS" BASIS,
+ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ See the License for the specific language governing permissions and
+ limitations under the License.
+
+ </legalnotice>
+
+ <title>Configuring algorithms in SSH</title>
+ <prepared></prepared>
+ <docno></docno>
+ <approved></approved>
+ <date></date>
+ <rev></rev>
+ <file>configure_algos.xml</file>
+ </header>
+
+ <section>
+ <marker id="introduction"/>
+ <title>Introduction</title>
+ <p>To fully understand how to configure the algorithms, we must understand partly both how the ssh protocol
+ works and how the OTP SSH app handles the corresponding items</p>
+
+ <p>The first subsection will give a short background about the ssh protocol while later sections describes
+ the implementation and provides many examples</p>
+
+ <section>
+ <title>Basics of the ssh protocol's algorithms handling</title>
+
+ <p>SSH uses different sets of algorithms in different phases of a session. Which
+ algorithms to use is negotiated by the client and the server at the beginning of a session.
+ See <url href="https://tools.ietf.org/html/rfc4253">RFC 4253</url>,
+ "The Secure Shell (SSH) Transport Layer Protocol" for details.
+ </p>
+
+ <p>The negotiation is simple: both peers sends their list of supported alghorithms to the other part.
+ The first algorithm on the client's list that also in on the server's list is selected. So it is the
+ client's orderering of the list that gives the priority for the algorithms.</p>
+
+ <p>There are five lists exchanged in the connection setup. Three of them are also divided in two
+ directions, to and from the server.</p>
+
+ <p>The lists are (named as in the SSH application's options):</p>
+ <taglist>
+ <tag><c>kex</c></tag>
+ <item>
+ <p>Key exchange.</p>
+ <p>An algorithm is selected for computing a secret encryption key. Among examples are:
+ the old nowadays week <c>'diffie-hellman-group-exchange-sha1'</c> and the very strong and modern
+ <c>'ecdh-sha2-nistp512'</c>.</p>
+ </item>
+
+ <tag><c>public_key</c></tag>
+ <item>
+ <p>Server host key</p>
+ <p>The asymetric encryption algorithm used in the server's private-public host key pair.
+ Examples include the well-known RSA <c>'ssh-rsa'</c> and elliptic curve <c>'ecdsa-sha2-nistp521'</c>.
+ </p>
+ </item>
+
+ <tag><c>cipher</c></tag>
+ <item>
+ <p>Symetric cipher algorithm used for the message encryption. This algorithm will use the key calculated
+ in the kex phase (together with other info) to genereate the actual key used. Examples are
+ tripple-DES <c>'3des-cbc'</c> and one of many AES variants <c>'aes192-ctr'</c>.
+ </p>
+ <p>This list is actually two - one for each direction server-to-client and client-to-server. Therefore it
+ is possible but rare to have different algorithms in the two directions in one connection.</p>
+ </item>
+
+ <tag><c>mac</c></tag>
+ <item>
+ <p>Message authentication code</p>
+ <p>"Check sum" of each message sent between the peers. Examples are SHA <c>'hmac-sha1'</c> and
+ SHA2 <c>'hmac-sha2-512'</c>.</p>
+ <p>This list is also divided into two for the both directions</p>
+ </item>
+
+ <tag><c>compression</c></tag>
+ <item>
+ <p>If and how to compress the message. Examples are <c>none</c>, that is, no compression and
+ <c>zlib</c>.</p>
+ <p>This list is also divided into two for the both directions</p>
+ </item>
+
+ </taglist>
+ </section>
+
+ <section>
+ <title>The SSH app's mechanism</title>
+ <p>The set of algorithms that the SSH app uses by default depends on the algoritms supported by the:</p>
+ <list>
+ <item><p><seealso marker="crypto:crypto">crypto</seealso> app,</p>
+ </item>
+ <item><p>The cryptolib OTP is linked with, usally the one the OS uses, probably OpenSSL,</p>
+ </item>
+ <item><p>and finaly what the SSH app implements</p>
+ </item>
+ </list>
+ <p>Due to this, it impossible to list in documentation what algorithms that are available in a certain installation.</p>
+ <p>There is an important commands to list the actual algorithms and their ordering:
+ <seealso marker="ssh#default_algorithms-0">ssh:default_algorithms/0</seealso>.</p>
+ <code type="erl">
+0> 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',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['[email protected]',
+ 'aes256-ctr','aes192-ctr','[email protected]',
+ 'aes128-ctr','aes128-cbc','3des-cbc']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ '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,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+
+ </code>
