gethostbyname(3)

NAME

       gethostbyname, gethostbyaddr, sethostent, endhostent, her­
       ror, hstrerror - get network host entry

SYNOPSIS

       #include <netdb.h>
       extern int h_errno;

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

       #include <sys/socket.h>        /* for AF_INET */
       struct hostent *gethostbyaddr(const char *addr, int len, int type);

       void sethostent(int stayopen);

       void endhostent(void);

       void herror(const char *s);

       const char * hstrerror(int err);

DESCRIPTION

       The gethostbyname() function returns a structure  of  type
       hostent  for  the  given host name.  Here name is either a
       host name, or an IPv4 address in standard dot notation, or
       an IPv6 address in colon (and possibly dot) notation. (See
       RFC 1884 for the description of IPv6 addresses.)  If  name
       is  an  IPv4  or  IPv6 address, no lookup is performed and
       gethostbyname() simply copies name into the  h_name  field
       and  its struct in_addr equivalent into the h_addr_list[0]
       field of the returned hostent structure.  If name  doesn't
       end  in  a dot and the environment variable HOSTALIASES is
       set, the alias file pointed to by HOSTALIASES  will  first
       be  searched  for  name (see hostname(7) for the file for­
       mat).  The current domain and  its  parents  are  searched
       unless  name ends in a dot.  The gethostbyname2() function
       works the same way, but permits  to  specify  the  address
       family to which the address must belong.

       The  gethostbyaddr()  function returns a structure of type
       hostent for the given host address addr of length len  and
       address  type  type.   The only valid address type is cur­
       rently AF_INET.

       The sethostent() function specifies, if stayopen  is  true
       (1),  that  a  connected TCP socket should be used for the
       name server queries and that the connection should  remain
       open  during  successive  queries.  Otherwise, name server
       queries will use UDP datagrams.

       The endhostent() function ends the use of a TCP connection
       for name server queries.




       The  (obsolete) herror() function prints the error message
       associated with the current value of h_errno on stderr.

       The (obsolete) hstrerror() function takes an error  number
       (typically  h_errno) and returns the corresponding message
       string.

       The domain name queries carried out by gethostbyname() and
       gethostbyaddr()  use  a  combination  of any or all of the
       name server named(8), a broken out line  from  /etc/hosts,
       and the Network Information Service (NIS or YP), depending
       upon the contents of the  order  line  in  /etc/host.conf.
       (See   resolv+(8)).    The  default  action  is  to  query
       named(8), followed by /etc/hosts.

       The hostent structure is defined in <netdb.h> as follows:

              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 */
              }
              #define h_addr  h_addr_list[0]  /* for backward compatibility */

       The members of the hostent structure are:

       h_name The official name of the host.

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

       h_addrtype
              The type of address; always AF_INETP at present.

       h_length
              The length of the address in bytes.

       h_addr_list
              A  zero-terminated  array  of network addresses for
              the host in network byte order.

       h_addr The first address in h_addr_list for backward  com­
              patibility.

RETURN VALUE

       The  gethostbyname()  and gethostbyaddr() functions return
       the hostent structure  or  a  NULL  pointer  if  an  error
       occurs.   On  error,  the  h_errno variable holds an error
       number.

ERRORS

       The variable h_errno can have the following values:

       HOST_NOT_FOUND
              The specified host is unknown.

       NO_ADDRESS or NO_DATA
              The requested name is valid but does not have an IP
              address.

       NO_RECOVERY
              A non-recoverable name server error occurred.

       TRY_AGAIN
              A temporary error occurred on an authoritative name
              server.  Try again later.

FILES

       /etc/host.conf
              resolver configuration file

       /etc/hosts
              host database file

CONFORMING TO

       BSD 4.3.  The SUS-v2 declares the len parameter to  be  of
       type size_t.

RUMOURS

       The future glibc2.2 will follow SUS-v2.

SEE ALSO

       resolver(3), hosts(5), hostname(7), resolv+(8), named(8)