Given node and service, which identify an Internet host and a
service, getaddrinfo() returns one or more addrinfo structures,
each of which contains an Internet address that can be specified
in a call to bind(2) or connect(2). The getaddrinfo() function
combines the functionality provided by the gethostbyname(3) and
getservbyname(3) functions into a single interface, but unlike
the latter functions, getaddrinfo() is reentrant and allows
programs to eliminate IPv4-versus-IPv6 dependencies.
The addrinfo structure used by getaddrinfo() contains the
following fields:
struct addrinfo {
int ai_flags;
int ai_family;
int ai_socktype;
int ai_protocol;
socklen_t ai_addrlen;
struct sockaddr *ai_addr;
char *ai_canonname;
struct addrinfo *ai_next;
};
The hints argument points to an addrinfo structure that specifies
criteria for selecting the socket address structures returned in
the list pointed to by res. If hints is not NULL it points to an
addrinfo structure whose ai_family, ai_socktype, and ai_protocol
specify criteria that limit the set of socket addresses returned
by getaddrinfo(), as follows:
ai_family
This field specifies the desired address family for the
returned addresses. Valid values for this field include
AF_INET and AF_INET6. The value AF_UNSPEC indicates that
getaddrinfo() should return socket addresses for any
address family (either IPv4 or IPv6, for example) that can
be used with node and service.
ai_socktype
This field specifies the preferred socket type, for
example SOCK_STREAM or SOCK_DGRAM. Specifying 0 in this
field indicates that socket addresses of any type can be
returned by getaddrinfo().
ai_protocol
This field specifies the protocol for the returned socket
addresses. Specifying 0 in this field indicates that
socket addresses with any protocol can be returned by
getaddrinfo().
ai_flags
This field specifies additional options, described below.
Multiple flags are specified by bitwise OR-ing them
together.
All the other fields in the structure pointed to by hints must
contain either 0 or a null pointer, as appropriate.
Specifying hints as NULL is equivalent to setting ai_socktype and
ai_protocol to 0; ai_family to AF_UNSPEC; and ai_flags to
(AI_V4MAPPED | AI_ADDRCONFIG). (POSIX specifies different
defaults for ai_flags; see NOTES.) node specifies either a
numerical network address (for IPv4, numbers-and-dots notation as
supported by inet_aton(3); for IPv6, hexadecimal string format as
supported by inet_pton(3)), or a network hostname, whose network
addresses are looked up and resolved. If hints.ai_flags contains
the AI_NUMERICHOST flag, then node must be a numerical network
address. The AI_NUMERICHOST flag suppresses any potentially
lengthy network host address lookups.
If the AI_PASSIVE flag is specified in hints.ai_flags, and node
is NULL, then the returned socket addresses will be suitable for
bind(2)ing a socket that will accept(2) connections. The
returned socket address will contain the "wildcard address"
(INADDR_ANY for IPv4 addresses, IN6ADDR_ANY_INIT for IPv6
address). The wildcard address is used by applications
(typically servers) that intend to accept connections on any of
the host's network addresses. If node is not NULL, then the
AI_PASSIVE flag is ignored.
If the AI_PASSIVE flag is not set in hints.ai_flags, then the
returned socket addresses will be suitable for use with
connect(2), sendto(2), or sendmsg(2). If node is NULL, then the
network address will be set to the loopback interface address
(INADDR_LOOPBACK for IPv4 addresses, IN6ADDR_LOOPBACK_INIT for
IPv6 address); this is used by applications that intend to
communicate with peers running on the same host.
service sets the port in each returned address structure. If
this argument is a service name (see services(5)), it is
translated to the corresponding port number. This argument can
also be specified as a decimal number, which is simply converted
to binary. If service is NULL, then the port number of the
returned socket addresses will be left uninitialized. If
AI_NUMERICSERV is specified in hints.ai_flags and service is not
NULL, then service must point to a string containing a numeric
port number. This flag is used to inhibit the invocation of a
name resolution service in cases where it is known not to be
required.
Either node or service, but not both, may be NULL.
