gen.subproj/gethostbyname.3

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  28 .\"     From: @(#)gethostbyname.3       8.4 (Berkeley) 5/25/95
  29 .\" $FreeBSD: src/lib/libc/net/gethostbyname.3,v 1.38 2007/01/09 00:28:02 imp Exp $
  30 .\"
  31 .Dd May 12, 2006
  32 .Dt GETHOSTBYNAME 3
  33 .Os
  34 .Sh NAME
  35 .Nm gethostbyname ,
  36 .Nm gethostbyname2 ,
  37 .Nm gethostbyaddr ,
  38 .Nm gethostent ,
  39 .Nm sethostent ,
  40 .Nm endhostent ,
  41 .Nm herror ,
  42 .Nm hstrerror
  43 .Nd get network host entry
  44 .Sh LIBRARY
  45 .Lb libc
  46 .Sh SYNOPSIS
  47 .In netdb.h
  48 .Vt int h_errno ;
  49 .Ft struct hostent *
  50 .Fn gethostbyname "const char *name"
  51 .Ft struct hostent *
  52 .Fn gethostbyname2 "const char *name" "int af"
  53 .Ft struct hostent *
  54 .Fn gethostbyaddr "const void *addr" "socklen_t len" "int type"
  55 .Ft struct hostent *
  56 .Fn gethostent void
  57 .Ft void
  58 .Fn sethostent "int stayopen"
  59 .Ft void
  60 .Fn endhostent void
  61 .Ft void
  62 .Fn herror "const char *string"
  63 .Ft const char *
  64 .Fn hstrerror "int err"
  65 .Sh DESCRIPTION
  66 .Bf -symbolic
  67 The
  68 .Xr getaddrinfo 3
  69 and
  70 .Xr getnameinfo 3
  71 functions are preferred over the
  72 .Fn gethostbyname ,
  73 .Fn gethostbyname2 ,
  74 and
  75 .Fn gethostbyaddr
  76 functions.
  77 .Ef
  78 .Pp
  79 The
  80 .Fn gethostbyname ,
  81 .Fn gethostbyname2
  82 and
  83 .Fn gethostbyaddr
  84 functions
  85 each return a pointer to an object with the
  86 following structure describing an internet host
  87 referenced by name or by address, respectively.
  88 .Pp
  89 The
  90 .Fa name
  91 argument passed to
  92 .Fn gethostbyname
  93 or
  94 .Fn gethostbyname2
  95 should point to a
  96 .Dv NUL Ns -terminated
  97 hostname.
  98 The
  99 .Fa addr
 100 argument passed to
 101 .Fn gethostbyaddr
 102 should point to an address which is
 103 .Fa len
 104 bytes long,
 105 in binary form
 106 (i.e., not an IP address in human readable
 107 .Tn ASCII
 108 form).
 109 The
 110 .Fa type
 111 argument specifies the address family
 112 (e.g.\&
 113 .Dv AF_INET , AF_INET6 ,
 114 etc.) of this address.
 115 .Pp
 116 The structure returned contains information obtained from
 117 .Xr mDNSResponder 8 ,
 118 including records in
 119 .Pa /etc/hosts .
 120 .\"The structure returned contains either the information obtained from the name
 121 .\"server,
 122 .\".Xr named 8 ,
 123 .\"broken-out fields from a line in
 124 .\".Pa /etc/hosts ,
 125 .\"or database entries supplied by the
 126 .\".Xr yp 8
 127 .\"system.
 128 .\"The order of the lookups is controlled by the
 129 .\".Sq hosts
 130 .\"entry in
 131 .\".Xr nsswitch.conf 5 .
