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| <h1 align="CENTER">An In-Depth Discussion of Virtual Host |
| Matching</h1> |
| |
| <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 <a |
| href="../mod/core.html#namevirtualhost"><samp>NameVirtualHost</samp></a> |
| 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> |
| |
| <h3>Config File Parsing</h3> |
| |
| <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 <a |
| href="../mod/core.html#virtualhost"><samp><VirtualHost></samp></a> |
| sections.</p> |
| |
| <p>The directives <a |
| href="../mod/mpm_common.html#listen"><samp>Listen</samp></a>, <a |
| href="../mod/core.html#servername"><samp>ServerName</samp></a>, |
| <a |
| href="../mod/core.html#serverpath"><samp>ServerPath</samp></a>, |
| and <a |
| href="../mod/core.html#serveralias"><samp>ServerAlias</samp></a> |
| 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 servers 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 <samp>*</samp> |
| indicates a wildcard that matches any port. Collectively the |
| entire set of addresses (including multiple <samp>A</samp> |
| record results from DNS lookups) are called the vhost's |
| <em>address set</em>.</p> |
| |
| <p>Unless a <a |
| href="../mod/core.html#namevirtualhost">NameVirtualHost</a> |
| 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> |
| <pre> |
| | |
| NameVirtualHost 111.22.33.44 | <VirtualHost 111.22.33.44> |
| <VirtualHost 111.22.33.44> | # server A |
| # server A | </VirtualHost> |
| ... | <VirtualHost 111.22.33.55> |
| </VirtualHost> | # server C |
| <VirtualHost 111.22.33.44> | ... |
| # server B | </VirtualHost> |
| ... | <VirtualHost 111.22.33.44> |
| </VirtualHost> | # server B |
| | ... |
| NameVirtualHost 111.22.33.55 | </VirtualHost> |
| <VirtualHost 111.22.33.55> | <VirtualHost 111.22.33.55> |
| # server C | # server D |
| ... | ... |
| </VirtualHost> | </VirtualHost> |
| <VirtualHost 111.22.33.55> | |
| # server D | NameVirtualHost 111.22.33.44 |
| ... | NameVirtualHost 111.22.33.55 |
| </VirtualHost> | |
| | |
| </pre> |
| |
| <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 <a |
| href="../mod/core.html#serveradmin"><code>ServerAdmin</code></a>, |
| <a |
| href="../mod/core.html#resourceconfig"><code>ResourceConfig</code></a>, |
| <a |
| href="../mod/core.html#accessconfig"><code>AccessConfig</code></a>, |
| <a href="../mod/core.html#timeout"><code>Timeout</code></a>, |
| <a |
| href="../mod/core.html#keepalivetimeout"><code>KeepAliveTimeout</code></a>, |
| <a |
| href="../mod/core.html#keepalive"><code>KeepAlive</code></a>, |
| <a |
| href="../mod/core.html#maxkeepaliverequests"><code>MaxKeepAliveRequests</code></a>, |
| or <a |
| href="../mod/core.html#sendbuffersize"><code>SendBufferSize</code></a> |
| 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> |
| 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>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 <samp>_default_</samp> |
| wildcard is given the same <code>ServerName</code> as the |
| main_server.</p> |
| |
| <h3>Virtual Host Matching</h3> |
| |
| <p>The server determines which vhost to use for a request as |
| follows:</p> |
| |
| <h4>Hash table lookup</h4> |
| |
| <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 <samp>_default_</samp> vhost if |
| there is such a vhost for the port to which the client sent the |
| request. If there is no matching <samp>_default_</samp> 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> |
| |
| <h4>IP-based vhost</h4> |
| |
| <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> |
| |
| <h4>Name-based vhost</h4> |
| |
| <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> |
| |
| <h4>Persistent connections</h4> |
| 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. |
| |
| <h4>Absolute URI</h4> |
| |
| <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>Observations</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 <samp>_default_</samp> |
| 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> |
| |
| <h3>Tips</h3> |
| |
| <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> |
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