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<!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="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"> var DOCUMENTATION_OPTIONS = { URL_ROOT: '../', VERSION: '1.15.1', COLLAPSE_INDEX: false, FILE_SUFFIX: '.html', HAS_SOURCE: true }; </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="copyright" title="Copyright" href="../copyright.html" /> <link rel="top" title="MIT Kerberos Documentation" href="../index.html" /> <link rel="up" title="Kerberos V5 concepts" href="index.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"> <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"><em>keytab</em></a> interfaces, uses <cite>type:value</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="http://tools.ietf.org/html/rfc4120.html#section-10"><strong>RFC 4120</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="default-rcache-type"> <h2>Default rcache type<a class="headerlink" href="#default-rcache-type" title="Permalink to this headline">¶</a></h2> <p>There is currently only one implemented kind of replay cache, called <strong>dfl</strong>. It stores replay data in one file, occasionally rewriting it to purge old, expired entries.</p> <p>The default type can be overridden by the <strong>KRB5RCACHETYPE</strong> environment variable.</p> <p>The placement of the replay cache file is determined by the following:</p> <ol class="arabic simple"> <li>The <strong>KRB5RCACHEDIR</strong> environment variable;</li> <li>If KRB5RCACHEDIR is unspecified, on UNIX, the library will fall back to the environment variable <strong>TMPDIR</strong>, and then to a temporary directory determined at configuration time such as <em>/tmp</em> or <em>/var/tmp</em>; on Windows, it will check the environment variables <em>TEMP</em> and <em>TMP</em>, and fall back to the directory C:\.</li> </ol> </div> <div class="section" id="performance-issues"> <h2>Performance issues<a class="headerlink" href="#performance-issues" title="Permalink to this headline">¶</a></h2> <p>Several known minor performance issues that may occur when replay cache is enabled on the Kerberos system include: delays due to writing the authenticator data to disk slowing down response time for very heavily loaded servers, and delays during the rewrite that may be unacceptable to high-performance services.</p> <p>For use cases where replays are adequately defended against for all protocols using a given service principal name, or where performance or other considerations outweigh the risk of replays, the special replay cache type “none” can be specified:</p> <div class="highlight-python"><div class="highlight"><pre><span class="n">KRB5RCACHETYPE</span><span class="o">=</span><span class="n">none</span> </pre></div> </div> <p>It doesn’t record any information about authenticators, and reports that any authenticator seen is not a replay.</p> </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="#default-rcache-type">Default rcache type</a></li> <li><a class="reference internal" href="#performance-issues">Performance issues</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><ul class="simple"> </ul> </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 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