1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
|
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en"><head><!--
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
This file is generated from xml source: DO NOT EDIT
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
-->
<title>An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server</title>
<link href="../style/css/manual.css" rel="stylesheet" media="all" type="text/css" title="Main stylesheet" />
<link href="../style/css/manual-loose-100pc.css" rel="alternate stylesheet" media="all" type="text/css" title="No Sidebar - Default font size" />
<link href="../style/css/manual-print.css" rel="stylesheet" media="print" type="text/css" />
<link href="../images/favicon.ico" rel="shortcut icon" /></head>
<body id="manual-page"><div id="page-header">
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p>
<p class="apache">Apache HTTP Server Version 2.1</p>
<img alt="" src="../images/feather.gif" /></div>
<div class="up"><a href="./"><img title="<-" alt="<-" src="../images/left.gif" /></a></div>
<div id="path">
<a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="../">Version 2.1</a> > <a href="./">Virtual Hosts</a></div><div id="page-content"><div id="preamble"><h1>An In-Depth Discussion of Virtual Host Matching</h1>
<div class="toplang">
<p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English"> en </a> |
<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
</div>
<p>The virtual host code was completely rewritten in
<strong>Apache 1.3</strong>. This document attempts to explain
exactly what Apache does when deciding what virtual host to
serve a hit from. With the help of the new
<code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
directive virtual host configuration should be a lot easier and
safer than with versions prior to 1.3.</p>
<p>If you just want to <cite>make it work</cite> without
understanding how, here are <a href="examples.html">some
examples</a>.</p>
</div>
<div id="quickview"><ul id="toc"><li><img alt="" src="../images/down.gif" /> <a href="#configparsing">Config File Parsing</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#hostmatching">Virtual Host Matching</a></li>
<li><img alt="" src="../images/down.gif" /> <a href="#tips">Tips</a></li>
</ul></div>
<div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="configparsing" id="configparsing">Config File Parsing</a></h2>
<p>There is a <em>main_server</em> which consists of all the
definitions appearing outside of
<code><VirtualHost></code> sections. There are virtual
servers, called <em>vhosts</em>, which are defined by
<code class="directive"><a href="../mod/core.html#virtualhost"><VirtualHost></a></code>
sections.</p>
<p>The directives
<code class="directive"><a href="../mod/mpm_common.html#listen">Listen</a></code>,
<code class="directive"><a href="../mod/core.html#servername">ServerName</a></code>,
<code class="directive"><a href="../mod/core.html#serverpath">ServerPath</a></code>,
and <code class="directive"><a href="../mod/core.html#serveralias">ServerAlias</a></code>
can appear anywhere within the definition of a server. However,
each appearance overrides the previous appearance (within that
server).</p>
<p>The default value of the <code>Listen</code> field for
main_server is 80. The main_server has no default
<code>ServerPath</code>, or <code>ServerAlias</code>. The
default <code>ServerName</code> is deduced from the server's IP
address.</p>
<p>The main_server Listen directive has two functions. One
function is to determine the default network port Apache will
bind to. The second function is to specify the port number
which is used in absolute URIs during redirects.</p>
<p>Unlike the main_server, vhost ports <em>do not</em> affect
what ports Apache listens for connections on.</p>
<p>Each address appearing in the <code>VirtualHost</code>
directive can have an optional port. If the port is unspecified
it defaults to the value of the main_server's most recent
<code>Listen</code> statement. The special port <code>*</code>
indicates a wildcard that matches any port. Collectively the
entire set of addresses (including multiple <code>A</code>
record results from DNS lookups) are called the vhost's
<em>address set</em>.</p>
<p>Unless a <code class="directive"><a href="../mod/core.html#namevirtualhost">NameVirtualHost</a></code>
directive is used for a specific IP address the first vhost
with that address is treated as an IP-based vhost. The IP
address can also be the wildcard <code>*</code>.</p>
<p>If name-based vhosts should be used a
<code>NameVirtualHost</code> directive <em>must</em> appear
with the IP address set to be used for the name-based vhosts.
