GETADDRINFO(3)

NAME
getaddrinfo, freeaddrinfo, gai_strerror - nodename-to-address translation in protocol-independent manner

LIBRARY
libc

SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int
getaddrinfo(const char *nodename, const char *servname, const struct addrinfo *hints, struct addrinfo **res);
void
freeaddrinfo(struct addrinfo *ai);
char *
gai_strerror(int ecode);

DESCRIPTION
The getaddrinfo() function is defined for protocol-independent nodename-to-address translation. It performs the functionality of gethostbyname(3) and getservbyname(3), but in a more sophisticated manner.

The addrinfo structure is defined as a result of including the <netdb.h> header:

    struct addrinfo {
         int     ai_flags;     /* AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST */
         int     ai_family;    /* PF_xxx */
         int     ai_socktype;  /* SOCK_xxx */
         int     ai_protocol;  /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
         size_t  ai_addrlen;   /* length of ai_addr */
         char   *ai_canonname; /* canonical name for nodename */
         struct sockaddr  *ai_addr; /* binary address */
         struct addrinfo  *ai_next; /* next structure in linked list */
    };

The nodename and servname arguments are pointers to null-terminated strings or NULL. One or both of these two arguments must be a non-NULL pointer. In the normal client scenario, both the nodename and servname are specified. In the normal server scenario, only the servname is specified. A non-NULL nodename string can be either a node name or a numeric host address string (i.e., a dotted-decimal IPv4 address or an IPv6 hex address). A non-NULL servname string can be either a service name or a decimal port number.

The caller can optionally pass an addrinfo structure, pointed to by the third argument, to provide hints concerning the type of socket that the caller supports. In this hints structure all members other than ai_flags, ai_family, ai_socktype, and ai_protocol must be zero or a NULL pointer. A value of PF_UNSPEC for ai_family means the caller will accept any protocol family. A value of 0 for ai_socktype means the caller will accept any socket type. A value of 0 for ai_protocol means the caller will accept any protocol. For example, if the caller handles only TCP and not UDP, then the ai_socktype member of the hints structure should be set to SOCK_STREAM when getaddrinfo() is called. If the caller handles only IPv4 and not IPv6, then the ai_family member of the hints structure should be set to PF_INET when getaddrinfo() is called. If the third argument to getaddrinfo() is a NULL pointer, this is the same as if the caller had filled in an addrinfo structure initialized to zero with ai_family set to PF_UNSPEC.

Upon successful return a pointer to a linked list of one or more addrinfo structures is returned through the final argument. The caller can process each addrinfo structure in this list by following the ai_next pointer, until a NULL pointer is encountered. In each returned addrinfo structure the three members ai_family, ai_socktype, and ai_protocol are the corresponding arguments for a call to the socket() function. In each addrinfo structure the ai_addr member points to a filled-in socket address structure whose length is specified by the ai_addrlen member.

If the AI_PASSIVE bit is set in the ai_flags member of the hints structure, then the caller plans to use the returned socket address structure in a call to bind(). In this case, if the nodename argument is a 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 in the ai_flags member of the hints structure, then the returned socket address structure will be ready for a call to connect() (for a connection-oriented protocol) or either connect(), sendto(), or sendmsg() (for a connectionless protocol). In this case, if the nodename argument is a NULL pointer, then the IP address portion of the socket address structure will be set to the loopback address.

If the AI_CANONNAME bit is set in the ai_flags member of the hints structure, then upon successful return the ai_canonname member of the first addrinfo structure in the linked list will point to a null-terminated string containing the canonical name of the specified nodename.

If the AI_NUMERICHOST bit is set in the ai_flags member of the hints structure, then a non-NULL nodename string must be a numeric host address string. Otherwise an error of EAI_NONAME is returned. This flag prevents any type of name resolution service (e.g., the DNS) from being called.

The arguments to getaddrinfo() must be sufficiently consistent and unambiguous. Here are some problem cases you may encounter:

•   The getaddrinfo() function will fail if the members in the hints structure are not consistent. For example, for internet address families, getaddrinfo() will fail if you specify SOCK_STREAM to ai_socktype while you specify IPPROTO_UDP to ai_protocol.

•   If you specify a servname which is defined only for certain ai_socktype, getaddrinfo() will fail because the arguments are not consistent. For example, getaddrinfo() will return an error if you ask for “tftp” service on SOCK_STREAM.

