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1 .\" $NetBSD: gethostbyname.3,v 1.29 2013/08/19 07:18:42 christos Exp $
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30 .\" @(#)gethostbyname.3 8.4 (Berkeley) 5/25/95
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32 .Dd August 19, 2013
33 .Dt GETHOSTBYNAME 3
34 .Os
35 .Sh NAME
36 .Nm gethostbyname ,
37 .Nm gethostbyname2 ,
38 .Nm gethostbyaddr ,
39 .Nm gethostent ,
40 .Nm sethostent ,
41 .Nm endhostent ,
42 .Nm herror ,
43 .Nm hstrerror
44 .Nd get network host entry
45 .Sh LIBRARY
46 .Lb libc
47 .Sh SYNOPSIS
48 .In netdb.h
49 .Fd extern int h_errno;
50 .Ft struct hostent *
51 .Fn gethostbyname "const char *name"
52 .Ft struct hostent *
53 .Fn gethostbyname2 "const char *name" "int af"
54 .Ft struct hostent *
55 .Fn gethostbyaddr "const void *addr" "socklen_t len" "int type"
56 .Ft struct hostent *
57 .Fn gethostent void
58 .Ft void
59 .Fn sethostent "int stayopen"
60 .Ft void
61 .Fn endhostent void
62 .Ft void
63 .Fn herror "const char *string"
64 .Ft const char *
65 .Fn hstrerror "int err"
66 .Sh DESCRIPTION
67 The
68 .Fn gethostbyname ,
69 .Fn gethostbyname2
70 and
71 .Fn gethostbyaddr
72 functions
73 each return a pointer to an object with the
74 following structure describing an internet host.
75 .Bd -literal
76 struct hostent {
77 char *h_name; /* official name of host */
78 char **h_aliases; /* alias list */
79 int h_addrtype; /* host address type */
80 int h_length; /* length of address */
81 char **h_addr_list; /* list of addresses from name server */
82 };
83 #define h_addr h_addr_list[0] /* address, for backward compatibility */
84 .Ed
85 .Pp
86 The members of this structure are:
87 .Bl -tag -width h_addr_list
88 .It Fa h_name
89 Official name of the host.
90 .It Fa h_aliases
91 A NULL-terminated array of alternative names for the host.
92 .It Fa h_addrtype
93 The type of address being returned; currently always
94 .Dv AF_INET .
95 .It Fa h_length
96 The length, in bytes, of the address.
97 .It Fa h_addr_list
98 A NULL-terminated array of network addresses for the host.
99 Host addresses are returned in network byte order.
100 .It Fa h_addr
101 The first address in
102 .Fa h_addr_list ;
103 this is for backward compatibility.
104 .El
105 .Pp
106 In the case of
107 .Fn gethostbyname
108 and
109 .Fn gethostbyname2 ,
110 the host is specified by name,
111 or using a string representation of a numeric address.
112 In the case of
113 .Fn gethostbyaddr ,
114 the host is specified using a binary representation of an address.
115 .Pp
116 The returned
117 .Ft "struct hostent"
118 structure may contain
119 the result of a simple string to binary conversion,
120 information obtained from the domain name resolver (see
121 .Xr resolver 3 ) ,
122 broken-out fields from a line in
123 .Pa /etc/hosts ,
124 or database entries supplied by the
125 .Xr yp 8
126 system.
127 The order of the lookups is controlled by the
128 .Sq hosts
129 entry in
130 .Xr nsswitch.conf 5 .
131 .Pp
132 When using the domain name resolver,
133 .Fn gethostbyname
134 and
135 .Fn gethostbyname2
136 will search for the named host in the current domain and its parents
137 unless the name ends in a dot.
138 If the name contains no dot, and if the environment variable
139 .Dq Ev HOSTALIASES
140 contains the name of an alias file, the alias file will first be searched
141 for an alias matching the input name.
142 See
143 .Xr hostname 7
144 for the domain search procedure and the alias file format.
145 .Pp
146 The
147 .Fn gethostbyname2
148 function is an evolution of
149 .Fn gethostbyname
150 which is intended to allow lookups in address families other than
151 .Dv AF_INET ,
152 for example
153 .Dv AF_INET6 .
154 Currently the
155 .Fa af
156 argument must be specified as
157 .Dv AF_INET
158 or
159 .Dv AF_INET6 ,
160 else the function will return
161 .Dv NULL
162 after having set
163 .Va h_errno
164 to
165 .Dv NETDB_INTERNAL .
166 .Pp
167 The
168 .Fn gethostent
169 function
170 reads the next line of the
171 .Pa /etc/hosts
172 file, opening the file if necessary.
