NAME
getaddrinfo
,
freeaddrinfo
—
host and service name to socket address
structure
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 IP 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 or an IPv6 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; /* input flags */ int ai_family; /* address family for socket */ int ai_socktype; /* socket type */ int ai_protocol; /* protocol for socket */ socklen_t ai_addrlen; /* length of socket-address */ struct sockaddr *ai_addr; /* socket-address for socket */ char *ai_canonname; /* canonical name for service location */ struct addrinfo *ai_next; /* pointer to next in 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
, orSOCK_RAW
. When ai_socktype is zero the caller will accept any socket type. - ai_protocol
- Indicates which transport protocol is desired,
IPPROTO_UDP
orIPPROTO_TCP
. If ai_protocol is zero the caller will accept any protocol. - ai_flags
- ai_flags is formed by OR'ing the following values:
AI_ADDRCONFIG
- If the
AI_ADDRCONFIG
bit is set, IPv4 addresses will be returned only if an IPv4 address is configured on an interface, and IPv6 addresses will be returned only if an IPv6 address is configured on an interface. Addresses on a loopback interface and link-local IPv6 addresses are not considered valid as configured addresses. This bit is only considered when determining whether a DNS query should be performed or not. AI_CANONNAME
- If the
AI_CANONNAME
bit is set, a successful call togetaddrinfo
() will return a NUL-terminated string containing the canonical name of the specified host name in the ai_canonname element of the firstaddrinfo
structure returned. AI_FQDN
- If the
AI_FQDN
bit is set, a successful call togetaddrinfo
() will return a NUL-terminated string containing the fully qualified domain name of the specified host name in the ai_canonname element of the firstaddrinfo
structure returned.This is different from the
AI_CANONNAME
bit flag that returns the canonical name registered in DNS, which may be different from the fully qualified domain name that the host name resolved to. Only one of theAI_FQDN
andAI_CANONNAME
bits can be set. 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, it indicates that servname should be treated as a numeric port string and no service name resolution should be attempted. 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 toINADDR_ANY
for an IPv4 address orIN6ADDR_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 andAI_PASSIVE
is not set.
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
, ai_flags set to
AI_ADDRCONFIG
, 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. The three members ai_family,
ai_socktype, and ai_protocol in
each returned addrinfo
structure are suitable for a
call to socket(2). 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
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 an
addrinfo
structure created by a call to
getaddrinfo
().
RETURN VALUES
getaddrinfo
() returns zero on success or
one of the error codes listed in
gai_strerror(3) if an error occurs. If an error occurs, no
memory is allocated by getaddrinfo
(), therefore it
is not necessary to release the addrinfo
structure(s).
EXAMPLES
The following code tries to connect to
“www.kame.net
” service
“www
” 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 save_errno; 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", "www", &hints, &res0); if (error) errx(1, "%s", gai_strerror(error)); s = -1; for (res = res0; res; res = res->ai_next) { s = socket(res->ai_family, res->ai_socktype, res->ai_protocol); if (s == -1) { cause = "socket"; continue; } if (connect(s, res->ai_addr, res->ai_addrlen) == -1) { cause = "connect"; save_errno = errno; close(s); errno = save_errno; s = -1; continue; } break; /* okay we got one */ } if (s == -1) err(1, "%s", cause); freeaddrinfo(res0);
The following example tries to open a wildcard listening socket
onto service “www
”, for all the
address families available.
struct addrinfo hints, *res, *res0; int error; int save_errno; 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, "www", &hints, &res0); if (error) errx(1, "%s", gai_strerror(error)); 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] == -1) { cause = "socket"; continue; } if (bind(s[nsock], res->ai_addr, res->ai_addrlen) == -1) { cause = "bind"; save_errno = errno; close(s[nsock]); errno = save_errno; continue; } (void) listen(s[nsock], 5); nsock++; } if (nsock == 0) err(1, "%s", cause); freeaddrinfo(res0);
SEE ALSO
bind(2), connect(2), send(2), socket(2), gai_strerror(3), gethostbyname(3), getnameinfo(3), getservbyname(3), resolver(3), hosts(5), resolv.conf(5), services(5), hostname(7)
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 1003.1g-2000 (“POSIX.1g”)
draft specification and documented in RFC 3493.
The AI_FQDN
flag bit first appeared in
Windows 7.
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.