getaddrinfo(3) - Linux man page

Name

getaddrinfo, freeaddrinfo, gai_strerror - network address and service translation

Synopsis

#include <sys/types.h>#include <sys/socket.h>#include <netdb.h>int getaddrinfo(const char *node, const char *service,                const struct addrinfo *hints,                struct addrinfo **res);void freeaddrinfo(struct addrinfo *res);const char *gai_strerror(int errcode);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

getaddrinfo(), freeaddrinfo(),gai_strerror():

_POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

Description

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 tobind(2) orconnect(2). Thegetaddrinfo() function combinesthe functionality provided by thegetservbyname(3) andgetservbyport(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;    size_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 listpointed to byres. If hints is not NULL it points to an addrinfo structure whoseai_family, ai_socktype, andai_protocol specify criteria that limit the set of socket addresses returned bygetaddrinfo(), as follows:

ai_family

This field specifies the desired address family for the returned addresses. Valid values for this field includeAF_INET and AF_INET6. Thevalue AF_UNSPEC indicates thatgetaddrinfo() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can beused withnode and service.

ai_socktype

This field specifies the preferred socket type, for example SOCK_STREAM orSOCK_DGRAM. Specifying 0 in this field indicates that socketaddresses of any type can be returned bygetaddrinfo().

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 bereturned bygetaddrinfo().

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. Specifyinghints as NULL isequivalent to setting ai_socktype and ai_protocol to 0;ai_family to AF_UNSPEC; and ai_flags to (AI_V4MAPPED |AI_ADDRCONFIG).

node specifies either a numerical network address (for IPv4, numbers-and-dots notation as supported byinet_aton(3); for IPv6, hexadecimalstring format as supported byinet_pton(3)), or a network hostname, whose network addresses are looked up and resolved. Ifhints.ai_flagscontains the AI_NUMERICHOST flag then node must be a numerical network address. The AI_NUMERICHOST flag suppresses any potentially lengthynetwork host address lookups.

If the AI_PASSIVE flag is specified in hints.ai_flags, andnode is NULL, then the returned socket addresses will be suitable forbind(2)ing a socket that willaccept(2) connections. The returned socket address will contain the "wildcard address" (INADDR_ANY for IPv4addresses,IN6ADDR_ANY_INIT for IPv6 address). The wildcard address is used by applications (typically servers) that intend to accept connections on anyof the hosts's network addresses. Ifnode 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 withconnect(2),sendto(2), orsendmsg(2). If node is NULL, then the network address will be set to the loopback interface address (INADDR_LOOPBACKfor IPv4 addresses,IN6ADDR_LOOPBACK_INIT for IPv6 address); this is used by applications that intend to communicate with peers running on the samehost.

service sets the port in each returned address structure. If this argument is a service name (seeservices(5)), it is translated to thecorresponding port number. This argument can also be specified as a decimal number, which is simply converted to binary. Ifservice is NULL, then theport number of the returned socket addresses will be left uninitialized. IfAI_NUMERICSERV is specified in hints.ai_flags and service isnot NULL, thenservice must point to a string containing a numeric port number. This flag is used to inhibit the invocation of a name resolution servicein 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 ofaddrinfo structures, one for each network address that matchesnode andservice, subject to any restrictions imposed by hints, and returns a pointer to the start of the list inres. The items inthe 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 overmultiple protocols (e.g. bothAF_INET and AF_INET6); or the same service is available from multiple socket types (oneSOCK_STREAM addressand another SOCK_DGRAM address, for example). Normally, the application should try using the addresses in the order in which they are returned. Thesorting function used withingetaddrinfo() 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 theai_canonname field of the first of the addrinfo structures in thereturned 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 asthe corresponding arguments ofsocket(2)). For example,ai_family might return AF_INET or AF_INET6;ai_socktype might returnSOCK_DGRAM or SOCK_STREAM; andai_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 theai_addrlenfield.

If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4 addresses are returned in the list pointed to byres only if the localsystem has at least one IPv4 address configured, and IPv6 addresses are only returned if the local system has at least one IPv6 address configured.

