gethostbyname

GETHOSTBYNAME(3)            Library Functions Manual          GETHOSTBYNAME(3)

NAME
     gethostbyname, gethostbyaddr, gethostent, sethostent, endhostent, herror
     — get network host entry

SYNOPSIS
     #include <netdb.h>

     extern int h_errno;

     struct hostent *
     gethostbyname(char *name);

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

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

     struct hostent *
     gethostent();

     sethostent(int stayopen);

     endhostent();

     herror(char *string);

DESCRIPTION
     Gethostbyname(), gethostbyname2(), and gethostbyaddr() each return a
     pointer to a hostent structure (see below) describing an internet host
     referenced by name or by address, as the function names indicate.  This
     structure contains either the information obtained from the name server,
     named(8), or broken-out fields from a line in /etc/hosts.  If the local
     name server is not running, these routines do a lookup 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 zero-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 zero-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() will search for the named host
     in each parent domain given in the “search” directive of resolv.conf(5)
     unless the name contains 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.

     Gethostbyname2() is an evolution of gethostbyname() 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 else the function
     will return NULL after having set h_errno to NETDB_INTERNAL.

     Sethostent() 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() or gethostbyaddr().  Otherwise,
     queries are performed using UDP datagrams.

     Endhostent() closes the TCP connection.

ENVIRONMENT
     HOSTALIASES    Name of file containing (host alias, full hostname) pairs.

FILES
     /etc/hosts     See hosts(5).

DIAGNOSTICS
     Error return status from gethostbyname() 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 message is printed
     with a trailing newline.

     h_errno can have the following values:

           NETDB_INTERNAL    This indicates an internal error in the library,
                             unrelated to the network or name service.  errno
                             will be valid in this case; see perror(2).

           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, as one might
                             expect.

           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
     hosts(5), hostname(7), named(8), resolver(3), resolver(5).

CAVEAT
     Gethostent() is defined, and sethostent() and endhostent() are redefined,
     when libc is built to use only the routines to lookup in /etc/hosts and
     not the name server:


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

           Sethostent() is redefined to open and rewind the file.  If the
           stayopen argument is non-zero, the hosts data base will not be
           closed after each call to gethostbyname() or gethostbyaddr().

           Endhostent() is redefined to close the file.

BUGS
     All information is contained in a static area so it must be copied if it
     is to be saved.  Only the Internet address format is currently
     understood.

4th Berkeley Distribution        June 23, 1990       4th Berkeley Distribution