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<!-- Copyright (C) 2003 Red Hat, Inc. --> <!-- This material may be distributed only subject to the terms --> <!-- and conditions set forth in the Open Publication License, v1.0 --> <!-- or later (the latest version is presently available at --> <!-- http://www.opencontent.org/openpub/). --> <!-- Distribution of the work or derivative of the work in any --> <!-- standard (paper) book form is prohibited unless prior --> <!-- permission is obtained from the copyright holder. --> <HTML ><HEAD ><TITLE >DNS</TITLE ><meta name="MSSmartTagsPreventParsing" content="TRUE"> <META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.76b+ "><LINK REL="HOME" TITLE="eCos Reference Manual" HREF="ecos-ref.html"><LINK REL="UP" TITLE="DNS for eCos and RedBoot" HREF="net-ns-dns.html"><LINK REL="PREVIOUS" TITLE="DNS for eCos and RedBoot" HREF="net-ns-dns.html"><LINK REL="NEXT" TITLE="Ethernet Device Drivers" HREF="io-eth-drv-generic.html"></HEAD ><BODY CLASS="CHAPTER" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >eCos Reference Manual</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="net-ns-dns.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="io-eth-drv-generic.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="CHAPTER" ><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 BORDER="5" BGCOLOR="#E0E0F0" WIDTH="70%" ><TR ><TD ><PRE CLASS="PROGRAMLISTING" >#include <network.h> 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 ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="net-ns-dns.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="ecos-ref.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="io-eth-drv-generic.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >DNS for eCos and RedBoot</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="net-ns-dns.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Ethernet Device Drivers</TD ></TR ></TABLE ></DIV ></BODY ></HTML >