man3/gethostbyname.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\"
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   7 .\" Permission is granted to copy and distribute modified versions of this
   8 .\" manual under the conditions for verbatim copying, provided that the
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  10 .\" permission notice identical to this one.
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  12 .\" Since the Linux kernel and libraries are constantly changing, this
  13 .\" manual page may be incorrect or out-of-date.  The author(s) assume no
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  15 .\" the use of the information contained herein.  The author(s) may not
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  17 .\" which is licensed free of charge, as they might when working
  18 .\" professionally.
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  20 .\" Formatted or processed versions of this manual, if unaccompanied by
  21 .\" the source, must acknowledge the copyright and authors of this work.
  22 .\"
  23 .\" References consulted:
  24 .\"     Linux libc source code
  25 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  26 .\"     386BSD man pages
  27 .\" Modified 1993-05-22, David Metcalfe
  28 .\" Modified 1993-07-25, Rik Faith (faith@cs.unc.edu)
  29 .\" Modified 1997-02-16, Andries Brouwer (aeb@cwi.nl)
  30 .\" Modified 1998-12-21, Andries Brouwer (aeb@cwi.nl)
  31 .\" Modified 2000-08-12, Andries Brouwer (aeb@cwi.nl)
  32 .\" Modified 2001-05-19, Andries Brouwer (aeb@cwi.nl)
  33 .\" Modified 2002-08-05, Michael Kerrisk
  34 .\" Modified 2004-10-31, Andries Brouwer
  35 .\"
  36 .TH GETHOSTBYNAME 3 2004-10-31 "" "Linux Programmer's Manual"
  37 .SH NAME
  38 gethostbyname, gethostbyaddr, sethostent, gethostend, endhostent,
  39 herror, hstrerror \- get network host entry
  40 .SH SYNOPSIS
  41 .nf
  42 .B #include <netdb.h>
  43 .B extern int h_errno;
  44 .sp
  45 .BI "struct hostent *gethostbyname(const char *" name );
  46 .sp
  47 .BR "#include <sys/socket.h>" "       /* for AF_INET */"
  48 .BI "struct hostent *"
  49 .br
  50 .BI "gethostbyaddr(const void *" addr ", int " len ", int " type );
  51 .sp
  52 .BI "void sethostent(int " stayopen );
  53 .sp
  54 .B void endhostent(void);
  55 .sp
  56 .BI "void herror(const char *" s );
  57 .sp
  58 .BI "const char *hstrerror(int " err );
  59 .sp 2
  60 /* SYSV/POSIX extension */
  61 .br
  62 .B struct hostent *gethostent(void);
  63 .sp 2
  64 /* GNU extensions */
  65 .br
  66 .BI "struct hostent *gethostbyname2(const char *" name ", int " af );
  67 .sp
  68 .BI "int gethostent_r("
  69 .BI "  struct hostent *" ret ", char *" buf ", size_t " buflen ,
  70 .BI "  struct hostent **" result ", int *" h_errnop );
  71 .sp
  72 .BI "int gethostbyname_r(const char *" name ,
  73 .BI "  struct hostent *" ret ", char *" buf ", size_t " buflen ,
  74 .BI "  struct hostent **" result ", int *" h_errnop );
  75 .sp
  76 .BI "int gethostbyname2_r(const char *" name ", int " af,
  77 .BI "  struct hostent *" ret ", char *" buf ", size_t " buflen ,
  78 .BI "  struct hostent **" result ", int *" h_errnop );
  79 .fi
  80 .SH DESCRIPTION
  81 The
  82 .BR gethostbyname ()
  83 function returns a structure of type
  84 .I hostent
  85 for the given host
  86 .IR name .
  87 Here
  88 .I name
  89 is either a host name, or an IPv4 address in standard dot notation,
  90 or an IPv6 address in colon (and possibly dot) notation.
  91 (See RFC 1884 for the description of IPv6 addresses.)