In other words, you must specify the IP address that holds the
hostname aliases (CNAMEs) for your name-based vhosts via a
<code>NameVirtualHost</code> directive in your configuration
file.</p>
<p>Multiple <code>NameVirtualHost</code> directives can be used
each with a set of <code>VirtualHost</code> directives but only
one <code>NameVirtualHost</code> directive should be used for
each specific IP:port pair.</p>
<p>The ordering of <code>NameVirtualHost</code> and
<code>VirtualHost</code> directives is not important which
makes the following two examples identical (only the order of
the <code>VirtualHost</code> directives for <em>one</em>
address set is important, see below):</p>
<table><tr>
<td><div class="example"><p><code>
NameVirtualHost 111.22.33.44<br />
<VirtualHost 111.22.33.44><br />
# server A<br />
...<br />
</VirtualHost><br />
<VirtualHost 111.22.33.44><br />
# server B<br />
...<br />
</VirtualHost><br />
<br />
NameVirtualHost 111.22.33.55<br />
<VirtualHost 111.22.33.55><br />
# server C<br />
...<br />
</VirtualHost><br />
<VirtualHost 111.22.33.55><br />
# server D<br />
...<br />
</VirtualHost>
</code></p></div></td>
<td><div class="example"><p><code>
<VirtualHost 111.22.33.44><br />
# server A<br />
</VirtualHost><br />
<VirtualHost 111.22.33.55><br />
# server C<br />
...<br />
</VirtualHost><br />
<VirtualHost 111.22.33.44><br />
# server B<br />
...<br />
</VirtualHost><br />
<VirtualHost 111.22.33.55><br />
# server D<br />
...<br />
</VirtualHost><br />
<br />
NameVirtualHost 111.22.33.44<br />
NameVirtualHost 111.22.33.55<br />
<br />
</code></p></div></td>
</tr></table>
<p>(To aid the readability of your configuration you should
prefer the left variant.)</p>
<p>After parsing the <code>VirtualHost</code> directive, the
vhost server is given a default <code>Listen</code> equal to the
port assigned to the first name in its <code>VirtualHost</code>
directive.</p>
<p>The complete list of names in the <code>VirtualHost</code>
directive are treated just like a <code>ServerAlias</code> (but
are not overridden by any <code>ServerAlias</code> statement)
if all names resolve to the same address set. Note that
subsequent <code>Listen</code> statements for this vhost will not
affect the ports assigned in the address set.</p>
<p>During initialization a list for each IP address is
generated and inserted into an hash table. If the IP address is
used in a <code>NameVirtualHost</code> directive the list
contains all name-based vhosts for the given IP address. If
there are no vhosts defined for that address the
<code>NameVirtualHost</code> directive is ignored and an error
is logged. For an IP-based vhost the list in the hash table is
empty.</p>
<p>Due to a fast hashing function the overhead of hashing an IP
address during a request is minimal and almost not existent.
Additionally the table is optimized for IP addresses which vary
in the last octet.</p>
<p>For every vhost various default values are set. In
particular:</p>
<ol>
<li>If a vhost has no <code class="directive"><a href="../mod/core.html#serveradmin">ServerAdmin</a></code>,
<code class="directive"><a href="../mod/core.html#resourceconfig">ResourceConfig</a></code>,
<code class="directive"><a href="../mod/core.html#accessconfig">AccessConfig</a></code>,
<code class="directive"><a href="../mod/core.html#timeout">Timeout</a></code>,
<code class="directive"><a href="../mod/core.html#keepalivetimeout">KeepAliveTimeout</a></code>,
<code class="directive"><a href="../mod/core.html#keepalive">KeepAlive</a></code>,
<code class="directive"><a href="../mod/core.html#maxkeepaliverequests">MaxKeepAliveRequests</a></code>,
or <code class="directive"><a href="../mod/core.html#sendbuffersize">SendBufferSize</a></code>
directive then the respective value is inherited from the
main_server. (That is, inherited from whatever the final
setting of that value is in the main_server.)</li>
<li>The "lookup defaults" that define the default directory
permissions for a vhost are merged with those of the
main_server. This includes any per-directory configuration
information for any module.</li>
<li>The per-server configs for each module from the
main_server are merged into the vhost server.</li>
</ol>
<p>Essentially, the main_server is treated as "defaults" or a
"base" on which to build each vhost. But the positioning of
these main_server definitions in the config file is largely
irrelevant -- the entire config of the main_server has been
parsed when this final merging occurs. So even if a main_server
definition appears after a vhost definition it might affect the
vhost definition.</p>
<p>If the main_server has no <code>ServerName</code> at this
point, then the hostname of the machine that httpd is running
on is used instead. We will call the <em>main_server address
set</em> those IP addresses returned by a DNS lookup on the
