dns.sgml

来自「ecos实时嵌入式操作系统」· SGML 代码 · 共 119 行

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<PART ID="net-ns-dns">
<TITLE>DNS for eCos and RedBoot</TITLE>
<PARTINTRO>
<PARA>
<productname>eCos</productname> and
<productname>RedBoot</productname>
can both use the DNS package to perform network name lookups.
</PARA>
</PARTINTRO>
<CHAPTER id="net-ns-dns-api">
<TITLE>DNS</TITLE>
<SECT1 id="net-ns-dns-api1">
<TITLE>DNS API</TITLE>
<PARA>The DNS client uses the normal BSD API for performing lookups:
<function>gethostbyname()</function> and
<FUNCTION>gethostbyaddr()</FUNCTION>.
</PARA>
<PARA>There are a few restrictions:</PARA>
<ITEMIZEDLIST>
<LISTITEM>
<PARA>Only IPv4 is supported, ie IPv6 addresses cannot be looked
up.</PARA>
</LISTITEM>
<LISTITEM>
<PARA>If the DNS server returns multiple authoritive records
for a host name, the hostent will only contain a record for the
first entry.</PARA>
</LISTITEM>
<LISTITEM>
<PARA>The code has been made thread safe. ie multiple threads
may call 
<FUNCTION>gethostbyname()</FUNCTION>
 without causing problems to the hostent structure returned. What
is not safe is one thread using both 
<FUNCTION>gethostbyname()</FUNCTION>
 and 
<FUNCTION>gethostbyaddr()</FUNCTION>.
A call to one will destroy the results from the previous call
to the other function.</PARA>
</LISTITEM>
</ITEMIZEDLIST>
<PARA>To initialise the DNS client the following function must be
called:</PARA>
<PROGRAMLISTING>#include &lt;network.h&gt;
int cyg_dns_res_init(struct in_addr *dns_server)</PROGRAMLISTING>
<PARA>where dns_server is the address of the DNS server
the client should query. On Error this function returns -1, otherwise
0 for success. If lookups are attemped before this function has
been called, they will fail and return NULL.</PARA>

<PARA>A default, hard coded, server may be specified in the CDL option
<literal>CYGDAT_NS_DNS_DEFAULT_SERVER</literal>. The use of this is
controlled by <literal>CYGPKG_NS_DNS_DEFAULT</literal>. If this is
enabled, <literal>init_all_network_interfaces</literal> will
initialize the resolver with the hard coded address. The DHCP client
or user code my override this address by calling
<literal>cyg_dns_res_init</literal> again. </PARA>

<PARA>The DNS client understands the concepts of the target being
in a domain. By default no domain will be used. Host name lookups
should be for fully qualified names. The domain name can be set
and retrieved using the functions:
<funcsynopsis>
  <funcprototype>
    <funcdef>int <function>getdomainname</function></funcdef>
    <paramdef>char *<parameter>name</parameter></paramdef>
    <paramdef>size_t <parameter>len</parameter></paramdef>
  </funcprototype>
  <funcprototype>
    <funcdef>int <function>setdomainname</function></funcdef>
    <paramdef>const char *<parameter>name</parameter></paramdef>
    <paramdef>size_t <parameter>len</parameter></paramdef>
  </funcprototype>
</funcsynopsis>
</para>

<PARA>Alternatively, a hard coded domain name can be set using CDL.
The boolean <literal>CYGPKG_NS_DNS_DOMAINNAME</literal> enables this
and the domain name is taken from
<literal>CYGPKG_NS_DNS_DOMAINNAME_NAME</literal>.</PARA>

<PARA>Once set, the DNS client will first perform a lookup with the domain
name appended. If this fails it will then perform a second lookup
without the appended domain name. </PARA>
</SECT1>
</CHAPTER>
</PART>