<CHAPTER ID="ZADM-DNSISSUES"><?dbhtml filename="zadm-dnsissues.html">
<TITLE>DNS and ZMailer</TITLE>

<PARA>
The cornerstone of everything in busy Internet email routing is a properly
working DNS server, and a modern resolver library. If you use the BIND 
nameserver, you should be using a recent version, at least BIND 4.8.
(As of this writing, bind is on version 8.1.2)
</PARA>

<PARA>
You can get improved DNS performance by installing local <COMMAND>named</>(8),
which does cache replies, including <EMPHASIS>negative</> replies.
</PARA>

<PARA>
For the file <FILENAME>/etc/resolv.conf</>:
<SCREEN>
  domain     your.domain
  nameserver 127.0.0.1
  nameserver (some other server)
</SCREEN>
</PARA>

<PARA>
For the local nameserver daemon (<APPLICATION>named</>(8)) you should have
at least following type of configuration:
<SCREEN>
For 4.* series:  /etc/named.boot
  forwarders 10.12.34.56  10.45.67.89
  options forward-only

For 8.* series:  /etc/named.conf
  options {
      forward only;
      forwarders {
             10.12.34.56;
             10.45.67.89;
      };
  };
</SCREEN>
which means that all the queries are attempted to be resolved
by the servers at IP addresses <EMPHASIS>10.12.34.56</> and
<EMPHASIS>10.45.67.89</>, and both the local server, and remote
servers will cache DNS responses.
</PARA>

<PARA>
For Solaris, Linux, and some other environments you propably have
file  <FILENAME>/etc/nsswitch.conf</>.  There the interesting line
is one referring with <QUOTE><EMPHASIS>hosts:</></> tag.
In most cases the default setup assumes you will use e.g. NIS(+)
in the system overriding DNS and/or local files.   In general that
is quite <EMPHASIS>bad</> thing to do &mdash; especially for DNS intensive
application, like mailers...  Suggested value:
<SCREEN>
  hosts: files dns
</SCREEN>
</PARA>

<PARA>
At DEC Tru64 there is another file with same purpose as
<FILENAME>nsswitch.conf</>, it is: <FILENAME>/etc/svc.conf</>.
</PARA>

<PARA>
At Solaris 2.6 (and after?) there is also a ``<COMMAND>nscd</>'' daemon
(name service cache daemon), which has appeared at times to <emphasis>harm</>
DNS lookup intensive systems.
At its configuration file <FILENAME>/etc/nscd.conf</>:
<SCREEN>
     enable-cache    hosts   no
</SCREEN>
</PARA>

</CHAPTER>