The getaddrinfo() function allocates and initializes a linked
list of addrinfo structures, one for each network address that
matches node and service, subject to any restrictions imposed by
hints, and returns a pointer to the start of the list in res.
The items in the linked list are linked by the ai_next field.
There are several reasons why the linked list may have more than
one addrinfo structure, including: the network host is
multihomed, accessible over multiple protocols (e.g., both
AF_INET and AF_INET6); or the same service is available from
multiple socket types (one SOCK_STREAM address and another
SOCK_DGRAM address, for example). Normally, the application
should try using the addresses in the order in which they are
returned. The sorting function used within getaddrinfo() is
defined in RFC 3484; the order can be tweaked for a particular
system by editing /etc/gai.conf (available since glibc 2.5).
If hints.ai_flags includes the AI_CANONNAME flag, then the
ai_canonname field of the first of the addrinfo structures in the
returned list is set to point to the official name of the host.
The remaining fields of each returned addrinfo structure are
initialized as follows:
* The ai_family, ai_socktype, and ai_protocol fields return the
socket creation parameters (i.e., these fields have the same
meaning as the corresponding arguments of socket(2)). For
example, ai_family might return AF_INET or AF_INET6;
ai_socktype might return SOCK_DGRAM or SOCK_STREAM; and
ai_protocol returns the protocol for the socket.
* A pointer to the socket address is placed in the ai_addr field,
and the length of the socket address, in bytes, is placed in
the ai_addrlen field.
If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4
addresses are returned in the list pointed to by res only if the
local system has at least one IPv4 address configured, and IPv6
addresses are returned only if the local system has at least one
IPv6 address configured. The loopback address is not considered
for this case as valid as a configured address. This flag is
useful on, for example, IPv4-only systems, to ensure that
getaddrinfo() does not return IPv6 socket addresses that would
always fail in connect(2) or bind(2).
If hints.ai_flags specifies the AI_V4MAPPED flag, and
hints.ai_family was specified as AF_INET6, and no matching IPv6
addresses could be found, then return IPv4-mapped IPv6 addresses
in the list pointed to by res. If both AI_V4MAPPED and AI_ALL
are specified in hints.ai_flags, then return both IPv6 and
IPv4-mapped IPv6 addresses in the list pointed to by res. AI_ALL
is ignored if AI_V4MAPPED is not also specified.
The freeaddrinfo() function frees the memory that was allocated
for the dynamically allocated linked list res.
Extensions to getaddrinfo() for Internationalized Domain Names
Starting with glibc 2.3.4, getaddrinfo() has been extended to
selectively allow the incoming and outgoing hostnames to be
transparently converted to and from the Internationalized Domain
Name (IDN) format (see RFC 3490, Internationalizing Domain Names
in Applications (IDNA)). Four new flags are defined:
AI_IDN If this flag is specified, then the node name given in
node is converted to IDN format if necessary. The source
encoding is that of the current locale.
If the input name contains non-ASCII characters, then the
IDN encoding is used. Those parts of the node name
(delimited by dots) that contain non-ASCII characters are
encoded using ASCII Compatible Encoding (ACE) before being
passed to the name resolution functions.
AI_CANONIDN
After a successful name lookup, and if the AI_CANONNAME
flag was specified, getaddrinfo() will return the
canonical name of the node corresponding to the addrinfo
structure value passed back. The return value is an exact
copy of the value returned by the name resolution
function.
If the name is encoded using ACE, then it will contain the
xn-- prefix for one or more components of the name. To
convert these components into a readable form the
AI_CANONIDN flag can be passed in addition to
AI_CANONNAME. The resulting string is encoded using the
current locale's encoding.
AI_IDN_ALLOW_UNASSIGNED, AI_IDN_USE_STD3_ASCII_RULES
Setting these flags will enable the IDNA_ALLOW_UNASSIGNED
(allow unassigned Unicode code points) and
IDNA_USE_STD3_ASCII_RULES (check output to make sure it is
a STD3 conforming hostname) flags respectively to be used
in the IDNA handling.
