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     17 <h1><img src="postfix-logo.jpg" width="203" height="98" ALT="">Postfix and Linux</h1>
     18 
     19 <hr>
     20 
     21 <h2> Host lookup issues </h2>
     22 
     23 <p> By default Linux /etc/hosts lookups do not support multiple IP
     24 addresses per hostname. This causes warnings from the Postfix SMTP
     25 server that "hostname XXX does not resolve to address YYY", and is
     26 especially a problem with hosts that have both IPv4 and IPv6
     27 addresses.  To fix this, turn on support for multiple IP addresses: </p>
     28 
     29 <blockquote>
     30 <pre>
     31 /etc/host.conf:
     32     ...
     33     # We have machines with multiple IP addresses.
     34     multi on
     35     ...
     36 </pre>
     37 </blockquote>
     38 
     39 <p> Alternatively, specify the RESOLV_MULTI environment variable
     40 in main.cf: </p>
     41 
     42 <blockquote>
     43 <pre>
     44 /etc/postfix/main.cf:
     45     import_environment = MAIL_CONFIG MAIL_DEBUG MAIL_LOGTAG TZ XAUTHORITY DISPLAY LANG=C RESOLV_MULTI=on
     46 </pre>
     47 </blockquote>
     48 
     49 <h2>Berkeley DB issues</h2>
     50 
     51 <p> If you can't compile Postfix because the file "db.h"
     52 isn't found, then you MUST install the Berkeley DB development
     53 package (name: db???-devel-???) that matches your system library.
     54 You can find out what is installed with the rpm command. For example:
     55 </p>
     56 
     57 <blockquote>
     58 <pre>
     59 $ <b>rpm -qf /usr/lib/libdb.so</b>
     60 db4-4.3.29-2
     61 </pre>
     62 </blockquote>
     63 
     64 <p> This means that you need to install db4-devel-4.3.29-2 (on
     65 some systems, specify "<b>rpm -qf /lib/libdb.so</b>" instead). </p>
     66 
     67 <p> DO NOT download some Berkeley DB version from the network.
     68 Every Postfix program will dump core when it is built with a different
     69 Berkeley DB version than the version that is used by the system
     70 library routines.  See the DB_README file for further information.
     71 </p>
     72 
     73 <h2>Procmail issues</h2>
     74 
     75 <p> On RedHat Linux 7.1 and later <b>procmail</b> no longer has
     76 permission
     77 to write to the mail spool directory. Workaround: </p>
     78 
     79 <blockquote>
     80 <pre>
     81 # chmod 1777 /var/spool/mail
     82 </pre>
     83 </blockquote>
     84 
     85 <h2>Logging in a container</h2>
     86 
     87 <p> When running Postfix inside a container, you can use stdout
     88 logging as described in MAILLOG_README. Alternatives: run syslogd
     89 inside the container, or mount the host's syslog socket inside the
     90 container. </p>
     91 
     92 <h2>Syslogd performance</h2>
     93 
     94 <p> LINUX <b>syslogd</b> uses synchronous writes by default. Because
     95 of this, <b>syslogd</b> can actually use more system resources than
     96 Postfix.  To avoid such badness, disable synchronous mail logfile
     97 writes by editing /etc/syslog.conf and by prepending a - to the
     98 logfile name:  </p>
     99 
    100 <blockquote>
    101 <pre>
    102 /etc/syslog.conf:
    103     mail.*                          -/var/log/mail.log
    104 </pre>
    105 </blockquote>
    106 
    107 <p> Send a "<b>kill -HUP</b>" to the <b>syslogd</b> to make the
    108 change effective.  </p>
    109 
    110 <h2>Other logging performance issues</h2>
    111 
    112 <p> LINUX <b>systemd</b> intercepts all logging and enforces its
    113 own rate limits before handing off requests to a backend such as
    114 <b>rsyslogd</b> or <b>syslog-ng</b>. On a busy mail server this can
    115 result in information loss. As a workaround, you can use Postfix's
    116 built-in logging as described in MAILLOG_README. </p>
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