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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>Principal names and DNS — 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="For administrators" href="index.html" /> <link rel="next" title="Encryption types" href="enctypes.html" /> <link rel="prev" title="OTP Preauthentication" href="otp.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="otp.html" title="OTP Preauthentication" accesskey="P">previous</a> | <a href="enctypes.html" title="Encryption types" 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__Principal names and DNS">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="principal-names-and-dns"> <h1>Principal names and DNS<a class="headerlink" href="#principal-names-and-dns" title="Permalink to this headline">¶</a></h1> <p>Kerberos clients can do DNS lookups to canonicalize service principal names. This can cause difficulties when setting up Kerberos application servers, especially when the client’s name for the service is different from what the service thinks its name is.</p> <div class="section" id="service-principal-names"> <h2>Service principal names<a class="headerlink" href="#service-principal-names" title="Permalink to this headline">¶</a></h2> <p>A frequently used kind of principal name is the host-based service principal name. This kind of principal name has two components: a service name and a hostname. For example, <tt class="docutils literal"><span class="pre">imap/imap.example.com</span></tt> is the principal name of the “imap” service on the host “imap.example.com”. Other possible service names for the first component include “host” (remote login services such as ssh), “HTTP”, and “nfs” (Network File System).</p> <p>Service administrators often publish well-known hostname aliases that they would prefer users to use instead of the canonical name of the service host. This gives service administrators more flexibility in deploying services. For example, a shell login server might be named “long-vanity-hostname.example.com”, but users will naturally prefer to type something like “login.example.com”. Hostname aliases also allow for administrators to set up load balancing for some sorts of services based on rotating <tt class="docutils literal"><span class="pre">CNAME</span></tt> records in DNS.</p> </div> <div class="section" id="service-principal-canonicalization"> <h2>Service principal canonicalization<a class="headerlink" href="#service-principal-canonicalization" title="Permalink to this headline">¶</a></h2> <p>MIT Kerberos clients currently always do forward resolution (looking up the IPv4 and possibly IPv6 addresses using <tt class="docutils literal"><span class="pre">getaddrinfo()</span></tt>) of the hostname part of a host-based service principal to canonicalize the hostname. They obtain the “canonical” name of the host when doing so. By default, MIT Kerberos clients will also then do reverse DNS resolution (looking up the hostname associated with the IPv4 or IPv6 address using <tt class="docutils literal"><span class="pre">getnameinfo()</span></tt>) of the hostname. Using the <a class="reference internal" href="conf_files/krb5_conf.html#krb5-conf-5"><em>krb5.conf</em></a> setting:</p> <div class="highlight-python"><pre>[libdefaults] rdns = false</pre> </div> <p>will disable reverse DNS lookup on clients. The default setting is “true”.</p> <p>Operating system bugs may prevent a setting of <tt class="docutils literal"><span class="pre">rdns</span> <span class="pre">=</span> <span class="pre">false</span></tt> from disabling reverse DNS lookup. Some versions of GNU libc have a bug in <tt class="docutils literal"><span class="pre">getaddrinfo()</span></tt> that cause them to look up <tt class="docutils literal"><span class="pre">PTR</span></tt> records even when not required. MIT Kerberos releases krb5-1.10.2 and newer have a workaround for this problem, as does the krb5-1.9.x series as of release krb5-1.9.4.</p> </div> <div class="section" id="reverse-dns-mismatches"> <h2>Reverse DNS mismatches<a class="headerlink" href="#reverse-dns-mismatches" title="Permalink to this headline">¶</a></h2> <p>Sometimes, an enterprise will have control over its forward DNS but not its reverse DNS. The reverse DNS is sometimes under the control of the Internet service provider of the enterprise, and the enterprise may not have much influence in setting up reverse DNS records for its address space. If there are difficulties with getting forward and reverse DNS to match, it is best to set <tt class="docutils literal"><span class="pre">rdns</span> <span class="pre">=</span> <span class="pre">false</span></tt> on client machines.</p> </div> <div class="section" id="overriding-application-behavior"> <h2>Overriding application behavior<a class="headerlink" href="#overriding-application-behavior" title="Permalink to this headline">¶</a></h2> <p>Applications can choose to use a default hostname component in their service principal name when accepting authentication, which avoids some sorts of hostname mismatches. Because not all relevant applications do this yet, using the <a class="reference internal" href="conf_files/krb5_conf.html#krb5-conf-5"><em>krb5.conf</em></a> setting:</p> <div class="highlight-python"><pre>[libdefaults] ignore_acceptor_hostname = true</pre> </div> <p>will allow the Kerberos library to override the application’s choice of service principal hostname and will allow a server program to accept incoming authentications using any key in its keytab that matches the service name and realm name (if given). This setting defaults to “false” and is available in releases krb5-1.10 and later.