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docker-mailserver/edge/config/best-practices/dkim/index.html
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<h1>DKIM</h1>
<p>DKIM is a security measure targeting email spoofing. It is greatly recommended one activates it.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>See <a href="https://en.wikipedia.org/wiki/DomainKeys_Identified_Mail">the Wikipedia page</a> for more details on DKIM.</p>
</div>
<h2 id="enabling-dkim-signature"><a class="toclink" href="#enabling-dkim-signature">Enabling DKIM Signature</a></h2>
<p>To enable DKIM signature, <strong>you must have created at least one email account</strong>. Once its done, just run the following command to generate the signature:</p>
<div class="highlight"><pre><span></span><code>./setup.sh config dkim
</code></pre></div>
<p>After generating DKIM keys, you should restart <code>docker-mailserver</code>. DNS edits may take a few minutes to hours to propagate.</p>
<p>The script should ideally be run with a volume for <em>config</em> attached (eg: <code>./docker-data/dms/config/:/tmp/docker-mailserver/</code>), otherwise by default it will mount <code>./config/:/tmp/docker-mailserver/</code>.</p>
<p>The default keysize when generating the signature is 4096 bits for now. If you need to change it (e.g. your DNS provider limits the size), then provide the size as the first parameter of the command:</p>
<div class="highlight"><pre><span></span><code>./setup.sh config dkim keysize &lt;keysize&gt;
</code></pre></div>
<p>For LDAP systems that do not have any directly created user account you can run the following command (since <code>8.0.0</code>) to generate the signature by additionally providing the desired domain name (if you have multiple domains use the command multiple times or provide a comma-separated list of domains):</p>
<div class="highlight"><pre><span></span><code>./setup.sh config dkim keysize &lt;key-size&gt; domain &lt;example.com&gt;<span class="o">[</span>,&lt;not-example.com&gt;<span class="o">]</span>
</code></pre></div>
<p>Now the keys are generated, you can configure your DNS server with DKIM signature, simply by adding a TXT record. If you have direct access to your DNS zone file, then it's only a matter of pasting the content of <code>docker-data/dms/config/opendkim/keys/example.com/mail.txt</code> in your <code>example.com.hosts</code> zone.</p>
<div class="highlight"><pre><span></span><code><span class="gp">$ </span>dig mail._domainkey.example.com TXT
<span class="go">---</span>
<span class="go">;; ANSWER SECTION</span>
<span class="go">mail._domainkey.&lt;DOMAIN&gt; 300 IN TXT &quot;v=DKIM1; k=rsa; p=AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN/AZERTYUIOPQSDFGHJKLMWXCVBN&quot;</span>
</code></pre></div>
<h2 id="configuration-using-a-web-interface"><a class="toclink" href="#configuration-using-a-web-interface">Configuration using a Web Interface</a></h2>
<ol>
<li>Generate a new record of the type <code>TXT</code>.</li>
<li>Paste <code>mail._domainkey</code> the <code>Name</code> txt field.</li>
<li>In the <code>Target</code> or <code>Value</code> field fill in <code>v=DKIM1; k=rsa; p=AZERTYUGHJKLMWX...</code>.</li>
<li>In <code>TTL</code> (time to live): Time span in seconds. How long the DNS server should cache the <code>TXT</code> record.</li>
<li>Save.</li>
</ol>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>Sometimes the key in <code>docker-data/dms/config/opendkim/keys/example.com/mail.txt</code> can be on multiple lines. If so then you need to concatenate the values in the TXT record:</p>
</div>
<div class="highlight"><pre><span></span><code><span class="gp">$ </span>dig mail._domainkey.example.com TXT
