feat: add NSD backend and Topology C (multi-instance with peer sync) 🏗️

- New NSDBackend: zone files + nsd-control reload, zone registration via
  nsd.conf.d include file; mirrors BIND backend interface exactly
- BackendRegistry now supports type "nsd"; config defaults for nsd.zones_dir
  and nsd.nsd_conf
- Dockerfile installs both NSD and BIND9 — entrypoint detects configured
  backend type(s) and starts only the required daemon; CoreDNS MySQL
  deployments start neither
- docker/nsd.conf: minimal NSD base config with remote-control and
  zones.conf include
- entrypoint.sh: reads config file + env vars to determine which daemon
  to start; runs nsd-control-setup on first boot
- 20 new NSD backend tests (117 total, all passing)
- README: Topology C (multi-instance + peer sync) documented as most robust
  HA option; NSD config reference; updated topology comparison table;
  NSD env-var-only compose examples; version 2.5.0

docker/entrypoint.sh

@@ -2,10 +2,11 @@
 set -e
 
 # ---------------------------------------------------------------------------
-# Detect whether any bind backend is configured and enabled.
+# Detect which DNS backend type(s) are configured and enabled.
 # Uses the same config search order as the application itself.
 # ---------------------------------------------------------------------------
-BIND_ENABLED=$(python3 - <<'EOF'
+detect_backend_types() {
+python3 - <<'EOF'
 import yaml, sys, os
 
 config_paths = [
@@ -17,6 +18,10 @@ config_paths = [
     "/app/config/app.yaml",
 ]
 
+# Also honour env-var-only deployments (no config file)
+bind_env = os.environ.get("DADNS_DNS_BACKENDS_BIND_ENABLED", "").lower() == "true"
+nsd_env  = os.environ.get("DADNS_DNS_BACKENDS_NSD_ENABLED", "").lower()  == "true"
+
 config = {}
 for path in config_paths:
     if os.path.exists(path):
@@ -25,20 +30,62 @@ for path in config_paths:
         break
 
 backends = config.get("dns", {}).get("backends", {})
+has_bind = bind_env
+has_nsd  = nsd_env
 for cfg in backends.values():
-    if isinstance(cfg, dict) and cfg.get("type") == "bind" and cfg.get("enabled", False):
-        print("true")
-        sys.exit(0)
-print("false")
-EOF
-)
+    if not isinstance(cfg, dict) or not cfg.get("enabled", False):
+        continue
+    btype = cfg.get("type", "")
+    if btype == "bind":
+        has_bind = True
+    elif btype == "nsd":
+        has_nsd = True
 
-if [ "$BIND_ENABLED" = "true" ]; then
-    echo "[entrypoint] BIND backend enabled — starting named"
-    /usr/sbin/named -u bind -f &
-else
-    echo "[entrypoint] No BIND backend configured — skipping named"
+types = []
+if has_bind:
+    types.append("bind")
+if has_nsd:
+    types.append("nsd")
+print(" ".join(types) if types else "none")
+EOF
+}
+
+BACKEND_TYPES=$(detect_backend_types)
+echo "[entrypoint] Detected DNS backend type(s): ${BACKEND_TYPES:-none}"
+
+# ---------------------------------------------------------------------------
+# Start BIND if a bind backend is configured
+# ---------------------------------------------------------------------------
+if echo "$BACKEND_TYPES" | grep -qw "bind"; then
+    if command -v named >/dev/null 2>&1; then
+        echo "[entrypoint] Starting BIND (named)"
+        /usr/sbin/named -u bind -f &
+    else
+        echo "[entrypoint] WARNING: bind backend configured but 'named' not found — skipping"
+    fi
 fi
 
-# Start the application
+# ---------------------------------------------------------------------------
+# Start NSD if an nsd backend is configured
+# ---------------------------------------------------------------------------
+if echo "$BACKEND_TYPES" | grep -qw "nsd"; then
+    if command -v nsd >/dev/null 2>&1; then
+        echo "[entrypoint] Starting NSD"
+        # Ensure nsd-control keys exist (generated on first run)
+        if [ ! -f /etc/nsd/nsd_server.key ]; then
+            nsd-control-setup 2>/dev/null || true
+        fi
+        /usr/sbin/nsd -d -c /etc/nsd/nsd.conf &
+    else
+        echo "[entrypoint] WARNING: nsd backend configured but 'nsd' not found — skipping"
+    fi
+fi
+
+if [ "$BACKEND_TYPES" = "none" ] || [ -z "$BACKEND_TYPES" ]; then
+    echo "[entrypoint] No local DNS daemon required (CoreDNS MySQL or similar)"
+fi
+
+# ---------------------------------------------------------------------------
+# Start the directdnsonly application
+# ---------------------------------------------------------------------------
 exec python -m directdnsonly