A Simple Example

In order for Apache 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 Apache must use DNS to find the address of www.abc.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 (prior to Apache version 1.2 the server would not even boot).

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

This time Apache needs to use reverse DNS to find the ServerName for this virtualhost. If that reverse lookup fails then it will partially disable the virtualhost (prior to Apache version 1.2 the server would not even boot). 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 Apache should ever have to generate a full URL for the server which includes the server name, then it will fail to generate a valid URL.

Here is a snippet that avoids both of these problems:

Denial of Service

There are (at least) two forms that denial of service can come in. If you are running a version of Apache prior to version 1.2 then your server will not even boot if one of the two DNS lookups mentioned above fails for any of your virtual hosts. In some cases this DNS lookup may not even be under your control; for example, if abc.dom is one of your customers and they control their own DNS, they can force your (pre-1.2) server to fail while booting simply by deleting the www.abc.dom record.

Another form is far more insidious. Consider this configuration snippet:

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

Requests coming in to 192.0.2.1 (including all those where users typed in URLs of the form http://www.abc.dom/whatever) will all be served by the def.dom virtual host. To better understand why this happens requires a more in-depth discussion of how Apache matches up incoming requests with the virtual host that will serve it. A rough document describing this is available.

The "main server" Address

The addition of name-based virtual host support in Apache 1.1 requires Apache 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 Apache 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

Appendix: Future Directions

The situation regarding DNS is highly undesirable. For Apache 1.2 we've attempted to make the server at least continue booting in the event of failed DNS, but it might not be the best we can do. In any event, requiring the use of explicit IP addresses in configuration files is highly undesirable in today's Internet where renumbering is a necessity.

A possible work around to the theft of service attack described above would be to perform a reverse DNS lookup on the IP address returned by the forward lookup and compare the two names -- in the event of a mismatch, the virtualhost would be disabled. This would require reverse DNS to be configured properly (which is something that most admins are familiar with because of the common use of "double-reverse" DNS lookups by FTP servers and TCP wrappers).

In any event, it doesn't seem possible to reliably boot a virtual-hosted web server when DNS has failed unless IP addresses are used. Partial solutions such as disabling portions of the configuration might be worse than not booting at all depending on what the webserver is supposed to accomplish.

As HTTP/1.1 is deployed and browsers and proxies start issuing the Host header it will become possible to avoid the use of IP-based virtual hosts entirely. In this case, a webserver has no requirement to do DNS lookups during configuration. But as of March 1997 these features have not been deployed widely enough to be put into use on critical webservers.