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.
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 Port, 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 Port
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 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
Listen
directives.
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 Port
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> <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 the left variant.)
After parsing the VirtualHost
directive, the vhost server
is given a default Port
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 Port
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:
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.)
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.
The server determines which vhost to use for a request as follows:
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.
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.
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).
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.
NameVirtualHost
directive.
ServerAlias
and ServerPath
checks are never
performed for an IP-based vhost.
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.
Host:
header field is never used during the matching process. Apache always
uses the real port to which the client sent the request.
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.)
_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.
This also applies to NameVirtualHost *
vhosts.
_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).
_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.
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.
In addition to the tips on the DNS Issues page, here are some further tips:
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.)
NameVirtualHost
and
VirtualHost
definitions in your configuration to ensure
better readability.
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").