Issues Regarding DNS and Apache HTTP Server

This page could be summarized with the statement: don't configure Apache HTTP Server in such a way that it relies on DNS resolution for parsing of the configuration files. If httpd requires DNS resolution to parse the configuration files then your server may be subject to reliability problems (ie. it might not start up), or denial and theft of service attacks (including virtual hosts able to steal hits from other virtual hosts).

A Simple Example

# This is a misconfiguration example, do not use on your server
<VirtualHost www.example.dom>
  ServerAdmin [email protected]
  DocumentRoot "/www/example"
</VirtualHost>

In order for the server to function properly, it absolutely needs to have two pieces of information about each virtual host: the ServerName and at least one IP address that the server will bind and respond to. The above example does not include the IP address, so httpd must use DNS to find the address of www.example.dom. If for some reason DNS is not available at the time your server is parsing its config file, then this virtual host will not be configured. It won't be able to respond to any hits to this virtual host.

Suppose that www.example.dom has address 192.0.2.1. Then consider this configuration snippet:

# This is a misconfiguration example, do not use on your server
<VirtualHost 192.0.2.1>
  ServerAdmin [email protected]
  DocumentRoot "/www/example"
</VirtualHost>

This time httpd needs to use reverse DNS to find the ServerName for this virtualhost. If that reverse lookup fails then it will partially disable the virtualhost. If the virtual host is name-based then it will effectively be totally disabled, but if it is IP-based then it will mostly work. However, if httpd should ever have to generate a full URL for the server which includes the server name (such as when a Redirect is issued), then it will fail to generate a valid URL.

Here is a snippet that avoids both of these problems:

<VirtualHost 192.0.2.1>
  ServerName www.example.dom
  ServerAdmin [email protected]
  DocumentRoot "/www/example"
</VirtualHost>

Denial of Service

Consider this configuration snippet:

<VirtualHost www.example1.dom>
  ServerAdmin [email protected]
  DocumentRoot "/www/example1"
</VirtualHost>
<VirtualHost www.example2.dom>
  ServerAdmin [email protected]
  DocumentRoot "/www/example2"
</VirtualHost>

Suppose that you've assigned 192.0.2.1 to www.example1.dom and 192.0.2.2 to www.example2.dom. Furthermore, suppose that example1.dom has control of their own DNS. With this config you have put example1.dom into a position where they can steal all traffic destined to example2.dom. To do so, all they have to do is set www.example1.dom to 192.0.2.2. Since they control their own DNS you can't stop them from pointing the www.example1.dom record wherever they wish.

Requests coming in to 192.0.2.2 (including all those where users typed in URLs of the form http://www.example2.dom/whatever) will all be served by the example1.dom virtual host. To better understand why this happens requires a more in-depth discussion of how httpd matches up incoming requests with the virtual host that will serve it. A rough document describing this is available.

The "main server" Address

Name-based virtual host support requires httpd to know the IP address(es) of the host that httpd is running on. To get this address it uses either the global ServerName (if present) or calls the C function gethostname (which should return the same as typing "hostname" at the command prompt). Then it performs a DNS lookup on this address. At present there is no way to avoid this lookup.

If you fear that this lookup might fail because your DNS server is down then you can insert the hostname in /etc/hosts (where you probably already have it so that the machine can boot properly). Then ensure that your machine is configured to use /etc/hosts in the event that DNS fails. Depending on what OS you are using this might be accomplished by editing /etc/resolv.conf, or maybe /etc/nsswitch.conf.

If your server doesn't have to perform DNS for any other reason then you might be able to get away with running httpd with the HOSTRESORDER environment variable set to "local". This all depends on what OS and resolver libraries you are using. It also affects CGIs unless you use mod_env to control the environment. It's best to consult the man pages or FAQs for your OS.

Tips to Avoid These Problems

• use IP addresses in VirtualHost
• use IP addresses in Listen
• ensure all virtual hosts have an explicit ServerName
• create a <VirtualHost _default_:*> server that has no pages to serve