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<!-- Copyright (C) 2003 Red Hat, Inc.                                -->
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><DIV
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><H1
><A
NAME="NET-NS-DNS-API">Chapter 45. DNS</H1
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="NET-NS-DNS-API1">DNS API</H1
><P
>The DNS client uses the normal BSD API for performing lookups:
<TT
CLASS="FUNCTION"
>gethostbyname()</TT
> and
<TT
CLASS="FUNCTION"
>gethostbyaddr()</TT
>.</P
><P
>There are a few restrictions:</P
><P
></P
><UL
><LI
><P
>Only IPv4 is supported, ie IPv6 addresses cannot be looked
up.</P
></LI
><LI
><P
>If the DNS server returns multiple authoritive records
for a host name, the hostent will only contain a record for the
first entry.</P
></LI
><LI
><P
>The code has been made thread safe. ie multiple threads
may call 
<TT
CLASS="FUNCTION"
>gethostbyname()</TT
>
 without causing problems to the hostent structure returned. What
is not safe is one thread using both 
<TT
CLASS="FUNCTION"
>gethostbyname()</TT
>
 and 
<TT
CLASS="FUNCTION"
>gethostbyaddr()</TT
>.
A call to one will destroy the results from the previous call
to the other function.</P
></LI
></UL
><P
>To initialise the DNS client the following function must be
called:</P
><TABLE
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><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>#include &lt;network.h&gt;
int cyg_dns_res_init(struct in_addr *dns_server)</PRE
></TD
></TR
></TABLE
><P
>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.</P
><P
>A default, hard coded, server may be specified in the CDL option
<TT
CLASS="LITERAL"
>CYGDAT_NS_DNS_DEFAULT_SERVER</TT
>. The use of this is
controlled by <TT
CLASS="LITERAL"
>CYGPKG_NS_DNS_DEFAULT</TT
>. If this is
enabled, <TT
CLASS="LITERAL"
>init_all_network_interfaces</TT
> will
initialize the resolver with the hard coded address. The DHCP client
or user code my override this address by calling
<TT
CLASS="LITERAL"
>cyg_dns_res_init</TT
> again. </P
><P
>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:
<DIV
CLASS="FUNCSYNOPSIS"
><A
NAME="AEN14685"><P
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int getdomainname</CODE
>(char *name, size_t len);</CODE
></P
><P
><CODE
><CODE
CLASS="FUNCDEF"
>int setdomainname</CODE
>(const char *name, size_t len);</CODE
></P
><P
></P
></DIV
></P
><P
>Alternatively, a hard coded domain name can be set using CDL.
The boolean <TT
CLASS="LITERAL"
>CYGPKG_NS_DNS_DOMAINNAME</TT
> enables this
and the domain name is taken from
<TT
CLASS="LITERAL"
>CYGPKG_NS_DNS_DOMAINNAME_NAME</TT
>.</P
><P
>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. </P
></DIV
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[/] [openrisc/] [trunk/] [rtos/] [ecos-2.0/] [doc/] [html/] [ref/] [net-ns-dns-api.html] - Rev 598
