lookup.subproj/getnameinfo.3

   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 $
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  19 .\" $FreeBSD: src/lib/libc/net/getnameinfo.3,v 1.25 2007/02/28 21:28:33 bms Exp $
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  21 .Dd February 28, 2007
  22 .Dt GETNAMEINFO 3
  23 .Os
  24 .Sh NAME
  25 .Nm getnameinfo
  26 .Nd socket address structure to hostname and service name
  27 .Sh SYNOPSIS
  28 .Fd #include <sys/types.h>
  29 .Fd #include <sys/socket.h>
  30 .Fd #include <netdb.h>
  31 .Ft int
  32 .Fo getnameinfo
  33 .Fa "const struct sockaddr *sa" "socklen_t salen" "char *host"
  34 .Fa "socklen_t hostlen" "char *serv" "socklen_t servlen" "int flags"
  35 .\".Fa "size_t hostlen" "char *serv" "size_t servlen" "int flags"
  36 .Fc
  37 .Sh DESCRIPTION
  38 The
  39 .Fn getnameinfo
  40 function is used to convert a
  41 .Li sockaddr
  42 structure to a pair of host name and service strings.
  43 It is a replacement for and provides more flexibility than the
  44 .Xr gethostbyaddr 3
  45 and
  46 .Xr getservbyport 3
  47 functions and is the converse of the
  48 .Xr getaddrinfo 3
  49 function.
  50 .Pp
  51 If a link-layer address is passed to
  52 .Fn getnameinfo ,
  53 its ASCII representation will be stored in
  54 .Fa host .
  55 The string pointed to by
  56 .Fa serv
  57 will be set to the empty string if non-NULL;
  58 .Fa flags
  59 will always be ignored.
  60 This is intended as a replacement for the legacy
  61 .Xr link_ntoa 3
  62 function.
  63 .Pp
  64 The
  65 .Li sockaddr
  66 structure
  67 .Fa sa
  68 should point to either a
  69 .Li sockaddr_in ,
  70 .Li sockaddr_in6
  71 or
  72 .Li sockaddr_dl
  73 structure (for IPv4, IPv6 or link-layer respectively) that is
  74 .Fa salen
  75 bytes long.
  76 .Pp
  77 The host and service names associated with
  78 .Fa sa
  79 are stored in
  80 .Fa host
  81 and
  82 .Fa serv
  83 which have length parameters
  84 .Fa hostlen
  85 and
  86 .Fa servlen .
  87 The maximum value for
  88 .Fa hostlen
  89 is
  90 .Dv NI_MAXHOST
  91 and
  92 the maximum value for
  93 .Fa servlen
  94 is
  95 .Dv NI_MAXSERV ,
  96 as defined by
  97 .Aq Pa netdb.h .
  98 If a length parameter is zero, no string will be stored.
  99 Otherwise, enough space must be provided to store the
 100 host name or service string plus a byte for the NUL terminator.
 101 .Pp
 102 The
 103 .Fa flags
 104 argument is formed by
 105 .Tn OR Ns 'ing
 106 the following values:
 107 .Bl -tag -width "NI_NUMERICHOSTXX"
 108 .It Dv NI_NOFQDN
 109 A fully qualified domain name is not required for local hosts.
 110 The local part of the fully qualified domain name is returned instead.
 111 .It Dv NI_NUMERICHOST
 112 Return the address in numeric form, as if calling
 113 .Xr inet_ntop 3 ,
 114 instead of a host name.
 115 .It Dv NI_NAMEREQD
 116 A name is required.
 117 If the host name cannot be found in DNS and this flag is set,
 118 a non-zero error code is returned.
 119 If the host name is not found and the flag is not set, the
 120 address is returned in numeric form.
 121 .It NI_NUMERICSERV
 122 The service name is returned as a digit string representing the port number.
 123 .It NI_DGRAM
 124 Specifies that the service being looked up is a datagram
 125 service, and causes
 126 .Xr getservbyport 3
 127 to be called with a second argument of
 128 .Dq udp
 129 instead of its default of
 130 .Dq tcp .
 131 This is required for the few ports (512\-514) that have different services
 132 for
 133 .Tn UDP
 134 and
 135 .Tn TCP .
 136 .El
 137 .Pp
 138 This implementation allows numeric IPv6 address notation with scope identifier,
 139 as documented in chapter 11 of draft-ietf-ipv6-scoping-arch-02.txt.
 140 IPv6 link-local address will appear as a string like
 141 .Dq Li fe80::1%ne0 .
 142 Refer to
 143 .Xr getaddrinfo 3
 144 for more information.
 145 .Sh RETURN VALUES
 146 .Fn getnameinfo
 147 returns zero on success or one of the error codes listed in
 148 .Xr gai_strerror 3
 149 if an error occurs.
 150 .Sh EXAMPLES
 151 The following code tries to get a numeric host name, and service name,
 152 for a given socket address.
