1GETNAMEINFO(3)             Linux Programmer's Manual            GETNAMEINFO(3)
2
3
4

NAME

6       getnameinfo  - address-to-name translation in protocol-independent man‐
7       ner
8

SYNOPSIS

10       #include <sys/socket.h>
11       #include <netdb.h>
12
13       int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,
14                       char *host, socklen_t hostlen,
15                       char *serv, socklen_t servlen, int flags);
16
17   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18
19       getnameinfo():
20           Since glibc 2.22: _POSIX_C_SOURCE >= 200112L
21           Glibc 2.21 and earlier: _POSIX_C_SOURCE
22

DESCRIPTION

24       The getnameinfo() function is the inverse of  getaddrinfo(3):  it  con‐
25       verts a socket address to a corresponding host and service, in a proto‐
26       col-independent manner.  It  combines  the  functionality  of  gethost‐
27       byaddr(3)  and  getservbyport(3),  but unlike those functions, getname‐
28       info() is reentrant and allows programs to  eliminate  IPv4-versus-IPv6
29       dependencies.
30
31       The  addr  argument  is a pointer to a generic socket address structure
32       (of type sockaddr_in or sockaddr_in6) of size addrlen  that  holds  the
33       input  IP  address  and  port  number.  The arguments host and serv are
34       pointers to caller-allocated buffers (of size hostlen and  servlen  re‐
35       spectively)  into  which  getnameinfo()  places null-terminated strings
36       containing the host and service names respectively.
37
38       The caller can specify that no hostname (or no  service  name)  is  re‐
39       quired  by  providing  a NULL host (or serv) argument or a zero hostlen
40       (or servlen) argument.  However, at least one of  hostname  or  service
41       name must be requested.
42
43       The flags argument modifies the behavior of getnameinfo() as follows:
44
45       NI_NAMEREQD
46              If  set, then an error is returned if the hostname cannot be de‐
47              termined.
48
49       NI_DGRAM
50              If set, then the service is datagram  (UDP)  based  rather  than
51              stream  (TCP)  based.   This  is  required  for  the  few  ports
52              (512–514) that have different services for UDP and TCP.
53
54       NI_NOFQDN
55              If set, return only the hostname part of the fully qualified do‐
56              main name for local hosts.
57
58       NI_NUMERICHOST
59              If  set,  then  the  numeric  form  of the hostname is returned.
60              (When not set, this will still happen in case  the  node's  name
61              cannot be determined.)
62
63       NI_NUMERICSERV
64              If  set,  then  the  numeric  form of the service address is re‐
65              turned.  (When not set, this will still happen in case the  ser‐
66              vice's name cannot be determined.)
67
68   Extensions to getnameinfo() for Internationalized Domain Names
69       Starting  with  glibc  2.3.4, getnameinfo() has been extended to selec‐
70       tively allow hostnames to be transparently converted to  and  from  the
71       Internationalized  Domain Name (IDN) format (see RFC 3490, Internation‐
72       alizing Domain Names in Applications (IDNA)).  Three new flags are  de‐
73       fined:
74
75       NI_IDN If  this flag is used, then the name found in the lookup process
76              is converted from IDN format to the locale's encoding if  neces‐
77              sary.   ASCII-only  names  are  not  affected by the conversion,
78              which makes this flag usable in existing programs  and  environ‐
79              ments.
80
81       NI_IDN_ALLOW_UNASSIGNED, NI_IDN_USE_STD3_ASCII_RULES
82              Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow
83              unassigned Unicode code  points)  and  IDNA_USE_STD3_ASCII_RULES
84              (check  output  to  make  sure it is a STD3 conforming hostname)
85              flags respectively to be used in the IDNA handling.
86

