feat: Use configured DNS name to lookup instance IP address (#514)

hessjcg · web-flow · commit 7655fd81d8b1 · 2025-12-17T15:46:40.000-07:00
When a custom DNS name is used to connect to a Cloud SQL instance, the dialer should first attempt to resolve the custom DNS name to an IP address and use that for the connection. If the lookup fails, the dialer should fall back to using the IP address from the instance metadata. Fixes #513
diff --git a/src/cloud-sql-instance.ts b/src/cloud-sql-instance.ts
@@ -18,6 +18,7 @@ import {
   isSameInstance,
   resolveInstanceName,
 } from './parse-instance-connection-name';
+import {resolveARecord} from './dns-lookup';
 import {InstanceMetadata} from './sqladmin-fetcher';
 import {generateKeys} from './crypto';
 import {RSAKeys} from './rsa-keys';
@@ -69,12 +70,13 @@ export class CloudSQLInstance {
   static async getCloudSQLInstance(
     options: CloudSQLInstanceOptions
   ): Promise<CloudSQLInstance> {
+    const instanceInfo = await resolveInstanceName(
+      options.instanceConnectionName,
+      options.domainName
+    );
     const instance = new CloudSQLInstance({
       options: options,
-      instanceInfo: await resolveInstanceName(
-        options.instanceConnectionName,
-        options.domainName
-      ),
+      instanceInfo,
     });
     await instance.refresh();
     return instance;
@@ -266,7 +268,20 @@ export class CloudSQLInstance {
       rsaKeys.publicKey,
       this.authType
     );
-    const host = selectIpAddress(metadata.ipAddresses, this.ipType);
+    let host;
+    if (this.instanceInfo && this.instanceInfo.domainName) {
+      try {
+        const ips = await resolveARecord(this.instanceInfo.domainName);
+        if (ips && ips.length > 0) {
+          host = ips[0];
+        }
+      } catch (e) {
+        // ignore error, fallback to metadata IP
+      }
+    }
+    if (!host) {
+      host = selectIpAddress(metadata.ipAddresses, this.ipType);
+    }
     const privateKey = rsaKeys.privateKey;
     const serverCaCert = metadata.serverCaCert;
     this.serverCaMode = metadata.serverCaMode;
diff --git a/src/dns-lookup.ts b/src/dns-lookup.ts
@@ -48,3 +48,15 @@ export async function resolveTxtRecord(name: string): Promise<string> {
     });
   });
 }
+
+export async function resolveARecord(name: string): Promise<string[]> {
+  return new Promise((resolve, reject) => {
+    dns.resolve4(name, (err, addresses) => {
+      if (err) {
+        reject(err);
+        return;
+      }
+      resolve(addresses);
+    });
+  });
+}
diff --git a/test/cloud-sql-instance-dns.ts b/test/cloud-sql-instance-dns.ts
@@ -0,0 +1,161 @@
+// Copyright 2025 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+import t from 'tap';
+import {IpAddressTypes} from '../src/ip-addresses';
+import {AuthTypes} from '../src/auth-types';
+import {CA_CERT, CLIENT_CERT, CLIENT_KEY} from './fixtures/certs';
+import {setupCredentials} from './fixtures/setup-credentials';
+
+t.test('CloudSQLInstance DNS Lookup', async t => {
+  setupCredentials(t);
+
+  const fetcher = {
+    async getInstanceMetadata() {
+      return {
+        ipAddresses: {
+          public: '127.0.0.1',
+        },
+        serverCaCert: {
+          cert: CA_CERT,
+          expirationTime: '2033-01-06T10:00:00.232Z',
+        },
+      };
+    },
+    async getEphemeralCertificate() {
+      return {
+        cert: CLIENT_CERT,
+        expirationTime: '2033-01-06T10:00:00.232Z',
+      };
+    },
+  };
+
+  let resolveARecordMock = async (): Promise<string[]> => {
+    return [];
+  };
+  let resolveTXTRecordMock = async (): Promise<string[]> => {
+    return [];
+  };
+
+  const {CloudSQLInstance} = t.mockRequire('../src/cloud-sql-instance', {
+    '../src/crypto': {
+      generateKeys: async () => ({
+        publicKey: '-----BEGIN PUBLIC KEY-----',
+        privateKey: CLIENT_KEY,
+      }),
+    },
+    '../src/time': {
+      getRefreshInterval() {
+        return 50;
+      },
+      isExpirationTimeValid() {
+        return true;
+      },
+    },
+    '../src/dns-lookup': {
+      resolveARecord: async (name: string) => resolveARecordMock(name),
+      resolveTxtRecord: async (name: string) => resolveTXTRecordMock(name),
+    },
+  });
+
+  t.test('should use resolved IP when domainName is present', async t => {
+    const expectedIp = '10.0.0.1';
+    const expectInstanceName = 'my-project:us-east1:my-instance';
+    resolveARecordMock = async (name: string) => {
+      t.equal(name, 'example.com');
+      return [expectedIp];
+    };
+    resolveTXTRecordMock = async (name: string) => {
+      t.equal(name, 'example.com');
+      return [expectInstanceName];
+    };
+
+    const instance = await CloudSQLInstance.getCloudSQLInstance({
+      ipType: IpAddressTypes.PUBLIC,
+      authType: AuthTypes.PASSWORD,
+      domainName: 'example.com',
+      sqlAdminFetcher: fetcher,
+    });
+    t.after(() => instance.close());
+
+    t.equal(instance.host, expectedIp, 'Host should match resolved IP');
+  });
+
+  t.test('should fallback to metadata IP when resolution fails', async t => {
+    resolveARecordMock = async () => {
+      throw new Error('DNS Error');
+    };
+    const expectInstanceName = 'my-project:us-east1:my-instance';
+    resolveTXTRecordMock = async (name: string) => {
+      t.equal(name, 'example.com');
+      return [expectInstanceName];
+    };
+
+    const instance = await CloudSQLInstance.getCloudSQLInstance({
+      ipType: IpAddressTypes.PUBLIC,
+      authType: AuthTypes.PASSWORD,
+      domainName: 'example.com',
+      sqlAdminFetcher: fetcher,
+    });
+    t.after(() => instance.close());
+
+    t.equal(instance.host, '127.0.0.1', 'Host should fallback to metadata IP');
+  });
+
+  t.test(
+    'should fallback to metadata IP when resolution returns empty',
+    async t => {
+      resolveARecordMock = async () => {
+        return [];
+      };
+      const expectInstanceName = 'my-project:us-east1:my-instance';
+      resolveTXTRecordMock = async (name: string) => {
+        t.equal(name, 'example.com');
+        return [expectInstanceName];
+      };
+
+      const instance = await CloudSQLInstance.getCloudSQLInstance({
+        ipType: IpAddressTypes.PUBLIC,
+        authType: AuthTypes.PASSWORD,
+        domainName: 'example.com',
+        sqlAdminFetcher: fetcher,
+      });
+      t.after(() => instance.close());
+
+      t.equal(
+        instance.host,
+        '127.0.0.1',
+        'Host should fallback to metadata IP'
+      );
+    }
+  );
+
+  t.test('should use metadata IP when domainName is not present', async t => {
+    // resolveARecord should not be called, but if it is, ensure it doesn't return the expected IP
+    resolveARecordMock = async () => {
+      t.fail('Should not attempt to resolve DNS');
+      return ['10.0.0.1'];
+    };
+
+    const instance = await CloudSQLInstance.getCloudSQLInstance({
+      ipType: IpAddressTypes.PUBLIC,
+      authType: AuthTypes.PASSWORD,
+      instanceConnectionName: 'my-project:us-east1:my-instance',
+      sqlAdminFetcher: fetcher,
+    });
+    t.after(() => instance.close());
+
+    t.equal(instance.host, '127.0.0.1', 'Host should use metadata IP');
+  });
+});
diff --git a/test/dns-lookup.ts b/test/dns-lookup.ts
@@ -15,32 +15,44 @@
 import t from 'tap';
 
