1 .\" $KAME: getnameinfo.3,v 1.37 2005/01/05 03:23:05 itojun Exp $
2 .\" $OpenBSD: getnameinfo.3,v 1.36 2004/12/21 09:48:20 jmc Exp $
4 .\" Copyright (C) 2004 Internet Systems Consortium, Inc. ("ISC")
5 .\" Copyright (C) 2000, 2001 Internet Software Consortium.
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26 .Nd socket address structure to hostname and service name
33 .Fa "const struct sockaddr *sa" "socklen_t salen" "char *host"
34 .Fa "size_t hostlen" "char *serv" "size_t servlen" "int flags"
39 function is used to convert a
41 structure to a pair of host name and service strings.
42 It is a replacement for and provides more flexibility than the
46 functions and is the converse of the
50 If a link-layer address is passed to
52 its ASCII representation will be stored in
54 The string pointed to by
56 will be set to the empty string if non-NULL;
58 will always be ignored.
59 This is intended as a replacement for the legacy
67 should point to either a
72 structure (for IPv4, IPv6 or link-layer respectively) that is
76 The host and service names associated with
82 which have length parameters
97 If a length parameter is zero, no string will be stored.
98 Otherwise, enough space must be provided to store the
99 host name or service string plus a byte for the NUL terminator.
103 argument is formed by
105 the following values:
106 .Bl -tag -width "NI_NUMERICHOSTXX"
108 A fully qualified domain name is not required for local hosts.
109 The local part of the fully qualified domain name is returned instead.
110 .It Dv NI_NUMERICHOST
111 Return the address in numeric form, as if calling
113 instead of a host name.
116 If the host name cannot be found in DNS and this flag is set,
117 a non-zero error code is returned.
118 If the host name is not found and the flag is not set, the
119 address is returned in numeric form.
121 The service name is returned as a digit string representing the port number.
123 Specifies that the service being looked up is a datagram
126 to be called with a second argument of
128 instead of its default of
130 This is required for the few ports (512\-514) that have different services
137 This implementation allows numeric IPv6 address notation with scope identifier,
138 as documented in chapter 11 of RFC 4007.
139 IPv6 link-local address will appear as a string like
143 for more information.
146 returns zero on success or one of the error codes listed in
150 The following code tries to get a numeric host name, and service name,
151 for a given socket address.
152 Observe that there is no hardcoded reference to a particular address family.
153 .Bd -literal -offset indent
154 struct sockaddr *sa; /* input */
155 char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
157 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
158 sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {
159 errx(1, "could not get numeric hostname");
162 printf("host=%s, serv=%s\en", hbuf, sbuf);
165 The following version checks if the socket address has a reverse address mapping:
166 .Bd -literal -offset indent
167 struct sockaddr *sa; /* input */
168 char hbuf[NI_MAXHOST];
170 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,
172 errx(1, "could not resolve hostname");
175 printf("host=%s\en", hbuf);
180 .Xr gethostbyaddr 3 ,
181 .Xr getservbyport 3 ,
196 .%T Basic Socket Interface Extensions for IPv6
206 .%T "IPv6 Scoped Address Architecture"
212 .%T Protocol Independence Using the Sockets API
213 .%B "Proceedings of the freenix track: 2000 USENIX annual technical conference"
219 function is defined by the
221 specification and documented in
223 .Dq Basic Socket Interface Extensions for IPv6 .
226 can return both numeric and FQDN forms of the address specified in
228 There is no return value that indicates whether the string returned in
230 is a result of binary to numeric-text translation (like
232 or is the result of a DNS reverse lookup.
233 Because of this, malicious parties could set up a PTR record as follows:
234 .Bd -literal -offset indent
235 1.0.0.127.in-addr.arpa. IN PTR 10.1.1.1
238 and trick the caller of
247 To prevent such attacks, the use of
249 is recommended when the result of
252 for access control purposes:
253 .Bd -literal -offset indent
256 char addr[NI_MAXHOST];
257 struct addrinfo hints, *res;
260 error = getnameinfo(sa, salen, addr, sizeof(addr),
261 NULL, 0, NI_NAMEREQD);
263 memset(&hints, 0, sizeof(hints));
264 hints.ai_socktype = SOCK_DGRAM; /*dummy*/
265 hints.ai_flags = AI_NUMERICHOST;
266 if (getaddrinfo(addr, "0", &hints, &res) == 0) {
267 /* malicious PTR record */
269 printf("bogus PTR record\en");
272 /* addr is FQDN as a result of PTR lookup */
274 /* addr is numeric string */
275 error = getnameinfo(sa, salen, addr, sizeof(addr),
276 NULL, 0, NI_NUMERICHOST);
281 .\"intentionally uses a different
285 .\"suggests, to avoid buffer length handling mistakes.