kerrigan: configure radvd for IPv6 forwarding

ops/nixos/kerrigan/default.nix

@@ -30,6 +30,12 @@ in
   boot.kernelParams = [ "console=ttyS0,115200n8" ];
   boot.initrd.kernelModules = [ "phy-mvebu-cp110-utmi" ];
 
+  boot.kernel.sysctl = {
+    "net.ipv4.ip_forward" = "1";
+    "net.ipv6.conf.default.forwarding" = "1";
+    "net.ipv6.conf.all.forwarding" = "1";
+  };
+
   networking = {
     hostName = "kerrigan";
     domain = "as205479.net";
@@ -75,5 +81,21 @@ in
     ${populate-boot-builder} -c ${config.system.build.toplevel} -d ./files/boot
   '';
 
+  services.radvd = {
+    enable = true;
+    config = ''
+      interface br0 {
+        AdvSendAdvert on;
+        AdvLinkMTU 1280;
+
+        prefix ffff:ffff:ffff:ffff::/64 {
+          Base6Interface wwan0;
+          AdvValidLifetime 600;
+          AdvPreferredLifetime 300;
+        }
+      }
+    '';
+  };
+
   system.stateVersion = "23.05";
 }