<span class="go">---</span>
<span class="go">;; ANSWER SECTION</span>
<span class="go">mail._domainkey.&lt;DOMAIN&gt; 300 IN TXT &quot;v=DKIM1; k=rsa; &quot;</span>
<span class="go"> &quot;p=AZERTYUIOPQSDF...&quot;</span>
<span class="go"> &quot;asdfQWERTYUIOPQSDF...&quot;</span>
</code></pre></div>
<p>The target (or value) field must then have all the parts together: <code>v=DKIM1; k=rsa; p=AZERTYUIOPQSDF...asdfQWERTYUIOPQSDF...</code></p>
<h2 id="verify-only"><a class="toclink" href="#verify-only">Verify-Only</a></h2>
<p>If you want DKIM to only <em>verify</em> incoming emails, the following version of <code>/etc/opendkim.conf</code> may be useful (right now there is no easy mechanism for installing it other than forking the repo):</p>
<div class="highlight"><pre><span></span><code><span class="c1"># This is a simple config file verifying messages only</span><span class="w"></span>
<span class="c1">#LogWhy yes</span><span class="w"></span>
<span class="na">Syslog yes</span><span class="w"></span>
<span class="na">SyslogSuccess yes</span><span class="w"></span>
<span class="na">Socket inet:12301@localhost</span><span class="w"></span>
<span class="na">PidFile /var/run/opendkim/opendkim.pid</span><span class="w"></span>
<span class="na">ReportAddress postmaster@example.com</span><span class="w"></span>
<span class="na">SendReports yes</span><span class="w"></span>
<span class="na">Mode v</span><span class="w"></span>
</code></pre></div>
<h2 id="switch-off-dkim"><a class="toclink" href="#switch-off-dkim">Switch Off DKIM</a></h2>
<p>Simply remove the DKIM key by recreating (not just relaunching) the <code>docker-mailserver</code> container.</p>
<h2 id="debugging"><a class="toclink" href="#debugging">Debugging</a></h2>
<ul>
<li><a href="https://addons.mozilla.org/en-US/thunderbird/addon/dkim-verifier">DKIM-verifer</a>: A add-on for the mail client Thunderbird.</li>
<li>You can debug your TXT records with the <code>dig</code> tool.</li>
</ul>
<div class="highlight"><pre><span></span><code><span class="gp">$ </span>dig TXT mail._domainkey.example.com
<span class="go">---</span>
<span class="go">; &lt;&lt;&gt;&gt; DiG 9.10.3-P4-Debian &lt;&lt;&gt;&gt; TXT mail._domainkey.example.com</span>
<span class="go">;; global options: +cmd</span>
<span class="go">;; Got answer:</span>
<span class="go">;; -&gt;&gt;HEADER&lt;&lt;- opcode: QUERY, status: NOERROR, id: 39669</span>
<span class="go">;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1</span>
<span class="go">;; OPT PSEUDOSECTION:</span>
<span class="go">; EDNS: version: 0, flags:; udp: 512</span>
<span class="go">;; QUESTION SECTION:</span>
<span class="go">;mail._domainkey.example.com. IN TXT</span>
<span class="go">;; ANSWER SECTION:</span>
<span class="go">mail._domainkey.example.com. 3600 IN TXT &quot;v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCxBSjG6RnWAdU3oOlqsdf2WC0FOUmU8uHVrzxPLW2R3yRBPGLrGO1++yy3tv6kMieWZwEBHVOdefM6uQOQsZ4brahu9lhG8sFLPX4MaKYN/NR6RK4gdjrZu+MYSdfk3THgSbNwIDAQAB&quot;</span>
<span class="go">;; Query time: 50 msec</span>
<span class="go">;; SERVER: 127.0.1.1#53(127.0.1.1)</span>
<span class="go">;; WHEN: Wed Sep 07 18:22:57 CEST 2016</span>
<span class="go">;; MSG SIZE rcvd: 310</span>
</code></pre></div>
<hr />
<div class="admonition warning">
<p class="admonition-title">Key sizes &gt;=4096-bit</p>
<p>Keys of 4096 bits could de denied by some mail-servers. According to <a href="https://tools.ietf.org/html/rfc6376">https://tools.ietf.org/html/rfc6376</a> keys are preferably between 512 and 2048 bits. See issue <a href="https://github.com/docker-mailserver/docker-mailserver/issues/1854">#1854</a>.</p>
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