</p> </div> <div class="section" id="provisioning-keytabs"> <h2>Provisioning keytabs<a class="headerlink" href="#provisioning-keytabs" title="Permalink to this headline">¶</a></h2> <p>One service principal entry that should be in the keytab is a principal whose hostname component is the canonical hostname that <tt class="docutils literal"><span class="pre">getaddrinfo()</span></tt> reports for all known aliases for the host. If the reverse DNS information does not match this canonical hostname, an additional service principal entry should be in the keytab for this different hostname.</p> </div> <div class="section" id="specific-application-advice"> <h2>Specific application advice<a class="headerlink" href="#specific-application-advice" title="Permalink to this headline">¶</a></h2> <div class="section" id="secure-shell-ssh"> <h3>Secure shell (ssh)<a class="headerlink" href="#secure-shell-ssh" title="Permalink to this headline">¶</a></h3> <p>Setting <tt class="docutils literal"><span class="pre">GSSAPIStrictAcceptorCheck</span> <span class="pre">=</span> <span class="pre">no</span></tt> in the configuration file of modern versions of the openssh daemon will allow the daemon to try any key in its keytab when accepting a connection, rather than looking for the keytab entry that matches the host’s own idea of its name (typically the name that <tt class="docutils literal"><span class="pre">gethostname()</span></tt> returns). This requires krb5-1.10 or later.</p> </div> </div> </div> </div> </div> </div> </div> <div class="sidebar"> <h2>On this page</h2> <ul> <li><a class="reference internal" href="#">Principal names and DNS</a><ul> <li><a class="reference internal" href="#service-principal-names">Service principal names</a></li> <li><a class="reference internal" href="#service-principal-canonicalization">Service principal canonicalization</a></li> <li><a class="reference internal" href="#reverse-dns-mismatches">Reverse DNS mismatches</a></li> <li><a class="reference internal" href="#overriding-application-behavior">Overriding application behavior</a></li> <li><a class="reference internal" href="#provisioning-keytabs">Provisioning keytabs</a></li> <li><a class="reference internal" href="#specific-application-advice">Specific application advice</a><ul> <li><a class="reference internal" href="#secure-shell-ssh">Secure shell (ssh)</a></li> </ul> </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 current"><a class="reference internal" href="index.html">For administrators</a><ul class="current"> <li class="toctree-l2"><a class="reference internal" href="install.html">Installation guide</a></li> <li class="toctree-l2"><a class="reference internal" href="conf_files/index.html">Configuration Files</a></li> <li class="toctree-l2"><a class="reference internal" href="realm_config.html">Realm configuration decisions</a></li> <li class="toctree-l2"><a class="reference internal" href="database.html">Database administration</a></li> <li class="toctree-l2"><a class="reference internal" href="lockout.html">Account lockout</a></li> <li class="toctree-l2"><a class="reference internal" href="conf_ldap.html">Configuring Kerberos with OpenLDAP back-end</a></li> <li class="toctree-l2"><a class="reference internal" href="appl_servers.html">Application servers</a></li> <li class="toctree-l2"><a class="reference internal" href="host_config.html">Host configuration</a></li> <li class="toctree-l2"><a class="reference internal" href="backup_host.html">Backups of secure hosts</a></li> <li class="toctree-l2"><a class="reference internal" href="pkinit.html">PKINIT configuration</a></li> <li class="toctree-l2"><a class="reference internal" href="otp.html">OTP Preauthentication</a></li> <li class="toctree-l2 current"><a class="current reference internal" href="">Principal names and DNS</a><ul class="simple"> </ul> </li> <li class="toctree-l2"><a class="reference internal" href="enctypes.html">Encryption types</a></li> <li class="toctree-l2"><a class="reference internal" href="https.html">HTTPS proxy configuration</a></li> <li class="toctree-l2"><a class="reference internal" href="auth_indicator.html">Authentication indicators</a></li> <li class="toctree-l2"><a class="reference internal" href="admin_commands/index.html">Administration programs</a></li> <li class="toctree-l2"><a class="reference internal" href="../mitK5defaults.html">MIT Kerberos defaults</a></li> <li class="toctree-l2"><a class="reference internal" href="env_variables.html">Environment variables</a></li> <li class="toctree-l2"><a class="reference internal" href="troubleshoot.html">Troubleshooting</a></li> <li class="toctree-l2"><a class="reference internal" href="advanced/index.html">Advanced topics</a></li> <li class="toctree-l2"><a class="reference internal" href="various_envs.html">Various links</a></li> </ul> </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"><a class="reference internal" href="../basic/index.html">Kerberos V5 concepts</a></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> 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