man3/getnameinfo.3

.\" %%%LICENSE_START(PUBLIC_DOMAIN)
.\" This page is in the public domain.
.\" %%%LICENSE_END
.\"
.\" Almost all details are from RFC 2553.
.\"
.\" 2004-12-14, mtk, Added EAI_OVERFLOW error
.\" 2004-12-14 Fixed description of error return
.\"
.TH GETNAMEINFO 3 2021-03-22 "GNU" "Linux Programmer's Manual"
.SH NAME
getnameinfo \- address-to-name translation in protocol-independent manner
.SH SYNOPSIS
.nf
.B #include <sys/socket.h>
.B #include <netdb.h>
.PP
.BI "int getnameinfo(const struct sockaddr *restrict " addr \
", socklen_t " addrlen ,
.BI "                char *restrict " host ", socklen_t " hostlen ,
.BI "                char *restrict " serv ", socklen_t " servlen \
", int " flags );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR getnameinfo ():
.nf
    Since glibc 2.22:
        _POSIX_C_SOURCE >= 200112L
    Glibc 2.21 and earlier:
        _POSIX_C_SOURCE
.fi
.SH DESCRIPTION
The
.BR getnameinfo ()
function is the inverse of
.BR getaddrinfo (3):
it converts a socket address to a corresponding host and service,
in a protocol-independent manner.
It combines the functionality of
.BR gethostbyaddr (3)
and
.BR getservbyport (3),
but unlike those functions,
.BR getnameinfo ()
is reentrant and allows programs to eliminate
IPv4-versus-IPv6 dependencies.
.PP
The
.I addr
argument is a pointer to a generic socket address structure
(of type
.I sockaddr_in
or
.IR sockaddr_in6 )
of size
.I addrlen
that holds the input IP address and port number.
The arguments
.I host
and
.I serv
are pointers to caller-allocated buffers (of size
.I hostlen
and
.I servlen
respectively) into which
.BR getnameinfo ()
places null-terminated strings containing the host and
service names respectively.
.PP
The caller can specify that no hostname (or no service name)
is required by providing a NULL
.I host
(or
.IR serv )
argument or a zero
.I hostlen
(or
.IR servlen )
argument.
However, at least one of hostname or service name
must be requested.
.PP
The
.I flags
argument modifies the behavior of
.BR getnameinfo ()
as follows:
.TP
.B NI_NAMEREQD
If set, then an error is returned if the hostname cannot be determined.
.TP
.B NI_DGRAM
If set, then the service is datagram (UDP) based rather than
stream (TCP) based.
This is required for the few ports (512\(en514)
that have different services for UDP and TCP.
.TP
.B NI_NOFQDN
If set, return only the hostname part of the fully qualified domain name
for local hosts.
.TP
.B NI_NUMERICHOST
If set, then the numeric form of the hostname is returned.
.\" For example, by calling
.\" .BR inet_ntop ()
.\" instead of
.\" .BR gethostbyaddr ().
(When not set, this will still happen in case the node's name
cannot be determined.)
.\" POSIX.1-2001 TC1 has NI_NUMERICSCOPE, but glibc doesn't have it.
.TP
.B 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.)
.SS Extensions to getnameinfo() for Internationalized Domain Names
Starting with glibc 2.3.4,
.BR getnameinfo ()
has been extended to selectively allow
hostnames to be transparently converted to and from the
Internationalized Domain Name (IDN) format (see RFC 3490,
.IR "Internationalizing Domain Names in Applications (IDNA)" ).
Three new flags are defined:
.TP
.B 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.
.TP
.BR 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.
.SH RETURN VALUE
.\" FIXME glibc defines the following additional errors, some which
.\" can probably be returned by getnameinfo(); they need to
.\" be documented.
.\"
.\"     #ifdef __USE_GNU
.\"     #define EAI_INPROGRESS  -100  /* Processing request in progress.  */
.\"     #define EAI_CANCELED    -101  /* Request canceled.  */
.\"     #define EAI_NOTCANCELED -102  /* Request not canceled.  */
.\"     #define EAI_ALLDONE     -103  /* All requests done.  */
.\"     #define EAI_INTR        -104  /* Interrupted by a signal.  */
.\"     #define EAI_IDN_ENCODE  -105  /* IDN encoding failed.  */
.\"     #endif
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:
.TP
.B EAI_AGAIN
The name could not be resolved at this time.
