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.
7 .\" Permission to use, copy, modify, and distribute this software for any
8 .\" purpose with or without fee is hereby granted, provided that the above
9 .\" copyright notice and this permission notice appear in all copies.
11 .\" THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
12 .\" REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
13 .\" AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
14 .\" INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
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19 .\" $FreeBSD: src/lib/libc/net/getnameinfo.3,v 1.25 2007/02/28 21:28:33 bms Exp $
26 .Nd socket address structure to hostname and service name
35 .Fa "const struct sockaddr *sa" "socklen_t salen" "char *host"
36 .Fa "size_t hostlen" "char *serv" "size_t servlen" "int flags"
41 function is used to convert a
43 structure to a pair of host name and service strings.
44 It is a replacement for and provides more flexibility than the
48 functions and is the converse of the
52 If a link-layer address is passed to
54 its ASCII representation will be stored in
56 The string pointed to by
58 will be set to the empty string if non-NULL;
60 will always be ignored.
61 This is intended as a replacement for the legacy
69 should point to either a
74 structure (for IPv4, IPv6 or link-layer respectively) that is
78 The host and service names associated with
84 which have length parameters
99 If a length parameter is zero, no string will be stored.
100 Otherwise, enough space must be provided to store the
101 host name or service string plus a byte for the NUL terminator.
105 argument is formed by
107 the following values:
108 .Bl -tag -width ".Dv NI_NUMERICHOST"
110 A fully qualified domain name is not required for local hosts.
111 The local part of the fully qualified domain name is returned instead.
112 .It Dv NI_NUMERICHOST
113 Return the address in numeric form, as if calling
115 instead of a host name.
118 If the host name cannot be found in DNS and this flag is set,
119 a non-zero error code is returned.
120 If the host name is not found and the flag is not set, the
121 address is returned in numeric form.
122 .It Dv NI_NUMERICSERV
123 The service name is returned as a digit string representing the port number.
125 Specifies that the service being looked up is a datagram
128 to be called with a second argument of
130 instead of its default of
132 This is required for the few ports (512\-514) that have different services
139 This implementation allows numeric IPv6 address notation with scope identifier,
140 as documented in chapter 11 of draft-ietf-ipv6-scoping-arch-02.txt.
141 IPv6 link-local address will appear as a string like
145 for more information.
148 returns zero on success or one of the error codes listed in
152 The following code tries to get a numeric host name, and service name,
153 for a given socket address.
154 Observe that there is no hardcoded reference to a particular address family.
155 .Bd -literal -offset indent
156 struct sockaddr *sa; /* input */
157 char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
159 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
160 sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {
161 errx(1, "could not get numeric hostname");
164 printf("host=%s, serv=%s\en", hbuf, sbuf);
167 The following version checks if the socket address has a reverse address mapping:
168 .Bd -literal -offset indent
169 struct sockaddr *sa; /* input */
170 char hbuf[NI_MAXHOST];
172 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,
174 errx(1, "could not resolve hostname");
177 printf("host=%s\en", hbuf);
182 .Xr gethostbyaddr 3 ,
183 .Xr getservbyport 3 ,
197 .%T Basic Socket Interface Extensions for IPv6
207 .%T "IPv6 Scoped Address Architecture"
209 .%N draft-ietf-ipv6-scoping-arch-02.txt
210 .%O work in progress material
214 .%T Protocol Independence Using the Sockets API
215 .%B "Proceedings of the freenix track: 2000 USENIX annual technical conference"
221 function is defined by the
223 draft specification and documented in
225 .Dq Basic Socket Interface Extensions for IPv6 .
228 can return both numeric and FQDN forms of the address specified in
230 There is no return value that indicates whether the string returned in
232 is a result of binary to numeric-text translation (like
234 or is the result of a DNS reverse lookup.
235 Because of this, malicious parties could set up a PTR record as follows:
236 .Bd -literal -offset indent
237 1.0.0.127.in-addr.arpa. IN PTR 10.1.1.1
240 and trick the caller of
249 To prevent such attacks, the use of
251 is recommended when the result of
254 for access control purposes:
255 .Bd -literal -offset indent
258 char addr[NI_MAXHOST];
259 struct addrinfo hints, *res;
262 error = getnameinfo(sa, salen, addr, sizeof(addr),
263 NULL, 0, NI_NAMEREQD);
265 memset(&hints, 0, sizeof(hints));
266 hints.ai_socktype = SOCK_DGRAM; /*dummy*/
267 hints.ai_flags = AI_NUMERICHOST;
268 if (getaddrinfo(addr, "0", &hints, &res) == 0) {
269 /* malicious PTR record */
271 printf("bogus PTR record\en");
274 /* addr is FQDN as a result of PTR lookup */
276 /* addr is numeric string */
277 error = getnameinfo(sa, salen, addr, sizeof(addr),
278 NULL, 0, NI_NUMERICHOST);
283 .\"intentionally uses a different
287 .\"suggests, to avoid buffer length handling mistakes.