Edit file File name : rcache_def.html Content : <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>replay cache — MIT Kerberos Documentation</title> <link rel="stylesheet" href="../_static/agogo.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <link rel="stylesheet" href="../_static/kerb.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="stash file" href="stash_file_def.html" /> <link rel="prev" title="keytab" href="keytab_def.html" /> </head><body> <div class="header-wrapper"> <div class="header"> <h1><a href="../index.html">MIT Kerberos Documentation</a></h1> <div class="rel"> <a href="../index.html" title="Full Table of Contents" accesskey="C">Contents</a> | <a href="keytab_def.html" title="keytab" accesskey="P">previous</a> | <a href="stash_file_def.html" title="stash file" accesskey="N">next</a> | <a href="../genindex.html" title="General Index" accesskey="I">index</a> | <a href="../search.html" title="Enter search criteria" accesskey="S">Search</a> | <a href="mailto:krb5-bugs@mit.edu?subject=Documentation__replay cache">feedback</a> </div> </div> </div> <div class="content-wrapper"> <div class="content"> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="replay-cache"> <span id="rcache-definition"></span><h1>replay cache<a class="headerlink" href="#replay-cache" title="Permalink to this headline">¶</a></h1> <p>A replay cache (or “rcache”) keeps track of all authenticators recently presented to a service. If a duplicate authentication request is detected in the replay cache, an error message is sent to the application program.</p> <p>The replay cache interface, like the credential cache and <a class="reference internal" href="keytab_def.html#keytab-definition"><span class="std std-ref">keytab</span></a> interfaces, uses <cite>type:residual</cite> strings to indicate the type of replay cache and any associated cache naming data to use.</p> <div class="section" id="background-information"> <h2>Background information<a class="headerlink" href="#background-information" title="Permalink to this headline">¶</a></h2> <p>Some Kerberos or GSSAPI services use a simple authentication mechanism where a message is sent containing an authenticator, which establishes the encryption key that the client will use for talking to the service. But nothing about that prevents an eavesdropper from recording the messages sent by the client, establishing a new connection, and re-sending or “replaying” the same messages; the replayed authenticator will establish the same encryption key for the new session, and the following messages will be decrypted and processed. The attacker may not know what the messages say, and can’t generate new messages under the same encryption key, but in some instances it may be harmful to the user (or helpful to the attacker) to cause the server to see the same messages again a second time. For example, if the legitimate client sends “delete first message in mailbox”, a replay from an attacker may delete another, different “first” message. (Protocol design to guard against such problems has been discussed in <span class="target" id="index-0"></span><a class="rfc reference external" href="https://tools.ietf.org/html/rfc4120.html#section-10"><strong>RFC 4120#section-10</strong></a>.)</p> <p>Even if one protocol uses further protection to verify that the client side of the connection actually knows the encryption keys (and thus is presumably a legitimate user), if another service uses the same service principal name, it may be possible to record an authenticator used with the first protocol and “replay” it against the second.</p> <p>The replay cache mitigates these attacks somewhat, by keeping track of authenticators that have been seen until their five-minute window expires. Different authenticators generated by multiple connections from the same legitimate client will generally have different timestamps, and thus will not be considered the same.</p> <p>This mechanism isn’t perfect. If a message is sent to one application server but a man-in-the-middle attacker can prevent it from actually arriving at that server, the attacker could then use the authenticator (once!) against a different service on the same host. This could be a problem if the message from the client included something more than authentication in the first message that could be useful to the attacker (which is uncommon; in most protocols the server has to indicate a successful authentication before the client sends additional messages), or if the simple act of presenting the authenticator triggers some interesting action in the service being attacked.</p> </div> <div class="section" id="replay-cache-types"> <h2>Replay cache types<a class="headerlink" href="#replay-cache-types" title="Permalink to this headline">¶</a></h2> <p>Unlike the credential cache and keytab interfaces, replay cache types are in lowercase. The following types are defined:</p> <ol class="arabic simple"> <li><strong>none</strong> disables the replay cache. The residual value is ignored.</li> <li><strong>file2</strong> (new in release 1.18) uses a hash-based format to store replay records. The file may grow to accommodate hash collisions. The residual value is the filename.