lwres_gethostent.docbook

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<refentry>

<refentryinfo>
<date>Jun 30, 2000</date>
</refentryinfo>

<refmeta>
<refentrytitle>lwres_gethostent</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>BIND9</refmiscinfo>
</refmeta>

<refnamediv>
<refname>lwres_gethostbyname</refname>
<refname>lwres_gethostbyname2</refname>
<refname>lwres_gethostbyaddr</refname>
<refname>lwres_gethostent</refname>
<refname>lwres_sethostent</refname>
<refname>lwres_endhostent</refname>
<refname>lwres_gethostbyname_r</refname>
<refname>lwres_gethostbyaddr_r</refname>
<refname>lwres_gethostent_r</refname>
<refname>lwres_sethostent_r</refname>
<refname>lwres_endhostent_r</refname>
<refpurpose>lightweight resolver get network host entry</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include &lt;lwres/netdb.h&gt;</funcsynopsisinfo>
<funcprototype>
<funcdef>
struct hostent *
<function>lwres_gethostbyname</function></funcdef>
<paramdef>const char *name</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
struct hostent *
<function>lwres_gethostbyname2</function></funcdef>
<paramdef>const char *name</paramdef>
<paramdef>int af</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
struct hostent *
<function>lwres_gethostbyaddr</function></funcdef>
<paramdef>const char *addr</paramdef>
<paramdef>int len</paramdef>
<paramdef>int type</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
struct hostent *
<function>lwres_gethostent</function></funcdef>
<paramdef>void</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
void
<function>lwres_sethostent</function></funcdef>
<paramdef>int stayopen</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
void
<function>lwres_endhostent</function></funcdef>
<paramdef>void</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
struct hostent *
<function>lwres_gethostbyname_r</function></funcdef>
<paramdef>const char *name</paramdef>
<paramdef>struct hostent *resbuf</paramdef>
<paramdef>char *buf</paramdef>
<paramdef>int buflen</paramdef>
<paramdef>int *error</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
struct hostent  *
<function>lwres_gethostbyaddr_r</function></funcdef>
<paramdef>const char *addr</paramdef>
<paramdef>int len</paramdef>
<paramdef>int type</paramdef>
<paramdef>struct hostent *resbuf</paramdef>
<paramdef>char *buf</paramdef>
<paramdef>int buflen</paramdef>
<paramdef>int *error</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
struct hostent  *
<function>lwres_gethostent_r</function></funcdef>
<paramdef>struct hostent *resbuf</paramdef>
<paramdef>char *buf</paramdef>
<paramdef>int buflen</paramdef>
<paramdef>int *error</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
void
<function>lwres_sethostent_r</function></funcdef>
<paramdef>int stayopen</paramdef>
</funcprototype>
<funcprototype>
<funcdef>
void
<function>lwres_endhostent_r</function></funcdef>
<paramdef>void</paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>

<refsect1>
<title>DESCRIPTION</title>
<para>
These functions provide hostname-to-address and
address-to-hostname lookups by means of the lightweight resolver.
They are similar to the standard
<citerefentry>
<refentrytitle>gethostent</refentrytitle><manvolnum>3
</manvolnum>
</citerefentry>
functions provided by most operating systems.
They use a
<type>struct hostent</type>
which is usually defined in
<filename>&lt;namedb.h&gt;</filename>.