 t.test('lookup dns with mock responses', async t => {
-  const {resolveTxtRecord} = t.mockRequire('../src/dns-lookup.ts', {
-    'node:dns': {
-      resolveTxt: (name, callback) => {
-        switch (name) {
-          case 'db.example.com':
-            callback(null, [['my-project:region-1:instance']]);
-            return;
-          case 'multiple.example.com':
-            callback(null, [
-              ['my-project:region-1:instance'],
-              ['another-project:region-1:instance'],
-            ]);
-            return;
-          case 'split.example.com':
-            callback(null, [['my-project:', 'region-1:instance']]);
-            return;
-          case 'empty.example.com':
+  const {resolveTxtRecord, resolveARecord} = t.mockRequire(
+    '../src/dns-lookup.ts',
+    {
+      'node:dns': {
+        resolveTxt: (name, callback) => {
+          switch (name) {
+            case 'db.example.com':
+              callback(null, [['my-project:region-1:instance']]);
+              return;
+            case 'multiple.example.com':
+              callback(null, [
+                ['my-project:region-1:instance'],
+                ['another-project:region-1:instance'],
+              ]);
+              return;
+            case 'split.example.com':
+              callback(null, [['my-project:', 'region-1:instance']]);
+              return;
+            case 'empty.example.com':
+              callback(null, []);
+              return;
+            default:
+              callback(new Error('not found'), null);
+              return;
+          }
+        },
+        resolve4: (name, callback) => {
+          if (name === 'example.com') {
+            callback(null, ['10.0.0.1']);
+          } else if (name === 'empty.example.com') {
             callback(null, []);
-            return;
-          default:
-            callback(new Error('not found'), null);
-            return;
-        }
+          } else {
+            callback(new Error('not found'));
+          }
+        },
       },
-    },
-  });
+    }
+  );
 
   t.same(
     await resolveTxtRecord('db.example.com'),
@@ -67,6 +79,23 @@ t.test('lookup dns with mock responses', async t => {
     {code: 'EDOMAINNAMELOOKUPFAILED'},
     'should throw type error if an extra item is provided'
   );
+
+  // resolveARecord tests
+  t.same(
+    await resolveARecord('example.com'),
+    ['10.0.0.1'],
+    'should resolve A record'
+  );
+  t.same(
+    await resolveARecord('empty.example.com'),
+    [],
+    'should return empty array'
+  );
+  t.rejects(
+    async () => await resolveARecord('not-found'),
+    /not found/,
+    'should reject on error'
+  );
 });
 
 t.test('lookup dns with real responses', async t => {
