path: root/newlib/libc/sys/linux/net/gethostbyname.3

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, int 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 parameter passed to gethostbyname() or gethostbyname2() should point to a
     NUL-terminated hostname.  The addr parameter 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 parameter specifies the address family (e.g. AF_INET, AF_INET6, etc.)
     of this address.

     The structure returned contains either the information obtained from the name server,
     named(8), 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).

     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.  If the name con-
     tains 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.

     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 pa-
     rameter 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
     /etc/nsswitch.conf
     /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 char *)&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 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.

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

CAVEAT
     The gethostent() function is defined, and sethostent() and endhostent() are redefined, when
     libc(3) 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 argu-
     ment 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 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.

     gethostbyname2() cannot perform AF_INET6 lookups over NIS.  getaddrinfo(3) must be used in-
     stead.

BSD                                        May 25, 1995                                       BSD