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| |
| <manualpage metafile="dns-caveats.xml.meta"> |
| |
| <title>Issues Regarding DNS and Apache HTTP Server</title> |
| |
| <summary> |
| <p>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).</p> |
| </summary> |
| |
| <section id="example"> |
| <title>A Simple Example</title> |
| |
| <highlight language="config"> |
| # This is a misconfiguration example, do not use on your server |
| <VirtualHost www.example.dom> |
| ServerAdmin webgirl@example.dom |
| DocumentRoot "/www/example" |
| </VirtualHost> |
| </highlight> |
| |
| <p>In order for the server to function properly, it absolutely needs |
| to have two pieces of information about each virtual host: the |
| <directive module="core">ServerName</directive> 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 <code>www.example.dom</code>. If for some |
| reason DNS is not available at the time your server is parsing |
| its config file, then this virtual host <strong>will not be |
| configured</strong>. It won't be able to respond to any hits |
| to this virtual host.</p> |
| |
| <p>Suppose that <code>www.example.dom</code> has address 192.0.2.1. |
| Then consider this configuration snippet:</p> |
| |
| <highlight language="config"> |
| # This is a misconfiguration example, do not use on your server |
| <VirtualHost 192.0.2.1> |
| ServerAdmin webgirl@example.dom |
| DocumentRoot "/www/example" |
| </VirtualHost> |
| </highlight> |
| |
| <p>This time httpd needs to use reverse DNS to find the |
| <code>ServerName</code> 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.</p> |
| |
| <p>Here is a snippet that avoids both of these problems:</p> |
| |
| <highlight language="config"> |
| <VirtualHost 192.0.2.1> |
| ServerName www.example.dom |
| ServerAdmin webgirl@example.dom |
| DocumentRoot "/www/example" |
| </VirtualHost> |
| </highlight> |
| </section> |
| |
| <section id="denial"> |
| <title>Denial of Service</title> |
| |
| <p>Consider this configuration snippet:</p> |
| |
| <highlight language="config"> |
| <VirtualHost www.example1.dom> |
| ServerAdmin webgirl@example1.dom |
| DocumentRoot "/www/example1" |
| </VirtualHost> |
| <VirtualHost www.example2.dom> |
| ServerAdmin webguy@example2.dom |
| DocumentRoot "/www/example2" |
| </VirtualHost> |
| </highlight> |
| |
| <p>Suppose that you've assigned 192.0.2.1 to |
| <code>www.example1.dom</code> and 192.0.2.2 to |
| <code>www.example2.dom</code>. Furthermore, suppose that |
| <code>example1.dom</code> has control of their own DNS. With this |
| config you have put <code>example1.dom</code> into a position where |
| they can steal all traffic destined to <code>example2.dom</code>. To |
| do so, all they have to do is set <code>www.example1.dom</code> to |
| 192.0.2.2. Since they control their own DNS you can't stop them |
| from pointing the <code>www.example1.dom</code> record wherever they |
| wish.</p> |
| |
| <p>Requests coming in to 192.0.2.2 (including all those where |
| users typed in URLs of the form |
| <code>http://www.example2.dom/whatever</code>) will all be served by |
| the <code>example1.dom</code> 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 <a |
| href="vhosts/details.html">is available</a>.</p> |
| </section> |
| |
| <section id="main"> |
| <title>The "main server" Address</title> |
| |
| <p><a href="vhosts/name-based.html">Name-based |
| virtual host support</a> requires httpd to know |
| the IP address(es) of the host that <program>httpd</program> |
| is running on. To get this address it uses either the global |
| <directive module="core">ServerName</directive> |
| (if present) or calls the C function <code>gethostname</code> |
| (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.</p> |
| |
| <p>If you fear that this lookup might fail because your DNS |
| server is down then you can insert the hostname in |
| <code>/etc/hosts</code> (where you probably already have it so |
| that the machine can boot properly). Then ensure that your |
| machine is configured to use <code>/etc/hosts</code> in the |
| event that DNS fails. Depending on what OS you are using this |
| might be accomplished by editing <code>/etc/resolv.conf</code>, |
| or maybe <code>/etc/nsswitch.conf</code>.</p> |
| |
| <p>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 <code>HOSTRESORDER</code> environment variable set to |
| "local". This all depends on what OS and resolver libraries you |
| are using. It also affects CGIs unless you use |
| <module>mod_env</module> to control the environment. It's best |
| to consult the man pages or FAQs for your OS.</p> |
| </section> |
| |
| <section id="tips"> |
| <title>Tips to Avoid These Problems</title> |
| |
| <ul> |
| <li> |
| use IP addresses in |
| <directive module="core">VirtualHost</directive> |
| </li> |
| |
| <li> |
| use IP addresses in |
| <directive module="mpm_common">Listen</directive> |
| </li> |
| |
| <li> |
| ensure all virtual hosts have an explicit |
| <directive module="core">ServerName</directive> |
| </li> |
| |
| <li>create a <code><VirtualHost _default_:*></code> |
| server that has no pages to serve</li> |
| </ul> |
| </section> |
| |
| </manualpage> |