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>

     int h_errno;

     struct hostent *
     gethostbyname(const char *name);

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

     struct hostent *
     gethostbyaddr(const void *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 getaddrinfo(3) and getnameinfo(3) functions are preferred over the gethostbyname(), gethostbyname2(), and gethostbyaddr() functions.

     The gethostbyname(), gethostbyname2() and gethostbyaddr() functions each return a pointer to an object with the following structure describ-
     ing an internet host referenced by name or by address, respectively.

     The name argument passed to gethostbyname() or gethostbyname2() should point to a NUL-terminated hostname.  The addr argument passed to
     gethostbyaddr() should point to an address which is len bytes long, in binary form (i.e., not an IP address in human readable ASCII form).
     The type argument specifies the address family (e.g. AF_INET, AF_INET6, etc.) of this address.

     The structure returned contains information obtained from mDNSResponder(8), including records in /etc/hosts.

     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 alternate names for the host.

     h_addrtype   The type of address being returned; usually 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.

     When using the nameserver, gethostbyname() and gethostbyname2() will search for the named host in the current domain and its parents unless
     the name ends in a dot.

     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.

     The herror() function writes a message to the diagnostic output consisting of the string argument string, 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 argument.

     Historically, passing a host's own hostname to gethostbyname() or gethostbyname2() has been a popular technique for determining that host's
     IP address(es), but this is fragile, and doesn't work reliably in all cases. The appropriate way for software to discover the IP address(es)
     of the host it is running on is to use getifaddrs(3).

FILES

     /etc/hosts
     /etc/resolv.conf

EXAMPLES

     Print out the hostname associated with a specific IP address:

	   const char *ipstr = "127.0.0.1";
	   struct in_addr ip;
	   struct hostent *hp;

	   if (!inet_aton(ipstr, &ip))
		   errx(1, "can't parse IP address %s", ipstr);

	   if ((hp = gethostbyaddr((const void *)&ip,
	       sizeof ip, AF_INET)) == NULL)
		   errx(1, "no name associated with %s", ipstr);

	   printf("name associated with %s is %s
", ipstr, hp->h_name);

DIAGNOSTICS

     Error return status from gethostbyname(), gethostbyname2() and gethostbyaddr() is indicated by return of a NULL pointer.  The 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 message
     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 example, a mail-forwarder may be registered for this domain.

SEE ALSO

     getaddrinfo(3), getnameinfo(3), inet_aton(3), resolver(3), hosts(5), hostname(7), mDNSResponder(8)

CAVEAT

     The gethostent() function is defined, and sethostent() and endhostent() are redefined, when Standard C Library (libc, -lc) is built to use
     only the routines to lookup in /etc/hosts and not the name server.

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

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

     The endhostent() function closes the file.

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 version 4.9.4.

BUGS

     These functions use a thread-specific data storage; if the data is needed for future use, it should be copied before any subsequent calls
     overwrite it.

     Though these functions are thread-safe, still it is recommended to use the getaddrinfo(3) family of functions, instead.

     Only the Internet address format is currently understood.

BSD
								   May 12, 2006 							       BSD