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<h1>Create a mail server with Postfix, Dovecot, SpamAssassin and OpenDKIM</h1>
<p>The entry is going to be long because it’s a <em>tedious</em> process. This is also based on <a href="https://github.com/LukeSmithxyz/emailwiz">Luke Smith’s script</a>, but adapted to Arch Linux (his script works on debian-based distributions). This entry is mostly so I can record all the notes required while I’m in the process of installing/configuring the mail server on a new VPS of mine; also I’m going to be writing a script that does everything in one go (for Arch Linux), that will be hosted <a href="https://git.luevano.xyz/server_scripts.git">here</a>.</p>
<p>This configuration works for local users (users that appear in <code>/etc/passwd</code>), and does not use any type of SQL Database. And note that most if not all commands executed here are run with root privileges.</p>
<h2 id="prerequisites">Prerequisites</h2>
<p>Basically the same as with the <a href="https://blog.luevano.xyz/a/website_with_nginx.html">website with Nginx and Certbot</a>, with the extras:</p>
<ul>
<li>You will need a <strong>CNAME</strong> for “mail” and (optionally) “www.mail”, or whatever you want to call the sub-domains (although the <a href="https://tools.ietf.org/html/rfc2181#section-10.3">RFC 2181</a> states that it NEEDS to be an <strong>A</strong> record, fuck the police).</li>
<li>An SSL certificate. You can use the SSL certificate obtained following my last post using <code>certbot</code> (just create a <code>mail.conf</code> and run <code>certbot --nginx</code> again).</li>
<li>Ports 25, 587 (SMTP), 465 (SMTPS), 143 (IMAP) and 993 (IMAPS) open on the firewall.</li>
</ul>
<h2 id="postfix">Postfix</h2>
<p><a href="https://wiki.archlinux.org/title/postfix">Postfix</a> is a “mail transfer agent” which is the component of the mail server that receives and sends emails via SMTP.</p>
<p>Install the <code>postfix</code> package:</p>
<pre><code class="language-sh">pacman -S postfix
</code></pre>
<p>We have two main files to configure (inside <code>/etc/postfix</code>): <code>master.cf</code> (<a href="https://man.archlinux.org/man/master.5">master(5)</a>) and <code>main.cf</code> (<a href="https://man.archlinux.org/man/postconf.5">postconf(5)</a>). We’re going to edit <code>main.cf</code> first either by using the command <code>postconf -e 'setting'</code> or by editing the file itself (I prefer to edit the file).</p>
<p>Note that the default file itself has a lot of comments with description on what each thing does (or you can look up the manual, linked above), I used what Luke’s script did plus some other settings that worked for me.</p>
<p>Now, first locate where your website cert is, mine is at the default location <code>/etc/letsencrypt/live/</code>, so my <code>certdir</code> is <code>/etc/letsencrypt/live/luevano.xyz</code>. Given this information, change <code>{yourcertdir}</code> on the corresponding lines. The configuration described below has to be appended in the <code>main.cf</code> configuration file.</p>
<p>Certificates and ciphers to use for authentication and security:</p>
<pre><code class="language-apache">smtpd_tls_key_file = {yourcertdir}/privkey.pem
smtpd_tls_cert_file = {yourcertdir}/fullchain.pem
smtpd_use_tls = yes
smtpd_tls_auth_only = yes
smtp_tls_security_level = may
smtp_tls_loglevel = 1
smtp_tls_CAfile = {yourcertdir}/cert.pem
smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1
tls_preempt_cipherlist = yes
smtpd_tls_exclude_ciphers = aNULL, LOW, EXP, MEDIUM, ADH, AECDH, MD5,
DSS, ECDSA, CAMELLIA128, 3DES, CAMELLIA256,
RSA+AES, eNULL
smtp_tls_CApath = /etc/ssl/certs
smtpd_tls_CApath = /etc/ssl/certs
smtpd_relay_restrictions = permit_sasl_authenticated, permit_mynetworks, defer_unauth_destination
