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 >= 201112L
	   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 4.10 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				  2016-03-15			GETADDRINFO(3)