getaddrinfo(3)



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

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  dependen-
       cies.

       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  cri-
       teria  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 crite-
       ria 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_NUMERI-
       CHOST 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  connec-
       tions  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, accessi-
       ble 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  initial-
       ized 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 config-
       ured 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 speci-
       fied.

       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 selec-
       tively 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  reso-
              lution 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 ser-
              vice 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 ser-
              vice available only on stream sockets), and either hints.ai_pro-
              tocol  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 sup-
              port 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).

       +----------------+---------------+--------------------+
       |Interface       | Attribute     | Value              |
       +----------------+---------------+--------------------+
       |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  con-
       sidered an improvement on the specification.

EXAMPLE
       The  following  programs demonstrate the use of getaddrinfo(), gai_str-
       error(), 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/.

GNU                               2017-09-15                    GETADDRINFO(3)

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