GETNAMEINFO(3) | Linux Programmer's Manual | GETNAMEINFO(3) |
getnameinfo - address-to-name translation in protocol-independent manner
#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
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:
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:
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 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.
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
getnameinfo () | Thread safety | MT-Safe env locale |
POSIX.1-2001, POSIX.1-2008, RFC 2553.
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.
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).
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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2017-09-15 | GNU |