+ <p>To change this listing, there are two options which can be used in
+ <seealso marker="ssh#connect-3">ssh:default_algorithms/2,3,4</seealso>
+ and
+ <seealso marker="ssh#daemon-2">ssh:daemon/2,3</seealso>. The options could of course
+ be used in all other functions that initiates connections.</p>
+
+ <p>The options are <c>preferred_algorithms</c> and <c>modify_algorithms</c>. The first one
+ replaces the default set, while the latter modifies the default set.</p>
+ </section>
+ </section>
+
+ <section>
+ <title>Replacing the default set: preferred_algorithms</title>
+ <p>See the <seealso marker="ssh#option_preferred_algorithms">Reference Manual</seealso> for details</p>
+
+ <p>Here follows a series of examples ranging from simple to more complex.</p>
+
+ <p>The experimental function <c>ssh:chk_algos_opts(Opts)</c> mangles the options <c>preferred_algorithms</c>
+ and <c>modify_algorithms</c> as <c>ssh:dameon</c>, <c>ssh:connect</c> and others does.</p>
+
+ <section>
+ <title>Example 1</title>
+ <p>Replace the kex algorithms list with the single algorithm <c>'diffie-hellman-group14-sha256'</c>:</p>
+ <code>
+1> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{kex, ['diffie-hellman-group14-sha256']}
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group14-sha256']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['[email protected]',
+ 'aes256-ctr','aes192-ctr','[email protected]',
+ 'aes128-ctr','aes128-cbc','3des-cbc']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ '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,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+ </code>
+ <p>Note that the unmentioned lists (<c>public_key</c>, <c>cipher</c>, <c>mac</c> and <c>compression</c>)
+ are un-changed.</p>
+ </section>
+
+ <section>
+ <title>Example 2</title>
+ <p>In the lists that are divided in two for the two directions (c.f <c>cipher</c>) it is possible
+ to change both directions at once:</p>
+ <code>
+2> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,['aes128-ctr']}
+ ]
+ }
+ ]).
+[{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',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['aes128-ctr']}]},
+ {mac,[{client2server,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']},
+ {server2client,['hmac-sha2-256','hmac-sha2-512',
+ 'hmac-sha1']}]},
+ {compression,[{client2server,[none,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+ </code>
+ <p>Note that both lists in <c>cipher</c> has been changed to the provided value (<c>'aes128-ctr'</c>).</p>
+ </section>
+
+ <section>
+ <title>Example 3</title>
+ <p>In the lists that are divided in two for the two directions (c.f <c>cipher</c>) it is possible
+ to change only one of the directions:</p>
+ <code>
+3> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,[{client2server,['aes128-ctr']}]}
+ ]
+ }
+ ]).
+[{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',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ '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,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+ </code>
+ </section>
+
+ <section>
+ <title>Example 4</title>
+ <p>It is of course possible to change more than one list:</p>
+ <code>
+4> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,['aes128-ctr']},
+ {mac,['hmac-sha2-256']},
+ {kex,['ecdh-sha2-nistp384']},
+ {public_key,['ssh-rsa']},
+ {compression,[{server2client,[none]},
+ {client2server,[zlib]}]}
+ ]
+ }
+ ]).
+[{kex,['ecdh-sha2-nistp384']},
+ {public_key,['ssh-rsa']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['aes128-ctr']}]},
+ {mac,[{client2server,['hmac-sha2-256']},
+ {server2client,['hmac-sha2-256']}]},
+ {compression,[{client2server,[zlib]},
+ {server2client,[none]}]}]
+
+ </code>
+ <p>Note that the ordering of the tuples in the lists didn't matter.</p>
+ </section>
+ </section>
+
+ <section>
+ <title>Modifying the default set: modify_algorithms</title>
+ <p>The option <c>preferred_algorithms</c> is complicated to use for adding or removing single algorithms. One has
+ to first list them with <c>ssh:default_algorithms()</c> and then do substitutions in the lists. A situation
+ when it might be useful to add an algorithm is when one need to use a supported but disabled one. An example
+ is the kex <c>'diffie-hellman-group1-sha1'</c> which nowadays is very unsecure and therefore disabled. It is
+ however still supported and might be used.</p>
+
+ <p>To facilitate addition or removal of algorithms the option <c>modify_algorithms</c> is available.
