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
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
The main_server has no default
ServerName is deduced from the server's IP
Port numbers specified in the
VirtualHost directive do
not influence what port numbers Apache will listen on, they only discriminate between
VirtualHost will be selected to handle a request.
Each address appearing in the
directive can have an optional port. If the port is unspecified
it is treated as a wildcard port. The special port
indicates a wildcard that matches any port. Collectively the
entire set of addresses (including multiple
record results from DNS lookups) are called the vhost's
directive is used for the exact IP address and port pair in the
VirtualHost directive, Apache selects the best match
only on the basis of the IP address (or wildcard) and port number.
If there are multiple identical best matches, the first
appearing in the configuration file will be selected.
If you want Apache to further discriminate on the basis of the
Host header supplied by the client, the
NameVirtualHost directive must appear
with the exact IP address (or wildcard) and port pair used in a corresponding
The name-based virtual host selection occurs only after a single IP-based virtual host has been selected, and only considers the set of virtual hosts that carry an identical IP address and port pair.
Hostnames can be used in place of IP addresses in a virtual host definition, but it is resolved at startup and is not recommended.
NameVirtualHost directives can be used
each with a set of
VirtualHost directives but only
NameVirtualHost directive should be used for
each specific IP:port pair.
The ordering of
VirtualHost directives is not important which
makes the following two examples identical (only the order of
VirtualHost directives for one
address set is important, see below):
(To aid the readability of your configuration you should prefer the left variant.)
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
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:
- If a vhost has no
SendBufferSizedirective 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.)
- 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.
- 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
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
wildcard is given the same
ServerName as the
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
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
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
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
ServerAlias is taken
and the request is served from that vhost. A
header field can contain a port number, but Apache always
matches against the real port to which the client sent the
The complete list of names in the
directive are treated just like a (non wildcard)
(but are not overridden by any
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).
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.
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.
- 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
ServerPathchecks are never performed for an IP-based vhost.
- The order of name-/IP-based, the
_default_vhost and the
NameVirtualHostdirective 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.
- If a
ServerPathdirective exists which is a prefix of another
ServerPathdirective 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.
_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
Listenby default. A wildcard port can be specified (i.e.,
_default_:*) to catch requests to any available port. This also applies to
NameVirtualHost *vhosts. Note that this is simply an extension of the "best match" principle, as a specific and exact match is favored over a wildcard.
- 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).
_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
- You should never specify DNS names in
VirtualHostdirectives 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.
ServerNameshould 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:
- Place all main_server definitions before any
VirtualHostdefinitions. (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
VirtualHostdefinitions in your configuration to ensure better readability.
ServerPathswhich 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").