Try again later.
.TP
.B EAI_BADFLAGS
The
.I flags
argument has an invalid value.
.TP
.B EAI_FAIL
A nonrecoverable error occurred.
.TP
.B EAI_FAMILY
The address family was not recognized,
or the address length was invalid for the specified family.
.TP
.B EAI_MEMORY
Out of memory.
.TP
.B EAI_NONAME
The name does not resolve for the supplied arguments.
.B NI_NAMEREQD
is set and the host's name cannot be located,
or neither hostname nor service name were requested.
.TP
.B EAI_OVERFLOW
The buffer pointed to by
.I host
or
.I serv
was too small.
.TP
.B EAI_SYSTEM
A system error occurred.
The error code can be found in
.IR errno .
.PP
The
.BR gai_strerror (3)
function translates these error codes to a human readable string,
suitable for error reporting.
.SH FILES
.I /etc/hosts
.br
.I /etc/nsswitch.conf
.br
.I /etc/resolv.conf
.SH VERSIONS
.BR getnameinfo ()
is provided in glibc since version 2.1.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.ad l
.nh
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.BR getnameinfo ()
T}	Thread safety	MT-Safe env locale
.TE
.hy
.ad
.sp 1
.SH CONFORMING TO
POSIX.1-2001, POSIX.1-2008, RFC\ 2553.
.SH NOTES
In order to assist the programmer in choosing reasonable sizes
for the supplied buffers,
.I <netdb.h>
defines the constants
.PP
.in +4n
.EX
#define NI_MAXHOST      1025
#define NI_MAXSERV      32
.EE
.in
.PP
Since glibc 2.8,
these definitions are exposed only if suitable
feature test macros are defined, namely:
.BR _GNU_SOURCE ,
.BR _DEFAULT_SOURCE
(since glibc 2.19),
or (in glibc versions up to and including 2.19)
.BR _BSD_SOURCE
or
.BR _SVID_SOURCE .
.PP
The former is the constant
.B MAXDNAME
in recent versions of BIND's
.I <arpa/nameser.h>
header file.
The latter is a guess based on the services listed
in the current Assigned Numbers RFC.
.PP
Before glibc version 2.2, the
.I hostlen
and
.I servlen
arguments were typed as
.IR size_t .
.SH 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.
.PP
.in +4n
.EX
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\en", hbuf, sbuf);
.EE
.in
.PP
The following version checks if the socket address has a
reverse address mapping.
.PP
.in +4n
.EX
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\en", hbuf);
.EE
.in
.PP
An example program using
.BR getnameinfo ()
can be found in
.BR getaddrinfo (3).
.SH SEE ALSO
.BR accept (2),
.BR getpeername (2),
.BR getsockname (2),
.BR recvfrom (2),
.BR socket (2),
.BR getaddrinfo (3),
.BR gethostbyaddr (3),
.BR getservbyname (3),
.BR getservbyport (3),
.BR inet_ntop (3),
.BR hosts (5),
.BR services (5),
.BR hostname (7),
.BR named (8)
.PP
R.\& Gilligan, S.\& Thomson, J.\& Bound and W.\& Stevens,
.IR "Basic Socket Interface Extensions for IPv6" ,
RFC\ 2553, March 1999.
.PP
Tatsuya Jinmei and Atsushi Onoe,
.IR "An Extension of Format for IPv6 Scoped Addresses" ,
internet draft, work in progress
.UR ftp://ftp.ietf.org\:/internet\-drafts\:/draft\-ietf\-ipngwg\-scopedaddr\-format\-02.txt
.UE .
.PP
Craig Metz,
.IR "Protocol Independence Using the Sockets API" ,
Proceedings of the freenix track:
2000 USENIX annual technical conference, June 2000
.ad l
.UR http://www.usenix.org\:/publications\:/library\:/proceedings\:/usenix2000\:/freenix\:/metzprotocol.html
.UE .