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


Standard C library (libc, -lc)


#include <sys/socket.h>
#include <netdb.h>
int getnameinfo(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)):


    Since glibc 2.22:
        _POSIX_C_SOURCE >= 200112L
    glibc 2.21 and earlier:


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:


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


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.


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


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.)


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:


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.


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.


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:


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


The flags argument has an invalid value.


A nonrecoverable error occurred.


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


Out of memory.


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.


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


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.


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");
    printf("host=%s\n", hbuf);

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




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

Interface Attribute Value


Thread safety MT-Safe env locale


POSIX.1-2008. RFC 2553.


glibc 2.1. POSIX.1-2001.

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


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.


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">.

Craig Metz, Protocol Independence Using the Sockets API, Proceedings of the freenix track: 2000 USENIX annual technical conference, June 2000">.