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getipnodebyaddr(3)

NAME

     getipnodebyname, getipnodebyaddr, freehostent -- nodename-to-address and
     address-to-nodename translation

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netdb.h>

     struct hostent *
     getipnodebyname(const char *name, int af, int flags, int *error_num);

     struct hostent *
     getipnodebyaddr(const void *src, size_t len, int af, int *error_num);

     void
     freehostent(struct hostent *ptr);

DESCRIPTION

     The getipnodebyname() and getipnodebyaddr() functions are very similar to
     gethostbyname(3), gethostbyname2(3) and gethostbyaddr(3).	The functions
     cover all the functionalities provided by the older ones, and provide
     better interface to programmers.  The functions require additional argu-
     ments, af, and flags, for specifying address family and operation mode.
     The additional arguments allow programmer to get address for a nodename,
     for specific address family (such as AF_INET or AF_INET6).  The functions
     also require an additional pointer argument, error_num to return the
     appropriate error code, to support thread safe error code returns.

     The type and usage of the return value, struct hostent is described in
     gethostbyname(3).

     For getipnodebyname(), the name argument can be either a node name or a
     numeric address string (i.e., a dotted-decimal IPv4 address or an IPv6
     hex address).  The af argument specifies the address family, either
     AF_INET or AF_INET6.  The flags argument specifies the types of addresses
     that are searched for, and the types of addresses that are returned.  We
     note that a special flags value of AI_DEFAULT (defined below) should han-
     dle most applications.  That is, porting simple applications to use IPv6
     replaces the call

	hptr = gethostbyname(name);

     with

	hptr = getipnodebyname(name, AF_INET6, AI_DEFAULT, &error_num);

     Applications desiring finer control over the types of addresses searched
     for and returned, can specify other combinations of the flags argument.

     A flags of 0 implies a strict interpretation of the af argument:

     o	 If flags is 0 and af is AF_INET, then the caller wants only IPv4
	 addresses.  A query is made for A records.  If successful, the IPv4
     Other constants can be logically-ORed into the flags argument, to modify
     the behavior of the function.

     o	 If the AI_V4MAPPED flag is specified along with an af of AF_INET6,
	 then the caller will accept IPv4-mapped IPv6 addresses.  That is, if
	 no AAAA records are found then a query is made for A records and any
	 found are returned as IPv4-mapped IPv6 addresses (h_length will be
	 16).  The AI_V4MAPPED flag is ignored unless af equals AF_INET6.

     o	 The AI_V4MAPPED_CFG flag is exact same as the AI_V4MAPPED flag only
	 if the kernel supports IPv4-mapped IPv6 address.

     o	 If the AI_ALL flag is used in conjunction with the AI_V4MAPPED flag,
	 and only used with the IPv6 address family.  When AI_ALL is logically
	 or'd with AI_V4MAPPED flag then the caller wants all addresses: IPv6
	 and IPv4-mapped IPv6.	A query is first made for AAAA records and if
	 successful, the IPv6 addresses are returned.  Another query is then
	 made for A records and any found are returned as IPv4-mapped IPv6
	 addresses.  h_length will be 16.  Only if both queries fail does the
	 function return a NULL pointer.  This flag is ignored unless af
	 equals AF_INET6.  If both AI_ALL and AI_V4MAPPED are specified,
	 AI_ALL takes precedence.

     o	 The AI_ADDRCONFIG flag specifies that a query for AAAA records should
	 occur only if the node has at least one IPv6 source address config-
	 ured and a query for A records should occur only if the node has at
	 least one IPv4 source address configured.

	 For example, if the node has no IPv6 source addresses configured, and
	 af equals AF_INET6, and the node name being looked up has both AAAA
	 and A records, then: (a) if only AI_ADDRCONFIG is specified, the
	 function returns a NULL pointer; (b) if AI_ADDRCONFIG | AI_V4MAPPED
	 is specified, the A records are returned as IPv4-mapped IPv6
	 addresses;

     The special flags value of AI_DEFAULT is defined as

	#define  AI_DEFAULT  (AI_V4MAPPED_CFG | AI_ADDRCONFIG)

     We noted that the getipnodebyname() function must allow the name argument
     to be either a node name or a literal address string (i.e., a dotted-dec-
     imal IPv4 address or an IPv6 hex address).  This saves applications from
     having to call inet_pton(3) to handle literal address strings.  When the
     name argument is a literal address string, the flags argument is always
     ignored.

     There are four scenarios based on the type of literal address string and
     the value of the af argument.  The two simple cases are when name is a
     dotted-decimal IPv4 address and af equals AF_INET, or when name is an
     IPv6 hex address and af equals AF_INET6.  The members of the returned
     hostent structure are: h_name points to a copy of the name argument,
     h_aliases is a NULL pointer, h_addrtype is a copy of the af argument,
     h_length is either 4 (for AF_INET) or 16 (for AF_INET6), h_addr_list[0]
     is a pointer to the 4-byte or 16-byte binary address, and h_addr_list[1]
     is a NULL pointer.

     When name is a dotted-decimal IPv4 address and af equals AF_INET6, and
     AI_V4MAPPED is specified, an IPv4-mapped IPv6 address is returned: h_name

     The getipnodebyaddr() function takes almost the same argument as
     gethostbyaddr(3), but adds a pointer to return an error number.  Addi-
     tionally it takes care of IPv4-mapped IPv6 addresses, and IPv4-compatible
     IPv6 addresses.

     The getipnodebyname() and getipnodebyaddr() functions dynamically allo-
     cate the structure to be returned to the caller.  The freehostent() func-
     tion reclaims memory region allocated and returned by getipnodebyname()
     or getipnodebyaddr().

FILES

     /etc/hosts
     /etc/nsswitch.conf
     /etc/resolv.conf

DIAGNOSTICS

     The getipnodebyname() and getipnodebyaddr() functions returns NULL on
     errors.  The integer values pointed to by error_num may then be checked
     to see whether this is a temporary failure or an invalid or unknown host.
     The meanings of each error code are described in gethostbyname(3).

SEE ALSO

     getaddrinfo(3), gethostbyaddr(3), gethostbyname(3), getnameinfo(3),
     hosts(5), nsswitch.conf(5), services(5), hostname(7), named(8)

     R. Gilligan, S. Thomson, J. Bound, and W. Stevens, Basic Socket Interface
     Extensions for IPv6, RFC2553, March 1999.

HISTORY

     The implementation first appeared in KAME advanced networking kit.

STANDARDS

     The getipnodebyname() and getipnodebyaddr() functions are documented in
     ``Basic Socket Interface Extensions for IPv6'' (RFC2553).

BUGS

     Although the current implementation is otherwise thread-safe, using it in
     conjunction with gethostby*() (see gethostbyname(3)) or yp(8) breaks the
     thread-safety of both.

     The getipnodebyname() and getipnodebyaddr() functions do not handle
     scoped IPv6 address properly.  If you use these functions, your program
     will not be able to handle scoped IPv6 addresses.	For IPv6 address
     manipulation, getaddrinfo(3) and getnameinfo(3) are recommended.

     The text was shamelessly copied from RFC2553.

FreeBSD 5.4			August 6, 2004			   FreeBSD 5.4

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