getnameinfo

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



NAME
       getnameinfo - address-to-name translation in protocol-independent
       manner

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
       converts a socket address to a corresponding host and service, in a
       protocol-independent manner.  It combines the functionality of
       gethostbyaddr(3) and getservbyport(3), but unlike those functions,
       getnameinfo() 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
       selectively allow hostnames to be transparently converted to and from
       the Internationalized Domain Name (IDN) format (see RFC 3490,
       Internationalizing 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
              necessary.  ASCII-only names are not affected by the conversion,
              which makes this flag usable in existing programs and
              environments.

       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
       services 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).

SEE ALSO
       accept(2), getpeername(2), getsockname(2), recvfrom(2), socket(2),
       getaddrinfo(3), gethostbyaddr(3), getservbyname(3), getservbyport(3),
       inet_ntop(3), hosts(5), services(5), hostname(7), named(8)

       R. Gilligan, S. Thomson, J. Bound and W. Stevens, Basic Socket
       Interface Extensions for IPv6, RFC 2553, March 1999.

       Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
       Addresses, internet draft, work in progress ⟨ftp://ftp.ietf.org
       /internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt⟩.

       Craig Metz, Protocol Independence Using the Sockets API, Proceedings of
       the freenix track: 2000 USENIX annual technical conference, June 2000
       ⟨http://www.usenix.org/publications/library/proceedings/usenix2000
       /freenix/metzprotocol.html⟩.

COLOPHON
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       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
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GNU                               2019-03-06                    GETNAMEINFO(3)