<h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix and Linux</h1>
<hr>
<h2> Host lookup issues </h2>
<p> By default Linux /etc/hosts lookups do not support multiple IP
addresses per hostname. This causes warnings from the Postfix SMTP
server that "hostname XXX does not resolve to address YYY", and is
especially a problem with hosts that have both IPv4 and IPv6
addresses. To fix this, turn on support for multiple IP addresses: </p>
<blockquote>
<pre>
/etc/host.conf:
...
# We have machines with multiple IP addresses.
multi on
...
</pre>
</blockquote>
<p> Alternatively, specify the RESOLV_MULTI environment variable
in main.cf: </p>
<p> If you can't compile Postfix because the file "db.h"
isn't found, then you MUST install the Berkeley DB development
package (name: db???-devel-???) that matches your system library.
You can find out what is installed with the rpm command. For example:
</p>
<p> This means that you need to install db4-devel-4.3.29-2 (on
some systems, specify "<b>rpm -qf /lib/libdb.so</b>" instead). </p>
<p> DO NOT download some Berkeley DB version from the network.
Every Postfix program will dump core when it is built with a different
Berkeley DB version than the version that is used by the system
library routines. See the DB_README file for further information.
</p>
<h2>Procmail issues</h2>
<p> On RedHat Linux 7.1 and later <b>procmail</b> no longer has
permission
to write to the mail spool directory. Workaround: </p>
<p> When running Postfix inside a container, you can use stdout
logging as described in MAILLOG_README. Alternatives: run syslogd
inside the container, or mount the host's syslog socket inside the
container. </p>
<h2>Syslogd performance</h2>
<p> LINUX <b>syslogd</b> uses synchronous writes by default. Because
of this, <b>syslogd</b> can actually use more system resources than
Postfix. To avoid such badness, disable synchronous mail logfile
writes by editing /etc/syslog.conf and by prepending a - to the
logfile name: </p>
<p> Send a "<b>kill -HUP</b>" to the <b>syslogd</b> to make the
change effective. </p>
<h2>Other logging performance issues</h2>
<p> LINUX <b>systemd</b> intercepts all logging and enforces its
own rate limits before handing off requests to a backend such as
<b>rsyslogd</b> or <b>syslog-ng</b>. On a busy mail server this can
result in information loss. As a workaround, you can use Postfix's
built-in logging as described in MAILLOG_README. </p>
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