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 describing 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 com-
                  patibility.

     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 consist-
     ing of the string argument string, the constant string ": ", and a mes-
     sage corresponding to the value of h_errno.

     The hstrerror() function returns a string which is the message text cor-
     responding 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\n", 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 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 authori-
                     tative 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() func-
     tions 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