+ See the <seealso marker="ssh#option_modify_algorithms">Reference Manual</seealso> for details.</p>
+
+ <p>The option takes a list with instructions to append, prepend or remove algorithms:</p>
+ <code type="erl">
+{modify_algorithms, [{append, ...},
+ {prepend, ...},
+ {rm, ...}
+ ]}
+ </code>
+ <p>Each of the <c>...</c> can be a <c>algs_list()</c> as the argument to the <c>preferred_algorithms</c> option.</p>
+ <section>
+ <title>Example 5</title>
+ <p>As an example let's add the Diffie-Hellman Group1 first in the kex list. It is supported according to
+ <seealso marker="SSH_app#supported_algos">Supported algoritms</seealso>.</p>
+ <code type="erl">
+5> ssh:chk_algos_opts(
+ [{modify_algorithms,
+ [{prepend,
+ [{kex,['diffie-hellman-group1-sha1']}]
+ }
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group1-sha1','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',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ 'ecdsa-sha2-nistp256','ssh-rsa','rsa-sha2-256',
+ 'rsa-sha2-512','ssh-dss']},
+ {cipher,[{client2server,['[email protected]',
+ 'aes256-ctr','aes192-ctr','[email protected]',
+ 'aes128-ctr','aes128-cbc','3des-cbc']},
+ {server2client,['[email protected]','aes256-ctr',
+ 'aes192-ctr','[email protected]','aes128-ctr',
+ '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,'[email protected]',zlib]},
+ {server2client,[none,'[email protected]',zlib]}]}]
+
+ </code>
+ <p>And the result shows that the Diffie-Hellman Group1 is added at the head of the kex list</p>
+ </section>
+
+ <section>
+ <title>Example 6</title>
+ <p>In next example, we also move the <c>'ecdh-sha2-nistp521'</c> to the end in the kex
+ list, that is, <c>append</c>.</p>
+ <code type="erl">
+6> ssh:chk_algos_opts(
+ [{modify_algorithms,
+ [{prepend,
+ [{kex, ['diffie-hellman-group1-sha1']}
+ ]},
+ {append,
+ [{kex, ['ecdh-sha2-nistp521']}
+ ]}
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group1-sha1','ecdh-sha2-nistp384',
+ 'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1','ecdh-sha2-nistp521']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ .....
+]
+ </code>
+ <p>Note that the appended algorithm is removed from its original place and then appended.</p>
+ </section>
+
+ <section>
+ <title>Example 7</title>
+ <p>In next example, we also move the <c>'ecdh-sha2-nistp521'</c> to the end in the kex
+ list, that is, <c>append</c>.</p>
+ <code type="erl">
+7> ssh:chk_algos_opts(
+ [{modify_algorithms,
+ [{prepend,
+ [{kex, ['diffie-hellman-group1-sha1']}
+ ]},
+ {append,
+ [{kex, ['ecdh-sha2-nistp521']}
+ ]}
+ ]
+ }
+ ]).
+[{kex,['diffie-hellman-group1-sha1','ecdh-sha2-nistp384',
+ 'ecdh-sha2-nistp256','diffie-hellman-group-exchange-sha256',
+ 'diffie-hellman-group16-sha512',
+ 'diffie-hellman-group18-sha512',
+ 'diffie-hellman-group14-sha256',
+ 'diffie-hellman-group14-sha1',
+ 'diffie-hellman-group-exchange-sha1','ecdh-sha2-nistp521']},
+ {public_key,['ecdsa-sha2-nistp384','ecdsa-sha2-nistp521',
+ .....
