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<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>
<blockquote>
<div><div class="highlight-python"><pre>[libdefaults]
rdns = false</pre>
</div>
</div></blockquote>
<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>
<blockquote>
<div><div class="highlight-python"><pre>[libdefaults]
ignore_acceptor_hostname = true</pre>
</div>
</div></blockquote>
<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>
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<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>
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