173 .Pp
174 The
175 .Fn sethostent
176 function
177 may be used to request the use of a connected
178 .Tn TCP
179 socket for queries.
180 If the
181 .Fa stayopen
182 flag is non-zero,
183 this sets the option to send all queries to the name server using
184 .Tn TCP
185 and to retain the connection after each call to
186 .Fn gethostbyname ,
187 .Fn gethostbyname2 ,
188 or
189 .Fn gethostbyaddr .
190 Otherwise, queries are performed using
191 .Tn UDP
192 datagrams.
193 .Pp
194 The
195 .Fn endhostent
196 function
197 closes the
198 .Tn TCP
199 connection.
200 .Pp
201 The
202 .Fn herror
203 function writes a message to the diagnostic output consisting of the
204 string parameter
205 .Fa s ,
206 the constant string ": ", and a message corresponding to the value of
207 .Va h_errno .
208 .Pp
209 The
210 .Fn hstrerror
211 function returns a string which is the message text corresponding to the
212 value of the
213 .Fa err
214 parameter.
215 .Sh FILES
216 .Bl -tag -width /etc/hosts -compact
217 .It Pa /etc/hosts
218 .El
219 .Sh DIAGNOSTICS
220 Error return status from
221 .Fn gethostbyent ,
222 .Fn gethostbyname ,
223 .Fn gethostbyname2 ,
224 and
225 .Fn gethostbyaddr
226 is indicated by return of a null pointer.
227 The external integer
228 .Va h_errno
229 may then be checked to see whether this is a temporary failure
230 or an invalid or unknown host.
231 The routine
232 .Fn herror
233 can be used to print an error message describing the failure.
234 If its argument
235 .Fa string
236 is
237 .Pf non Dv -NULL ,
238 it is printed, followed by a colon and a space.
239 The error message is printed with a trailing newline.
240 .Pp
241 The variable
242 .Va h_errno
243 can have the following values:
244 .Bl -tag -width HOST_NOT_FOUND
245 .It Dv HOST_NOT_FOUND
246 No such host is known.
247 .It Dv TRY_AGAIN
248 This is usually a temporary error
249 and means that the local server did not receive
250 a response from an authoritative server.
251 A retry at some later time may succeed.
252 .It Dv NO_RECOVERY
253 Some unexpected server failure was encountered.
254 This is a non-recoverable error.
255 .It Dv NO_DATA
256 The requested name is valid but does not have an IP address;
257 this is not a temporary error.
258 This means that the name is known to the name server but there is no address
259 associated with this name.
260 Another type of request to the name server using this domain name
261 will result in an answer;
262 for example, a mail-forwarder may be registered for this domain.
263 .El
264 .Sh SEE ALSO
265 .Xr resolver 3 ,
266 .Xr hosts 5 ,
267 .Xr nsswitch.conf 5 ,
268 .Xr hostname 7 ,
269 .Xr named 8
270 .Sh HISTORY
271 The
272 .Fn herror
273 function appeared in
274 .Bx 4.3 .
275 The
276 .Fn endhostent ,
277 .Fn gethostbyaddr ,
278 .Fn gethostbyname ,
279 .Fn gethostent ,
280 and
281 .Fn sethostent
282 functions appeared in
283 .Bx 4.2 .
284 The
285 .Fn gethostbyname2
286 function first appeared in bind-4.9.4.
287 IPv6 support was implemented in WIDE Hydrangea IPv6 protocol stack kit.
288 .Sh CAVEATS
289 If the search routines specified in
290 .Xr nsswitch.conf 5
291 decide to read the
292 .Pa /etc/hosts
293 file,
294 .Fn gethostbyname ,
295 .Fn gethostbyname2 ,
296 and
297 .Fn gethostbyaddr
298 will
299 read the next line of the file,
300 re-opening the file if necessary.
301 .Pp
302 The
303 .Fn sethostent
304 function
305 opens and/or rewinds the file
306 .Pa /etc/hosts .
307 If the
308 .Fa stayopen
309 argument is non-zero,
310 the file will not be closed after each call to
311 .Fn gethostbyname ,
312 .Fn gethostbyname2 ,
313 .Fn gethostbyaddr ,
314 or
315 .Fn gethostent .
316 .Pp
317 The
318 .Fn endhostent
319 function closes the file.
320 .Sh BUGS
321 These functions use static data storage;
322 if the data is needed for future use, it should be
323 copied before any subsequent calls overwrite it.
324 Only the Internet
325 address format is currently understood.
326 .Pp
327 The
328 .Fn gethostent
329 does not currently follow the search order specified in
330 .Xr nsswitch.conf 5
331 and only reads the
332 .Pa /etc/hosts
333 file.