+]
+ </code>
+ <p>Note that the appended algorithm first is removed from its original place and then appended.</p>
+ </section>
+
+
+ <section>
+ <title>Example 8</title>
+ <p>In this example, we use both options (<c>preferred_algorithms</c> and <c>modify_algorithms</c>) and
+ also try to prepend an unsupported algorithm. Any unsupported algorithm is quietly removed.</p>
+ <code type="erl">
+8> ssh:chk_algos_opts(
+ [{preferred_algorithms,
+ [{cipher,['aes128-ctr']},
+ {mac,['hmac-sha2-256']},
+ {kex,['ecdh-sha2-nistp384']},
+ {public_key,['ssh-rsa']},
+ {compression,[{server2client,[none]},
+ {client2server,[zlib]}]}
+ ]
+ },
+ {modify_algorithms,
+ [{prepend,
+ [{kex, ['some unsupported algorithm']}
+ ]},
+ {append,
+ [{kex, ['diffie-hellman-group1-sha1']}
+ ]}
+ ]
+ }
+ ]).
+[{kex,['ecdh-sha2-nistp384','diffie-hellman-group1-sha1']},
+ {public_key,['ssh-rsa']},
+ {cipher,[{client2server,['aes128-ctr']},
+ {server2client,['aes128-ctr']}]},
+ {mac,[{client2server,['hmac-sha2-256']},
+ {server2client,['hmac-sha2-256']}]},
+ {compression,[{client2server,[zlib]},
+ {server2client,[none]}]}]
+
+ </code>
+ <p>It is of course questionable why anyone would like to use the both options together, but it is possible
+ if the needed.</p>
+ </section>
+
+
+
+ </section>
+
+</chapter>
diff --git a/lib/ssh/doc/src/ssh.xml b/lib/ssh/doc/src/ssh.xml
index ea7e975ef5..48553bf503 100644
--- a/lib/ssh/doc/src/ssh.xml
+++ b/lib/ssh/doc/src/ssh.xml
@@ -108,6 +108,9 @@
<tag><c>double_algs() =</c></tag>
<item><p><c>[{client2serverlist,simple_algs()},{server2client,simple_algs()}] | simple_algs()</c></p></item>
+
+ <tag><c>modify_algs_list() =</c></tag>
+ <item><p><c>list( {append,algs_list()} | {prepend,algs_list()} | {rm,algs_list()} )</c></p></item>
</taglist>
</section>
@@ -254,7 +257,8 @@
</p>
</item>
- <tag><c><![CDATA[{preferred_algorithms, algs_list()}]]></c></tag>
+ <tag><marker id="option_preferred_algorithms"></marker>
+ <c><![CDATA[{preferred_algorithms, algs_list()}]]></c></tag>
<item>
<p>List of algorithms to use in the algorithm negotiation. The default <c>algs_list()</c> can
be obtained from <seealso marker="#default_algorithms/0">default_algorithms/0</seealso>.
@@ -275,6 +279,8 @@
for cipher but specifies the same algorithms for mac and compression in both directions.
The kex (key exchange) is implicit but public_key is set explicitly.</p>
+ <p>For background and more examples see the <seealso marker="configure_algos#introduction">User's Guide</seealso>.</p>
+
<warning>
<p>Changing the values can make a connection less secure. Do not change unless you
know exactly what you are doing. If you do not understand the values then you
@@ -282,6 +288,41 @@
</warning>
</item>
+ <tag><marker id="option_modify_algorithms"></marker>
+ <c><![CDATA[{modify_algorithms, modify_algs_list()}]]></c></tag>
+ <item>
+ <p>Modifies the list of algorithms to use in the algorithm negotiation. The modifications are
+ applied after the option <c>preferred_algorithms</c> is applied (if existing)</p>
+ <p>The possible modifications are to:</p>
+ <list>
+ <item><p>Append or prepend supported but not enabled algorithm(s) to the list of
+ algorithms.</p><p>If the wanted algorithms already are in the list of algorithms, they will first
+ be removed and then appended or prepended.