</li> <li><strong>dfl</strong> is the default type if no environment variable or configuration specifies a different type. It stores replay data in a file2 replay cache with a filename based on the effective uid. The residual value is ignored.</li> </ol> <p>For the dfl type, the location of the replay cache file is determined as follows:</p> <ol class="arabic simple"> <li>The directory is taken from the <strong>KRB5RCACHEDIR</strong> environment variable, or the <strong>TMPDIR</strong> environment variable, or a temporary directory determined at configuration time such as <code class="docutils literal notranslate"><span class="pre">/var/tmp</span></code>, in descending order of preference.</li> <li>The filename is <code class="docutils literal notranslate"><span class="pre">krb5_EUID.rcache2</span></code> where EUID is the effective uid of the process.</li> <li>The file is opened without following symbolic links, and ownership of the file is verified to match the effective uid.</li> </ol> <p>On Windows, the directory for the dfl type is the local appdata directory, unless overridden by the <strong>KRB5RCACHEDIR</strong> environment variable. The filename on Windows is <code class="docutils literal notranslate"><span class="pre">krb5.rcache2</span></code>, and the file is opened normally.</p> </div> <div class="section" id="default-replay-cache-name"> <h2>Default replay cache name<a class="headerlink" href="#default-replay-cache-name" title="Permalink to this headline">¶</a></h2> <p>The default replay cache name is determined by the following, in descending order of priority:</p> <ol class="arabic simple"> <li>The <strong>KRB5RCACHENAME</strong> environment variable (new in release 1.18).</li> <li>The <strong>KRB5RCACHETYPE</strong> environment variable. If this variable is set, the residual value is empty.</li> <li>The <strong>default_rcache_name</strong> profile variable in <a class="reference internal" href="../admin/conf_files/krb5_conf.html#libdefaults"><span class="std std-ref">[libdefaults]</span></a> (new in release 1.18).</li> <li>If none of the above are set, the default replay cache name is <code class="docutils literal notranslate"><span class="pre">dfl:</span></code>.</li> </ol> </div> </div> </div> </div> </div> </div> <div class="sidebar"> <h2>On this page</h2> <ul> <li><a class="reference internal" href="#">replay cache</a><ul> <li><a class="reference internal" href="#background-information">Background information</a></li> <li><a class="reference internal" href="#replay-cache-types">Replay cache types</a></li> <li><a class="reference internal" href="#default-replay-cache-name">Default replay cache name</a></li> </ul> </li> </ul> <br/> <h2>Table of contents</h2> <ul class="current"> <li class="toctree-l1"><a class="reference internal" href="../user/index.html">For users</a></li> <li class="toctree-l1"><a class="reference internal" href="../admin/index.html">For administrators</a></li> <li class="toctree-l1"><a class="reference internal" href="../appdev/index.html">For application developers</a></li> <li class="toctree-l1"><a class="reference internal" href="../plugindev/index.html">For plugin module developers</a></li> <li class="toctree-l1"><a class="reference internal" href="../build/index.html">Building Kerberos V5</a></li> <li class="toctree-l1 current"><a class="reference internal" href="index.html">Kerberos V5 concepts</a><ul class="current"> <li class="toctree-l2"><a class="reference internal" href="ccache_def.html">Credential cache</a></li> <li class="toctree-l2"><a class="reference internal" href="keytab_def.html">keytab</a></li> <li class="toctree-l2 current"><a class="current reference internal" href="#">replay cache</a></li> <li class="toctree-l2"><a class="reference internal" href="stash_file_def.html">stash file</a></li> <li class="toctree-l2"><a class="reference internal" href="date_format.html">Supported date and time formats</a></li> </ul> </li> <li class="toctree-l1"><a class="reference internal" href="../formats/index.html">Protocols and file formats</a></li> <li class="toctree-l1"><a class="reference internal" href="../mitK5features.html">MIT Kerberos features</a></li> <li class="toctree-l1"><a class="reference internal" href="../build_this.html">How to build this documentation from the source</a></li> <li class="toctree-l1"><a class="reference internal" href="../about.html">Contributing to the MIT Kerberos Documentation</a></li> <li class="toctree-l1"><a class="reference internal" href="../resources.html">Resources</a></li> </ul> <br/> <h4><a href="../index.html">Full Table of Contents</a></h4> <h4>Search</h4> <form class="search" action="../search.html" method="get"> <input type="text" name="q" size="18" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <div class="clearer"></div> </div> </div> <div class="footer-wrapper"> <div class="footer" > <div class="right" ><i>Release: 1.18.2</i><br /> © <a href="../copyright.html">Copyright</a> 1985-2020, MIT. </div> <div class="left"> <a href="../index.html" title="Full Table of Contents" >Contents</a> | <a href="keytab_def.html" title="keytab" >previous</a> | <a href="stash_file_def.html" title="stash file" >next</a> | <a href="../genindex.html" title="General Index" >index</a> | <a href="../search.html" title="Enter search criteria" >Search</a> | <a href="mailto:krb5-bugs@mit.edu?subject=Documentation__replay cache">feedback</a> </div> </div> </div> </body> </html>Save