documentation: DNAT examples

[steveej]

i'd like to add to the wishlist 🙂 e.g.

1. redirect a destination port to a different port on localhost
2. redirect a destination port to a different host on lan
3. redirect all traffic except for the ssh port to a host on lan

these should be relatively common use-cases and, being new to both nftables and this library, it's not obvious to me how to do this here.

as this is my first post in this repo, thank you for creating this library 🙌

Originally posted by @steveej in #12 (comment)

---

creating this issue to start contributing documentation. here's an example for exposing a host (case 3 and 1 from above)

networking.nftables = {
  firewall = {
    enable = true;
    zones = {
      wan.interfaces = ["wan" "lan0"];
    };
  };
  chains = {
    prerouting = {
      "redirectweb" = {
        after = ["hook"];
        rules = let
          wanInterfaces = builtins.concatStringsSep ", " config.networking.nftables.firewall.zones.wan.interfaces;
          exposedHost = "192.168.0.23";
         in [
          "iifname { ${wanInterfaces} } tcp dport 220 redirect to 22"
          "iifname { ${wanInterfaces} } dnat ip to ${exposedHost}"
        ];
      };
    };
  };
};