dns: add middleware property to root server to hijack queries

lib/dns/server.js

@@ -109,6 +109,7 @@ class RootServer extends DNSServer {
     this.host = '127.0.0.1';
     this.port = 5300;
     this.lookup = null;
+    this.middle = null;
     this.publicHost = '127.0.0.1';
 
     // Plugins can add or remove items from
@@ -397,6 +398,27 @@ class RootServer extends DNSServer {
     const {name, type} = qs;
     const tld = util.from(name, -1);
 
+    // Plugins can insert middleware here and hijack the
+    // lookup for special TLDs before checking Urkel tree.
+    // We also pass the entire question in case a plugin
+    // is able to return an authoritative (non-referral) answer.
+    if (typeof this.middle === 'function') {
+      let res;
+      try {
+        res = await this.middle(tld, req);
+      } catch (e) {
+        this.logger.warning(
+          'Root server middleware resolution failed for name: %s',
+          name
+        );
+        this.logger.debug(e.stack);
+      }
+
+      if (res) {
+        return res;
+      }
+    }
+
     // Hit the cache first.
     const cache = this.cache.get(name, type);
 
@@ -421,6 +443,11 @@ class RootServer extends DNSServer {
     return reserved.getByName(tld);
   }
 
+  // Intended to be called by plugin.
+  signRRSet(rrset, type) {
+    key.signZSK(rrset, type);
+  }
+
   resetCache() {
     this.cache.reset();
   }