1GETNAMEINFO(3) Linux Programmer's Manual GETNAMEINFO(3)
2
6 getnameinfo - address-to-name translation in protocol-independent man‐
7 ner
8
10 #include <sys/socket.h>
11 #include <netdb.h>
12 int getnameinfo(const struct sockaddr *restrict addr, socklen_t addrlen,
14 char *restrict host, socklen_t hostlen,
15 char *restrict serv, socklen_t servlen, int flags);
16 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
18 getnameinfo():
20 Since glibc 2.22:
21 _POSIX_C_SOURCE >= 200112L
22 Glibc 2.21 and earlier:
23 _POSIX_C_SOURCE
24
26 The getnameinfo() function is the inverse of getaddrinfo(3): it con‐
27 verts a socket address to a corresponding host and service, in a proto‐
28 col-independent manner. It combines the functionality of gethost‐
29 byaddr(3) and getservbyport(3), but unlike those functions, getname‐
30 info() is reentrant and allows programs to eliminate IPv4-versus-IPv6
31 dependencies.
32 The addr argument is a pointer to a generic socket address structure
34 (of type sockaddr_in or sockaddr_in6) of size addrlen that holds the
35 input IP address and port number. The arguments host and serv are
36 pointers to caller-allocated buffers (of size hostlen and servlen re‐
37 spectively) into which getnameinfo() places null-terminated strings
38 containing the host and service names respectively.
39 The caller can specify that no hostname (or no service name) is re‐
41 quired by providing a NULL host (or serv) argument or a zero hostlen
42 (or servlen) argument. However, at least one of hostname or service
43 name must be requested.
44 The flags argument modifies the behavior of getnameinfo() as follows:
46 NI_NAMEREQD
48 If set, then an error is returned if the hostname cannot be de‐
49 termined.
50 NI_DGRAM
52 If set, then the service is datagram (UDP) based rather than
53 stream (TCP) based. This is required for the few ports
54 (512–514) that have different services for UDP and TCP.
55 NI_NOFQDN
57 If set, return only the hostname part of the fully qualified do‐
58 main name for local hosts.
59 NI_NUMERICHOST
61 If set, then the numeric form of the hostname is returned.
62 (When not set, this will still happen in case the node's name
63 cannot be determined.)
64 NI_NUMERICSERV
66 If set, then the numeric form of the service address is re‐
67 turned. (When not set, this will still happen in case the ser‐
68 vice's name cannot be determined.)
69 Extensions to getnameinfo() for Internationalized Domain Names
71 Starting with glibc 2.3.4, getnameinfo() has been extended to selec‐
72 tively allow hostnames to be transparently converted to and from the
73 Internationalized Domain Name (IDN) format (see RFC 3490, Internation‐
74 alizing Domain Names in Applications (IDNA)). Three new flags are de‐
75 fined:
76 NI_IDN If this flag is used, then the name found in the lookup process
78 is converted from IDN format to the locale's encoding if neces‐
79 sary. ASCII-only names are not affected by the conversion,
80 which makes this flag usable in existing programs and environ‐
81 ments.
82 NI_IDN_ALLOW_UNASSIGNED, NI_IDN_USE_STD3_ASCII_RULES
84 Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow
85 unassigned Unicode code points) and IDNA_USE_STD3_ASCII_RULES
86 (check output to make sure it is a STD3 conforming hostname)
87 flags respectively to be used in the IDNA handling.
88
90 On success, 0 is returned, and node and service names, if requested,
91 are filled with null-terminated strings, possibly truncated to fit the
92 specified buffer lengths. On error, one of the following nonzero error
93 codes is returned:
94 EAI_AGAIN
96 The name could not be resolved at this time. Try again later.
97 EAI_BADFLAGS
99 The flags argument has an invalid value.
100 EAI_FAIL
102 A nonrecoverable error occurred.
103 EAI_FAMILY
105 The address family was not recognized, or the address length was
106 invalid for the specified family.
