gen.subproj/gethostbyname.3

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  32 .\"     From: @(#)gethostbyname.3       8.4 (Berkeley) 5/25/95
  33 .\" $FreeBSD: src/lib/libc/net/gethostbyname.3,v 1.12.2.6 2001/08/17 15:42:38 ru Exp $
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  35 .Dd May 25, 1995
  36 .Dt GETHOSTBYNAME 3
  37 .Os
  38 .Sh NAME
  39 .Nm gethostbyname ,
  40 .Nm gethostbyname2 ,
  41 .Nm gethostbyaddr ,
  42 .Nm gethostent ,
  43 .Nm sethostent ,
  44 .Nm endhostent ,
  45 .Nm herror ,
  46 .Nm hstrerror
  47 .Nd get network host entry
  48 .Sh LIBRARY
  49 .Lb libc
  50 .Sh SYNOPSIS
  51 .Fd #include <netdb.h>
  52 .Vt extern int h_errno ;
  53 .Ft struct hostent *
  54 .Fn gethostbyname "const char *name"
  55 .Ft struct hostent *
  56 .Fn gethostbyname2 "const char *name" "int af"
  57 .Ft struct hostent *
  58 .Fn gethostbyaddr "const char *addr" "int len" "int type"
  59 .Ft struct hostent *
  60 .Fn gethostent void
  61 .Ft void
  62 .Fn sethostent "int stayopen"
  63 .Ft void
  64 .Fn endhostent void
  65 .Ft void
  66 .Fn herror "const char *string"
  67 .Ft const char *
  68 .Fn hstrerror "int err"
  69 .Sh DESCRIPTION
  70 The
  71 .Fn gethostbyname ,
  72 .Fn gethostbyname2
  73 and
  74 .Fn gethostbyaddr
  75 functions
  76 each return a pointer to an object with the
  77 following structure describing an internet host
  78 referenced by name or by address, respectively.
  79 This structure contains either the information obtained from the name server,
  80 .Xr named 8 ,
  81 or broken-out fields from a line in
  82 .Pa /etc/hosts .
  83 If the local name server is not running these routines do a lookup in
  84 .Pa /etc/hosts .
  85 .Bd -literal
  86 struct  hostent {
  87         char    *h_name;        /* official name of host */
  88         char    **h_aliases;    /* alias list */
  89         int     h_addrtype;     /* host address type */
  90         int     h_length;       /* length of address */
  91         char    **h_addr_list;  /* list of addresses from name server */
  92 };
  93 #define h_addr  h_addr_list[0]  /* address, for backward compatibility */
  94 .Ed
  95 .Pp
  96 The members of this structure are:
  97 .Bl -tag -width h_addr_list
  98 .It Va h_name
  99 Official name of the host.
 100 .It Va h_aliases
 101 A
 102 .Dv NULL Ns -terminated
 103 array of alternate names for the host.
 104 .It Va h_addrtype
 105 The type of address being returned; usually
 106 .Dv AF_INET .
 107 .It Va h_length
 108 The length, in bytes, of the address.
 109 .It Va h_addr_list
 110 A
 111 .Dv NULL Ns -terminated
 112 array of network addresses for the host.
 113 Host addresses are returned in network byte order.
 114 .It Va h_addr
 115 The first address in
 116 .Va h_addr_list ;
 117 this is for backward compatibility.
 118 .El
 119 .Pp
 120 When using the nameserver,
 121 .Fn gethostbyname
 122 and
 123 .Fn gethostbyname2
 124 will search for the named host in the current domain and its parents
 125 unless the name ends in a dot.
 126 If the name contains no dot, and if the environment variable
 127 .Dq Ev HOSTALIASES
 128 contains the name of an alias file, the alias file will first be searched
 129 for an alias matching the input name.
 130 See
 131 .Xr hostname 7
 132 for the domain search procedure and the alias file format.
 133 .Pp
 134 The
 135 .Fn gethostbyname2
 136 function is an evolution of
 137 .Fn gethostbyname
 138 which is intended to allow lookups in address families other than
 139 .Dv AF_INET ,
 140 for example
 141 .Dv AF_INET6 .
 142 Both of these address families are supported in the Mac OS X implemention.
 143 .Pp
 144 The
 145 .Fn sethostent
 146 function
 147 may be used to request the use of a connected
 148 .Tn TCP
 149 socket for queries.
 150 If the
 151 .Fa stayopen
 152 flag is non-zero,
 153 this sets the option to send all queries to the name server using
 154 .Tn TCP
 155 and to retain the connection after each call to
 156 .Fn gethostbyname ,
 157 .Fn gethostbyname2
 158 or
 159 .Fn gethostbyaddr .