  92 If
  93 .I name
  94 is an IPv4 or IPv6 address, no lookup is performed and
  95 .BR gethostbyname ()
  96 simply copies
  97 .I name
  98 into the
  99 .I h_name
 100 field and its
 101 .I struct in_addr
 102 equivalent into the
 103 .I h_addr_list[0]
 104 field of the returned
 105 .I hostent
 106 structure.
 107 If
 108 .I name
 109 doesn't end in a dot and the environment variable
 110 .B HOSTALIASES
 111 is set, the alias file pointed to by
 112 .B HOSTALIASES
 113 will first be searched for
 114 .I name
 115 (see
 116 .BR hostname (7)
 117 for the file format).
 118 The current domain and its parents are searched unless \fIname\fP
 119 ends in a dot.
 120 .PP
 121 The \fBgethostbyaddr()\fP function returns a structure of type \fIhostent\fP
 122 for the given host address \fIaddr\fP of length \fIlen\fP and address type
 123 \fItype\fP.  Valid address types are
 124 .B AF_INET
 125 and
 126 .BR AF_INET6 .
 127 The host address argument is a pointer to a struct of a type depending
 128 on the address type, for example a \fBstruct in_addr *\fP (probably
 129 obtained via a call to \fIinet_addr()\fP) for address type AF_INET.
 130 .PP
 131 The \fBsethostent()\fP function specifies, if \fIstayopen\fP is true (1),
 132 that a connected TCP socket should be used for the name server queries and
 133 that the connection should remain open during successive queries.  Otherwise,
 134 name server queries will use UDP datagrams.
 135 .PP
 136 The \fBendhostent()\fP function ends the use of a TCP connection for name
 137 server queries.
 138 .PP
 139 The (obsolete) \fBherror()\fP function prints the error message associated
 140 with the current value of \fIh_errno\fP on stderr.
 141 .PP
 142 The (obsolete) \fBhstrerror()\fP function takes an error number
 143 (typically \fIh_errno\fP) and returns the corresponding message string.
 144 .PP
 145 The domain name queries carried out by \fBgethostbyname()\fP and
 146 \fBgethostbyaddr()\fP use a combination of any or all of the name server
 147 .BR named (8),
 148 a broken out line from \fI/etc/hosts\fP, and the Network
 149 Information Service (NIS or YP), depending upon the contents of the
 150 \fIorder\fP line in
 151 .IR /etc/host.conf .
 152 (See
 153 .BR resolv+ (8)).
 154 The default action is to query
 155 .BR named (8),
 156 followed by
 157 .IR /etc/hosts .
 158 .PP
 159 The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
 160 .sp
 161 .RS
 162 .nf
 163 .ne 7
 164 .ta 8n 16n 32n
 165 struct hostent {
 166         char    *h_name;                /* official name of host */
 167         char    **h_aliases;            /* alias list */
 168         int     h_addrtype;             /* host address type */
 169         int     h_length;               /* length of address */
 170         char    **h_addr_list;          /* list of addresses */
 171 }
 172 #define h_addr  h_addr_list[0]          /* for backward compatibility */
 173 .ta
 174 .fi
 175 .RE
 176 .PP
 177 The members of the \fIhostent\fP structure are:
 178 .TP
 179 .I h_name
 180 The official name of the host.
 181 .TP
 182 .I h_aliases
 183 A zero-terminated array of alternative names for the host.
 184 .TP
 185 .I h_addrtype
 186 The type of address; always
 187 .B AF_INET
 188 or
 189 .B AF_INET6
 190 at present.
 191 .TP
 192 .I h_length
 193 The length of the address in bytes.
 194 .TP
 195 .I h_addr_list
 196 A zero-terminated array of network addresses for the host in network byte
 197 order.
 198 .TP
 199 .I h_addr
 200 The first address in \fIh_addr_list\fP for backward compatibility.
 201 .SH "RETURN VALUE"
 202 The
 203 .BR gethostbyname ()
 204 and
 205 .BR gethostbyaddr()
 206 functions return the
 207 .I hostent
 208 structure or a NULL pointer if an error occurs.  On error, the
 209 .I h_errno
 210 variable holds an error number.