<programlisting>
struct  hostent {
        char    *h_name;        /* official name of host */
        char    **h_aliases;    /* alias list */
        int     h_addrtype;     /* host address type */
        int     h_length;       /* length of address */
        char    **h_addr_list;  /* list of addresses from name server */
};
#define h_addr  h_addr_list[0]  /* address, for backward compatibility */
</programlisting>
</para>
<para>
The members of this structure are:
<variablelist>
<varlistentry><term><constant>h_name</constant></term>
<listitem>
<para>
The official (canonical) name of the host.
</para>
</listitem></varlistentry>
<varlistentry><term><constant>h_aliases</constant></term>
<listitem>
<para>
A NULL-terminated array of alternate names (nicknames) for the host.
</para>
</listitem></varlistentry>
<varlistentry><term><constant>h_addrtype</constant></term>
<listitem>
<para>
The type of address being returned &mdash;
<type>PF_INET</type>
or
<type>PF_INET6</type>.
</para>
</listitem></varlistentry>
<varlistentry><term><constant>h_length</constant></term>
<listitem>
<para>
The length of the address in bytes.
</para>
</listitem></varlistentry>
<varlistentry><term><constant>h_addr_list</constant></term>
<listitem>
<para>
A <type>NULL</type>
terminated array of network addresses for the host.
Host addresses are returned in network byte order.
</para>
</listitem></varlistentry>
</variablelist>
</para>
<para>
For backward compatibility with very old software,
<constant>h_addr</constant>
is the first address in
<constant>h_addr_list.</constant>
</para>
<para>
<function>lwres_gethostent()</function>,
<function>lwres_sethostent()</function>,
<function>lwres_endhostent()</function>,
<function>lwres_gethostent_r()</function>,
<function>lwres_sethostent_r()</function>
and
<function>lwres_endhostent_r()</function>
provide iteration over the known host entries on systems that
provide such functionality through facilities like
<filename>/etc/hosts</filename>
or NIS.  The lightweight resolver does not currently implement
these functions; it only provides them as stub functions that always
return failure.
</para>

<para>
<function>lwres_gethostbyname()</function> and
<function>lwres_gethostbyname2()</function> look up the hostname
<parameter>name</parameter>.
<function>lwres_gethostbyname()</function> always looks for an IPv4
address while <function>lwres_gethostbyname2()</function> looks for an
address of protocol family <parameter>af</parameter>: either
<type>PF_INET</type> or <type>PF_INET6</type> &mdash; IPv4 or IPV6
addresses respectively.  Successful calls of the functions return a
<type>struct hostent</type>for the name that was looked up.
<type>NULL</type> is returned if the lookups by
<function>lwres_gethostbyname()</function> or
<function>lwres_gethostbyname2()</function> fail.
</para>

<para>
Reverse lookups of addresses are performed by
<function>lwres_gethostbyaddr()</function>.
<parameter>addr</parameter> is an address of length
<parameter>len</parameter> bytes and protocol family
<parameter>type</parameter> &mdash; <type>PF_INET</type> or
<type>PF_INET6</type>.
<function>lwres_gethostbyname_r()</function> is a thread-safe function
for forward lookups.  If an error occurs, an error code is returned in
<parameter>*error</parameter>.
<parameter>resbuf</parameter> is a pointer to a <type>struct
hostent</type> which is initialised by a successful call to
<function>lwres_gethostbyname_r()</function> .
<parameter>buf</parameter> is a buffer of length
<parameter>len</parameter> bytes which is used to store the
<constant>h_name</constant>, <constant>h_aliases</constant>, and
<constant>h_addr_list</constant> elements of the <type>struct
hostent</type> returned in <parameter>resbuf</parameter>.
Successful calls to <function>lwres_gethostbyname_r()</function>
return <parameter>resbuf</parameter>,
which is a pointer to the <type>struct hostent</type> it created.
</para>

<para>
<function>lwres_gethostbyaddr_r()</function> is a thread-safe function
that performs a reverse lookup of address <parameter>addr</parameter>
which is <parameter>len</parameter> bytes long and is of protocol
family <parameter>type</parameter> &mdash; <type>PF_INET</type> or
<type>PF_INET6</type>.  If an error occurs, the error code is returned
in <parameter>*error</parameter>.  The other function parameters are
identical to those in <function>lwres_gethostbyname_r()</function>.
<parameter>resbuf</parameter> is a pointer to a <type>struct
hostent</type> which is initialised by a successful call to
<function>lwres_gethostbyaddr_r()</function>.
<parameter>buf</parameter> is a buffer of length
<parameter>len</parameter> bytes which is used to store the
<constant>h_name</constant>, <constant>h_aliases</constant>, and
<constant>h_addr_list</constant> elements of the <type>struct
hostent</type> returned in <parameter>resbuf</parameter>.  Successful
calls to <function>lwres_gethostbyaddr_r()</function> return
<parameter>resbuf</parameter>, which is a pointer to the
<function>struct hostent()</function> it created.
</para>

