getnameinfo

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

int getnameinfo(const struct sockaddr *sa, socklen_t salen,
                char *host, size_t hostlen,
                char *serv, size_t servlen, int flags);

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

getnameinfo(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

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 sa argument is a pointer to a generic socket address structure (of type sockaddr_in or sockaddr_in6) of size salen 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.)

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.

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

getnameinfo() is provided in glibc since version 2.1.

RFC 2553, POSIX.1-2001.

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 one of the feature test macros _BSD_SOURCE, _SVID_SOURCE, or _GNU_SOURCE is defined.

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.

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 *sa;    /* input */
socklen_t len;         /* input */
char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
if (getnameinfo(sa, len, 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 *sa;    /* input */
socklen_t len;         /* input */
char hbuf[NI_MAXHOST];
if (getnameinfo(sa, len, 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).

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.

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