Apache HTTP Server Version 1.3

An In-Depth Discussion of Virtual Host Matching

The virtual host code was completely rewritten inApache 1.3.This document attempts to explain exactly what Apache does whendeciding what virtual host to serve a hit from. With the help of thenew NameVirtualHostdirective virtual host configuration should be a lot easier and saferthan with versions prior to 1.3.

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

Config File Parsing

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

The directivesPort,ServerName,ServerPath,andServerAliascan appear anywhere within the definition ofa server. However, each appearance overrides the previous appearance(within that server).

The default value of the Port field for main_serveris 80. The main_server has no default ServerPath, orServerAlias. The default ServerName isdeduced from the servers IP address.

The main_server Port directive has two functions due to legacycompatibility with NCSA configuration files. One function isto determine the default network port Apache will bind to. Thisdefault is overridden by the existence of anyListen directives.The second function is to specify the port number which is usedin absolute URIs during redirects.

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

Each address appearing in the VirtualHost directivecan have an optional port. If the port is unspecified it defaults tothe value of the main_server's most recent Port statement.The special port * indicates a wildcard that matches any port.Collectively the entire set of addresses (including multipleA recordresults from DNS lookups) are called the vhost's address set.

Unless a NameVirtualHostdirective is used for a specific IP address the first vhost withthat address is treated as an IP-based vhost.

If name-based vhosts should be used a NameVirtualHostdirective must appear with the IP address set to be used for thename-based vhosts. In other words, you must specify the IP address thatholds the hostname aliases (CNAMEs) for your name-based vhosts via aNameVirtualHost directive in your configuration file.

Multiple NameVirtualHost directives can be used eachwith a set of VirtualHost directives but only oneNameVirtualHost directive should be used for eachspecific IP:port pair.

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

                                |  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>	        |                                |

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

After parsing the VirtualHost directive, the vhost serveris given a default Port equal to the port assigned to thefirst name in its VirtualHost directive.

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

During initialization a list for each IP addressis generated an inserted into an hash table. If the IP address isused in a NameVirtualHost directive the list containsall name-based vhosts for the given IP address. If there are novhosts defined for that address the NameVirtualHost directiveis ignored and an error is logged. For an IP-based vhost the list in thehash table is empty.

Due to a fast hashing function the overhead of hashing an IP addressduring a request is minimal and almost not existent. Additionallythe 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.

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

For any undefined ServerName fields, a name-based vhostdefaults to the address given first in the VirtualHoststatement defining the vhost.

Any vhost that includes the magic _default_ wildcardis given the same ServerNameaccommodation in Paris as the main_server.

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 towhich the client connected is looked up in the internal IP hash table.

If the lookup fails (the IP address wasn't found) the request isserved from the _default_ vhost if there is such a vhostfor the port to which the client sent the request. If there is nomatching _default_ vhost the request is served from themain_server.

If the lookup succeeded (a corresponding list for the IP address wasfound) the next step is to decide if we have to deal with an IP-basedor a name-base vhost.

IP-based vhost

If the entry we found has an empty name list then we have found anIP-based vhost, no further actions are performed and the request isserved from that vhost.

Name-based vhost

If the entry corresponds to a name-based vhost the name list containsone or more vhost structures. This list contains the vhosts in the sameorder as the VirtualHost directives appear in the configfile.

The first vhost on this list (the first vhost in the config file withthe specified IP address) has the highest priority and catches any requestto an unknown server name or a request without a Host:header field.

If the client provided a Host: header field the list issearched for a matching vhost and the first hit on a ServerNameor ServerAlias is taken and the request is served fromthat vhost. A Host: header field can contain a port number, butApache always matches against the real port to which the client sentthe 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 andany existing ServerPath is matched against the URIfrom the request. The first matching path on the list is used and therequest is served from that vhost.

If no matching vhost could be found the request is served from thefirst vhost with a matching port number that is on the list for the IPto which the client connected (as already mentioned before).

Persistent connections

Absolute URI

If the URI from the request is an absolute URI, and its hostname andport match the main server or one of the configured virtual hostsand match the address and port to which the client sent the request,then the scheme/hostname/port prefix is stripped off and the remainingrelative URI is served by the corresponding main server or virtual host.If it does not match, then the URI remains untouched and the request istaken 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 Port by default. A wildcard port can be specified (i.e., _default_:*) to catch requests to any available port.

• 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.

Tips

In addition to the tips on the DNSIssues 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").

Apache HTTP Server Version 1.3