fix: filter out duplicate dns results

src/rfc-8252-http-server.spec.ts

@@ -9,6 +9,7 @@ import type { SinonSandbox } from 'sinon';
 import sinon from 'sinon';
 import { promisify } from 'util';
 import { randomBytes } from 'crypto';
+import { promises as dns } from 'dns';
 
 // node-fetch@3 is ESM-only...
 // eslint-disable-next-line @typescript-eslint/consistent-type-imports
@@ -485,4 +486,71 @@ describe('RFC8252HTTPServer', function () {
       });
     });
   });
+
+  context('with dns duplicates', function () {
+    const originalDnsLookup = dns.lookup;
+    let dnsLookupStub: sinon.SinonStub;
+
+    this.beforeEach(function () {
+      dnsLookupStub = sinon.stub();
+      dns.lookup = dnsLookupStub;
+    });
+
+    this.afterEach(function () {
+      dns.lookup = originalDnsLookup;
+    });
+
+    it('only filters exact duplicates', async function () {
+      dnsLookupStub.resolves([
+        { address: '127.0.0.1', family: 4 },
+        { address: '127.0.0.1', family: 4 },
+        { address: '[::1]', family: 6 },
+        { address: '[::1]', family: 6 },
+      ]);
+
+      const interfaces = await RFC8252HTTPServer['_getAllInterfaces'].call(
+        'localhost'
+      );
+
+      expect(interfaces).to.have.lengthOf(2);
+      expect(interfaces[0].address).to.equal('127.0.0.1');
+      expect(interfaces[1].address).to.equal('[::1]');
+      expect(interfaces[0].family).to.equal(4);
+      expect(interfaces[1].family).to.equal(6);
+    });
+
+    it('keeps same addresses, different family', async function () {
+      dnsLookupStub.resolves([
+        { address: '127.0.0.1', family: 4 },
+        { address: '127.0.0.1', family: 6 },
+      ]);
+
+      const interfaces = await RFC8252HTTPServer['_getAllInterfaces'].call(
+        'localhost'
+      );
+
+      expect(interfaces).to.have.lengthOf(2);
+      expect(interfaces[0].address).to.equal('127.0.0.1');
+      expect(interfaces[1].address).to.equal('127.0.0.1');
+      expect(interfaces[0].family).to.equal(4);
+      expect(interfaces[1].family).to.equal(6);
+    });
+
+    it('keeps same familes, different address', async function () {
+      dnsLookupStub.resolves([
+        { address: '127.0.0.1', family: 4 },
+        { address: '192.168.1.15', family: 4 },
+      ]);
+
+      const interfaces = await RFC8252HTTPServer['_getAllInterfaces'].call(
+        'localhost'
+      );
+
+      expect(interfaces).to.have.lengthOf(2);
+      expect(interfaces[0].address).to.equal('127.0.0.1');
+      expect(interfaces[1].address).to.equal('192.168.1.15');
+      expect(interfaces[0].family).to.equal(4);
+      expect(interfaces[1].family).to.equal(4);
+    });
+  });
 });

src/rfc-8252-http-server.ts

@@ -272,6 +272,25 @@ export class RFC8252HTTPServer {
     });
   };
 
+  private static async _getAllInterfaces(
+    hostname: string
+  ): Promise<{ address: string; family: number }[]> {
+    const dnsResults = await dns.lookup(hostname, {
+      all: true,
+      hints: ADDRCONFIG,
+    });
+
+    return dnsResults
+      .filter(
+        (dns, index, arr) =>
+          arr.findIndex(
+            (otherDns) =>
+              dns.address === otherDns.address && dns.family === otherDns.family
+          ) === index
+      )
+      .map(({ address, family }) => ({ address, family }));
+  }
+
   /**
    * Add a redirect from a local URL served on the server to an external URL.
    */
@@ -368,14 +387,11 @@ export class RFC8252HTTPServer {
     // to do what Node.js does by default when only a host is provided,
     // namely listening on all interfaces.
     let hostname = this.redirectUrl.hostname;
-    if (hostname.startsWith('[') && hostname.endsWith(']'))
+    if (hostname.startsWith('[') && hostname.endsWith(']')) {
       hostname = hostname.slice(1, -1);
-    const dnsResults = (
-      await dns.lookup(hostname, {
-        all: true,
-        hints: ADDRCONFIG,
-      })
-    ).map(({ address, family }) => ({ address, family }));
+    }
+
+    const dnsResults = await RFC8252HTTPServer._getAllInterfaces(hostname);
 
     this.logger.emit('mongodb-oidc-plugin:local-listen-resolved-hostname', {
       url: this.redirectUrl.toString(),