•   For internet address families, if you specify servname while you set ai_socktype to SOCK_RAW, getaddrinfo() will fail, because service names are not defined for the internet SOCK_RAW space.

•   If you specify numeric servname, while leaving ai_socktype and ai_protocol unspecified, getaddrinfo() will fail. This is because the numeric servname does not identify any socket type, and getaddrinfo() is not allowed to glob the argument in such case.

All of the information returned by getaddrinfo() is dynamically allocated: the addrinfo structures, the socket address structures, and canonical node name strings pointed to by the addrinfo structures. To return this information to the system the function freeaddrinfo() is called. The addrinfo structure pointed to by the ai argument is freed, along with any dynamic storage pointed to by the structure. This operation is repeated until a NULL ai_next pointer is encountered.

To aid applications in printing error messages based on the EAI_xxx codes returned by getaddrinfo(), gai_strerror() is defined. The argument is one of the EAI_xxx values defined earlier and the return value points to a string describing the error. If the argument is not one of the EAI_xxx values, the function still returns a pointer to a string whose contents indicate an unknown error.

EXTENSIONS
This implementation supports numeric IPv6 address notation with the experimental scope identifier. By appending a percent sign and scope identifier to the address, you can specify the value of the sin6_scope_id field of the socket address. This makes management of scoped address easier, and allows cut-and-paste input of scoped addresses.

At the moment the code supports only link-local addresses in this format. The scope identifier is hardcoded to name of hardware interface associated with the link, (such as ne0). For example, “fe80::1%ne0”, which means “ fe80::1 on the link associated with the ne0 interface ”.

This implementation is still very experimental and non-standard. The current implementation assumes a one-to-one relationship between interfaces and links, which is not necessarily true according to the specification.

EXAMPLES
The following code tries to connect to “www.kame.net” service “http”. via stream socket. It loops through all the addresses available, regardless of the 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 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 = PF_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;
    cause = "no addresses";
    errno = EADDRNOTAVAIL;
    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, 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 = PF_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;
    	}
    	if (listen(s[nsock], SOMAXCONN) < 0) {
    		cause = "listen";
    		close(s[nsock]);
    		continue;
    	}
    	nsock++;
    }
    if (nsock == 0) {
    	err(1, cause);
    	/*NOTREACHED*/
    }
    freeaddrinfo(res0);

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

DIAGNOSTICS
Error return status from getaddrinfo() is zero on success and non-zero on errors. Non-zero error codes are defined in <netdb.h>, and as follows:

EAI_AGAIN
    Temporary failure in name resolution.
EAI_BADFLAGS
    Invalid value for ai_flags.
EAI_FAIL
    Non-recoverable failure in name resolution.
EAI_FAMILY
    The ai_family address family is not supported.
EAI_MEMORY
    Memory allocation failure.
EAI_NONAME
    Neither nodename nor servname provided, or not known.
EAI_SERVICE
    The servname service name is not supported for ai_socktype.
EAI_SOCKTYPE
    The ai_socktype socket type is not supported.
EAI_SYSTEM
    System error returned in errno.
EAI_BADHINTS
    Invalid value for hints.
EAI_PROTOCOL
    Resolved protocol is unknown.
EAI_MAX
    Unknown error.

If called with an appropriate argument, gai_strerror() returns a pointer to a string describing the given error code. If the argument is not one of the EAI_xxx values, the function still returns a pointer to a string whose contents indicate an unknown error.

SEE ALSO
gethostbyname(3), getnameinfo(3), getservbyname(3), hosts(5), resolv.conf(5), services(5), hostname(7), named(8)

R. Gilligan, S. Thomson, J. Bound, W. Stevens, Basic Socket Interface Extensions for IPv6, RFC2553, March 1999. Tatsuya Jinmei, Atsushi Onoe, An Extension of Format for IPv6 Scoped Addresses, internet draft, draft-ietf-ipngwg-scopedaddr-format-02.txt, work in progress material. Craig Metz, Protocol Independence Using the Sockets API, Proceedings of the freenix track: 2000 USENIX annual technical conference, June 2000.

HISTORY
The implementation first appeared in WIDE Hydrangea IPv6 protocol stack kit.

STANDARDS
The getaddrinfo() function is defined in p1003.1g-2000, and documented in “Basic Socket Interface Extensions for IPv6” (RFC2553).

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

The text was shamelessly copied from RFC2553.