nixos/castle: Pin static DNS on lan-hi

castle took its resolver solely from the IPv6 RA (radvd RDNSS) on
lan-hi, so DNS broke whenever RA was absent. Since castle's own v6 is
RA/SLAAC-derived it has no usable v6 address in that case, so anchor
DNS on the always-present static v4 via the VRRP VIP, with the v6 VIP
as a bonus when v6 is up. Search domains reuse the centralised
lib.my.c.home.searchDomains list.

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

lib/constants.nix

@@ -307,6 +307,8 @@ rec {
 
   home = rec {
     domain = "h.${pubDomain}";
+    # Search domains advertised to clients (radvd DNSSL / kea domain-search)
+    searchDomains = [ domain "dyn.${domain}" colony.domain britway.domain ];
     vlans = {
       hi = 100;
       lo = 110;

nixos/boxes/home/castle/default.nix

@@ -2,7 +2,7 @@
 let
   inherit (lib.my) net;
   inherit (lib.my.c) networkd;
-  inherit (lib.my.c.home) domain vlans prefixes vips roceBootModules;
+  inherit (lib.my.c.home) domain searchDomains vlans prefixes vips roceBootModules;
 in
 {
   nixos.systems.castle = {
@@ -189,8 +189,16 @@ in
               };
               "40-lan-hi" = mkMerge [
                 (networkdAssignment "lan-hi" assignments.hi)
-                # So we don't drop the IP we use to connect to NVMe-oF!
-                { networkConfig.KeepConfiguration = "static"; }
+                {
+                  networkConfig = {
+                    # v6 is RA/SLAAC-derived, so when RA is absent we have no v6 at all;
+                    # anchor DNS on the always-present static v4 (the VRRP VIP follows the master)
+                    DNS = [ vips.hi.v4 vips.hi.v6 ];
+                    Domains = searchDomains;
+                    # So we don't drop the IP we use to connect to NVMe-oF!
+                    KeepConfiguration = "static";
+                  };
+                }
               ];
               "45-lan-lo" = {
                 matchConfig.Name = "lan-lo";

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

@@ -1,8 +1,8 @@
 index: { lib, pkgs, config, assignments, allAssignments, ... }:
 let
-  inherit (lib) mkForce;
+  inherit (lib) mkForce concatStringsSep;
   inherit (lib.my) net netbootKeaClientClasses;
-  inherit (lib.my.c.home) domain prefixes vips hiMTU;
+  inherit (lib.my.c.home) domain searchDomains prefixes vips hiMTU;
 
   dns-servers = [
     {
@@ -59,7 +59,7 @@ in
             }
             {
               name = "domain-search";
-              data = "${domain}, dyn.${domain}, ${lib.my.c.colony.domain}, ${lib.my.c.britway.domain}";
+              data = concatStringsSep ", " searchDomains;
               always-send = true;
             }
           ];

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

@@ -1,8 +1,8 @@
 index: { lib, pkgs, ... }:
 let
-  inherit (lib) mkForce concatMapStringsSep;
+  inherit (lib) mkForce concatMapStringsSep concatStringsSep;
   inherit (lib.my) net;
-  inherit (lib.my.c.home) domain prefixes vips;
+  inherit (lib.my.c.home) domain searchDomains prefixes vips;
 
   # untrusted uses external (Cloudflare) resolvers, matching the v4 kea config;
   # trusted VLANs use the internal recursor via its floating VRRP VIP
@@ -18,7 +18,7 @@ let
       AdvLinkMTU ${toString prefixes."${name}".mtu};
       prefix ${prefixes."${name}".v6} {};
       RDNSS ${rdnss name} {};
-      DNSSL ${domain} dyn.${domain} ${lib.my.c.colony.domain} ${lib.my.c.britway.domain} {};
+      DNSSL ${concatStringsSep " " searchDomains} {};
      };
   '';
 in