router(host): add nftables config for basic router functionality

inventories/chaosknoten/host_vars/router.yaml

@@ -1 +1,2 @@
 systemd_networkd__config_dir: 'resources/chaosknoten/router/systemd_networkd/'
+nftables__config: "{{ lookup('ansible.builtin.file', 'resources/chaosknoten/router/nftables/nftables.conf') }}"

inventories/chaosknoten/hosts.yaml

@@ -93,6 +93,7 @@ systemd_networkd_hosts:
     router:
 nftables_hosts:
   hosts:
+    router:
 docker_compose_hosts:
   hosts:
     ccchoir:

resources/chaosknoten/router/nftables/nftables.conf

@@ -0,0 +1,73 @@
+#!/usr/sbin/nft -f
+
+## Variables
+
+# Interfaces
+define if_net1_v4_wan = "net1"
+define if_net2_v6_wan = "net2"
+define if_net0_2_v4_nat = "net0.2"
+define if_net0_3_ci_runner = "net0.3"
+
+# Interface Groups
+define wan_ifs = { $if_net1_v4_wan,
+                   $if_net2_v6_wan }
+define lan_ifs = { $if_net0_2_v4_nat,
+                   $if_net0_3_ci_runner }
+
+
+## Rules
+
+table inet reverse-path-forwarding {
+    chain rpf-filter {
+        type filter hook prerouting priority mangle + 10; policy drop;
+
+        # Only allow packets if their source address is routed via their incoming interface.
+        # https://github.com/NixOS/nixpkgs/blob/d9d87c51960050e89c79e4025082ed965e770d68/nixos/modules/services/networking/firewall-nftables.nix#L100
+        fib saddr . mark . iif oif exists accept
+    }
+}
+
+table inet host {
+    chain input {
+        type filter hook input priority filter; policy drop;
+
+        iifname "lo" accept comment "allow loopback"
+
+        ct state invalid drop
+        ct state established,related accept
+
+		ip protocol icmp accept
+		ip6 nexthdr icmpv6 accept
+
+        # Allow SSH access.
+        tcp dport 22 accept comment "allow ssh access"
+
+        # Allow DHCP server access.
+		iifname $if_net0_3_ci_runner udp dport 67 accept comment "allow dhcp server access"
+    }
+}
+
+table ip v4nat {
+    chain prerouting {
+        type nat hook prerouting priority dstnat; policy accept;
+    }
+
+    chain postrouting {
+        type nat hook postrouting priority srcnat; policy accept;
+
+        oifname $if_net1_v4_wan masquerade
+    }
+}
+
+table inet forward {
+    chain forward {
+        type filter hook forward priority filter; policy drop;
+
+        ct state invalid drop
+        ct state established,related accept
+
+        # Allow internet access.
+		meta nfproto ipv6 iifname $lan_ifs oifname $if_net2_v6_wan accept comment "allow v6 internet access"
+		meta nfproto ipv4 iifname $lan_ifs oifname $if_net1_v4_wan accept comment "allow v4 internet access"
+    }
+}

resources/chaosknoten/router/systemd_networkd/21-net0.2-v4_nat.network

@@ -8,7 +8,7 @@ RequiredForOnline=no
 [Network]
 Description=v4-NAT
 
-IPMasquerade=ipv4
+# Masquerading done in nftables (nftables.conf).
 IPv6SendRA=yes
 
 [Address]

resources/chaosknoten/router/systemd_networkd/21-net0.3-ci_runners.network

@@ -8,7 +8,7 @@ RequiredForOnline=no
 [Network]
 Description=ci-runners
 
-IPMasquerade=ipv4
+# Masquerading done in nftables (nftables.conf).
 IPv6SendRA=yes
 
 DHCPServer=true