98 lines
6.3 KiB
HTML
98 lines
6.3 KiB
HTML
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<title>Example: Split Domain Name System over firewall</title>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Example: Split Domain Name System over firewall</h1>
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<div><p>This example depicts Domain Name System (DNS) operating over a
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firewall to protect internal data from the Internet, while allowing internal
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users to access data on the Internet.</p>
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<div class="section"><p>The following illustration depicts a simple subnet network that
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uses a firewall for security. OS/400<sup>®</sup> V5R1 DNS based on BIND 8 allows
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you to set up multiple DNS servers on a single iSeries™. Suppose that the company has
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an internal network with reserved IP space, and an external section of a network
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that is available to the public.</p>
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<p>The company wants its internal clients
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to be able to resolve external host names and to exchange mail with people
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on the outside. The company also wants its internal resolvers to have access
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to certain internal-only zones that are not available at all outside of the
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internal network. However, they do not want any outside resolvers to be able
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to access the internal network.</p>
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<p>To accomplish this, the company sets
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up two DNS server instances on the same iSeries server, one for the intranet
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and one for everything in its public domain. This is called<em> split DNS</em>.</p>
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<div class="fignone"><span class="figcap">Figure 1. Split DNS over firewall</span><br /><img src="rzakk504.gif" alt="Split DNS over firewall." /><br /></div>
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<p>The external server, DNSB, is configured with a primary zone mycompany.com.
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This zone data includes only the resource records that are intended to be
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part of the public domain. The internal server, DNSA, is configured with a
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primary zone mycompany.com, but the zone data defined on DNSA contains intranet
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resource records. The forwarders option is defined as 10.1.2.5. This forces
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DNSA to forward queries it cannot resolve to the DNSB server.</p>
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<p>If you
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are concerned about the integrity of your firewall or other security threats,
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you have the option of using the listen-on option to help protect internal
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data. To do this, you can configure the internal server to only allow queries
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to the internal mycompany.com zone from internal hosts. In order for all this
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to work properly, internal clients need to be configured to query only the
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DNSA server. You need to consider the following configuration settings to
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set up split DNS:</p>
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<ul><li>Listen-on<p>In previous examples, there has been only one DNS server on
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an iSeries.
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It is set to listen on all interface IP addresses. Whenever you have multiple
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DNS servers on an iSeries, you have to define the interface IP addresses
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that each one listens on. Two DNS servers cannot listen on the same address.
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In this case, assume that all queries coming in from the firewall are sent
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in on 10.1.2.5. These queries should be sent to the external server. Therefore,
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DNSB is configured to listen on 10.1.2.5. The internal server, DNSA, is configured
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to accept queries from anything on the 10.1.x.x interface IP addresses <em>except</em> 10.1.2.5.
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To effectively exclude this address, the Address Match List (AML) must have
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the excluded address listed before the included address prefix.</p>
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</li>
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<li>Address Match List (AML) order<p>The first element in the AML that a given
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address matches is used. For example, to allow all addresses on the 10.1.x.x
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network except 10.1.2.5, the ACL elements must be in the order (!10.1.2.5;
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10.1/16). In this case, the address 10.1.2.5 is compared to the first element
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and will immediately be denied.</p>
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<p>If the elements are reversed (10.1/16;
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!10.1.2.5), the IP address 10.1.2.5 will be allowed access because the server
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will compare it to the first element, which matches, and allow it without
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checking the rest of the rules.</p>
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</li>
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</ul>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzakkexampleparent.htm" title="You can use these examples to understand how to use Domain Name System (DNS) in your network.">Domain Name System examples</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="rzakkconceptbind.htm" title="Besides dynamic updates, BIND 8 offers several features to enhance performance of your Domain Name System (DNS) server.">BIND 8 features</a></div>
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</div>
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</div>
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</body>
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