firewalls: Add nftables subsection.

src/config/network/firewalls.md

@@ -53,3 +53,39 @@ and restore them by upping the relevant service, e.g.:
 ```
 # sv up ip6tables
 ```
+
+## nftables
+
+`nftables` replaces `iptables`, `ip6tables`, `arptables` and `ebtables`
+(collectively referred to as `xtables`). The [nftables
+wiki](https://wiki.nftables.org/wiki-nftables/index.php/Main_Page) describes
+[the main
+differences](https://wiki.nftables.org/wiki-nftables/index.php/Main_differences_with_iptables)
+from the `iptables` toolset.
+
+To use `nftables`, install the `nftables` package, which provides
+[nft(8)](https://man.voidlinux.org/nft.8). It also provides
+[iptables-translate(8)/ip6tables-translate(8)](https://man.voidlinux.org/iptables-translate.8)
+and
+[iptables-restore-translate(8)/ip6tables-restore-translate(8)](https://man.voidlinux.org/iptables-restore-translate.8),
+which convert `iptables` rules to `nftables` rules.
+
+### Applying the rules at boot
+
+To apply nftables rules at runit stage 1, install the `runit-nftables` package.
+This adds a core-service which restores the ruleset in `/etc/nftables.conf`.
+
+### Applying the rules at runtime
+
+The `nftables` package provides the `nftables` service, which uses rules from
+`/etc/nftables.conf`. To load the rules, run:
+
+```
+# sv up nftables
+```
+
+To flush the rules, run:
+
+```
+# sv down nftables
+```