getipnodebyname, getipnodebyaddr, freehostent -- nodename-to-address and
Standard C Library (libc, -lc)
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);
freehostent(struct hostent *ptr);
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 arguments,
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
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 handle
most applications. That is, porting simple applications to use IPv6
replaces the call
hptr = gethostbyname(name);
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
addresses are returned and the h_length member of the hostent structure
will be 4, else the function returns a NULL pointer.
+o If flags is 0 and if af is AF_INET6, then the caller wants only IPv6
addresses. A query is made for AAAA records. If successful, the
IPv6 addresses are returned and the h_length member of the hostent
structure will be 16, else the function returns a NULL pointer.
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 configured
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
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-decimal
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
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
is a pointer to the 4-byte or 16-byte binary address, and h_addr_list
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
points to an IPv6 hex address containing the IPv4-mapped IPv6 address,
h_aliases is a NULL pointer, h_addrtype is AF_INET6, h_length is 16,
h_addr_list is a pointer to the 16-byte binary address, and
h_addr_list is a NULL pointer.
It is an error when name is an IPv6 hex address and af equals AF_INET.
The function's return value is a NULL pointer and the value pointed to by
error_num equals HOST_NOT_FOUND.
The getipnodebyaddr() function takes almost the same argument as
gethostbyaddr(3), but adds a pointer to return an error number. Additionally
it takes care of IPv4-mapped IPv6 addresses, and IPv4-compatible
The getipnodebyname() and getipnodebyaddr() functions dynamically allocate
the structure to be returned to the caller. The freehostent() function
reclaims memory region allocated and returned by getipnodebyname()
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).
gethostbyaddr(3), gethostbyname(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.
The implementation first appeared in KAME advanced networking kit.
The getipnodebyname() and getipnodebyaddr() functions are documented in
``Basic Socket Interface Extensions for IPv6'' (RFC2553).
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 current implementation is not thread-safe.
The text was shamelessly copied from RFC2553.
FreeBSD 5.2.1 May 25, 1995 FreeBSD 5.2.1 [ Back ]