nixos/home: De-share Virgin Media modem specifics from routing-common

The modem management subnet is stream-specific (it shares stream's `wan`
interface), so it no longer belongs in the shared routing-common. Drop the
`modem` network assignment and configure the address directly on stream's
`wan` (modemV4). The firewall reject and DNS wan-A-record skipBroadcasts that
existed only because of the modem move to per-box options
`my.homeRouter.{firewall.untrustedRejectV4,dns.wanSkipBroadcasts}`, set solely
by stream. river needs neither: its ONT sits on a separate interface already
blocked by the interface-based forward rules.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nixos/boxes/home/routing-common/default.nix

@@ -11,12 +11,6 @@ in
 {
   nixos.systems."${name}" = {
     assignments = {
-      modem = {
-        ipv4 = {
-          address = net.cidr.host (254 - index) prefixes.modem.v4;
-          gateway = null;
-        };
-      };
       core = {
         name = "${name}-core";
         inherit domain;
@@ -100,9 +94,11 @@ in
 
     configuration = { lib, pkgs, config, assignments, allAssignments, ... }:
       let
-        inherit (lib) mkIf mkMerge mkForce;
+        inherit (lib) mkIf mkMerge mkForce optionalString concatStringsSep;
-        inherit (lib.my) networkdAssignment;
+        inherit (lib.my) mkOpt' networkdAssignment;
         inherit (lib.my.c) networkd;
+
+        cfg = config.my.homeRouter;
       in
       {
         imports = map (m: import m index) [
@@ -112,6 +108,20 @@ in
           ./kea.nix
         ];
 
+        # Per-box WAN-management specifics: the Virgin Media modem on stream lives on the `wan`
+        # interface itself, whereas river's ONT sits on its own interface. Declared as options the
+        # box sets so routing-common itself carries no modem/ONT knowledge.
+        options.my.homeRouter = with lib.types; {
+          dns.wanSkipBroadcasts = mkOpt' (listOf str) [ ] ''
+            Broadcast addresses to exclude when auto-selecting the router's own `wan` A record,
+            for extra static subnets that share the `wan` interface.
+          '';
+          firewall.untrustedRejectV4 = mkOpt' (listOf str) [ ] ''
+            IPv4 prefixes untrusted clients must be explicitly rejected from reaching. Only needed
+            for subnets sharing the `wan` interface, since `wan` egress is otherwise accepted.
+          '';
+        };
+
         config = {
           environment = {
             systemPackages = with pkgs; [
@@ -338,7 +348,7 @@ in
                     return
                   }
                   chain filter-untrusted {
-                    ip daddr ${prefixes.modem.v4} reject
+                    ${optionalString (cfg.firewall.untrustedRejectV4 != [ ]) "ip daddr { ${concatStringsSep ", " cfg.firewall.untrustedRejectV4} } reject"}
                     oifname wan accept
                     return
                   }

nixos/boxes/home/routing-common/dns.nix

@@ -206,7 +206,7 @@ in
             ${name} IN LUA ${lib.my.dns.ifaceA {
               inherit pkgs;
               iface = "wan";
-              skipBroadcasts = [ (lib.my.netBroadcast prefixes.modem.v4) ];
+              skipBroadcasts = config.my.homeRouter.dns.wanSkipBroadcasts;
             }}
             ${otherName} IN LUA ${lib.my.dns.lookupIP {
               inherit pkgs;

nixos/boxes/home/stream.nix

@@ -6,11 +6,16 @@
     nixpkgs = "mine";
     home-manager = "mine";
 
-    configuration = { lib, pkgs, config, assignments, ... }:
+    configuration = { lib, pkgs, config, ... }:
     let
       inherit (lib) mkMerge;
-      inherit (lib.my) networkdAssignment;
+      inherit (lib.my) net;
       inherit (lib.my.c) networkd;
+      inherit (lib.my.c.home) prefixes;
+
+      # Static address on the Virgin Media modem's management subnet. Kept as a plain interface
+      # address (not a network assignment) since it's local to this box's WAN uplink.
+      modemV4 = net.cidr.host 100 prefixes.modem.v4;
     in
     {
       imports = [ ./routing-common/mstpd.nix ];
@@ -77,7 +82,7 @@
           };
         };
 
-        # wan carries a permanent static modem-management address (assignments.modem)
+        # wan carries a permanent static modem-management address (modemV4)
         # alongside the DHCP public IP, so wait-online@wan reports "online" as soon as
         # the static address is up - before the DHCP lease arrives. ipsec's left= is the
         # public IP, so gating on wait-online lets it start unoriented and never connect.
@@ -190,26 +195,28 @@
                 CompensationMode=none
               '';
             };
-            "50-wan" = mkMerge [
+            "50-wan" = {
-              (networkdAssignment "wan" assignments.modem)
+              matchConfig.Name = "wan";
-              {
+              # Static modem-management address alongside the DHCP public lease. It has no
-                matchConfig.Name = "wan";
+              # gateway, so the wan-wait-online gate keys off the DHCP default route instead.
-                DHCP = "ipv4";
+              address = [ "${modemV4}/24" ];
-                dns = [ "127.0.0.1" "::1" ];
+              DHCP = "ipv4";
-                dhcpV4Config.UseDNS = false;
+              dns = [ "127.0.0.1" "::1" ];
+              dhcpV4Config.UseDNS = false;
+              # IPv4-only WAN (public IPv6 arrives over the tunnel, not this link).
+              networkConfig.IPv6AcceptRA = false;
 
-                qdiscConfig = {
+              qdiscConfig = {
-                  Parent = "ingress";
+                Parent = "ingress";
-                  Handle = "0xffff";
+                Handle = "0xffff";
-                };
+              };
-                extraConfig = ''
+              extraConfig = ''
-                  [CAKE]
+                [CAKE]
-                  Parent=root
+                Parent=root
-                  Bandwidth=48M
+                Bandwidth=48M
-                  RTTSec=50ms
+                RTTSec=50ms
-                '';
+              '';
-              }
+            };
-            ];
           };
         };
 
@@ -218,6 +225,12 @@
             key = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIPYTB4zeAqotrEJ8M+AiGm/s9PFsWlAodz3hYSROGuDb";
           };
           server.enable = true;
+          # The modem's management subnet shares the `wan` interface: skip its address when
+          # picking our own wan A record, and reject untrusted clients from reaching it.
+          homeRouter = {
+            dns.wanSkipBroadcasts = [ (lib.my.netBroadcast prefixes.modem.v4) ];
+            firewall.untrustedRejectV4 = [ prefixes.modem.v4 ];
+          };
           # deploy.node.hostname = "192.168.68.2";
         };
       };