getaddrinfo(3) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | ATTRIBUTES | CONFORMING TO | NOTES | EXAMPLES | SEE ALSO | COLOPHON

GETADDRINFO(3)          Linux Programmer's Manual         GETADDRINFO(3)

NAME         top

       getaddrinfo, freeaddrinfo, gai_strerror - network address and
       service translation

SYNOPSIS         top

       #include <sys/types.h>
       #include <sys/socket.h>
       #include <netdb.h>

       int getaddrinfo(const char *restrict node,
                       const char *restrict service,
                       const struct addrinfo *restrict hints,
                       struct addrinfo **restrict 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():
           Since glibc 2.22:
               _POSIX_C_SOURCE >= 200112L
           Glibc 2.21 and earlier:
               _POSIX_C_SOURCE

DESCRIPTION         top

       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.

RETURN VALUE         top

       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
              included AI_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 and service
              are NULL; or AI_NUMERICSERV was specified in
              hints.ai_flags and service 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 occur if service was
              "shell" (a service available only on stream sockets), and
              either hints.ai_protocol was IPPROTO_UDP, or
              hints.ai_socktype was SOCK_DGRAM; or the error could occur
              if service was not NULL, and hints.ai_socktype was
              SOCK_RAW (a socket type that does not support the concept
              of services).

       EAI_SOCKTYPE
              The requested socket type is not supported.  This could
              occur, for example, if hints.ai_socktype and
              hints.ai_protocol are inconsistent (e.g., SOCK_DGRAM and
              IPPROTO_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         top

       /etc/gai.conf

ATTRIBUTES         top

       For an explanation of the terms used in this section, see
       attributes(7).

       ┌───────────────────────────┬───────────────┬────────────────────┐
       │Interface                  Attribute     Value              │
       ├───────────────────────────┼───────────────┼────────────────────┤
       │getaddrinfo()              │ Thread safety │ MT-Safe env locale │
       ├───────────────────────────┼───────────────┼────────────────────┤
       │freeaddrinfo(),            │ Thread safety │ MT-Safe            │
       │gai_strerror()             │               │                    │
       └───────────────────────────┴───────────────┴────────────────────┘

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008.  The getaddrinfo() function is
       documented in RFC 2553.

NOTES         top

       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, 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.

EXAMPLES         top

       The following programs demonstrate the use of getaddrinfo(),
       gai_strerror(), freeaddrinfo(), and getnameinfo(3).  The programs
       are 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 500

       int
       main(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(hints));
           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);
           }

           freeaddrinfo(result);           /* No longer needed */

           if (rp == NULL) {               /* No address succeeded */
               fprintf(stderr, "Could not bind\n");
               exit(EXIT_FAILURE);
           }

           /* Read datagrams and echo them back to sender. */

           for (;;) {
               peer_addr_len = sizeof(peer_addr);
               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 %zd bytes from %s:%s\n",
                           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 500

       int
       main(int argc, char *argv[])
       {
           struct addrinfo hints;
           struct addrinfo *result, *rp;
           int sfd, s;
           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(hints));
           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);
           }

           freeaddrinfo(result);           /* No longer needed */

           if (rp == NULL) {               /* No address succeeded */
               fprintf(stderr, "Could not connect\n");
               exit(EXIT_FAILURE);
           }

           /* Send remaining command-line arguments as separate
              datagrams, and read responses from server. */

           for (int j = 3; j < argc; j++) {
               len = strlen(argv[j]) + 1;
                       /* +1 for terminating null byte */

               if (len > 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 %zd bytes: %s\n", nread, buf);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3),
       gai.conf(5), hostname(7), ip(7)

COLOPHON         top

       This page is part of release 5.12 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

GNU                            2021-03-22                 GETADDRINFO(3)

Pages that refer to this page: getent(1)pmdanetcheck(1)bind(2)connect(2)recv(2)send(2)socket(2)getaddrinfo_a(3)gethostbyname(3)getipnodebyname(3)getnameinfo(3)inet(3)inet_pton(3)resolver(3)gai.conf(5)resolv.conf(5)hostname(7)agetty(8)systemd-machined.service(8)systemd-resolved.service(8)