FreeBSD Man Pages

gethostbyaddr(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 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 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 {
     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.  If the name contains no dot, and if the environ-
     ment 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 for-
     mat.

     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 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.

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);

     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 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), named(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 static data storage; if the data is needed for future
     use, it should be copied before any subsequent calls overwrite it.
     Threaded applications should never use them, as they will also conflict
     with the getaddrinfo(3) and getipnodebyname(3) families of functions
     (which should be used instead).  Only the Internet address format is cur-
     rently understood.

SPONSORED LINKS