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man:getaddrinfo

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 4.16 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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