nixos/home: Anchor static hi clients' DNS on VIPs

Statically-addressed home servers on hi run no DHCP, so they learned a
resolver only from the v6 RA RDNSS and lost DNS whenever v6 (and thus
the RA) was absent. Factor the fix castle/palace applied inline into a
shared lib.my.c.home.vlanDns helper that points resolved at the VLAN's
VRRP VIPs (always-present static v4, plus v6 when up) and sets the
advertised search domains, then apply it to every statically-addressed
hi client: castle, palace, cellar, sfh and the sfh hass/unifi
containers. Document it under the router client DNS section.

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

docs/networking.md

@@ -321,6 +321,12 @@ family move together.
 serving disjoint pool halves. `radvd` advertises the v6 VIP as RDNSS (`untrusted` gets Cloudflare)
 and keepalived's `notify_master`/`notify_backup` hooks ensure that only the master sends RAs.
 
+Statically-addressed boxes (the servers on `hi`) don't run DHCP, so they'd otherwise learn a
+resolver only from the v6 RA RDNSS — which vanishes when v6 is disabled, taking DNS with it. They
+instead anchor DNS on the VIPs via the shared `lib.my.c.home.vlanDns "<vlan>"` fragment, which sets
+`DNS` to `vips.<vlan>.{v4,v6}` and `Domains` to the advertised search list; the always-present
+static v4 VIP keeps resolution working even with v6 down.
+
 #### DNS binding
 
 `pdns-recursor` binds the VIPs directly; see

lib/constants.nix

@@ -385,6 +385,16 @@ rec {
       };
     };
 
+    # networkConfig fragment anchoring a VLAN client's DNS on the router pair's
+    # VRRP VIPs rather than the RA RDNSS. v6 addresses here are RA/token-derived,
+    # so when RA is absent (e.g. v6 disabled) there is no v6 and no RDNSS at all;
+    # the always-present static v4 VIP keeps name resolution working, with the v6
+    # VIP as a bonus when v6 is up. Merge into the VLAN network's networkConfig.
+    vlanDns = vlan: {
+      DNS = [ vips.${vlan}.v4 vips.${vlan}.v6 ];
+      Domains = searchDomains;
+    };
+
     roceBootModules = [ "ib_core" "ib_uverbs" "mlx5_core" "mlx5_ib" ];
   };
 

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 searchDomains vlans prefixes vips roceBootModules;
+  inherit (lib.my.c.home) domain vlans prefixes vips vlanDns roceBootModules;
 in
 {
   nixos.systems.castle = {
@@ -190,11 +190,7 @@ in
               "40-lan-hi" = mkMerge [
                 (networkdAssignment "lan-hi" assignments.hi)
                 {
-                  networkConfig = {
+                  networkConfig = vlanDns "hi" // {
-                    # 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";
                   };

nixos/boxes/home/palace/default.nix

@@ -2,7 +2,7 @@
 let
   inherit (lib.my) net mkVLAN;
   inherit (lib.my.c) pubDomain;
-  inherit (lib.my.c.home) domain vlans prefixes vips hiMTU;
+  inherit (lib.my.c.home) domain vlans prefixes vips vlanDns hiMTU;
 in
 {
   imports = [ ./vms ];
@@ -203,7 +203,10 @@ in
                   MACAddress=52:54:00:90:34:95
                 '';
               };
-              "60-lan-hi" = networkdAssignment "lan-hi" assignments.hi;
+              "60-lan-hi" = mkMerge [
+                (networkdAssignment "lan-hi" assignments.hi)
+                { networkConfig = vlanDns "hi"; }
+              ];
 
               "50-lan-core-phy" = {
                 matchConfig.Name = "lan-core-phy";

nixos/boxes/home/palace/vms/cellar/default.nix

@@ -2,7 +2,7 @@
 let
   inherit (lib.my) net;
   inherit (lib.my.c) pubDomain;
-  inherit (lib.my.c.home) domain prefixes vips hiMTU;
+  inherit (lib.my.c.home) domain prefixes vips vlanDns hiMTU;
 in
 {
   nixos.systems.cellar = {
@@ -79,7 +79,10 @@ in
               };
 
               networks = {
-                "80-lan-hi" = networkdAssignment "lan-hi" assignments.hi;
+                "80-lan-hi" = mkMerge [
+                  (networkdAssignment "lan-hi" assignments.hi)
+                  { networkConfig = vlanDns "hi"; }
+                ];
               };
             };
 

nixos/boxes/home/palace/vms/sfh/containers/hass.nix

@@ -2,7 +2,7 @@
 let
   inherit (lib.my) net;
   inherit (lib.my.c) pubDomain;
-  inherit (lib.my.c.home) domain prefixes vips hiMTU;
+  inherit (lib.my.c.home) domain prefixes vips vlanDns hiMTU;
 in
 {
   nixos.systems.hass = { config, ... }: {
@@ -82,7 +82,10 @@ in
 
         systemd = {
           network.networks = {
-            "80-container-host0" = networkdAssignment "host0" assignments.hi;
+            "80-container-host0" = mkMerge [
+              (networkdAssignment "host0" assignments.hi)
+              { networkConfig = vlanDns "hi"; }
+            ];
             "80-container-lan-lo" = networkdAssignment "lan-lo" assignments.lo;
           };
         };

nixos/boxes/home/palace/vms/sfh/containers/unifi.nix

@@ -1,7 +1,7 @@
 { lib, ... }:
 let
   inherit (lib.my) net;
-  inherit (lib.my.c.home) domain prefixes vips hiMTU;
+  inherit (lib.my.c.home) domain prefixes vips vlanDns hiMTU;
 in
 {
   nixos.systems.unifi = { config, ... }: {
@@ -58,7 +58,10 @@ in
 
         systemd = {
           network.networks = {
-            "80-container-host0" = networkdAssignment "host0" assignments.hi;
+            "80-container-host0" = mkMerge [
+              (networkdAssignment "host0" assignments.hi)
+              { networkConfig = vlanDns "hi"; }
+            ];
             "80-lan-core" = networkdAssignment "lan-core" assignments.core;
           };
         };

nixos/boxes/home/palace/vms/sfh/default.nix

@@ -1,7 +1,7 @@
 { lib, ... }:
 let
   inherit (lib.my) net;
-  inherit (lib.my.c.home) domain prefixes vips hiMTU roceBootModules;
+  inherit (lib.my.c.home) domain prefixes vips vlanDns hiMTU roceBootModules;
 in
 {
   imports = [ ./containers ];
@@ -134,8 +134,10 @@ in
           networks = {
             "30-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 = vlanDns "hi" // { KeepConfiguration = "static"; };
+              }
             ];
             "30-lan-hi-ctrs" = {
               matchConfig.Name = "lan-hi-ctrs";