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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 to bind(2)
       or connect(2).  The getaddrinfo() function combines  the  functionality
       provided  by the getservbyname(3) and getservbyport(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
       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  logically  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).

       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 hosts'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 only
       returned if the local system has at least one IPv6 address  configured.

       If  hint.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_family,
       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

       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   only   available   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

       /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  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(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 500

       int
       main(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

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

COLOPHON

       This  page  is  part of release 3.24 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.