</refsect1>

<refsect1>
<title>RETURN VALUES</title>
<para>
The functions
<function>lwres_gethostbyname()</function>,
<function>lwres_gethostbyname2()</function>,
<function>lwres_gethostbyaddr()</function>,
and
<function>lwres_gethostent()</function>
return NULL to indicate an error.  In this case the global variable
<type>lwres_h_errno</type>
will contain one of the following error codes defined in
<filename>&lt;lwres/netdb.h&gt;</filename>:

<variablelist>
<varlistentry><term><constant>HOST_NOT_FOUND</constant></term>
<listitem>
<para>
The host or address was not found.
</para>
</listitem></varlistentry>
<varlistentry><term><constant>TRY_AGAIN</constant></term>
<listitem>
<para>
A recoverable error occurred, e.g., a timeout.
Retrying the lookup may succeed.
</para>
</listitem></varlistentry>
<varlistentry><term><constant>NO_RECOVERY</constant></term>
<listitem>
<para>
A non-recoverable error occurred.
</para>
</listitem></varlistentry>
<varlistentry><term><constant>NO_DATA</constant></term>
<listitem>
<para>
The name exists, but has no address information
associated with it (or vice versa in the case
of a reverse lookup).  The code NO_ADDRESS
is accepted as a synonym for NO_DATA for backwards
compatibility.
</para>
</listitem></varlistentry>
</variablelist>
</para>

<para>
<citerefentry>
<refentrytitle>lwres_hstrerror</refentrytitle><manvolnum>3
</manvolnum>
</citerefentry>
translates these error codes to suitable error messages.
</para>

<para>
<function>lwres_gethostent()</function>
and
<function>lwres_gethostent_r()</function>
always return
<type>NULL</type>.
</para>

<para>
Successful calls to <function>lwres_gethostbyname_r()</function> and
<function>lwres_gethostbyaddr_r()</function> return
<parameter>resbuf</parameter>, a pointer to the <type>struct
hostent</type> that was initialised by these functions.  They return
<type>NULL</type> if the lookups fail or if <parameter>buf</parameter>
was too small to hold the list of addresses and names referenced by
the <constant>h_name</constant>, <constant>h_aliases</constant>, and
<constant>h_addr_list</constant> elements of the <type>struct
hostent</type>.  If <parameter>buf</parameter> was too small, both
<function>lwres_gethostbyname_r()</function> and
<function>lwres_gethostbyaddr_r()</function> set the global variable
<type>errno</type> to <errorcode>ERANGE</errorcode>.
</para>

</refsect1>
<refsect1>
<title>SEE ALSO</title>
<para>
<citerefentry>
<refentrytitle>gethostent</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>,

<citerefentry>
<refentrytitle>lwres_getipnode</refentrytitle><manvolnum>3</manvolnum>
</citerefentry>,

<citerefentry>
<refentrytitle>lwres_hstrerror</refentrytitle><manvolnum>3
</manvolnum>
</citerefentry>
</para>
</refsect1>

<refsect1>
<title>BUGS</title>
<para>
<function>lwres_gethostbyname()</function>,
<function>lwres_gethostbyname2()</function>,
<function>lwres_gethostbyaddr()</function>
and
<function>lwres_endhostent()</function>
are not thread safe; they return pointers to static data and 
provide error codes through a global variable.
Thread-safe versions for name and address lookup are provided by
<function>lwres_gethostbyname_r()</function>,
and
<function>lwres_gethostbyaddr_r()</function>
respectively.
</para>
<para>
The resolver daemon does not currently support any non-DNS
name services such as 
<filename>/etc/hosts</filename>
or
<type>NIS</type>,
consequently the above functions don't, either.
</para>
</refsect1>
</refentry>