An In-Depth Discussion of Virtual Host Matching - Apache HTTP Server
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An In-Depth Discussion of Virtual Host Matching

The virtual host code was completely rewritten in Apache 1.3. 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 NameVirtualHost directive virtual host configuration should be a lot easier and safer than with versions prior to 1.3.

If you just want to make it work without understanding how, here are some examples.

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Config File Parsing

There is a main_server which consists of all the definitions appearing outside of <VirtualHost> sections. There are virtual servers, called vhosts, which are defined by <VirtualHost> sections.

The directives Listen, ServerName, ServerPath, and ServerAlias can appear anywhere within the definition of a server. However, each appearance overrides the previous appearance (within that server).

The default value of the Listen field for main_server is 80. The main_server has no default ServerPath, or ServerAlias. The default ServerName is deduced from the servers IP address.

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.

Unlike the main_server, vhost ports do not affect what ports Apache listens for connections on.

Each address appearing in the VirtualHost directive can have an optional port. If the port is unspecified it defaults to the value of the main_server's most recent Listen statement. The special port * indicates a wildcard that matches any port. Collectively the entire set of addresses (including multiple A record results from DNS lookups) are called the vhost's address set.

Unless a NameVirtualHost 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 *.

If name-based vhosts should be used a NameVirtualHost directive must 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 NameVirtualHost directive in your configuration file.

Multiple NameVirtualHost directives can be used each with a set of VirtualHost directives but only one NameVirtualHost directive should be used for each specific IP:port pair.

The ordering of NameVirtualHost and VirtualHost directives is not important which makes the following two examples identical (only the order of the VirtualHost directives for one address set is important, see below):

NameVirtualHost 111.22.33.44
<VirtualHost 111.22.33.44>
# server A
...
</VirtualHost>
<VirtualHost 111.22.33.44>
# server B
...
</VirtualHost>

NameVirtualHost 111.22.33.55
<VirtualHost 111.22.33.55>
# server C
...
</VirtualHost>
<VirtualHost 111.22.33.55>
# server D
...
</VirtualHost>

<VirtualHost 111.22.33.44>
# server A
</VirtualHost>
<VirtualHost 111.22.33.55>
# server C
...
</VirtualHost>
<VirtualHost 111.22.33.44>
# server B
...
</VirtualHost>
<VirtualHost 111.22.33.55>
# server D
...
</VirtualHost>

NameVirtualHost 111.22.33.44
NameVirtualHost 111.22.33.55

(To aid the readability of your configuration you should prefer the left variant.)

After parsing the VirtualHost directive, the vhost server is given a default Listen equal to the port assigned to the first name in its VirtualHost directive.

The complete list of names in the VirtualHost directive are treated just like a ServerAlias (but are not overridden by any ServerAlias statement) if all names resolve to the same address set. Note that subsequent Listen statements for this vhost will not affect the ports assigned in the address set.

During initialization a list for each IP address is generated and inserted into an hash table. If the IP address is used in a NameVirtualHost directive the list contains all name-based vhosts for the given IP address. If there are no vhosts defined for that address the NameVirtualHost directive is ignored and an error is logged. For an IP-based vhost the list in the hash table is empty.

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.

For every vhost various default values are set. In particular:

  1. If a vhost has no ServerAdmin, ResourceConfig, AccessConfig, Timeout, KeepAliveTimeout, KeepAlive, MaxKeepAliveRequests, or SendBufferSize 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.)
  2. 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.
  3. The per-server configs for each module from the main_server are merged into the vhost server.

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.

If the main_server has no ServerName at this point, then the hostname of the machine that httpd is running on is used instead. We will call the main_server address set those IP addresses returned by a DNS lookup on the ServerName of the main_server.

For any undefined ServerName fields, a name-based vhost defaults to the address given first in the VirtualHost statement defining the vhost.

Any vhost that includes the magic _default_ wildcard is given the same ServerName as the main_server.

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Virtual Host Matching

The server determines which vhost to use for a request as follows:

Hash table lookup

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.

If the lookup fails (the IP address wasn't found) the request is served from the _default_ vhost if there is such a vhost for the port to which the client sent the request. If there is no matching _default_ vhost the request is served from the main_server.

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 NameVirtualHost *, which is subsequently handled like other name-based vhosts.

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.

IP-based vhost

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.

Name-based vhost

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 VirtualHost directives appear in the config file.

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 Host: header field.

If the client provided a Host: header field the list is searched for a matching vhost and the first hit on a ServerName or ServerAlias is taken and the request is served from that vhost. A Host: header field can contain a port number, but Apache always matches against the real port to which the client sent the request.

If the client submitted a HTTP/1.0 request without Host: header field we don't know to what server the client tried to connect and any existing ServerPath 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.

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).

Persistent connections

The IP lookup described above is only done once for a particular TCP/IP session while the name lookup is done on every request during a KeepAlive/persistent connection. In other words a client may request pages from different name-based vhosts during a single persistent connection.

Absolute URI

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 and 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.

Observations

  • 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 NameVirtualHost directive.
  • ServerAlias and ServerPath checks are never performed for an IP-based vhost.
  • The order of name-/IP-based, the _default_ vhost and the NameVirtualHost 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.
  • For security reasons the port number given in a Host: header field is never used during the matching process. Apache always uses the real port to which the client sent the request.
  • If a ServerPath directive exists which is a prefix of another ServerPath 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 field was available to disambiguate the two.)
  • 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.
  • A _default_ vhost catches a request only if there is no other vhost with a matching IP address and 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 _default_ vhost which is your standard Listen by default. A wildcard port can be specified (i.e., _default_:*) to catch requests to any available port. This also applies to NameVirtualHost * vhosts.
  • 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 _default_ vhost). In other words the main_server only catches a request for an unspecified address/port combination (unless there is a _default_ vhost which matches that port).
  • A _default_ vhost or the main_server is never matched for a request with an unknown or missing Host: header field if the client connected to an address (and port) which is used for name-based vhosts, e.g., in a NameVirtualHost directive.
  • You should never specify DNS names in VirtualHost 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 more information available on this and the next two topics.
  • ServerName should always be set for each vhost. Otherwise A DNS lookup is required for each vhost.
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Tips

In addition to the tips on the DNS Issues page, here are some further tips:

  • Place all main_server definitions before any VirtualHost 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.)
  • Group corresponding NameVirtualHost and VirtualHost definitions in your configuration to ensure better readability.
  • Avoid ServerPaths which are prefixes of other ServerPaths. 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 (i.e., "ServerPath /abc" should appear after "ServerPath /abc/def").
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