 211 When non-NULL, the return value may point at static data, see the notes below.
 212 .SH ERRORS
 213 The variable \fIh_errno\fP can have the following values:
 214 .TP
 215 .B HOST_NOT_FOUND
 216 The specified host is unknown.
 217 .TP
 218 .BR NO_ADDRESS " or " NO_DATA
 219 The requested name is valid but does not have an IP address.
 220 .TP
 221 .B NO_RECOVERY
 222 A non-recoverable name server error occurred.
 223 .TP
 224 .B TRY_AGAIN
 225 A temporary error occurred on an authoritative name server.  Try again
 226 later.
 227 .SH FILES
 228 .TP
 229 .I /etc/host.conf
 230 resolver configuration file
 231 .TP
 232 .I /etc/hosts
 233 host database file
 234 .SH "CONFORMING TO"
 235 BSD 4.3.
 236 .SH "SYSV/POSIX EXTENSION"
 237 POSIX requires the
 238 .BR gethostent ()
 239 call, that should return the next entry in the host data base.
 240 When using DNS/BIND this does not make much sense, but it may
 241 be reasonable if the host data base is a file that can be read
 242 line by line. On many systems a routine of this name reads
 243 from the file
 244 .IR /etc/hosts .
 245 .\" e.g. Linux, FreeBSD, Unixware, HP-UX
 246 It may be available only when the library was built without DNS support.
 247 .\" e.g. FreeBSD, AIX
 248 The glibc version will ignore ipv6 entries. This function is not reentrant,
 249 and glibc adds a reentrant version
 250 .BR gethostent_r ().
 251 .SH "GNU EXTENSIONS"
 252 Glibc2 also has a
 253 .B gethostbyname2()
 254 that works like
 255 .BR gethostbyname() ,
 256 but permits to specify the address family to which the address must belong.
 257 .LP
 258 Glibc2 also has reentrant versions
 259 .B gethostbyname_r()
 260 and
 261 .BR gethostbyname2_r() .
 262 These return 0 on success and nonzero on error. The result of the call
 263 is now stored in the struct with address
 264 .IR ret .
 265 After the call,
 266 .RI * result
 267 will be NULL on error or point to the result on success.
 268 Auxiliary data is stored in the buffer
 269 .I buf
 270 of length
 271 .IR buflen .
 272 (If the buffer is too small, these functions will return
 273 .BR ERANGE .)
 274 No global variable
 275 .I h_errno
 276 is modified, but the address of a variable in which to store error numbers
 277 is passed in
 278 .IR h_errnop .
 279 .SH NOTES
 280 The functions
 281 .BR gethostbyname ()
 282 and
 283 .BR gethostbyaddr ()
 284 may return pointers to static data, which may be overwritten by
 285 later calls. Copying the
 286 .I struct hostent
 287 does not suffice, since it contains pointers - a deep copy is required.
 288 .LP
 289 The SUS-v2 standard is buggy and declares the
 290 .I len
 291 parameter of
 292 .B gethostbyaddr()
 293 to be of type
 294 .IR size_t .
 295 (That is wrong, because it has to be
 296 .IR int ,
 297 and
 298 .I size_t
 299 is not. POSIX 1003.1-2001 makes it
 300 .IR socklen_t ,
 301 which is OK.)
 302 .LP
 303 The BSD prototype for
 304 .B gethostbyaddr()
 305 uses
 306 .I const char *
 307 for the first argument.
 308 .LP
 309 POSIX 1003.1-2001 marks
 310 .B gethostbyaddr()
 311 and
 312 .B gethostbyname()
 313 obsolescent. See
 314 .BR getaddrinfo (3),
 315 .BR getnameinfo (3),
 316 .BR gai_strerror (3).
 317 .SH "SEE ALSO"
 318 .BR getaddrinfo (3),
 319 .BR getipnodebyaddr (3),
 320 .BR getipnodebyname (3),
 321 .BR getnameinfo (3),
 322 .BR inet_ntop (3),
 323 .BR inet_pton (3),
 324 .BR resolver (3),
 325 .BR hosts (5),
 326 .BR hostname (7),
 327 .BR named (8),
 328 .BR resolv+ (8)