107 EAI_MEMORY
109 Out of memory.
110 EAI_NONAME
112 The name does not resolve for the supplied arguments.
113 NI_NAMEREQD is set and the host's name cannot be located, or
114 neither hostname nor service name were requested.
115 EAI_OVERFLOW
117 The buffer pointed to by host or serv was too small.
118 EAI_SYSTEM
120 A system error occurred. The error code can be found in errno.
121 The gai_strerror(3) function translates these error codes to a human
123 readable string, suitable for error reporting.
124
126 /etc/hosts
127 /etc/nsswitch.conf
128 /etc/resolv.conf
129
131 getnameinfo() is provided in glibc since version 2.1.
132
134 For an explanation of the terms used in this section, see at‐
135 tributes(7).
136 ┌─────────────────────────────────┬───────────────┬────────────────────┐
138 │Interface │ Attribute │ Value │
139 ├─────────────────────────────────┼───────────────┼────────────────────┤
140 │getnameinfo() │ Thread safety │ MT-Safe env locale │
141 └─────────────────────────────────┴───────────────┴────────────────────┘
142
144 POSIX.1-2001, POSIX.1-2008, RFC 2553.
145
147 In order to assist the programmer in choosing reasonable sizes for the
148 supplied buffers, <netdb.h> defines the constants
149 #define NI_MAXHOST 1025
151 #define NI_MAXSERV 32
152 Since glibc 2.8, these definitions are exposed only if suitable feature
154 test macros are defined, namely: _GNU_SOURCE, _DEFAULT_SOURCE (since
155 glibc 2.19), or (in glibc versions up to and including 2.19)
156 _BSD_SOURCE or _SVID_SOURCE.
157 The former is the constant MAXDNAME in recent versions of BIND's
159 <arpa/nameser.h> header file. The latter is a guess based on the ser‐
160 vices listed in the current Assigned Numbers RFC.
161 Before glibc version 2.2, the hostlen and servlen arguments were typed
163 as size_t.
164
166 The following code tries to get the numeric hostname and service name,
167 for a given socket address. Note that there is no hardcoded reference
168 to a particular address family.
169 struct sockaddr *addr; /* input */
171 socklen_t addrlen; /* input */
172 char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
173 if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf), sbuf,
175 sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)
176 printf("host=%s, serv=%s\n", hbuf, sbuf);
177 The following version checks if the socket address has a reverse ad‐
179 dress mapping.
180 struct sockaddr *addr; /* input */
182 socklen_t addrlen; /* input */
183 char hbuf[NI_MAXHOST];
184 if (getnameinfo(addr, addrlen, hbuf, sizeof(hbuf),
186 NULL, 0, NI_NAMEREQD))
187 printf("could not resolve hostname");
188 else
189 printf("host=%s\n", hbuf);
190 An example program using getnameinfo() can be found in getaddrinfo(3).
192
194 accept(2), getpeername(2), getsockname(2), recvfrom(2), socket(2),
195 getaddrinfo(3), gethostbyaddr(3), getservbyname(3), getservbyport(3),
196 inet_ntop(3), hosts(5), services(5), hostname(7), named(8)
197 R. Gilligan, S. Thomson, J. Bound and W. Stevens, Basic Socket Inter‐
199 face Extensions for IPv6, RFC 2553, March 1999.
200 Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
202 Addresses, internet draft, work in progress ⟨ftp://ftp.ietf.org
203 /internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt⟩.
204 Craig Metz, Protocol Independence Using the Sockets API, Proceedings of
206 the freenix track: 2000 USENIX annual technical conference, June 2000
207 ⟨http://www.usenix.org/publications/library/proceedings/usenix2000
208 /freenix/metzprotocol.html⟩.
209
211 This page is part of release 5.13 of the Linux man-pages project. A
212 description of the project, information about reporting bugs, and the
213 latest version of this page, can be found at
214 https://www.kernel.org/doc/man-pages/.
215GNU 2021-03-22 GETNAMEINFO(3)