If hint.ai_flags specifies the AI_V4MAPPED flag, andhints.ai_family was specified as AF_INET6, and no matching IPv6 addressescould be found, then return IPv4-mapped IPv6 addresses in the list pointed to byres. If both AI_V4MAPPED and AI_ALL are specified inhints.ai_flags, then return both IPv6 and IPv4-mapped IPv6 addresses in the list pointed to byres. AI_ALL is ignored ifAI_V4MAPPED is not also specified.

The freeaddrinfo() function frees the memory that was allocated for the dynamically allocated linked listres.

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 toand from the Internationalized Domain Name (IDN) format (see RFC 3490,Internationalizing Domain Names in Applications (IDNA)). Four new flags aredefined:

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 currentlocale.

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-ASCIIcharacters 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 nodecorresponding to theaddrinfo 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 thesecomponents into a readable form theAI_CANONIDN flag can be passed in addition to AI_CANONNAME. The resulting string is encoded using the currentlocale'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 itis a STD3 conforming hostname) flags respectively to be used in the IDNA handling.

Return Value

getaddrinfo() returns 0 if it succeeds, or one of the following nonzero error codes:

EAI_ADDRFAMILY

The specified network host does not have any network addresses in the requested address family.

EAI_AGAIN

The name server returned a temporary failure indication. Try again later.

EAI_BADFLAGS

hints.ai_flags contains invalid flags; or, hints.ai_flags includedAI_CANONNAME and name was NULL.

EAI_FAIL

The name server returned a permanent failure indication.

EAI_FAMILY

The requested address family is not supported.

EAI_MEMORY

Out of memory.

EAI_NODATA

The specified network host exists, but does not have any network addresses defined.

EAI_NONAME

The node or service is not known; or both node andservice are NULL; or AI_NUMERICSERV was specified inhints.ai_flags andservice was not a numeric port-number string.

EAI_SERVICE

The requested service is not available for the requested socket type. It may be available through another socket type. For example, this error could occurifservice was "shell" (a service only available on stream sockets), and eitherhints.ai_protocol was IPPROTO_UDP, orhints.ai_socktype wasSOCK_DGRAM; or the error could occur if service was not NULL, andhints.ai_socktype was SOCK_RAW (asocket type that does not support the concept of services).

EAI_SOCKTYPE

The requested socket type is not supported. This could occur, for example, ifhints.ai_socktype and hints.ai_protocol are inconsistent (e.g.,SOCK_DGRAM andIPPROTO_TCP, respectively).

EAI_SYSTEM

Other system error, check errno for details.

The gai_strerror() function translates these error codes to a human readable string, suitable for error reporting.

Files

/etc/gai.conf

Conforming To

POSIX.1-2001. The getaddrinfo() function is documented in RFC 2553.

Notes

getaddrinfo() supports the address%scope-id notation for specifying the IPv6 scope-ID.

AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc 2.3.3.AI_NUMERICSERV is available since glibc 2.3.4.

According to POSIX.1-2001, specifying hints as NULL should cause ai_flags to be assumed as 0. The GNU C library instead assumes a value of(AI_V4MAPPED | AI_ADDRCONFIG) for this case, since this value is considered an improvement on the specification.

Example

The following programs demonstrate the use of getaddrinfo(), gai_strerror(), freeaddrinfo(), and getnameinfo(3). The programsare an echo server and client for UDP datagrams.

