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GETHOSTBYNAME(3)		   BSD Library Functions Manual 		 GETHOSTBYNAME(3)

NAME
     gethostbyname, gethostbyname2, gethostbyaddr, gethostent, sethostent, endhostent, herror,
     hstrerror -- get network host entry

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <netdb.h>
     extern int h_errno;

     struct hostent *
     gethostbyname(const char *name);

     struct hostent *
     gethostbyname2(const char *name, int af);

     struct hostent *
     gethostbyaddr(const char *addr, socklen_t len, int type);

     struct hostent *
     gethostent(void);

     void
     sethostent(int stayopen);

     void
     endhostent(void);

     void
     herror(const char *string);

     const char *
     hstrerror(int err);

DESCRIPTION
     The gethostbyname(), gethostbyname2() and gethostbyaddr() functions each return a pointer to
     an object with the following structure describing an internet host.

     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 */

     The members of this structure are:

     h_name	  Official name of the host.

     h_aliases	  A NULL-terminated array of alternative names for the host.

     h_addrtype   The type of address being returned; currently always AF_INET.

     h_length	  The length, in bytes, of the address.

     h_addr_list  A NULL-terminated array of network addresses for the host.  Host addresses are
		  returned in network byte order.

     h_addr	  The first address in h_addr_list; this is for backward compatibility.

     In the case of gethostbyname() and gethostbyname2(), the host is specified by name, or using
     a string representation of a numeric address.  In the case of gethostbyaddr(), the host is
     specified using a binary representation of an address.

     The returned struct hostent structure may contain the result of a simple string to binary
     conversion, information obtained from the domain name resolver (see resolver(3)), broken-out
     fields from a line in /etc/hosts, or database entries supplied by the yp(8) system.  The
     order of the lookups is controlled by the 'hosts' entry in nsswitch.conf(5).

     When using the domain name resolver, gethostbyname() and gethostbyname2() will search for
     the named host in the current domain and its parents unless the name ends in a dot.  If the
     name contains no dot, and if the environment variable ``HOSTALIASES'' contains the name of
     an alias file, the alias file will first be searched for an alias matching the input name.
     See hostname(7) for the domain search procedure and the alias file format.

     The gethostbyname2() function is an evolution of gethostbyname() which is intended to allow
     lookups in address families other than AF_INET, for example AF_INET6.  Currently the af
     argument must be specified as AF_INET or AF_INET6, else the function will return NULL after
     having set h_errno to NETDB_INTERNAL.

     The gethostent() function reads the next line of the /etc/hosts file, opening the file if
     necessary.

     The sethostent() function may be used to request the use of a connected TCP socket for
     queries.  If the stayopen flag is non-zero, this sets the option to send all queries to the
     name server using TCP and to retain the connection after each call to gethostbyname(),
     gethostbyname2(), or gethostbyaddr().  Otherwise, queries are performed using UDP datagrams.

     The endhostent() function closes the TCP connection.

     The herror() function writes a message to the diagnostic output consisting of the string
     parameter s, the constant string ": ", and a message corresponding to the value of h_errno.

     The hstrerror() function returns a string which is the message text corresponding to the
     value of the err parameter.

FILES
     /etc/hosts

DIAGNOSTICS
     Error return status from gethostbyent(), gethostbyname(), gethostbyname2(), and
     gethostbyaddr() is indicated by return of a null pointer.	The external integer h_errno may
     then be checked to see whether this is a temporary failure or an invalid or unknown host.
     The routine herror() can be used to print an error message describing the failure.  If its
     argument string is non-NULL, it is printed, followed by a colon and a space.  The error mes-
     sage is printed with a trailing newline.

     The variable h_errno can have the following values:

     HOST_NOT_FOUND  No such host is known.

     TRY_AGAIN	     This is usually a temporary error and means that the local server did not
		     receive a response from an authoritative server.  A retry at some later time
		     may succeed.

     NO_RECOVERY     Some unexpected server failure was encountered.  This is a non-recoverable
		     error.

     NO_DATA	     The requested name is valid but does not have an IP address; this is not a
		     temporary error.  This means that the name is known to the name server but
		     there is no address associated with this name.  Another type of request to
		     the name server using this domain name will result in an answer; for exam-
		     ple, a mail-forwarder may be registered for this domain.

SEE ALSO
     resolver(3), hosts(5), nsswitch.conf(5), hostname(7), named(8)

HISTORY
     The herror() function appeared in 4.3BSD.	The endhostent(), gethostbyaddr(),
     gethostbyname(), gethostent(), and sethostent() functions appeared in 4.2BSD.  The
     gethostbyname2() function first appeared in bind-4.9.4.  IPv6 support was implemented in
     WIDE Hydrangea IPv6 protocol stack kit.

CAVEATS
     If the search routines specified in nsswitch.conf(5) decide to read the /etc/hosts file,
     gethostbyname(), gethostbyname2(), and gethostbyaddr() will read the next line of the file,
     re-opening the file if necessary.

     The sethostent() function opens and/or rewinds the file /etc/hosts.  If the stayopen argu-
     ment is non-zero, the file will not be closed after each call to gethostbyname(),
     gethostbyname2(), gethostbyaddr(), or gethostent().

     The endhostent() function closes the file.

BUGS
     These functions use static data storage; if the data is needed for future use, it should be
     copied before any subsequent calls overwrite it.  Only the Internet address format is cur-
     rently understood.

     The gethostent() does not currently follow the search order specified in nsswitch.conf(5)
     and only reads the /etc/hosts file.

BSD					 October 7, 2006				      BSD
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