RETURN VALUE

88       On success, 0 is returned, and node and service  names,  if  requested,
89       are  filled with null-terminated strings, possibly truncated to fit the
90       specified buffer lengths.  On error, one of the following nonzero error
91       codes is returned:
92
93       EAI_AGAIN
94              The name could not be resolved at this time.  Try again later.
95
96       EAI_BADFLAGS
97              The flags argument has an invalid value.
98
99       EAI_FAIL
100              A nonrecoverable error occurred.
101
102       EAI_FAMILY
103              The address family was not recognized, or the address length was
104              invalid for the specified family.
105
106       EAI_MEMORY
107              Out of memory.
108
109       EAI_NONAME
110              The  name  does  not  resolve  for   the   supplied   arguments.
111              NI_NAMEREQD  is  set  and  the host's name cannot be located, or
112              neither hostname nor service name were requested.
113
114       EAI_OVERFLOW
115              The buffer pointed to by host or serv was too small.
116
117       EAI_SYSTEM
118              A system error occurred.  The error code can be found in errno.
119
120       The gai_strerror(3) function translates these error codes  to  a  human
121       readable string, suitable for error reporting.
122

FILES

124       /etc/hosts
125       /etc/nsswitch.conf
126       /etc/resolv.conf
127

VERSIONS

129       getnameinfo() is provided in glibc since version 2.1.
130

ATTRIBUTES

132       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
133       tributes(7).
134
135       ┌──────────────┬───────────────┬────────────────────┐
136Interface     Attribute     Value              
137       ├──────────────┼───────────────┼────────────────────┤
138getnameinfo() │ Thread safety │ MT-Safe env locale │
139       └──────────────┴───────────────┴────────────────────┘
140

CONFORMING TO

142       POSIX.1-2001, POSIX.1-2008, RFC 2553.
143

NOTES

145       In order to assist the programmer in choosing reasonable sizes for  the
146       supplied buffers, <netdb.h> defines the constants
147
148           #define NI_MAXHOST      1025
149           #define NI_MAXSERV      32
150
151       Since glibc 2.8, these definitions are exposed only if suitable feature
152       test macros are defined, namely:  _GNU_SOURCE,  _DEFAULT_SOURCE  (since
153       glibc   2.19),  or  (in  glibc  versions  up  to  and  including  2.19)
154       _BSD_SOURCE or _SVID_SOURCE.
155
156       The former is the  constant  MAXDNAME  in  recent  versions  of  BIND's
157       <arpa/nameser.h>  header file.  The latter is a guess based on the ser‐
158       vices listed in the current Assigned Numbers RFC.
159
160       Before glibc version 2.2, the hostlen and servlen arguments were  typed
161       as size_t.
162

EXAMPLES

164       The  following code tries to get the numeric hostname and service name,
165       for a given socket address.  Note that there is no hardcoded  reference
166       to a particular address family.
167
168           struct sockaddr *addr;     /* input */
169           socklen_t addrlen;         /* input */
170           char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
171
172           if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), sbuf,
173                       sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)
174               printf("host=%s, serv=%s\n", hbuf, sbuf);
175
176       The  following  version  checks if the socket address has a reverse ad‐
177       dress mapping.
178
179           struct sockaddr *addr;     /* input */
180           socklen_t addrlen;         /* input */
181           char hbuf[NI_MAXHOST];
182
183           if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf),
184                       NULL, 0, NI_NAMEREQD))
185               printf("could not resolve hostname");
186           else
187               printf("host=%s\n", hbuf);
188
189       An example program using getnameinfo() can be found in getaddrinfo(3).
190

SEE ALSO

192       accept(2),  getpeername(2),  getsockname(2),  recvfrom(2),   socket(2),
193       getaddrinfo(3),  gethostbyaddr(3),  getservbyname(3), getservbyport(3),
194       inet_ntop(3), hosts(5), services(5), hostname(7), named(8)
195
196       R. Gilligan, S. Thomson, J. Bound and W. Stevens, Basic  Socket  Inter‐
197       face Extensions for IPv6, RFC 2553, March 1999.
198
199       Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
200       Addresses,  internet  draft,  work  in   progress   ⟨ftp://ftp.ietf.org
201       /internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt⟩.
202
203       Craig Metz, Protocol Independence Using the Sockets API, Proceedings of
204       the freenix track: 2000 USENIX annual technical conference, June 2000
205http://www.usenix.org/publications/library/proceedings/usenix2000
206       /freenix/metzprotocol.html⟩.
207

COLOPHON

209       This page is part of release 5.10 of the Linux man-pages project.  A
210       description of the project, information about reporting bugs, and the
211       latest version of this page, can be found at
212       https://www.kernel.org/doc/man-pages/.
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216GNU                               2020-06-09                    GETNAMEINFO(3)
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