虚拟主机就是在单台机器上运行多个网站的实现。apache虚拟主机可以是IP-Based，也可以是name-based。
"IP-based", meaning that you have a different IP address for every web site.每个网站有不同的IP地址（或端口号）
"name-based", meaning that you have multiple names running on each IP address.每个IP可以有多个不同的网站。
对于用户来说，这些网站是否运行在同一个物理机器上是透明的，不清楚的。
虚拟主机的配置指令主要有：
<VirtualHost> ServerName ServerAlias ServerPath
如果希望调试虚拟主机的配置可以使用下面的方法命令来实现：
/usr/local/apache2/bin/httpd -S
这个命令将导出apache如何解析配置文件的描述。
官网示例：
场景一：Running several name-based web sites on a single IP address. 一个IP,多个网站。
# Ensure that Apache listens on port 80
Listen 80
<VirtualHost *:80>
DocumentRoot "/www/example1"
ServerName www.example.com
# Other directives here
</VirtualHost>
<VirtualHost *:80>
DocumentRoot "/www/example2"
ServerName www.example.org
# Other directives here
</VirtualHost>
场景二：Name-based hosts on more than one IP address.一个或多个IP,多个虚拟主机.
Listen 80
# This is the "main" server running on 172.20.30.40
ServerName server.example.com
DocumentRoot "/www/mainserver"
<VirtualHost 172.20.30.50>
DocumentRoot "/www/example1"
ServerName www.example.com
# Other directives here ...
</VirtualHost>
<VirtualHost 172.20.30.50>
DocumentRoot "/www/example2"
ServerName www.example.org
# Other directives here ...
</VirtualHost>
场景三：Serving the same content on different IP addresses (such as an internal and external address). 不同IP，相同网站内容
<VirtualHost 192.168.1.1 172.20.30.40>
DocumentRoot "/www/server1"
ServerName server.example.com
ServerAlias server
</VirtualHost>
场景四：Running different sites on different ports. 不同端口，不同网站。
Listen 80
Listen 8080
<VirtualHost 172.20.30.40:80>
ServerName www.example.com
DocumentRoot "/www/domain-80"
</VirtualHost>
<VirtualHost 172.20.30.40:8080>
ServerName www.example.com
DocumentRoot "/www/domain-8080"
</VirtualHost>
<VirtualHost 172.20.30.40:80>
ServerName www.example.org
DocumentRoot "/www/otherdomain-80"
</VirtualHost>
<VirtualHost 172.20.30.40:8080>
ServerName www.example.org
DocumentRoot "/www/otherdomain-8080"
</VirtualHost>
场景五：IP-based virtual hosting 基于IP的虚拟主机
Listen 80
<VirtualHost 172.20.30.40>
DocumentRoot "/www/example1"
ServerName www.example.com
</VirtualHost>
<VirtualHost 172.20.30.50>
DocumentRoot "/www/example2"
ServerName www.example.org
</VirtualHost>
场景六：Mixed port-based and IP-based virtual hosts 混合型基于端口和IP的虚拟机配置
Listen 172.20.30.40:80
Listen 172.20.30.40:8080
Listen 172.20.30.50:80
Listen 172.20.30.50:8080
<VirtualHost 172.20.30.40:80>
DocumentRoot "/www/example1-80"
ServerName www.example.com
</VirtualHost>
<VirtualHost 172.20.30.40:8080>
DocumentRoot "/www/example1-8080"
ServerName www.example.com
</VirtualHost>
<VirtualHost 172.20.30.50:80>
DocumentRoot "/www/example2-80"
ServerName www.example.org
</VirtualHost>
<VirtualHost 172.20.30.50:8080>
DocumentRoot "/www/example2-8080"
ServerName www.example.org
</VirtualHost>
场景七：Mixed name-based and IP-based vhosts 混合的基于名称和基于IP的虚拟机
Listen 80
<VirtualHost 172.20.30.40>
DocumentRoot "/www/example1"
ServerName www.example.com
</VirtualHost>
<VirtualHost 172.20.30.40>
DocumentRoot "/www/example2"
ServerName www.example.org
</VirtualHost>
<VirtualHost 172.20.30.40>
DocumentRoot "/www/example3"
ServerName www.example.net
</VirtualHost>
# IP-based
<VirtualHost 172.20.30.50>
DocumentRoot "/www/example4"
ServerName www.example.edu
</VirtualHost>
<VirtualHost 172.20.30.60>
DocumentRoot "/www/example5"
ServerName www.example.gov
</VirtualHost>
场景八：Using Virtual_host and mod_proxy together
<VirtualHost *:*>
ProxyPreserveHost On
ProxyPass "/" "http://192.168.111.2/"
ProxyPassReverse "/" "http://192.168.111.2/"
ServerName hostname.example.com
</VirtualHost>
场景九：Using _default_ vhosts
<VirtualHost _default_:*>
DocumentRoot "/www/default"
</VirtualHost>
<VirtualHost _default_:80>
DocumentRoot "/www/default80"
# ...
</VirtualHost>
<VirtualHost _default_:*>
DocumentRoot "/www/default"
# ...
</VirtualHost>
场景十：Migrating a name-based vhost to an IP-based vhost
Listen 80
ServerName www.example.com
DocumentRoot "/www/example1"
<VirtualHost 172.20.30.40 172.20.30.50>
DocumentRoot "/www/example2"
ServerName www.example.org
# ...
</VirtualHost>
<VirtualHost 172.20.30.40>
DocumentRoot "/www/example3"
ServerName www.example.net
ServerAlias *.example.net
# ...
</VirtualHost>
场景11：Using the ServerPath directive
<VirtualHost 172.20.30.40>
# primary vhost
DocumentRoot "/www/subdomain"
RewriteEngine On
RewriteRule "." "/www/subdomain/index.html"
# ...
</VirtualHost>
<VirtualHost 172.20.30.40>
DocumentRoot "/www/subdomain/sub1"
ServerName www.sub1.domain.tld
ServerPath "/sub1/"
RewriteEngine On
RewriteRule "^(/sub1/.*)" "/www/subdomain$1"
# ...
</VirtualHost>
<VirtualHost 172.20.30.40>
DocumentRoot "/www/subdomain/sub2"
ServerName www.sub2.domain.tld
ServerPath "/sub2/"
RewriteEngine On
RewriteRule "^(/sub2/.*)" "/www/subdomain$1"
# ...
</VirtualHost>