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<h3 class="section">3.9 The domain name service</h3>
<p><a name="index-DNS-151"></a><a name="index-name-server-152"></a><a name="index-gethostbyname-153"></a><a name="index-resolver-154"></a><a name="index-IPv6-155"></a><a name="index-dnsMaxTimeout-156"></a><a name="index-dnsUseGethostbyname-157"></a><a name="index-dnsNameServer-158"></a><a name="index-dnsNegativeTtl-159"></a><a name="index-dnsGethostbynameTtl-160"></a><a name="index-dnsQueryIPv6-161"></a>
The low-level protocols beneath HTTP identify machines by IP
addresses, sequences of four 8-bit integers such as
‘<samp><span class="samp">199.232.41.10</span></samp>’<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>. HTTP, on the other hand, and most application
protocols, manipulate host names, strings such as ‘<samp><span class="samp">www.polipo.org</span></samp>’.
<p>The <dfn>domain name service</dfn> (DNS) is a distributed database that
maps host names to IP addresses. When an application wants to make
use of the DNS, it invokes a <dfn>resolver</dfn>, a local library or
process that contacts remote name servers.
<p>Polipo usually tries to speak the DNS protocol itself rather than
using the system resolver<a rel="footnote" href="#fn-2" name="fnd-2"><sup>2</sup></a>. Its
precise behaviour is controlled by the value of
<code>dnsUseGethostbyname</code>. If <code>dnsUseGethostbyname</code> is
<code>false</code>, Polipo never uses the system resolver. If it is
<code>reluctantly</code> (the default), Polipo tries to speak DNS and falls
back to the system resolver if a name server could not be contacted.
If it is <code>happily</code>, Polipo tries to speak DNS, and falls back to
the system resolver if the host couldn't be found for any reason (this
is not a good idea for shared proxies). Finally, if
<code>dnsUseGethostbyname</code> is <code>true</code>, Polipo never tries to speak
DNS itself and uses the system resolver straight away (this is not
recommended).
<p>If the internal DNS support is used, Polipo must be given a recursive
name server to speak to. By default, this information is taken from
the ‘<samp><span class="samp">/etc/resolv.conf</span></samp>’ file; however, if you wish to use
a different name server, you may set the variable <code>dnsNameServer</code>
to an IP address<a rel="footnote" href="#fn-3" name="fnd-3"><sup>3</sup></a>.
<p>When the reply to a DNS request is late to come, Polipo will retry
multiple times using an exponentially increasing timeout. The maximum
timeout used before Polipo gives up is defined by <code>dnsMaxTimeout</code>
(default 60<span class="dmn">s</span>); the total time before Polipo gives up on a DNS
query will be roughly twice <code>dnsMaxTimeout</code>.
<p>The variable <code>dnsNegativeTtl</code> specifies the time during which
negative DNS information (information that a host <em>doesn't</em>
exist) will be cached; this defaults to 120<span class="dmn">s</span>. Increasing this
value reduces both latency and network traffic but may cause a failed
host not to be noticed when it comes back up.
<p>The variable <code>dnsQueryIPv6</code> specifies whether to query for IPv4
or IPv6 addresses. If <code>dnsQueryIPv6</code> is <code>false</code>, only IPv4
addresses are queried. If <code>dnsQueryIPv6</code> is <code>reluctantly</code>,
both types of addresses are queried, but IPv4 addresses are preferred.
If <code>dnsQueryIPv6</code> is <code>happily</code> (the default), IPv6 addresses
are preferred. Finally, if <code>dnsQueryIPv6</code> is <code>true</code>, only
IPv6 addresses are queried.
<p>If the system resolver is used, the value <code>dnsGethostbynameTtl</code>
specifies the time during which a <code>gethostbyname</code> reply will be
cached (default 5 minutes).
<div class="footnote">
<hr>
<h4>Fußnoten</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Or sequences of eight 16-bit integers if
you are running IPv6.</p>
<p class="footnote"><small>[<a name="fn-2" href="#fnd-2">2</a>]</small> The Unix interface to the resolver
is provided by the <code>gethostbyname</code>(3) library call
(<code>getaddrinfo</code>(3) on recent systems), which was designed at
a time when a host lookup consisted in searching for one of five hosts
in a ‘<samp><span class="samp">HOSTS.TXT</span></samp>’ file. The <code>gethostbyname</code> call is
<dfn>blocking</dfn>, meaning that all activity must cease while a host
lookup is in progress. When the call eventually returns, it doesn't
provide a <dfn>time to live</dfn> (TTL) value to indicate how long the
address may be cached. For these reasons, <code>gethostbyname</code> is
hardly useful for programs that need to contact more than a few hosts.
(Recent systems replace <code>gethostbyname</code>(3) by
<code>getaddrinfo</code>(3), which is reentrant. While this removes one
important problem that multi-threaded programs encounter, it doesn't
solve any of the other issues with <code>gethostbyname</code>.)</p>
<p class="footnote"><small>[<a name="fn-3" href="#fnd-3">3</a>]</small> While Polipo does its own caching of DNS
data, I recommend that you run a local caching name server. I am very
happy with <a href="http://home.t-online.de/home/Moestl/"><code>pdnsd</code></a>,
notwithstanding its somewhat bizarre handling of TCP connections.</p>
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