dns.sgml

来自「eCos操作系统源码」· SGML 代码 · 共 227 行

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<!--     dns.sgml                                                    -->
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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>
<FUNCTION>gethostbyaddr()</FUNCTION>,
<FUNCTION>getaddrinfo()</function>,
<FUNCTION>getnameinfo()</function>. 
</PARA>
<PARA>There are a few restrictions:</PARA>
<ITEMIZEDLIST>
<LISTITEM>
<PARA>If the DNS server returns multiple authoritive records
for a host name to <function>gethostbyname</function>, the hostent will only contain a record for the
first entry. If multiple records are desired, use 
<function>getaddrinfo</function>, which will return multiple results.</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. <FUNCTION>getaddrinfo()</FUNCTION> and
<FUNCTION>getnameinfo()</FUNCTION> are thread
safe and so these are the preferred interfaces. They are also address
family independent so making it easier to port code to IPv6.</PARA>
</LISTITEM>
<LISTITEM><PARA>The DNS client will only return IPv4 addresses to
RedBoot. At the moment this is not really a limitation,
since RedBoot only supports IPv4 and not IPv6.</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_start(char * dns_server)</PROGRAMLISTING>

<PARA>Where dns_server is the address of the DNS server. The address
must be in numeric form and can be either an IPv4 or an IPv6 address.
</PARA>
<PARA>There also exists a deprecated function to start the DNS
client:</PARA>
<PROGRAMLISTING>
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. The address should be in network order and
can only be an IPv4 address.
</PARA>

<PARA> On error both this function returns -1, otherwise
0 for success. If lookups are attemped before this function has
been called, they will fail and return NULL, unless numeric host addresses
are passed. In this cause, the address will be converted and returned
without the need for a lookup.</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 use some simple heuristics when
      deciding how to use the domainname. If the name given to the
      client ends with a "." it is assumed to be a FQDN and the domain
      name will not be used. If the name contains a "." somewhere
      within it, first a lookup will be performed without the
      domainname. If that fails the domainname will be appended and
      looked up. If the name does not contain a ".", the domainname is
      appended and used for the first query. If that fails, the
      unadorned name is lookup.
</PARA>
      <PARA>The <FUNCTION>getaddrinfo</FUNCTION> will return both IPv4 and
        IPv6 addresses for a given host name, when IPv6 is enabled in
        the eCos configuration.  The CDL option
        <LITERAL>CYGOPT_NS_DNS_FIRST_FAMILY</LITERAL> controls the order
        IPv6 and IPv4 addresses are returned in the linked list of
        <LITERAL>addrinfo</LITERAL> structures. If the value
        <LITERAL>AF_INET</LITERAL> is used, the IPv4 addresses will be
        first. If the value <LITERAL>AF_INET6</LITERAL>, which is the
        default, is used, IPv6 address will be first. This ordering will
        control how clients attempt to connect to servers, ie using IPv6
        or IPv4 first. 
      </PARA>
    </SECT1>
    <SECT1 id="net-ns-dns-testing">
      <TITLE>DNS Client Testing</TITLE> 
      
      <PARA>The DNS client has a test program, dns1.c, which tests many of
        the features of the DNS client and the functions
        <FUNCTION>gethostbyname(),</FUNCTION>
        <FUNCTION>gethostbyaddr()</FUNCTION>,
        <FUNCTION>getaddrinfo()</FUNCTION>,
        <FUNCTION>getnameinfo()</FUNCTION>.
      </PARA>
      <PARA>In order for this test to work, a DNS server must be configured
        with a number of names and addresses. The following is an example
        forward address resolution database for bind v9, which explains the
        requirements.</para>
      
      <PROGRAMLISTING>
        $TTL            680400
        @               IN      SOA     lunn.org.       andrew.lunn.lunn.org (
        2003041801      ; serial
        10800           ; refresh
        1800            ; retry
        3600000         ; expire
        259200)         ; mimimum
        IN      NS      londo.lunn.org.
        
        hostnamev4      IN      A       192.168.88.1
        cnamev4         IN      CNAME   hostnamev4
        hostnamev6      IN      AAAA    fec0::88:4:3:2:1
        cnamev6         IN      CNAME   hostnamev6
        hostnamev46     IN      A       192.168.88.2
        hostnamev46     IN      AAAA    fec0::88:4:3:2:2
        cnamev46        IN      CNAME   hostnamev46
      </PROGRAMLISTING>
      <PARA>The actual names and addresses do not matter, since they are
        configurable in the test. What is important is the relationship
        between the names and the addresses and there family. ie
        hostnamev4 should map to one IPv4 address. hostnamev46 should
        map to both an IPv4 and an IPv6 address. cnamev4 should be a
        CNAME record for hostname4. Reverse lookup information is also
        needed by the test.
      </PARA>
      <PARA>The information placed into the DNS server is also need in the
        test case. A structure is defined to hold this
        information:</PARA>
      <PROGRAMLISTING>
        struct test_info_s {
            char * dns_server_v4;
            char * dns_server_v6;
            char * domain_name;
            char * hostname_v4;
            char * cname_v4;
            char * ip_addr_v4;
            char * hostname_v6;
            char * cname_v6;
            char * ip_addr_v6;
            char * hostname_v46;
            char * cname_v46;
            char * ip_addr_v46_v4;
            char * ip_addr_v46_v6;
        };
        </PROGRAMLISTING>
      <PARA>The test program may hold a number of such structures for
        different DNS server. The test will use each structure in turn
        to perform the tests.  If IPv6 is not enabled in the eCos
        configuration, the entries which use IPv6 may be assigned to
        NULL.
        </PARA>
    </SECT1>
  </CHAPTER>
</PART>