getnameinfo(3)

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

LIBRARY
Standard C library (libc, -lc)

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

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

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

getnameinfo():
    Since glibc 2.22:
        _POSIX_C_SOURCE >= 200112L
    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

ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value
 | Thread safety | MT-Safe env locale

STANDARDS
POSIX.1-2008. RFC 2553.

HISTORY
glibc 2.1. POSIX.1-2001.

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

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.

EXAMPLES
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.