 160 Otherwise, queries are performed using
 161 .Tn UDP
 162 datagrams.
 163 .Pp
 164 The
 165 .Fn endhostent
 166 function
 167 closes the
 168 .Tn TCP
 169 connection.
 170 .Pp
 171 The
 172 .Fn herror
 173 function writes a message to the diagnostic output consisting of the
 174 string parameter
 175 .Fa s ,
 176 the constant string
 177 .Qq Li ":\ " ,
 178 and a message corresponding to the value of
 179 .Va h_errno .
 180 .Pp
 181 The
 182 .Fn hstrerror
 183 function returns a string which is the message text corresponding to the
 184 value of the
 185 .Fa err
 186 parameter.
 187 .Sh FILES
 188 .Bl -tag -width /etc/resolv.conf -compact
 189 .It Pa /etc/hosts
 190 .It Pa /etc/host.conf
 191 .It Pa /etc/resolv.conf
 192 .El
 193 .Sh DIAGNOSTICS
 194 Error return status from
 195 .Fn gethostbyname ,
 196 .Fn gethostbyname2
 197 and
 198 .Fn gethostbyaddr
 199 is indicated by return of a
 200 .Dv NULL
 201 pointer.
 202 The external integer
 203 .Va h_errno
 204 may then be checked to see whether this is a temporary failure
 205 or an invalid or unknown host.
 206 The routine
 207 .Fn herror
 208 can be used to print an error message describing the failure.
 209 If its argument
 210 .Fa string
 211 is
 212 .Pf non- Dv NULL ,
 213 it is printed, followed by a colon and a space.
 214 The error message is printed with a trailing newline.
 215 .Pp
 216 The variable
 217 .Va h_errno
 218 can have the following values:
 219 .Bl -tag -width HOST_NOT_FOUND
 220 .It Dv HOST_NOT_FOUND
 221 No such host is known.
 222 .It Dv TRY_AGAIN
 223 This is usually a temporary error
 224 and means that the local server did not receive
 225 a response from an authoritative server.
 226 A retry at some later time may succeed.
 227 .It Dv NO_RECOVERY
 228 Some unexpected server failure was encountered.
 229 This is a non-recoverable error.
 230 .It Dv NO_DATA
 231 The requested name is valid but does not have an IP address;
 232 this is not a temporary error.
 233 This means that the name is known to the name server but there is no address
 234 associated with this name.
 235 Another type of request to the name server using this domain name
 236 will result in an answer;
 237 for example, a mail-forwarder may be registered for this domain.
 238 .El
 239 .Sh SEE ALSO
 240 .Xr getaddrinfo 3 ,
 241 .Xr resolver 3 ,
 242 .Xr hosts 5 ,
 243 .Xr hostname 7 ,
 244 .Xr named 8
 245 .Sh CAVEAT
 246 The
 247 .Fn gethostent
 248 function
 249 is defined, and
 250 .Fn sethostent
 251 and
 252 .Fn endhostent
 253 are redefined,
 254 when libc is built to use only the routines to lookup in
 255 .Pa /etc/hosts
 256 and not the name server.
 257 .Pp
 258 The
 259 .Fn gethostent
 260 function
 261 reads the next line of
 262 .Pa /etc/hosts ,
 263 opening the file if necessary.
 264 .Pp
 265 The
 266 .Fn sethostent
 267 function
 268 opens and/or rewinds the file
 269 .Pa /etc/hosts .
 270 If the
 271 .Fa stayopen
 272 argument is non-zero,
 273 the file will not be closed after each call to
 274 .Fn gethostbyname ,
 275 .Fn gethostbyname2
 276 or
 277 .Fn gethostbyaddr .
 278 .Pp
 279 The
 280 .Fn endhostent
 281 function
 282 closes the file.
 283 .Sh HISTORY
 284 The
 285 .Fn herror
 286 function appeared in
 287 .Bx 4.3 .
 288 The
 289 .Fn endhostent ,
 290 .Fn gethostbyaddr ,
 291 .Fn gethostbyname ,
 292 .Fn gethostent ,
 293 and
 294 .Fn sethostent
 295 functions appeared in
 296 .Bx 4.2 .
 297 The
 298 .Fn gethostbyname2
 299 function first appeared in
 300 .Tn BIND
 301 version 4.9.4.
 302 .Sh BUGS
 303 These functions use static data storage;
 304 if the data is needed for future use, it should be
 305 copied before any subsequent calls overwrite it.
 306 Only the Internet
 307 address format is currently understood.