 132 .Bd -literal
 133 struct  hostent {
 134         char    *h_name;        /* official name of host */
 135         char    **h_aliases;    /* alias list */
 136         int     h_addrtype;     /* host address type */
 137         int     h_length;       /* length of address */
 138         char    **h_addr_list;  /* list of addresses from name server */
 139 };
 140 #define h_addr  h_addr_list[0]  /* address, for backward compatibility */
 141 .Ed
 142 .Pp
 143 The members of this structure are:
 144 .Bl -tag -width h_addr_list
 145 .It Va h_name
 146 Official name of the host.
 147 .It Va h_aliases
 148 A
 149 .Dv NULL Ns -terminated
 150 array of alternate names for the host.
 151 .It Va h_addrtype
 152 The type of address being returned; usually
 153 .Dv AF_INET .
 154 .It Va h_length
 155 The length, in bytes, of the address.
 156 .It Va h_addr_list
 157 A
 158 .Dv NULL Ns -terminated
 159 array of network addresses for the host.
 160 Host addresses are returned in network byte order.
 161 .It Va h_addr
 162 The first address in
 163 .Va h_addr_list ;
 164 this is for backward compatibility.
 165 .El
 166 .Pp
 167 When using the nameserver,
 168 .Fn gethostbyname
 169 and
 170 .Fn gethostbyname2
 171 will search for the named host in the current domain and its parents
 172 unless the name ends in a dot.
 173 .\"If the name contains no dot, and if the environment variable
 174 .\".Dq Ev HOSTALIASES
 175 .\"contains the name of an alias file, the alias file will first be searched
 176 .\"for an alias matching the input name.
 177 .\"See
 178 .\".Xr hostname 7
 179 .\"for the domain search procedure and the alias file format.
 180 .Pp
 181 The
 182 .Fn gethostbyname2
 183 function is an evolution of
 184 .Fn gethostbyname
 185 which is intended to allow lookups in address families other than
 186 .Dv AF_INET ,
 187 for example
 188 .Dv AF_INET6 .
 189 .Pp
 190 .\"The
 191 .\".Fn sethostent
 192 .\"function
 193 .\"may be used to request the use of a connected
 194 .\".Tn TCP
 195 .\"socket for queries.
 196 .\"If the
 197 .\".Fa stayopen
 198 .\"flag is non-zero,
 199 .\"this sets the option to send all queries to the name server using
 200 .\".Tn TCP
 201 .\"and to retain the connection after each call to
 202 .\".Fn gethostbyname ,
 203 .\".Fn gethostbyname2
 204 .\"or
 205 .\".Fn gethostbyaddr .
 206 .\"Otherwise, queries are performed using
 207 .\".Tn UDP
 208 .\"datagrams.
 209 .\".Pp
 210 .\"The
 211 .\".Fn endhostent
 212 .\"function
 213 .\"closes the
 214 .\".Tn TCP
 215 .\"connection.
 216 .\".Pp
 217 The
 218 .Fn herror
 219 function writes a message to the diagnostic output consisting of the
 220 string argument
 221 .Fa string ,
 222 the constant string
 223 .Qq Li ":\ " ,
 224 and a message corresponding to the value of
 225 .Va h_errno .
 226 .Pp
 227 The
 228 .Fn hstrerror
 229 function returns a string which is the message text corresponding to the
 230 value of the
 231 .Fa err
 232 argument.
 233 .Sh FILES
 234 .Bl -tag -width /etc/nsswitch.conf -compact
 235 .It Pa /etc/hosts
 236 .\".It Pa /etc/nsswitch.conf
 237 .It Pa /etc/resolv.conf
 238 .El
 239 .Sh EXAMPLES
 240 Print out the hostname associated with a specific IP address:
 241 .Bd -literal -offset indent
 242 const char *ipstr = "127.0.0.1";
 243 struct in_addr ip;
 244 struct hostent *hp;
 245
 246 if (!inet_aton(ipstr, &ip))
 247         errx(1, "can't parse IP address %s", ipstr);
 248
 249 if ((hp = gethostbyaddr((const void *)&ip,
 250     sizeof ip, AF_INET)) == NULL)
 251         errx(1, "no name associated with %s", ipstr);
 252
 253 printf("name associated with %s is %s\en", ipstr, hp->h_name);
 254 .Ed
 255 .Sh DIAGNOSTICS
 256 Error return status from
 257 .Fn gethostbyname ,
 258 .Fn gethostbyname2
 259 and
 260 .Fn gethostbyaddr
 261 is indicated by return of a
 262 .Dv NULL
 263 pointer.