 153 Observe that there is no hardcoded reference to a particular address family.
 154 .Bd -literal -offset indent
 155 struct sockaddr *sa;    /* input */
 156 char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
 157
 158 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,
 159     sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {
 160         errx(1, "could not get numeric hostname");
 161         /*NOTREACHED*/
 162 }
 163 printf("host=%s, serv=%s\en", hbuf, sbuf);
 164 .Ed
 165 .Pp
 166 The following version checks if the socket address has a reverse address mapping:
 167 .Bd -literal -offset indent
 168 struct sockaddr *sa;    /* input */
 169 char hbuf[NI_MAXHOST];
 170
 171 if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,
 172     NI_NAMEREQD)) {
 173         errx(1, "could not resolve hostname");
 174         /*NOTREACHED*/
 175 }
 176 printf("host=%s\en", hbuf);
 177 .Ed
 178 .Sh SEE ALSO
 179 .Xr gai_strerror 3 ,
 180 .Xr getaddrinfo 3 ,
 181 .Xr gethostbyaddr 3 ,
 182 .Xr getservbyport 3 ,
 183 .Xr inet_ntop 3 ,
 184 .Xr link_ntoa 3 ,
 185 .Xr resolver 3 ,
 186 .Xr hosts 5 ,
 187 .Xr resolv.conf 5 ,
 188 .Xr services 5 ,
 189 .Xr hostname 7 ,
 190 .Xr named 8
 191 .Rs
 192 .%A R. Gilligan
 193 .%A S. Thomson
 194 .%A J. Bound
 195 .%A W. Stevens
 196 .%T Basic Socket Interface Extensions for IPv6
 197 .%R RFC 2553
 198 .%D March 1999
 199 .Re
 200 .Rs
 201 .%A S. Deering
 202 .%A B. Haberman
 203 .%A T. Jinmei
 204 .%A E. Nordmark
 205 .%A B. Zill
 206 .%T "IPv6 Scoped Address Architecture"
 207 .%R internet draft
 208 .%N draft-ietf-ipv6-scoping-arch-02.txt
 209 .%O work in progress material
 210 .Re
 211 .Rs
 212 .%A Craig Metz
 213 .%T Protocol Independence Using the Sockets API
 214 .%B "Proceedings of the freenix track: 2000 USENIX annual technical conference"
 215 .%D June 2000
 216 .Re
 217 .Sh STANDARDS
 218 The
 219 .Fn getnameinfo
 220 function is defined by the
 221 .St -p1003.1g-2000
 222 draft specification and documented in
 223 .Tn "RFC 2553" ,
 224 .Dq Basic Socket Interface Extensions for IPv6 .
 225 .Sh CAVEATS
 226 .Fn getnameinfo
 227 can return both numeric and FQDN forms of the address specified in
 228 .Fa sa .
 229 There is no return value that indicates whether the string returned in
 230 .Fa host
 231 is a result of binary to numeric-text translation (like
 232 .Xr inet_ntop 3 ) ,
 233 or is the result of a DNS reverse lookup.
 234 Because of this, malicious parties could set up a PTR record as follows:
 235 .Bd -literal -offset indent
 236 1.0.0.127.in-addr.arpa. IN PTR  10.1.1.1
 237 .Ed
 238 .Pp
 239 and trick the caller of
 240 .Fn getnameinfo
 241 into believing that
 242 .Fa sa
 243 is
 244 .Li 10.1.1.1
 245 when it is actually
 246 .Li 127.0.0.1 .
 247 .Pp
 248 To prevent such attacks, the use of
 249 .Dv NI_NAMEREQD
 250 is recommended when the result of
 251 .Fn getnameinfo
 252 is used
 253 for access control purposes:
 254 .Bd -literal -offset indent
 255 struct sockaddr *sa;
 256 socklen_t salen;
 257 char addr[NI_MAXHOST];
 258 struct addrinfo hints, *res;
 259 int error;
 260
 261 error = getnameinfo(sa, salen, addr, sizeof(addr),
 262     NULL, 0, NI_NAMEREQD);
 263 if (error == 0) {
 264         memset(&hints, 0, sizeof(hints));
 265         hints.ai_socktype = SOCK_DGRAM; /*dummy*/
 266         hints.ai_flags = AI_NUMERICHOST;
 267         if (getaddrinfo(addr, "0", &hints, &res) == 0) {
 268                 /* malicious PTR record */
 269                 freeaddrinfo(res);
 270                 printf("bogus PTR record\en");
 271                 return -1;
 272         }
 273         /* addr is FQDN as a result of PTR lookup */
 274 } else {
 275         /* addr is numeric string */
 276         error = getnameinfo(sa, salen, addr, sizeof(addr),
 277             NULL, 0, NI_NUMERICHOST);
 278 }
 279 .Ed
 280 .\".Pp
 281 .\".Ox
 282 .\"intentionally uses a different
 283 .\".Dv NI_MAXHOST
 284 .\"value from what
 285 .\".Tn "RFC 2553"
 286 .\"suggests, to avoid buffer length handling mistakes.