| <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> |
| <html><head> |
| <title>An In-Depth Discussion of Virtual Host Matching</title> |
| </head> |
| |
| <!-- Background white, links blue (unvisited), navy (visited), red (active) --> |
| <BODY |
| BGCOLOR="#FFFFFF" |
| TEXT="#000000" |
| LINK="#0000FF" |
| VLINK="#000080" |
| ALINK="#FF0000" |
| > |
| <!--#include virtual="header.html" --> |
| <h1 ALIGN="CENTER">An In-Depth Discussion of Virtual Host Matching</h1> |
| |
| <p>The virtual host code was completely rewritten in <B>Apache 1.3</B>. |
| 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>If you just want to <cite>make it work</cite> without understanding |
| how, here are <a href="examples.html">some examples</a>. |
| |
| <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>The directives |
| <A HREF="../mod/core.html#port"><SAMP>Port</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>The default value of the <code>Port</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>The main_server Port directive has two functions due to legacy |
| compatibility with NCSA configuration files. One function is |
| to determine the default network port Apache will bind to. This |
| default is overridden by the existence of any |
| <A HREF="../mod/core.html#listen"><code>Listen</code></A> directives. |
| The second function is to specify the port number which is used |
| in absolute URIs during redirects. |
| |
| <p>Unlike the main_server, vhost ports <em>do not</em> affect what |
| ports Apache listens for connections on. |
| |
| <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>Port</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>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. |
| |
| <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>Multiple <code>NameVirtualHost</code> directives can be used each |
| with a set of <code>VirtualHost</code> directives. |
| |
| <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): |
| |
| <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> After parsing the <code>VirtualHost</code> directive, the vhost server |
| is given a default <code>Port</code> equal to the port assigned to the |
| first name in its <code>VirtualHost</code> directive. |
| |
| <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>Port</code> statements for this vhost will not affect |
| the ports assigned in the address set. |
| |
| <p>During initialization a list for each IP address |
| is generated an 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>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>For every vhost various default values are set. In particular: |
| |
| <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>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>The per-server configs for each module from the main_server are |
| merged into the vhost server. |
| </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> 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>Any vhost that includes the magic <SAMP>_default_</SAMP> wildcard |
| is given the same <code>ServerName</code> as the main_server. |
| |
| |
| <h3>Virtual Host Matching</h3> |
| |
| <p>The server determines which vhost to use for a request as follows: |
| |
| <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>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>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. |
| |
| <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. |
| |
| <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>The first vhost on this list (the first vhost that appears after the |
| corresponding <code>NameVirtualHost</code> directive in the config file) |
| has the highest priority and catches any request to an unknown |
| server name or a request without a <code>Host:</code> header. |
| |
| <p>If the client provided a <code>Host:</code> header 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 can contain a port number, but |
| Apache always matches against the real port to which the client sent |
| the request. |
| |
| <p>If the client submitted a HTTP/1.0 request without <code>Host:</code> |
| header 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>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). |
| |
| <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. |
| |
| |
| <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. |
| <p> |
| |
| <li><code>ServerAlias</code> and <code>ServerPath</code> checks are never |
| performed for an IP-based vhost. |
| <p> |
| |
| <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. |
| <p> |
| |
| <li>For security reasons the port number given in a <code>Host:</code> |
| header is never used during the matching process. Apache always |
| uses the real port to which the client sent the request. |
| <p> |
| |
| <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 |
| Host header was available to disambiguate the two.) |
| <p> |
| |
| <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. |
| <p> |
| |
| <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 catched 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>Port</code> |
| by default. A wildcard port can be specified (i.e. |
| <code>_default_:*</code>) to catch requests to any available port. |
| <p> |
| |
| <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). |
| <p> |
| |
| <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 |
| if the client connected to an address (and port) which is used |
| for name-based vhosts, e.g. in a <code>NameVirtualHost</code> directive. |
| <p> |
| |
| <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. |
| <p> |
| |
| <li><code>ServerName</code> should always be set for each vhost. Otherwise |
| A DNS lookup is required for each vhost. |
| <p> |
| |
| </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: |
| |
| <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.) |
| <p> |
| |
| <li>Group corresponding <code>NameVirtualHost</code> and |
| <code>VirtualHost</code> definitions in your configuration to ensure |
| better readability. |
| <p> |
| |
| <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"). |
| <p> |
| |
| </ul> |
| |
| <!--#include virtual="footer.html" --> |
| </BODY> |
| </HTML> |