 264 The integer
 265 .Va h_errno
 266 may then be checked to see whether this is a temporary failure
 267 or an invalid or unknown host.
 268 The routine
 269 .Fn herror
 270 can be used to print an error message describing the failure.
 271 If its argument
 272 .Fa string
 273 is
 274 .Pf non- Dv NULL ,
 275 it is printed, followed by a colon and a space.
 276 The error message is printed with a trailing newline.
 277 .Pp
 278 The variable
 279 .Va h_errno
 280 can have the following values:
 281 .Bl -tag -width HOST_NOT_FOUND
 282 .It Dv HOST_NOT_FOUND
 283 No such host is known.
 284 .It Dv TRY_AGAIN
 285 This is usually a temporary error
 286 and means that the local server did not receive
 287 a response from an authoritative server.
 288 A retry at some later time may succeed.
 289 .It Dv NO_RECOVERY
 290 Some unexpected server failure was encountered.
 291 This is a non-recoverable error.
 292 .It Dv NO_DATA
 293 The requested name is valid but does not have an IP address;
 294 this is not a temporary error.
 295 This means that the name is known to the name server but there is no address
 296 associated with this name.
 297 Another type of request to the name server using this domain name
 298 will result in an answer;
 299 for example, a mail-forwarder may be registered for this domain.
 300 .El
 301 .Sh SEE ALSO
 302 .Xr getaddrinfo 3 ,
 303 .Xr getnameinfo 3 ,
 304 .Xr inet_aton 3 ,
 305 .Xr resolver 3 ,
 306 .Xr hosts 5 ,
 307 .Xr hostname 7 ,
 308 .Xr mDNSResponder 8
 309 .\".Xr named 8
 310 .Sh CAVEAT
 311 The
 312 .Fn gethostent
 313 function
 314 is defined, and
 315 .Fn sethostent
 316 and
 317 .Fn endhostent
 318 are redefined,
 319 when
 320 .Lb libc
 321 is built to use only the routines to lookup in
 322 .Pa /etc/hosts
 323 and not the name server.
 324 .Pp
 325 The
 326 .Fn gethostent
 327 function
 328 reads the next line of
 329 .Pa /etc/hosts ,
 330 opening the file if necessary.
 331 .Pp
 332 The
 333 .Fn sethostent
 334 function
 335 opens and/or rewinds the file
 336 .Pa /etc/hosts .
 337 If the
 338 .Fa stayopen
 339 argument is non-zero,
 340 the file will not be closed after each call to
 341 .Fn gethostbyname ,
 342 .Fn gethostbyname2
 343 or
 344 .Fn gethostbyaddr .
 345 .Pp
 346 The
 347 .Fn endhostent
 348 function
 349 closes the file.
 350 .Sh HISTORY
 351 The
 352 .Fn herror
 353 function appeared in
 354 .Bx 4.3 .
 355 The
 356 .Fn endhostent ,
 357 .Fn gethostbyaddr ,
 358 .Fn gethostbyname ,
 359 .Fn gethostent ,
 360 and
 361 .Fn sethostent
 362 functions appeared in
 363 .Bx 4.2 .
 364 The
 365 .Fn gethostbyname2
 366 function first appeared in
 367 .Tn BIND
 368 version 4.9.4.
 369 .Sh BUGS
 370 These functions use a thread-specific data storage;
 371 if the data is needed for future use, it should be
 372 copied before any subsequent calls overwrite it.
 373 .Pp
 374 Though these functions are thread-safe,
 375 still it is recommended to use the
 376 .Xr getaddrinfo 3
 377 family of functions, instead.
 378 .Pp
 379 Only the Internet
 380 address format is currently understood.