</code></pre>
<p>Also, for the <em>connection</em> with <code>dovecot</code>, append the next few lines (telling postfix that <code>dovecot</code> will use user/password for authentication):</p>
<pre><code class="language-apache">smtpd_sasl_auth_enable = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth
smtpd_sasl_security_options = noanonymous, noplaintext
smtpd_sasl_tls_security_options = noanonymous
</code></pre>
<p>Specify the mailbox home (this is going to be a directory inside your user’s home containing the actual mail files):</p>
<pre><code class="language-apache">home_mailbox = Mail/Inbox/
</code></pre>
<p>Pre-configuration to work seamlessly with <code>dovecot</code> and <code>opendkim</code>:</p>
<pre><code class="language-apache">myhostname = {yourdomainname}
mydomain = localdomain
mydestination = $myhostname, localhost.$mydomain, localhost
milter_default_action = accept
milter_protocol = 6
smtpd_milters = inet:127.0.0.1:8891
non_smtpd_milters = inet:127.0.0.1:8891
mailbox_command = /usr/lib/dovecot/deliver
</code></pre>
<p>Where <code>{yourdomainname}</code> is <code>luevano.xyz</code> in my case, or if you have <code>localhost</code> configured to your domain, then use <code>localhost</code> for <code>myhostname</code> (<code>myhostname = localhost</code>).</p>
<p>Lastly, if you don’t want the sender’s IP and user agent (application used to send the mail), add the following line:</p>
<pre><code class="language-apache">smtp_header_checks = regexp:/etc/postfix/smtp_header_checks
</code></pre>
<p>And create the <code>/etc/postfix/smtp_header_checks</code> file with the following content:</p>
<pre><code class="language-coffee">/^Received: .*/ IGNORE
/^User-Agent: .*/ IGNORE
</code></pre>
<p>That’s it for <code>main.cf</code>, now we have to configure <code>master.cf</code>. This one is a bit more tricky.</p>
<p>First look up lines (they’re uncommented) <code>smtp inet n - n - - smtpd</code>, <code>smtp unix - - n - - smtp</code> and <code>-o syslog_name=postfix/$service_name</code> and either delete or uncomment them… or just run <code>sed -i "/^\s*-o/d;/^\s*submission/d;/\s*smtp/d" /etc/postfix/master.cf</code> as stated in Luke’s script.</p>
<p>Lastly, append the following lines to complete postfix setup and pre-configure for <code>spamassassin</code>.</p>
<pre><code class="language-txt">smtp unix - - n - - smtp
smtp inet n - y - - smtpd
-o content_filter=spamassassin
submission inet n - y - - smtpd
-o syslog_name=postfix/submission
-o smtpd_tls_security_level=encrypt
-o smtpd_sasl_auth_enable=yes
-o smtpd_tls_auth_only=yes
smtps inet n - y - - smtpd
-o syslog_name=postfix/smtps
-o smtpd_tls_wrappermode=yes
-o smtpd_sasl_auth_enable=yes
spamassassin unix - n n - - pipe
user=spamd argv=/usr/bin/vendor_perl/spamc -f -e /usr/sbin/sendmail -oi -f \${sender} \${recipient}
</code></pre>
<p>Now, I ran into some problems with postfix, one being <a href="https://www.faqforge.com/linux/fix-for-opensuse-error-postfixmaster-fatal-0-0-0-0smtps-servname-not-supported-for-ai_socktype/">smtps: Servname not supported for ai_socktype</a>, to fix it, as <em>Till</em> posted in that site, edit <code>/etc/services</code> and add:</p>
<pre><code class="language-apache">smtps 465/tcp
smtps 465/udp
</code></pre>
<p>Before starting the <code>postfix</code> service, you need to run <code>newaliases</code> first, but you can do a bit of configuration beforehand editing the file <code>/etc/postfix/aliases</code>. I only change the <code>root: you</code> line (where <code>you</code> is the account that will be receiving “root” mail). After you’re done, run:</p>
<pre><code class="language-sh">postalias /etc/postfix/aliases
newaliases
</code></pre>
<p>At this point you’re done configuring <code>postfix</code> and you can already start/enable the <code>postfix</code> service:</p>