Server program

#include <sys/types.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <string.h>#include <sys/socket.h>#include <netdb.h>#define BUF_SIZE 500intmain(int argc, char *argv[]){    struct addrinfo hints;    struct addrinfo *result, *rp;    int sfd, s;    struct sockaddr_storage peer_addr;    socklen_t peer_addr_len;    ssize_t nread;    char buf[BUF_SIZE];   if (argc != 2) {        fprintf(stderr, "Usage: %s port\n", argv[0]);        exit(EXIT_FAILURE);    }   memset(&hints, 0, sizeof(struct addrinfo));    hints.ai_family = AF_UNSPEC;    /* Allow IPv4 or IPv6 */    hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */    hints.ai_flags = AI_PASSIVE;    /* For wildcard IP address */    hints.ai_protocol = 0;          /* Any protocol */    hints.ai_canonname = NULL;    hints.ai_addr = NULL;    hints.ai_next = NULL;   s = getaddrinfo(NULL, argv[1], &hints, &result);    if (s != 0) {        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));        exit(EXIT_FAILURE);    }   /* getaddrinfo() returns a list of address structures.       Try each address until we successfully bind(2).       If socket(2) (or bind(2)) fails, we (close the socket       and) try the next address. */   for (rp = result; rp != NULL; rp = rp->ai_next) {        sfd = socket(rp->ai_family, rp->ai_socktype,                rp->ai_protocol);        if (sfd == -1)            continue;       if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)            break;                  /* Success */       close(sfd);    }   if (rp == NULL) {               /* No address succeeded */        fprintf(stderr, "Could not bind\n");        exit(EXIT_FAILURE);    }   freeaddrinfo(result);           /* No longer needed */   /* Read datagrams and echo them back to sender */   for (;;) {        peer_addr_len = sizeof(struct sockaddr_storage);        nread = recvfrom(sfd, buf, BUF_SIZE, 0,                (struct sockaddr *) &peer_addr, &peer_addr_len);        if (nread == -1)            continue;               /* Ignore failed request */       char host[NI_MAXHOST], service[NI_MAXSERV];       s = getnameinfo((struct sockaddr *) &peer_addr,                        peer_addr_len, host, NI_MAXHOST,                        service, NI_MAXSERV, NI_NUMERICSERV);       if (s == 0)            printf("Received %ld bytes from %s:%s\n",                    (long) nread, host, service);        else            fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s));       if (sendto(sfd, buf, nread, 0,                    (struct sockaddr *) &peer_addr,                    peer_addr_len) != nread)            fprintf(stderr, "Error sending response\n");    }}

Client program

#include <sys/types.h>#include <sys/socket.h>#include <netdb.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>#include <string.h>#define BUF_SIZE 500intmain(int argc, char *argv[]){    struct addrinfo hints;    struct addrinfo *result, *rp;    int sfd, s, j;    size_t len;    ssize_t nread;    char buf[BUF_SIZE];   if (argc < 3) {        fprintf(stderr, "Usage: %s host port msg...\n", argv[0]);        exit(EXIT_FAILURE);    }   /* Obtain address(es) matching host/port */   memset(&hints, 0, sizeof(struct addrinfo));    hints.ai_family = AF_UNSPEC;    /* Allow IPv4 or IPv6 */    hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */    hints.ai_flags = 0;    hints.ai_protocol = 0;          /* Any protocol */   s = getaddrinfo(argv[1], argv[2], &hints, &result);    if (s != 0) {        fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));        exit(EXIT_FAILURE);    }   /* getaddrinfo() returns a list of address structures.       Try each address until we successfully connect(2).       If socket(2) (or connect(2)) fails, we (close the socket       and) try the next address. */   for (rp = result; rp != NULL; rp = rp->ai_next) {        sfd = socket(rp->ai_family, rp->ai_socktype,                     rp->ai_protocol);        if (sfd == -1)            continue;       if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1)            break;                  /* Success */       close(sfd);    }   if (rp == NULL) {               /* No address succeeded */        fprintf(stderr, "Could not connect\n");        exit(EXIT_FAILURE);    }   freeaddrinfo(result);           /* No longer needed */   /* Send remaining command-line arguments as separate       datagrams, and read responses from server */   for (j = 3; j < argc; j++) {        len = strlen(argv[j]) + 1;                /* +1 for terminating null byte */       if (len + 1 > BUF_SIZE) {            fprintf(stderr,                    "Ignoring long message in argument %d\n", j);            continue;        }       if (write(sfd, argv[j], len) != len) {            fprintf(stderr, "partial/failed write\n");            exit(EXIT_FAILURE);        }       nread = read(sfd, buf, BUF_SIZE);        if (nread == -1) {            perror("read");            exit(EXIT_FAILURE);        }       printf("Received %ld bytes: %s\n", (long) nread, buf);    }   exit(EXIT_SUCCESS);}

See Also

getaddrinfo_a(3),gethostbyname(3),getnameinfo(3),inet(3), hostname(7),ip(7)

Referenced By

cnetdb(3),gai.conf(5),getipnodebyname(3),inet_addr(3),inn.conf(5),libbind-getnameinfo(3),libval_shim(3),lwres_gai_strerror(3),mailx(1),name2addr(1),recv(2),send(2),val_getaddrinfo(3),xhost(1)