+ </p>
+ </item>
+ <item><p>Remove (rm) one or more algorithms from the list of algorithms.</p></item>
+ </list>
+ <p>If an unsupported algorithm is in the list, it will be silently ignored</p>
+
+ <p>Here is an example of this option:</p>
+ <code>
+{modify_algorithms,
+ [{prepend, [{kex, ['diffie-hellman-group1-sha1']}],
+ {rm, [{compression, [none]}]}
+ ]
+}
+</code>
+ <p>The example specifies that:</p>
+ <list>
+ <item><p>the old key exchange algorithm 'diffie-hellman-group1-sha1' should be
+ the main alternative. It will be the main alternative since it is prepened to the list</p>
+ </item>
+ <item><p>The compression algorithm none (= no compression) is removed so compression is enforced</p>
+ </item>
+ </list>
+ <p>For background and more examples see the <seealso marker="configure_algos#introduction">User's Guide</seealso>.</p>
+ </item>
+
<tag><c><![CDATA[{dh_gex_limits,{Min=integer(),I=integer(),Max=integer()}}]]></c></tag>
<item>
<p>Sets the three diffie-hellman-group-exchange parameters that guides the connected server in choosing a group.
@@ -555,6 +596,8 @@
for cipher but specifies the same algorithms for mac and compression in both directions.
The kex (key exchange) is implicit but public_key is set explicitly.</p>
+ <p>For background and more examples see the <seealso marker="configure_algos#introduction">User's Guide</seealso>.</p>
+
<warning>
<p>Changing the values can make a connection less secure. Do not change unless you
know exactly what you are doing. If you do not understand the values then you
@@ -562,6 +605,41 @@
</warning>
</item>
+ <tag><marker id="option_modify_algorithms"></marker>
+ <c><![CDATA[{modify_algorithms, modify_algs_list()}]]></c></tag>
+ <item>
+ <p>Modifies the list of algorithms to use in the algorithm negotiation. The modifications are
+ applied after the option <c>preferred_algorithms</c> is applied (if existing)</p>
+ <p>The possible modifications are to:</p>
+ <list>
+ <item><p>Append or prepend supported but not enabled algorithm(s) to the list of
+ algorithms.</p><p>If the wanted algorithms already are in the list of algorithms, they will first
+ be removed and then appended or prepended.
+ </p>
+ </item>
+ <item><p>Remove (rm) one or more algorithms from the list of algorithms.</p></item>
+ </list>
+ <p>If an unsupported algorithm is in the list, it will be silently ignored</p>
+
+ <p>Here is an example of this option:</p>
+ <code>
+{modify_algorithms,
+ [{prepend, [{kex, ['diffie-hellman-group1-sha1']}],
+ {rm, [{compression, [none]}]}
+ ]
+}
+</code>
+ <p>The example specifies that:</p>
+ <list>
+ <item><p>the old key exchange algorithm 'diffie-hellman-group1-sha1' should be
+ the main alternative. It will be the main alternative since it is prepened to the list</p>
+ </item>
+ <item><p>The compression algorithm none (= no compression) is removed so compression is enforced</p>
+ </item>
+ </list>
+ <p>For background and more examples see the <seealso marker="configure_algos#introduction">User's Guide</seealso>.</p>
+ </item>
+
<tag><c><![CDATA[{dh_gex_groups, [{Size=integer(),G=integer(),P=integer()}] | {file,filename()} {ssh_moduli_file,filename()} }]]></c></tag>
<item>
<p>Defines the groups the server may choose among when diffie-hellman-group-exchange is negotiated.
diff --git a/lib/ssh/doc/src/ssh_app.xml b/lib/ssh/doc/src/ssh_app.xml
index 33ec7aaee0..1cbbdfcf38 100644
--- a/lib/ssh/doc/src/ssh_app.xml
+++ b/lib/ssh/doc/src/ssh_app.xml
@@ -97,7 +97,7 @@
<p>The <c>known_hosts</c> file contains a list of approved servers and
their public keys. Once a server is listed, it can be verified
without user interaction.
- </p>
+ </p>
</section>
<section>
<title>Authorized Keys</title>
@@ -135,7 +135,7 @@
</p>
<p>Supported algorithms are:</p>
-
+ <marker id="supported_algos"></marker>
<taglist>
<tag>Key exchange algorithms</tag>
<item>
diff --git a/lib/ssh/doc/src/usersguide.xml b/lib/ssh/doc/src/usersguide.xml
index 70051ba771..d902df6848 100644
--- a/lib/ssh/doc/src/usersguide.xml
+++ b/lib/ssh/doc/src/usersguide.xml
@@ -36,4 +36,5 @@
</description>
<xi:include href="introduction.xml"/>
<xi:include href="using_ssh.xml"/>
+ <xi:include href="configure_algos.xml"/>
</part>