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Issues Regarding DNS and Apache HTTP Server - Apache HTTP Server Version 2.4








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Apache HTTP Server Version 2.4



Apache > HTTP Server > Documentation > Version 2.4Issues Regarding DNS and Apache HTTP Server

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    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
 Denial of Service
 The "main server" Address
 Tips to Avoid These Problems
See alsoComments


A Simple Example
    

    # This is a misconfiguration example, do not use on your server
<VirtualHost www.example.dom>
  ServerAdmin webgirl@example.dom
  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 webgirl@example.dom
  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 webgirl@example.dom
  DocumentRoot "/www/example"
</VirtualHost>

  

Denial of Service
    

    Consider this configuration snippet:

    <VirtualHost www.example1.dom>
  ServerAdmin webgirl@example1.dom
  DocumentRoot "/www/example1"
</VirtualHost>
<VirtualHost www.example2.dom>
  ServerAdmin webguy@example2.dom
  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
    
  

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CommentsNotice:This is not a Q&A section. Comments placed here should be pointed towards suggestions on improving the documentation or server, and may be removed again by our moderators if they are either implemented or considered invalid/off-topic. Questions on how to manage the Apache HTTP Server should be directed at either our IRC channel, #httpd, on Freenode, or sent to our mailing lists.

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