<pre><code class="language-sh">systemctl start postfix.service
systemctl enable postfix.service
</code></pre>
<h2 id="dovecot">Dovecot</h2>
<p><a href="https://wiki.archlinux.org/title/Dovecot">Dovecot</a> is an IMAP and POP3 server, which is what lets an email application retrieve the mail.</p>
<p>Install the <code>dovecot</code> and <code>pigeonhole</code> (sieve for <code>dovecot</code>) packages:</p>
<pre><code class="language-sh">pacman -S dovecot pigeonhole
</code></pre>
<p>On arch, by default, there is no <code>/etc/dovecot</code> directory with default configurations set in place, but the package does provide the example configuration files. Create the <code>dovecot</code> directory under <code>/etc</code> and, optionally, copy the <code>dovecot.conf</code> file and <code>conf.d</code> directory under the just created <code>dovecot</code> directory:</p>
<pre><code class="language-sh">mkdir /etc/dovecot
cp /usr/share/doc/dovecot/example-config/dovecot.conf /etc/dovecot/dovecot.conf
cp -r /usr/share/doc/dovecot/example-config/conf.d /etc/dovecot
</code></pre>
<p>As Luke stated, <code>dovecot</code> comes with a lot of “modules” (under <code>/etc/dovecot/conf.d/</code> if you copied that folder) for all sorts of configurations that you can include, but I do as he does and just edit/create the whole <code>dovecot.conf</code> file; although, I would like to check each of the separate configuration files <code>dovecot</code> provides I think the options Luke provides are more than good enough.</p>
<p>I’m working with an empty <code>dovecot.conf</code> file. Add the following lines for SSL and login configuration (also replace <code>{yourcertdir}</code> with the same certificate directory described in the Postfix section above, note that the <code><</code> is required):</p>
<pre><code class="language-apache">ssl = required
ssl_cert = <{yourcertdir}/fullchain.pem
ssl_key = <{yourcertdir}/privkey.pem
ssl_min_protocol = TLSv1.2
ssl_cipher_list = ALL:!RSA:!CAMELLIA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SHA1:!SHA256:!SHA384:!LOW@STRENGTH
ssl_prefer_server_ciphers = yes
ssl_dh = </etc/dovecot/dh.pem
auth_mechanisms = plain login
auth_username_format = %n
protocols = $protocols imap
</code></pre>
<p>You may notice we specify a file we don’t have under <code>/etc/dovecot</code>: <code>dh.pem</code>. We need to create it with <code>openssl</code> (you should already have it installed if you’ve been following this entry and the one for <code>nginx</code>). Just run (might take a few minutes):</p>
<pre><code class="language-sh">openssl dhparam -out /etc/dovecot/dh.pem 4096
</code></pre>
<p>After that, the next lines define what a “valid user is” (really just sets the database for users and passwords to be the local users with their password):</p>
<pre><code class="language-apache">userdb {
driver = passwd
}
passdb {
driver = pam
}
</code></pre>
<p>Next, comes the mail directory structure (has to match the one described in the Postfix section). Here, the <code>LAYOUT</code> option is important so the boxes are <code>.Sent</code> instead of <code>Sent</code>. Add the next lines (plus any you like):</p>
<pre><code class="language-apache">mail_location = maildir:~/Mail:INBOX=~/Mail/Inbox:LAYOUT=fs
namespace inbox {
inbox = yes
mailbox Drafts {
special_use = \Drafts
auto = subscribe
}
mailbox Junk {
special_use = \Junk
auto = subscribe
autoexpunge = 30d
}
mailbox Sent {
special_use = \Sent
auto = subscribe
}
mailbox Trash {
special_use = \Trash
}
mailbox Archive {
special_use = \Archive
}
}
</code></pre>
<p>Also include this so Postfix can use Dovecot’s authentication system:</p>
<pre><code class="language-apache">service auth {
unix_listener /var/spool/postfix/private/auth {
