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                <ol class="chapter"><li class="chapter-item expanded affix "><a href="../intro.html">Introduction</a></li><li class="chapter-item expanded "><a href="../tools/index.html"><strong aria-hidden="true">1.</strong> Tools</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../tools/zsh.html"><strong aria-hidden="true">1.1.</strong> zsh</a></li><li class="chapter-item expanded "><a href="../tools/bash.html"><strong aria-hidden="true">1.2.</strong> bash</a></li><li class="chapter-item expanded "><a href="../tools/fish.html"><strong aria-hidden="true">1.3.</strong> fish</a></li><li class="chapter-item expanded "><a href="../tools/tmux.html"><strong aria-hidden="true">1.4.</strong> tmux</a></li><li class="chapter-item expanded "><a href="../tools/git.html"><strong aria-hidden="true">1.5.</strong> git</a></li><li class="chapter-item expanded "><a href="../tools/awk.html"><strong aria-hidden="true">1.6.</strong> awk</a></li><li class="chapter-item expanded 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href="../tools/ffmpeg.html"><strong aria-hidden="true">1.15.</strong> ffmpeg</a></li><li class="chapter-item expanded "><a href="../tools/column.html"><strong aria-hidden="true">1.16.</strong> column</a></li><li class="chapter-item expanded "><a href="../tools/sort.html"><strong aria-hidden="true">1.17.</strong> sort</a></li><li class="chapter-item expanded "><a href="../tools/sed.html"><strong aria-hidden="true">1.18.</strong> sed</a></li></ol></li><li class="chapter-item expanded "><a href="../monitor/index.html"><strong aria-hidden="true">2.</strong> Resource analysis & monitor</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../monitor/lsof.html"><strong aria-hidden="true">2.1.</strong> lsof</a></li><li class="chapter-item expanded "><a href="../monitor/ss.html"><strong aria-hidden="true">2.2.</strong> ss</a></li><li class="chapter-item expanded "><a href="../monitor/pidstat.html"><strong aria-hidden="true">2.3.</strong> pidstat</a></li><li class="chapter-item expanded "><a href="../monitor/pgrep.html"><strong aria-hidden="true">2.4.</strong> pgrep</a></li><li class="chapter-item expanded "><a href="../monitor/pmap.html"><strong aria-hidden="true">2.5.</strong> pmap</a></li><li class="chapter-item expanded "><a href="../monitor/pstack.html"><strong aria-hidden="true">2.6.</strong> pstack</a></li></ol></li><li class="chapter-item expanded "><a href="../trace_profile/index.html"><strong aria-hidden="true">3.</strong> Trace and Profile</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../trace_profile/time.html"><strong aria-hidden="true">3.1.</strong> time</a></li><li class="chapter-item expanded "><a href="../trace_profile/strace.html"><strong aria-hidden="true">3.2.</strong> strace</a></li><li class="chapter-item expanded "><a href="../trace_profile/ltrace.html"><strong aria-hidden="true">3.3.</strong> ltrace</a></li><li class="chapter-item expanded "><a href="../trace_profile/perf.html"><strong aria-hidden="true">3.4.</strong> perf</a></li><li class="chapter-item expanded "><a href="../trace_profile/oprofile.html"><strong aria-hidden="true">3.5.</strong> OProfile</a></li><li class="chapter-item expanded "><a href="../trace_profile/callgrind.html"><strong aria-hidden="true">3.6.</strong> callgrind</a></li></ol></li><li class="chapter-item expanded "><a href="../binary/index.html"><strong aria-hidden="true">4.</strong> Binary</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../binary/od.html"><strong aria-hidden="true">4.1.</strong> od</a></li><li class="chapter-item expanded "><a href="../binary/xxd.html"><strong aria-hidden="true">4.2.</strong> xxd</a></li><li class="chapter-item expanded "><a href="../binary/readelf.html"><strong aria-hidden="true">4.3.</strong> readelf</a></li><li class="chapter-item expanded "><a href="../binary/objdump.html"><strong aria-hidden="true">4.4.</strong> objdump</a></li><li class="chapter-item expanded "><a href="../binary/nm.html"><strong aria-hidden="true">4.5.</strong> nm</a></li></ol></li><li class="chapter-item expanded "><a href="../development/index.html"><strong aria-hidden="true">5.</strong> Development</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../development/c++filt.html"><strong aria-hidden="true">5.1.</strong> c++filt</a></li><li class="chapter-item expanded "><a href="../development/c++.html"><strong aria-hidden="true">5.2.</strong> c++</a></li><li class="chapter-item expanded "><a href="../development/glibc.html"><strong aria-hidden="true">5.3.</strong> glibc</a></li><li class="chapter-item expanded "><a href="../development/gcc.html"><strong aria-hidden="true">5.4.</strong> gcc</a></li><li class="chapter-item expanded "><a href="../development/cmake.html"><strong aria-hidden="true">5.5.</strong> cmake</a></li><li class="chapter-item expanded "><a href="../development/make.html"><strong aria-hidden="true">5.6.</strong> make</a></li><li 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chartjs</a></li></ol></li><li class="chapter-item expanded "><a href="../arch/index.html"><strong aria-hidden="true">9.</strong> Arch</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="../arch/x86_64.html"><strong aria-hidden="true">9.1.</strong> x86_64</a></li><li class="chapter-item expanded "><a href="../arch/arm64.html"><strong aria-hidden="true">9.2.</strong> arm64</a></li><li class="chapter-item expanded "><a href="../arch/armv7.html"><strong aria-hidden="true">9.3.</strong> armv7</a></li><li class="chapter-item expanded "><a href="../arch/riscv.html"><strong aria-hidden="true">9.4.</strong> riscv</a></li></ol></li></ol>
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                    <h1 class="menu-title">Notes</h1>

