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 or UNIX-domain 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 For a link-layer address,
60 this can be used as a replacement of the legacy
68 should point to either a
78 or UNIX-domain respectively
84 The host and service names associated with
90 which have length parameters
105 If a length parameter is zero, no string will be stored.
106 Otherwise, enough space must be provided to store the
107 host name or service string plus a byte for the NUL terminator.
111 argument is formed by
113 the following values:
114 .Bl -tag -width "NI_NUMERICSCOPEXX"
116 A fully qualified domain name is not required for local hosts.
117 The local part of the fully qualified domain name is returned instead.
118 .It Dv NI_NUMERICHOST
119 Return the address in numeric form, as if calling
121 instead of a host name.
124 If the host name cannot be found in DNS and this flag is set,
125 a non-zero error code is returned.
126 If the host name is not found and the flag is not set, the
127 address is returned in numeric form.
129 The service name is returned as a digit string representing the port number.
131 The scope identifier is returned as a digit string.
133 Specifies that the service being looked up is a datagram
136 to be called with a second argument of
138 instead of its default of
140 This is required for the few ports (512\-514) that have different services
147 This implementation allows numeric IPv6 address notation with scope identifier,
148 as documented in chapter 11 of RFC 4007.
149 IPv6 link-local address will appear as a string like
153 for more information.
156 returns zero on success or one of the error codes listed in
160 The following code tries to get a numeric host name, and service name,
161 for a given socket address.
162 Observe that there is no hardcoded reference to a particular address family.
163 .Bd -literal -offset indent
164 struct sockaddr *sa; /* input */
165 char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
167 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
168 sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {
169 errx(1, "could not get numeric hostname");
172 printf("host=%s, serv=%s\en", hbuf, sbuf);
175 The following version checks if the socket address has a reverse address mapping:
176 .Bd -literal -offset indent
177 struct sockaddr *sa; /* input */
178 char hbuf[NI_MAXHOST];
180 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,
182 errx(1, "could not resolve hostname");
185 printf("host=%s\en", hbuf);
190 .Xr gethostbyaddr 3 ,
191 .Xr getservbyport 3 ,
209 .%T Basic Socket Interface Extensions for IPv6
219 .%T "IPv6 Scoped Address Architecture"
225 .%T Protocol Independence Using the Sockets API
226 .%B "Proceedings of the freenix track: 2000 USENIX annual technical conference"
232 function is defined by the
234 specification and documented in
236 .Dq Basic Socket Interface Extensions for IPv6 .
239 can return both numeric and FQDN forms of the address specified in
241 There is no return value that indicates whether the string returned in
243 is a result of binary to numeric-text translation (like
245 or is the result of a DNS reverse lookup.
246 Because of this, malicious parties could set up a PTR record as follows:
247 .Bd -literal -offset indent
248 1.0.0.127.in-addr.arpa. IN PTR 10.1.1.1
251 and trick the caller of
260 To prevent such attacks, the use of
262 is recommended when the result of
265 for access control purposes:
266 .Bd -literal -offset indent
269 char addr[NI_MAXHOST];
270 struct addrinfo hints, *res;
273 error = getnameinfo(sa, salen, addr, sizeof(addr),
274 NULL, 0, NI_NAMEREQD);
276 memset(&hints, 0, sizeof(hints));
277 hints.ai_socktype = SOCK_DGRAM; /*dummy*/
278 hints.ai_flags = AI_NUMERICHOST;
279 if (getaddrinfo(addr, "0", &hints, &res) == 0) {
280 /* malicious PTR record */
282 printf("bogus PTR record\en");
285 /* addr is FQDN as a result of PTR lookup */
287 /* addr is numeric string */
288 error = getnameinfo(sa, salen, addr, sizeof(addr),
289 NULL, 0, NI_NUMERICHOST);
294 .\"intentionally uses a different
298 .\"suggests, to avoid buffer length handling mistakes.