mode = 0660
user = postfix
group = postfix
}
}
</code></pre>
<p>Lastly (for Dovecot at least), the plugin configuration for <code>sieve</code> (<code>pigeonhole</code>):</p>
<pre><code class="language-apache">protocol lda {
mail_plugins = $mail_plugins sieve
}
protocol lmtp {
mail_plugins = $mail_plugins sieve
}
plugin {
sieve = ~/.dovecot.sieve
sieve_default = /var/lib/dovecot/sieve/default.sieve
sieve_dir = ~/.sieve
sieve_global_dir = /var/lib/dovecot/sieve/
</code></pre>
<p>Where <code>/var/lib/dovecot/sieve/default.sieve</code> doesn’t exist yet. Create the folders:</p>
<pre><code class="language-sh">mkdir -p /var/lib/dovecot/sieve
</code></pre>
<p>And create the file <code>default.sieve</code> inside that just created folder with the content:</p>
<pre><code class="language-nginx">require ["fileinto", "mailbox"];
if header :contains "X-Spam-Flag" "YES" {
fileinto "Junk";
}
</code></pre>
<p>Now, if you don’t have a <code>vmail</code> (virtual mail) user, create one and change the ownership of the <code>/var/lib/dovecot</code> directory to this user:</p>
<pre><code class="language-sh">grep -q "^vmail:" /etc/passwd || useradd -m vmail -s /usr/bin/nologin
chown -R vmail:vmail /var/lib/dovecot
</code></pre>
<p>Note that I also changed the shell for <code>vmail</code> to be <code>/usr/bin/nologin</code>. After that, to compile the configuration file run:</p>
<pre><code class="language-sh">sievec /var/lib/dovecot/sieve/default.sieve
</code></pre>
<p>A <code>default.svbin</code> file will be created next to <code>default.sieve</code>.</p>
<p>Next, add the following lines to <code>/etc/pam.d/dovecot</code> if not already present (shouldn’t be there if you’ve been following these notes):</p>
<pre><code class="language-txt">auth required pam_unix.so nullok
account required pam_unix.so
</code></pre>
<p>That’s it for Dovecot, at this point you can start/enable the <code>dovecot</code> service:</p>
<pre><code class="language-sh">systemctl start dovecot.service
systemctl enable dovecot.service
</code></pre>
<h2 id="opendkim">OpenDKIM</h2>
<p><a href="https://wiki.archlinux.org/title/OpenDKIM">OpenDKIM</a> is needed so services like G**gle (we don’t mention that name here [[[this is a meme]]]) don’t throw the mail to the trash. DKIM stands for “DomainKeys Identified Mail”.</p>
<p>Install the <code>opendkim</code> package:</p>
<pre><code class="language-sh">pacman -S opendkim
</code></pre>
<p>Generate the keys for your domain:</p>
<pre><code class="language-sh">opendkim-genkey -D /etc/opendkim -d {yourdomain} -s {yoursubdomain} -r -b 2048
</code></pre>
<p>Where you need to change <code>{yourdomain}</code> and <code>{yoursubdomain}</code> (doesn’t really need to be the sub-domain, could be anything that describes your key) accordingly, for me it’s <code>luevano.xyz</code> and <code>mail</code>, respectively. After that, we need to create some files inside the <code>/etc/opendkim</code> directory. First, create the file <code>KeyTable</code> with the content:</p>
<pre><code class="language-txt">{yoursubdomain}._domainkey.{yourdomain} {yourdomain}:{yoursubdomain}:/etc/opendkim/{yoursubdomain}.private
</code></pre>
<p>So, for me it would be:</p>
<pre><code class="language-txt">mail._domainkey.luevano.xyz luevano.xyz:mail:/etc/opendkim/mail.private
</code></pre>
<p>Next, create the file <code>SigningTable</code> with the content:</p>
<pre><code class="language-txt">*@{yourdomain} {yoursubdomain}._domainkey.{yourdomain}
</code></pre>
<p>Again, for me it would be:</p>
<pre><code class="language-txt">*@luevano.xyz mail._domainkey.luevano.xyz
</code></pre>
<p>And, lastly create the file <code>TrustedHosts</code> with the content:</p>
<pre><code class="language-txt">127.0.0.1
::1
10.1.0.0/16
1.2.3.4/24
localhost
{yourserverip}
...
