GETADDRINFO(3) FreeBSD Library Functions Manual GETADDRINFO(3)
NAME
getaddrinfo, freeaddrinfo - socket address structure to host and service
name
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int
getaddrinfo(const char *hostname, const char *servname,
const struct addrinfo *hints, struct addrinfo **res);
void
freeaddrinfo(struct addrinfo *ai);
DESCRIPTION
The getaddrinfo() function is used to get a list of addresses and port
numbers for host hostname and service servname. It is a replacement for
and provides more flexibility than the gethostbyname(3) and
getservbyname(3) functions.
The hostname and servname arguments are either pointers to NUL-terminated
strings or the null pointer. An acceptable value for hostname is either
a valid host name or a numeric host address string consisting of a dotted
decimal IPv4 address, an IPv6 address, or a UNIX-domain address. The
servname is either a decimal port number or a service name listed in
services(5). At least one of hostname and servname must be non-null.
hints is an optional pointer to a struct addrinfo, as defined by
<netdb.h>:
struct addrinfo {
int ai_flags; /* AI_PASSIVE, AI_CANONNAME, .. */
int ai_family; /* AF_xxx */
int ai_socktype; /* SOCK_xxx */
int ai_protocol; /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
socklen_t ai_addrlen; /* length of ai_addr */
char *ai_canonname; /* canonical name for hostname */
struct sockaddr *ai_addr; /* binary address */
struct addrinfo *ai_next; /* next structure in linked list */
};
This structure can be used to provide hints concerning the type of socket
that the caller supports or wishes to use. The caller can supply the
following structure elements in hints:
ai_family The address family that should be used. When ai_family is
set to AF_UNSPEC, it means the caller will accept any
address family supported by the operating system.
ai_socktype Denotes the type of socket that is wanted: SOCK_STREAM,
SOCK_DGRAM, SOCK_SEQPACKET, or SOCK_RAW. When ai_socktype
is zero the caller will accept any socket type.
ai_protocol Indicates which transport protocol is desired,
IPPROTO_UDP, IPPROTO_TCP, IPPROTO_SCTP, or
IPPROTO_UDPLITE. If ai_protocol is zero the caller will
accept any protocol.
ai_flags The ai_flags field to which the hints parameter points
shall be set to zero or be the bitwise-inclusive OR of one
or more of the values AI_ADDRCONFIG, AI_ALL, AI_CANONNAME,
AI_NUMERICHOST, AI_NUMERICSERV, AI_PASSIVE and
AI_V4MAPPED. For a UNIX-domain address, ai_flags is
ignored.
AI_ADDRCONFIG If the AI_ADDRCONFIG bit is set, IPv4
addresses shall be returned only if an
IPv4 address is configured on the local
system, and IPv6 addresses shall be
returned only if an IPv6 address is
configured on the local system.
AI_ALL If the AI_ALL flag is used with the
AI_V4MAPPED flag, then getaddrinfo() shall
return all matching IPv6 and IPv4
addresses.
For example, when using the DNS, queries
are made for both AAAA records and A
records, and getaddrinfo() returns the
combined results of both queries. Any
IPv4 addresses found are returned as
IPv4-mapped IPv6 addresses.
The AI_ALL flag without the AI_V4MAPPED
flag is ignored.
AI_CANONNAME If the AI_CANONNAME bit is set, a
successful call to getaddrinfo() will
return a NUL-terminated string containing
the canonical name of the specified
hostname in the ai_canonname element of
the first addrinfo structure returned.
AI_NUMERICHOST If the AI_NUMERICHOST bit is set, it
indicates that hostname should be treated
as a numeric string defining an IPv4 or
IPv6 address and no name resolution should
be attempted.
AI_NUMERICSERV If the AI_NUMERICSERV bit is set, then a
non-null servname string supplied shall be
a numeric port string. Otherwise, an
EAI_NONAME error shall be returned. This
bit shall prevent any type of name
resolution service (for example, NIS+)
from being invoked.
AI_PASSIVE If the AI_PASSIVE bit is set it indicates
that the returned socket address structure
is intended for use in a call to bind(2).
In this case, if the hostname argument is
the null pointer, then the IP address
portion of the socket address structure
will be set to INADDR_ANY for an IPv4
address or IN6ADDR_ANY_INIT for an IPv6
address.
If the AI_PASSIVE bit is not set, the
returned socket address structure will be
ready for use in a call to connect(2) for
a connection-oriented protocol or
connect(2), sendto(2), or sendmsg(2) if a
connectionless protocol was chosen. The
IP address portion of the socket address
structure will be set to the loopback
address if hostname is the null pointer
and AI_PASSIVE is not set.
AI_V4MAPPED If the AI_V4MAPPED flag is specified along
with an ai_family of AF_INET6, then
getaddrinfo() shall return IPv4-mapped
IPv6 addresses on finding no matching IPv6
addresses ( ai_addrlen shall be 16).
For example, when using the DNS, if no
AAAA records are found then a query is
made for A records and any found are
returned as IPv4-mapped IPv6 addresses.
The AI_V4MAPPED flag shall be ignored
unless ai_family equals AF_INET6.
