nixos-config/machines/koyomi/services/hypervisor.nix

# SPDX-FileCopyrightText: 2024 Simon Bruder <simon@sbruder.de>
#
# SPDX-License-Identifier: AGPL-3.0-or-later

{ lib, pkgs, ... }:
let
  guests = {
    forgejo-actions-runner = {
      mac = "42:80:00:00:00:02";
      v4 = "10.80.32.2";
      v6 = "2a01:4f8:151:712d:1::2";
    };
    hiroshi = {
      mac = "42:80:00:00:00:03";
      v4 = "10.80.32.3";
      v6 = "2a01:4f8:151:712d:1::3";
    };
  };

  # port forwarding for IPv4
  portForwards = {
    tcp = { };
    udp = { };
  };
in
{
  virtualisation.libvirtd = {
    enable = true;
    qemu.package = pkgs.qemu_kvm;
  };

  boot.kernel.sysctl = {
    "net.ipv4.conf.all.forwarding" = true;
    "net.ipv6.conf.all.forwarding" = true;
  };

  systemd.network = {
    enable = true;
    netdevs = {
      br-virt = {
        netdevConfig = {
          Name = "br-virt";
          Kind = "bridge";
        };
      };
    };
    networks = {
      br-virt = {
        name = "br-virt";
        address = [ "10.80.32.1/24" "2a01:4f8:151:712d:1::1/80" ];
      };
    };
  };
  services.resolved.enable = false;

  services.dnsmasq = {
    enable = true;

    settings = {
      interface = [ "br-virt" ];

      bind-interfaces = true; # do not bind to the wildcard interface
      bogus-priv = true; # do not forward revese lookups of internal addresses
      dhcp-fqdn = true; # only insert qualified names of DHCP clients into DNS
      domain-needed = true; # do not forward names without domain
      no-hosts = true; # do not resolve hosts from /etc/hosts
      no-resolv = true; # only use explicitly configured resolvers

      domain = [ "koyomi.sbruder.de" ];

      enable-ra = true; # required to tell clients to use DHCPv6

      # Force static configuration
      dhcp-range = [
        "10.80.32.0,static,255.255.255.0"
        "2a01:4f8:151:712d:1::,static,80"
      ];

      dhcp-host = lib.flatten (lib.mapAttrsToList
        (name: { mac, v4, v6 }: [
          "${mac},${v4},${name}"
          "${mac},[${v6}],${name}"
        ])
        guests);

      # Hetzner recursive name servers
      # https://docs.hetzner.com/dns-console/dns/general/recursive-name-servers/
      server = [
        "185.12.64.1"
        "185.12.64.2"
        "2a01:4ff:ff00::add:1"
        "2a01:4ff:ff00::add:2"
      ];
    };
  };

  networking.firewall = {
    allowedTCPPorts = map lib.toInt (lib.attrNames portForwards.tcp);
    allowedUDPPorts = map lib.toInt (lib.attrNames portForwards.udp);

    interfaces.br-virt = {
      allowedTCPPorts = [ 53 ]; # EDNS
      allowedUDPPorts = [ 53 67 547 ]; # DNS / DHCP / DHCPv6
    };
  };

  networking.nftables = {
    enable = true;
    ruleset = ''
      # only IPv4
      table ip hypervisor-nat {
        chain postrouting {
          type nat hook postrouting priority filter; policy accept
          oifname eth0 masquerade
        }

        chain prerouting {
          type nat hook prerouting priority dstnat; policy accept
          ${lib.concatStrings (lib.mapAttrsToList (port: guest: ''
            iifname eth0 tcp dport ${port} dnat to ${guests.${guest}.v4}
          '') portForwards.tcp)}
          ${lib.concatStrings (lib.mapAttrsToList (port: guest: ''
            iifname eth0 udp dport ${port} dnat to ${guests.${guest}.v4}
          '') portForwards.udp)}
        }
      }

      table inet hypervisor-filter {
        chain forward {
          type filter hook forward priority filter; policy drop

          iifname br-virt oifname eth0 counter accept
          iifname eth0 oifname br-virt counter accept
        }
      }
    '';
  };
}