</code></pre>
<p>And more, make sure to include your server IP and something like <code>subdomain.domainname</code>.</p>
<p>Next, edit <code>/etc/opendkim/opendkim.conf</code> to reflect the changes (or rather, addition) of these files, as well as some other configuration. You can look up the example configuration file located at <code>/usr/share/doc/opendkim/opendkim.conf.sample</code>, but I’m creating a blank one with the contents:</p>
<pre><code class="language-apache">Domain {yourdomain}
Selector {yoursubdomain}
Syslog Yes
UserID opendkim
KeyFile /etc/opendkim/{yoursubdomain}.private
Socket inet:8891@localhost
</code></pre>
<p>Now, change the permissions for all the files inside <code>/etc/opendkim</code>:</p>
<pre><code class="language-sh">chown -R root:opendkim /etc/opendkim
chmod g+r /etc/postfix/dkim/*
</code></pre>
<p>I’m using <code>root:opendkim</code> so <code>opendkim</code> doesn’t complain about the <code>{yoursubdomani}.private</code> being insecure (you can change that by using the option <code>RequireSafeKeys False</code> in the <code>opendkim.conf</code> file, as stated <a href="http://lists.opendkim.org/archive/opendkim/users/2014/12/3331.html">here</a>).</p>
<p>That’s it for the general configuration, but you could go more in depth and be more secure with some extra configuration.</p>
<p>Now, just start/enable the <code>opendkim</code> service:</p>
<pre><code class="language-sh">systemctl start opendkim.service
systemctl enable opendkim.service
</code></pre>
<p>And don’t forget to add the following <strong>TXT</strong> records on your domain registrar (these examples are for Epik):</p>
<ol>
<li><em>DKIM</em> entry: look up your <code>{yoursubdomain}.txt</code> file, it should look something like:</li>
</ol>
<pre><code class="language-txt">{yoursubdomain}._domainkey IN TXT ( "v=DKIM1; k=rsa; s=email; "
"p=..."
"..." ) ; ----- DKIM key mail for {yourdomain}
</code></pre>
<p>In the TXT record you will place <code>{yoursubdomain}._domainkey</code> as the “Host” and <code>"v=DKIM1; k=rsa; s=email; " "p=..." "..."</code> in the “TXT Value” (replace the dots with the actual value you see in your file).</p>
<ol start="2">
<li>
<p><em>DMARC</em> entry: just <code>_dmarc.{yourdomain}</code> as the “Host” and <code>"v=DMARC1; p=reject; rua=mailto:dmarc@{yourdomain}; fo=1"</code> as the “TXT Value”.</p>
</li>
<li>
<p><em>SPF</em> entry: just <code>@</code> as the “Host” and <code>"v=spf1 mx a:{yoursubdomain}.{yourdomain} - all"</code> as the “TXT Value”.</p>
</li>
</ol>
<p>And at this point you could test your mail for spoofing and more.</p>
<h2 id="spamassassin">SpamAssassin</h2>
<p><a href="https://wiki.archlinux.org/title/SpamAssassin">SpamAssassin</a> is just <em>a mail filter to identify spam</em>.</p>
<p>Install the <code>spamassassin</code> package (which will install a bunch of ugly <code>perl</code> packages…):</p>
<pre><code class="language-sh">pacman -S spamassassin
</code></pre>
<p>For some reason, the permissions on all <code>spamassassin</code> stuff are all over the place. First, change owner of the executables, and directories:</p>
<pre><code class="language-sh">chown spamd:spamd /usr/bin/vendor_perl/sa-*
chown spamd:spamd /usr/bin/vendor_perl/spam*
chwown -R spamd:spamd /etc/mail/spamassassin
</code></pre>
<p>Then, you can edit <code>local.cf</code> (located in <code>/etc/mail/spamassassin</code>) to fit your needs (I only uncommented the <code>rewrite_header Subject ...</code> line). And then you can run the following command to update the patterns and compile them:</p>
<pre><code class="language-sh">sudo -u spamd sa-update
sudo -u spamd sa-compile
</code></pre>
<p>And since this should be run periodically, create the service <code>spamassassin-update.service</code> under <code>/etc/systemd/system</code> with the following content:</p>
<pre><code class="language-ini">[Unit]
Description=SpamAssassin housekeeping
After=network.target
[Service]
User=spamd
Group=spamd
Type=oneshot
ExecStart=/usr/bin/vendor_perl/sa-update --allowplugins
SuccessExitStatus=1
ExecStart=/usr/bin/vendor_perl/sa-compile