All other elements of the addrinfo structure passed via hints must be
zero or the null pointer.
If hints is the null pointer, getaddrinfo() behaves as if the caller
provided a struct addrinfo with ai_family set to AF_UNSPEC and all other
elements set to zero or NULL.
After a successful call to getaddrinfo(), *res is a pointer to a linked
list of one or more addrinfo structures. The list can be traversed by
following the ai_next pointer in each addrinfo structure until a null
pointer is encountered. Each returned addrinfo structure contains three
members that are suitable for a call to socket(2): ai_family,
ai_socktype, and ai_protocol. For each addrinfo structure in the list,
the ai_addr member points to a filled-in socket address structure of
length ai_addrlen.
This implementation of getaddrinfo() allows numeric IPv6 address notation
with scope identifier, as documented in chapter 11 of RFC 4007. By
appending the percent character and scope identifier to addresses, one
can fill the sin6_scope_id field for addresses. This would make
management of scoped addresses easier and allows cut-and-paste input of
scoped addresses.
At this moment the code supports only link-local addresses with the
format. The scope identifier is hardcoded to the name of the hardware
interface associated with the link (such as ne0). An example is
"fe80::1%ne0", which means "fe80::1 on the link associated with the ne0
interface".
The current implementation assumes a one-to-one relationship between the
interface and link, which is not necessarily true from the specification.
All of the information returned by getaddrinfo() is dynamically
allocated: the addrinfo structures themselves as well as the socket
address structures and the canonical host name strings included in the
addrinfo structures.
Memory allocated for the dynamically allocated structures created by a
successful call to getaddrinfo() is released by the freeaddrinfo()
function. The ai pointer should be a addrinfo structure created by a
call to getaddrinfo().
IMPLEMENTATION NOTES
The behavior of freeadrinfo(NULL) is left unspecified by both Version 4
of the Single UNIX Specification ("SUSv4") and RFC 3493. The current
implementation ignores a NULL argument for compatibility with programs
that rely on the implementation details of other operating systems.
RETURN VALUES
getaddrinfo() returns zero on success or one of the error codes listed in
gai_strerror(3) if an error occurs.
EXAMPLES
The following code tries to connect to "www.kame.net" service "http" via
a stream socket. It loops through all the addresses available,
regardless of address family. If the destination resolves to an IPv4
address, it will use an AF_INET socket. Similarly, if it resolves to
IPv6, an AF_INET6 socket is used. Observe that there is no hardcoded
reference to a particular address family. The code works even if
getaddrinfo() returns addresses that are not IPv4/v6.
struct addrinfo hints, *res, *res0;
int error;
int s;
const char *cause = NULL;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
error = getaddrinfo("www.kame.net", "http", &hints, &res0);
if (error) {
errx(1, "%s", gai_strerror(error));
/* NOTREACHED */
}
s = -1;
for (res = res0; res; res = res->ai_next) {
s = socket(res->ai_family, res->ai_socktype,
res->ai_protocol);
if (s < 0) {
cause = "socket";
continue;
}
if (connect(s, res->ai_addr, res->ai_addrlen) < 0) {
cause = "connect";
close(s);
s = -1;
continue;
}
break; /* okay we got one */
}
if (s < 0) {
err(1, "%s", cause);
/* NOTREACHED */
}
freeaddrinfo(res0);
The following example tries to open a wildcard listening socket onto
service "http", for all the address families available.
struct addrinfo hints, *res, *res0;
int error;
int s[MAXSOCK];
int nsock;
const char *cause = NULL;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
error = getaddrinfo(NULL, "http", &hints, &res0);
if (error) {
errx(1, "%s", gai_strerror(error));
/* NOTREACHED */
}
nsock = 0;
for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) {
s[nsock] = socket(res->ai_family, res->ai_socktype,
res->ai_protocol);
if (s[nsock] < 0) {
cause = "socket";
continue;
}
if (bind(s[nsock], res->ai_addr, res->ai_addrlen) < 0) {
cause = "bind";
close(s[nsock]);
continue;
}
(void) listen(s[nsock], 5);
nsock++;
}
if (nsock == 0) {
err(1, "%s", cause);
/* NOTREACHED */
}
freeaddrinfo(res0);
SEE ALSO
bind(2), connect(2), send(2), socket(2), gai_strerror(3),
gethostbyname(3), getnameinfo(3), getservbyname(3), resolver(3), inet(4),
inet6(4), unix(4), hosts(5), resolv.conf(5), services(5), hostname(7),
named(8)
R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic
Socket Interface Extensions for IPv6, RFC 3493, February 2003.
S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, IPv6 Scoped
Address Architecture, RFC 4007, March 2005.
Craig Metz, "Protocol Independence Using the Sockets API", Proceedings of
the freenix track: 2000 USENIX annual technical conference, June 2000.
STANDARDS
The getaddrinfo() function is defined by the IEEE Std 1003.1-2004
("POSIX.1") specification and documented in RFC 3493, "Basic Socket
Interface Extensions for IPv6".
FreeBSD 13.1-RELEASE-p6 February 10, 2019 FreeBSD 13.1-RELEASE-p6
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