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<base href="https://bugzilla.netfilter.org/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Reverse path filtering using "fib" needs better documentation"
href="https://bugzilla.netfilter.org/show_bug.cgi?id=1220">1220</a>
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<th>Summary</th>
<td>Reverse path filtering using "fib" needs better documentation
</td>
</tr>
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<th>Product</th>
<td>nftables
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>All
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>minor
</td>
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<th>Priority</th>
<td>P5
</td>
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<th>Component</th>
<td>nft
</td>
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<th>Assignee</th>
<td>pablo@netfilter.org
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<th>Reporter</th>
<td>f30@f30.me
</td>
</tr></table>
<p>
<div>
<pre>Reverse path filtering means dropping a packet if connections to its source IP
wouldn't use the same interface the packet arrived on. The nftables wiki states
[1] that this can be implemented like:
nft add rule x prerouting fib saddr . iif oif eq 0 drop
Slides by Florian Westphal [2] suggest to use:
nft add rule ... fib saddr . oif oif = 0 drop
But this fails with "Invalid argument" even when replacing "=" by "==" or "eq".
`fib saddr . iif oif eq 0` achieves the desired goal (except for bugs like the
one I'm going to report next), but I'm having a hard time figuring out why:
As far as I understand, `fib saddr . iif` looks up routing information based on
source address and input interface. `oif` then selects the output interface
from the looked up information.
Most other examples compare that result with an interface name, which is pretty
straight-forward. But I can't tell why and when `oif` would return 0 and why
this can be used for reverse path filtering.
I therefore suggest to add better documentation covering that aspect.
[1] <a href="https://wiki.nftables.org/wiki-nftables/index.php/Routing_information#fib">https://wiki.nftables.org/wiki-nftables/index.php/Routing_information#fib</a>
[2] <a href="http://www.strlen.de/talks/netdev12_nft_fib.pdf">http://www.strlen.de/talks/netdev12_nft_fib.pdf</a></pre>
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</p>
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