ExecStart=/usr/bin/systemctl -q --no-block try-restart spamassassin.service
</code></pre>
<p>And you could also execute <code>sa-learn</code> to train <code>spamassassin</code>‘s bayes filter, but this works for me. Then create the timer <code>spamassassin-update.timer</code> under the same directory, with the content:</p>
<pre><code class="language-ini">[Unit]
Description=SpamAssassin housekeeping
[Timer]
OnCalendar=daily
Persistent=true
[Install]
WantedBy=timers.target
</code></pre>
<p>You can now start/enable the <code>spamassassin-update</code> timer:</p>
<pre><code class="language-sh">systemctl start spamassassin-update.timer
systemctl enable spamassassin-update.timer
</code></pre>
<p>Next, you may want to edit the <code>spamassassin</code> service before starting and enabling it, because by default, it could <a href="https://rimuhosting.com/howto/memory.jsp">spawn a lot of “childs”</a> eating a lot of resources and you really only need one child. Append <code>--max-children=1</code> to the line <code>ExecStart=...</code> in <code>/usr/bin/systemd/system/spamassassin.service</code>:</p>
<pre><code class="language-ini">...
ExecStart=/usr/bin/vendor_perl/spamd -x -u spamd -g spamd --listen=/run/spamd/spamd.sock --listen=localhost --max-children=1
...
</code></pre>
<p>Finally, start and enable the <code>spamassassin</code> service:</p>
<pre><code class="language-sh">systemctl start spamassassin.service
systemctl enable spamassassin.service
</code></pre>
<h2 id="wrapping-up">Wrapping up</h2>
<p>We should have a working mail server by now. Before continuing check your journal logs (<code>journalctl -xe --unit={unit}</code>, where <code>{unit}</code> could be <code>spamassassin.service</code> for example) to see if there was any error whatsoever and try to debug it, it should be a typo somewhere (the logs are generally really descriptive) because all the settings and steps detailed here just (literally just finished doing everything on a new server as of the writing of this text) worked <em>(((it just werks on my machine)))</em>.</p>
<p>Now, to actually use the mail service: first of all, you need a <em>normal</em> account (don’t use root) that belongs to the <code>mail</code> group (<code>gpasswd -a user group</code> to add a user <code>user</code> to group <code>group</code>) and that has a password.</p>
<p>Next, to actually login into a mail app/program/whateveryouwanttocallit, you will use the following settings, at least for <code>thunderdbird</code>(I tested in windows default mail app and you don’t need a lot of settings):</p>
<ul>
<li>* server: subdomain.domain (mail.luevano.xyz in my case)</li>
<li><strong>SMTP</strong> port: 587</li>
<li><strong>SMTPS</strong> port: 465 (I use this one)</li>
<li><strong>IMAP</strong> port: 143</li>
<li><strong>IMAPS</strong> port: 993 (again, I use this one)</li>
<li>Connection/security: SSL/TLS</li>
<li>Authentication method: Normal password</li>
<li>Username: just your <code>user</code>, not the whole email (<code>david</code> in my case)</li>
<li>Password: your <code>user</code> password (as in the password you use to login to the server with that user)</li>
</ul>
<p>All that’s left to do is test your mail server for spoofing, and to see if everything is setup correctly. Go to <a href="https://www.appmaildev.com/en/dkim">DKIM Test</a> and follow the instructions (basically click next, and send an email with whatever content to the email that they provide). After you send the email, you should see something like:</p>
<figure id="__yafg-figure-3">
<img alt="DKIM Test successful" src="images/b/notes/mail/dkim_test_successful.png" title="DKIM Test successful">
<figcaption>DKIM Test successful</figcaption>
</figure>
<p>Finally, that’s actually it for this entry, if you have any problem whatsoever you can <a href="https://luevano.xyz/contact.html">contact me</a>.</p>
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<div class="article-info">
<p>By David Luévano</p>
<p>Created: Sun, Mar 21, 2021 @ 04:05 UTC</p>
<p>Modified: Tue, Jun 08, 2021 @ 07:29 UTC</p>
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