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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 > 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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