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                        <h1 id="nftables"><a class="header" href="#nftables">nftables</a></h1>
<p><a href="https://nftables.org/projects/nftables/index.html">Nftables</a> is a stateful Linux firewall which uses the
<a href="https://nftables.org">netfilter</a> kernel hooks.
It is used for stateless, stateful packet filtering and all sorts of NAT.</p>
<p>Nftables is the successor to <a href="https://nftables.org/projects/iptables/index.html">iptables</a>.</p>
<p>In nftables, <code>rules</code> are organized with <code>chains</code> and <code>tables</code>.</p>
<ul>
<li><code>chain</code>: Orders <code>rules</code>. Chains exist in two kinds:
<ul>
<li><code>base chain</code>: Entry point from netfilter hooks (network stack).</li>
<li><code>regular chain</code>: Can be used as jump target to group rules for better organization.</li>
</ul>
</li>
<li><code>table</code>: Groups chains together. Tables are defined by a <code>name</code> and an
<code>address family</code> (eg <code>inet</code>, <code>ip</code>, <code>ip6</code>, ..).</li>
</ul>
<h2 id="ruleset"><a class="header" href="#ruleset">Ruleset</a></h2>
<pre><code class="language-bash">nft list ruleset        # List all tables/chains/rules (whole ruleset).
nft flush ruleset       # Clear whole ruleset.
</code></pre>
<h2 id="examples-save-rules-to-files-and-re-apply"><a class="header" href="#examples-save-rules-to-files-and-re-apply">Examples: Save rules to files and re-apply</a></h2>
<pre><code class="language-bash">nft list ruleset &gt; nft.rules
nft flush ruleset
nft -f nft.rules
</code></pre>
<h2 id="example-fully-trace-evaluation-of-nftables-rules"><a class="header" href="#example-fully-trace-evaluation-of-nftables-rules">Example: Fully trace evaluation of nftables rules</a></h2>
<pre><code class="language-text">table ip traceall {
    chain filter_prerouting {
        # Install chain with higher priority as the RAW standard priority.
        type filter hook prerouting priority raw - 50; policy accept;
        # Trace each and every packet (very verbose).
        #meta nftrace set 1;
        # Trace packet to port 80/81/8081 from localhost.
        tcp dport { 80, 81, 8081 } ip saddr 127.0.0.1 meta nftrace set 1;
    }
}
</code></pre>
<p>Use <code>nft monitor trace</code> to get trace output on tty.</p>
<h2 id="example-ipv4-port-forwarding"><a class="header" href="#example-ipv4-port-forwarding">Example: IPv4 port forwarding</a></h2>
<pre><code class="language-text">table ip fwd {
    chain nat_preroute {
        # Register this chain to the PREROUTE:NAT hook (stateful packet tracking via conntrack).
        type nat hook prerouting priority dstnat + 10 ; policy accept;
        meta nfproto ipv4 tcp dport 81 redirect to :8081
    }
}
</code></pre>
<h2 id="example-base-vs-regular-chain"><a class="header" href="#example-base-vs-regular-chain">Example: Base vs regular chain</a></h2>
<pre><code class="language-text"># Table named 'playground' handling 'ip' (ipv4) address family.
table ip playground {
    # Base chain.
    chain filter_input_base {
        # Register this chain to the INPUT:FILTER hook in the netfilter package flow.
        # Specify a prioirty relative to the inbuilt 'filter' priority (smaller
        # number means higher priority).
        # Set the default policy to ACCEPT, to let every packet pass by default.
        type filter hook input priority filter - 10; policy accept;

        # Create a rule for tcp packets arriving on either port 8000 or 8100.
        tcp dport { 8000, 8100 } jump input_reg_log;
        # Create a rule for tcp packets arriving on port 8200.
        tcp dport 8200 jump input_reg_log_all;
    }

    # Regular chain.
    chain input_reg_log {
        # Log every packet traversing this chain.
        # Message lands in the kernel ring buffer.
        log;
    }

    # Regular chain.
    chain input_reg_log_all {
        # Log every packet with all flags traversing this chain.
        log flags all;
    }
}
</code></pre>
<pre><code class="language-bash"># Load the nf rules.
sudo nft -f playground.rules
# Create test servers.
nc -lk 0.0.0.0 8000
nc -lk 0.0.0.0 8100
nc -lk 0.0.0.0 8200
# See the nftables logging in the kernel ring buffer.
sudo dmesg -w
# Make some client connections.
nc localhost 8000
nc localhost 8200
</code></pre>
<h2 id="mental-model-for-netfilter-packet-flow"><a class="header" href="#mental-model-for-netfilter-packet-flow">Mental model for netfilter packet flow</a></h2>
<p><img src="assets/nf_pkt_flow.png" alt="nf_pkt_flow.png" /></p>

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