GETNAMEINFO(3) Linux Programmer's Manual GETNAMEINFO(3)
getnameinfo - address-to-name translation in protocol-independent man-
int getnameinfo(const struct sockaddr *sa, socklen_t salen,
char *host, socklen_t hostlen,
char *serv, socklen_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 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
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:
If set, then an error is returned if the hostname cannot be
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 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
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-
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.
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 fea-
ture 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 ser-
vices listed in the current Assigned Numbers RFC.
Before glibc version 2.2, the hostlen and servlen arguments were typed
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
struct sockaddr *sa; /* input */
socklen_t len; /* input */
if (getnameinfo(sa, len, hbuf, sizeof(hbuf),
NULL, 0, NI_NAMEREQD))
printf("could not resolve hostname");
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 Inter-
face 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
Craig Metz, Protocol Independence Using the Sockets API, Proceedings of
the freenix track: 2000 USENIX annual technical conference, June 2000
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GNU 2014-05-28 GETNAMEINFO(3)