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Only NAT packets you can deliver responses for
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:slug: masquerade-with-filter
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:date: 2024-05-08
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:tags: linux
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:category: til
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:keywords: nat, nftables, network
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:lang: en
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:translation: false
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:status: published
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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 ``iifname eth-inside
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masquerade`` use something like ``iifname eth-inside ip saddr 198.51.100.0/24
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masquerade``.
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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 *default*
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route, sending the packet back to the ISP.
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The result: IDS triggered by *many* 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.)
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