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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():
           Since glibc 2.22: _POSIX_C_SOURCE >= 200112L
           Glibc 2.21 and earlier: _POSIX_C_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 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

       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

       /etc/gai.conf

ATTRIBUTES

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

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

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008.  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, 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 %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, 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 %zd bytes: %s\n", nread, buf);
           }

           exit(EXIT_SUCCESS);
       }

SEE ALSO

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

COLOPHON

       This  page  is  part of release 4.13 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/.