<code>ServerName</code> of the main_server.</p>
<p>For any undefined <code>ServerName</code> fields, a
name-based vhost defaults to the address given first in the
<code>VirtualHost</code> statement defining the vhost.</p>
<p>Any vhost that includes the magic <code>_default_</code>
wildcard is given the same <code>ServerName</code> as the
main_server.</p>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="hostmatching" id="hostmatching">Virtual Host Matching</a></h2>
<p>The server determines which vhost to use for a request as
follows:</p>
<h3><a name="hashtable" id="hashtable">Hash table lookup</a></h3>
<p>When the connection is first made by a client, the IP
address to which the client connected is looked up in the
internal IP hash table.</p>
<p>If the lookup fails (the IP address wasn't found) the
request is served from the <code>_default_</code> vhost if
there is such a vhost for the port to which the client sent the
request. If there is no matching <code>_default_</code> vhost
the request is served from the main_server.</p>
<p>If the IP address is not found in the hash table then the
match against the port number may also result in an entry
corresponding to a <code>NameVirtualHost *</code>, which is
subsequently handled like other name-based vhosts.</p>
<p>If the lookup succeeded (a corresponding list for the IP
address was found) the next step is to decide if we have to
deal with an IP-based or a name-base vhost.</p>
<h3><a name="ipbased" id="ipbased">IP-based vhost</a></h3>
<p>If the entry we found has an empty name list then we have
found an IP-based vhost, no further actions are performed and
the request is served from that vhost.</p>
<h3><a name="namebased" id="namebased">Name-based vhost</a></h3>
<p>If the entry corresponds to a name-based vhost the name list
contains one or more vhost structures. This list contains the
vhosts in the same order as the <code>VirtualHost</code>
directives appear in the config file.</p>
<p>The first vhost on this list (the first vhost in the config
file with the specified IP address) has the highest priority
and catches any request to an unknown server name or a request
without a <code>Host:</code> header field.</p>
<p>If the client provided a <code>Host:</code> header field the
list is searched for a matching vhost and the first hit on a
<code>ServerName</code> or <code>ServerAlias</code> is taken
and the request is served from that vhost. A <code>Host:</code>
header field can contain a port number, but Apache always
matches against the real port to which the client sent the
request.</p>
<p>If the client submitted a HTTP/1.0 request without
<code>Host:</code> header field we don't know to what server
the client tried to connect and any existing
<code>ServerPath</code> is matched against the URI from the
request. The first matching path on the list is used and the
request is served from that vhost.</p>
<p>If no matching vhost could be found the request is served
from the first vhost with a matching port number that is on the
list for the IP to which the client connected (as already
mentioned before).</p>
<h3><a name="persistent" id="persistent">Persistent connections</a></h3>
<p>The IP lookup described above is only done <em>once</em> for a
particular TCP/IP session while the name lookup is done on
<em>every</em> request during a KeepAlive/persistent
connection. In other words a client may request pages from
different name-based vhosts during a single persistent
connection.</p>
<h3><a name="absoluteURI" id="absoluteURI">Absolute URI</a></h3>
<p>If the URI from the request is an absolute URI, and its
hostname and port match the main server or one of the
configured virtual hosts <em>and</em> match the address and
port to which the client sent the request, then the
scheme/hostname/port prefix is stripped off and the remaining
relative URI is served by the corresponding main server or
virtual host. If it does not match, then the URI remains
untouched and the request is taken to be a proxy request.</p>
<h3><a name="observations" id="observations">Observations</a></h3>
<ul>
<li>A name-based vhost can never interfere with an IP-base
vhost and vice versa. IP-based vhosts can only be reached
through an IP address of its own address set and never
through any other address. The same applies to name-based
vhosts, they can only be reached through an IP address of the
corresponding address set which must be defined with a
<code>NameVirtualHost</code> directive.</li>
<li><code>ServerAlias</code> and <code>ServerPath</code>
checks are never performed for an IP-based vhost.</li>
<li>The order of name-/IP-based, the <code>_default_</code>
vhost and the <code>NameVirtualHost</code> directive within
the config file is not important. Only the ordering of
name-based vhosts for a specific address set is significant.
