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

NAME

     getnameinfo  - address-to-name translation in protocol-independent man-
     ner

SYNOPSIS

     #include <sys/socket.h>
     #include <netdb.h>

     int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,
                     char *host, socklen_t hostlen,
                     char *serv, socklen_t servlen, int flags);

 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

     getnameinfo():
         Since glibc 2.22: _POSIX_C_SOURCE >= 201112L
         Glibc 2.21 and earlier: _POSIX_C_SOURCE

DESCRIPTION

     The getnameinfo() function is the inverse of  getaddrinfo(3):  it  con-
     verts a socket address to a corresponding host and service, in a proto-
     col-independent manner.  It  combines  the  functionality  of  gethost-
     byaddr(3)  and  getservbyport(3),  but unlike those functions, getname-
     info() is reentrant and allows programs to  eliminate  IPv4-versus-IPv6
     dependencies.

     The  addr  argument  is a pointer to a generic socket address structure
     (of type sockaddr_in or sockaddr_in6) of size addrlen  that  holds  the
     input  IP  address  and  port  number.  The arguments host and serv are
     pointers to caller-allocated  buffers  (of  size  hostlen  and  servlen
     respectively)  into  which getnameinfo() places null-terminated strings
     containing the host and service names respectively.

     The caller can specify  that  no  hostname  (or  no  service  name)  is
     required  by providing a NULL host (or serv) argument or a zero hostlen
     (or servlen) argument.  However, at least one of  hostname  or  service
     name must be requested.

     The flags argument modifies the behavior of getnameinfo() as follows:

     NI_NAMEREQD
            If  set,  then  an  error  is returned if the hostname cannot be
            determined.

     NI_DGRAM
            If set, then the service is datagram  (UDP)  based  rather  than
            stream  (TCP)  based.   This  is  required  for  the  few  ports
            (512-514) that have different services for UDP and TCP.

     NI_NOFQDN
            If set, return only the hostname part  of  the  fully  qualified
            domain name for local hosts.

     NI_NUMERICHOST
            If  set,  then  the  numeric  form  of the hostname is returned.
            (When not set, this will still happen in case  the  node's  name
            cannot be determined.)

     NI_NUMERICSERV
            If  set,  then  the  numeric  form  of  the  service  address is
            returned.  (When not set, this will still  happen  in  case  the
            service's name cannot be determined.)

 Extensions to getnameinfo() for Internationalized Domain Names
     Starting  with  glibc  2.3.4, getnameinfo() has been extended to selec-
     tively allow hostnames to be transparently converted to  and  from  the
     Internationalized  Domain Name (IDN) format (see RFC 3490, Internation-
     alizing Domain Names in Applications  (IDNA)).   Three  new  flags  are
     defined:

     NI_IDN If  this flag is used, then the name found in the lookup process
            is converted from IDN format to the locale's encoding if  neces-
            sary.   ASCII-only  names  are  not  affected by the conversion,
            which makes this flag usable in existing programs  and  environ-
            ments.

     NI_IDN_ALLOW_UNASSIGNED, NI_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

     On success, 0 is returned, and node and service  names,  if  requested,
     are  filled with null-terminated strings, possibly truncated to fit the
     specified buffer lengths.  On error, one of the following nonzero error
     codes is returned:

     EAI_AGAIN
            The name could not be resolved at this time.  Try again later.

     EAI_BADFLAGS
            The flags argument has an invalid value.

     EAI_FAIL
            A nonrecoverable error occurred.

     EAI_FAMILY
            The address family was not recognized, or the address length was
            invalid for the specified family.

     EAI_MEMORY
            Out of memory.

     EAI_NONAME
            The  name  does  not  resolve  for   the   supplied   arguments.
            NI_NAMEREQD  is  set  and  the host's name cannot be located, or
            neither hostname nor service name were requested.

     EAI_OVERFLOW
            The buffer pointed to by host or serv was too small.

     EAI_SYSTEM
            A system error occurred.  The error code can be found in  errno.

     The  gai_strerror(3)  function  translates these error codes to a human
     readable string, suitable for error reporting.

FILES

     /etc/hosts
     /etc/nsswitch.conf
     /etc/resolv.conf

VERSIONS

     getnameinfo() is provided in glibc since version 2.1.

ATTRIBUTES

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

     +--------------+---------------+--------------------+
     |Interface     | Attribute     | Value              |
     +--------------+---------------+--------------------+
     |getnameinfo() | Thread safety | MT-Safe env locale |
     +--------------+---------------+--------------------+

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008, RFC 2553.

NOTES

     In  order to assist the programmer in choosing reasonable sizes for the
     supplied buffers, <netdb.h> defines the constants

         #define NI_MAXHOST      1025 #define NI_MAXSERV      32

     Since glibc 2.8, these definitions are exposed only if suitable feature
     test  macros  are  defined, namely: _GNU_SOURCE, _DEFAULT_SOURCE (since
     glibc  2.19),  or  (in  glibc  versions  up  to  and  including   2.19)
     _BSD_SOURCE or _SVID_SOURCE.

     The  former  is  the  constant  MAXDNAME  in  recent versions of BIND's
     <arpa/nameser.h> header file.  The latter is a guess based on the  ser-
     vices listed in the current Assigned Numbers RFC.

     Before  glibc version 2.2, the hostlen and servlen arguments were typed
     as size_t.

EXAMPLE

     The following code tries to get the numeric hostname and service  name,
     for  a given socket address.  Note that there is no hardcoded reference
     to a particular address family.

         struct  sockaddr  *addr;      /*  input   */   socklen_t   addrlen;
         /* input */ char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

         if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), sbuf,
                     sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)
             printf("host=%s, serv=%s\n", hbuf, sbuf);

     The  following  version  checks  if  the  socket  address has a reverse
     address mapping.

         struct  sockaddr  *addr;      /*  input   */   socklen_t   addrlen;
         /* input */ char hbuf[NI_MAXHOST];

         if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf),
                     NULL, 0, NI_NAMEREQD))
             printf("could not resolve hostname"); else
             printf("host=%s\n", hbuf);

     An  example program using getnameinfo() can be found in getaddrinfo(3).