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<title>Only NAT packets you can deliver responses for – LEdoian's Blog</title>
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<h1>LEdoian's Blog</h1>
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<h1>Only NAT packets you can deliver responses for</h1>
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<p>When setting up a masquerading nat, it is worth considering masquerading only
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packets from known networks. That is, instead of rule like <tt class="docutils literal">iifname <span class="pre">eth-inside</span>
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masquerade</tt> use something like <tt class="docutils literal">iifname <span class="pre">eth-inside</span> ip saddr 198.51.100.0/24
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masquerade</tt>.</p>
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<p>I learned the hard way: my laptop in a masqueraded network picked a wrong
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source address from a subnet the router had no knowledge about. The outbound
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packets passed through right, but the responses came in, undergone translation,
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and since the destination was unknown to the router, it used the <em>default</em>
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route, sending the packet back to the ISP.</p>
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<p>The result: IDS triggered by <em>many</em> packets from my router with source IP
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addresses from all around the Internet, all with destination to my private
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address. (The ISP was not happy about that.)</p>
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