The one name-based vhosts that comes first in the
configuration file has the highest priority for its
corresponding address set.</li>
<li>For security reasons the port number given in a
<code>Host:</code> header field is never used during the
matching process. Apache always uses the real port to which
the client sent the request.</li>
<li>If a <code>ServerPath</code> directive exists which is a
prefix of another <code>ServerPath</code> directive that
appears later in the configuration file, then the former will
always be matched and the latter will never be matched. (That
is assuming that no <code>Host:</code> header field was
available to disambiguate the two.)</li>
<li>If two IP-based vhosts have an address in common, the
vhost appearing first in the config file is always matched.
Such a thing might happen inadvertently. The server will give
a warning in the error logfile when it detects this.</li>
<li>A <code>_default_</code> vhost catches a request only if
there is no other vhost with a matching IP address
<em>and</em> a matching port number for the request. The
request is only caught if the port number to which the client
sent the request matches the port number of your
<code>_default_</code> vhost which is your standard
<code>Listen</code> by default. A wildcard port can be
specified (<em>i.e.</em>, <code>_default_:*</code>) to catch
requests to any available port. This also applies to
<code>NameVirtualHost *</code> vhosts.</li>
<li>The main_server is only used to serve a request if the IP
address and port number to which the client connected is
unspecified and does not match any other vhost (including a
<code>_default_</code> vhost). In other words the main_server
only catches a request for an unspecified address/port
combination (unless there is a <code>_default_</code> vhost
which matches that port).</li>
<li>A <code>_default_</code> vhost or the main_server is
<em>never</em> matched for a request with an unknown or
missing <code>Host:</code> header field if the client
connected to an address (and port) which is used for
name-based vhosts, <em>e.g.</em>, in a
<code>NameVirtualHost</code> directive.</li>
<li>You should never specify DNS names in
<code>VirtualHost</code> directives because it will force
your server to rely on DNS to boot. Furthermore it poses a
security threat if you do not control the DNS for all the
domains listed. There's <a href="../dns-caveats.html">more
information</a> available on this and the next two
topics.</li>
<li><code>ServerName</code> should always be set for each
vhost. Otherwise A DNS lookup is required for each
vhost.</li>
</ul>
</div><div class="top"><a href="#page-header"><img alt="top" src="../images/up.gif" /></a></div>
<div class="section">
<h2><a name="tips" id="tips">Tips</a></h2>
<p>In addition to the tips on the <a href="../dns-caveats.html#tips">DNS Issues</a> page, here are
some further tips:</p>
<ul>
<li>Place all main_server definitions before any
<code>VirtualHost</code> definitions. (This is to aid the
readability of the configuration -- the post-config merging
process makes it non-obvious that definitions mixed in around
virtual hosts might affect all virtual hosts.)</li>
<li>Group corresponding <code>NameVirtualHost</code> and
<code>VirtualHost</code> definitions in your configuration to
ensure better readability.</li>
<li>Avoid <code>ServerPaths</code> which are prefixes of
other <code>ServerPaths</code>. If you cannot avoid this then
you have to ensure that the longer (more specific) prefix
vhost appears earlier in the configuration file than the
shorter (less specific) prefix (<em>i.e.</em>, "ServerPath
/abc" should appear after "ServerPath /abc/def").</li>
</ul>
</div></div>
<div class="bottomlang">
<p><span>Available Languages: </span><a href="../en/vhosts/details.html" title="English"> en </a> |
<a href="../ko/vhosts/details.html" hreflang="ko" rel="alternate" title="Korean"> ko </a></p>
</div><div id="footer">
<p class="apache">Copyright 1999-2004 The Apache Software Foundation.<br />Licensed under the <a href="http://www.apache.org/licenses/LICENSE-2.0">Apache License, Version 2.0</a>.</p>
<p class="menu"><a href="../mod/">Modules</a> | <a href="../mod/directives.html">Directives</a> | <a href="../faq/">FAQ</a> | <a href="../glossary.html">Glossary</a> | <a href="../sitemap.html">Sitemap</a></p></div>
</body></html>
|
