feat: Use configured DNS name to lookup instance IP address

Bug Description
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.

This change modifies the dialer to:

Use the configured resolver to look up the host's IP address.
Use the IP address from the A record of the custom DNS name if available.
Fall back to the IP address from the instance metadata if the A record is not available.
See also GoogleCloudPlatform/cloud-sql-go-connector#1053

Fixes: #2243

Author: hessjcg

build.sh

@@ -39,8 +39,12 @@ function test() {
   if [[ "$(uname -s)" == "Darwin" ]]; then
     echo "macOS detected. Setting up IP aliases for tests."
     echo "You may be prompted for your password to run sudo."
-    sudo ifconfig lo0 alias 127.0.0.2 up
-    sudo ifconfig lo0 alias 127.0.0.3 up
+    if ! ifconfig lo0 | grep -q 127.0.0.2 ; then
+      sudo ifconfig lo0 alias 127.0.0.2 up
+    fi
+    if ! ifconfig lo0 | grep -q 127.0.0.3 ; then
+      sudo ifconfig lo0 alias 127.0.0.3 up
+    fi
   fi
   mvn -P coverage test
 }
@@ -91,7 +95,7 @@ function write_e2e_env(){
   secret_vars=(
     MYSQL_CONNECTION_NAME=MYSQL_CONNECTION_NAME
     MYSQL_USER=MYSQL_USER
-    MYSQL_USER_IAM=MYSQL_USER_IAM_GO
+    IMPERSONATED_USER=IMPERSONATED_USER
     MYSQL_PASS=MYSQL_PASS
     MYSQL_DB=MYSQL_DB
     MYSQL_MCP_CONNECTION_NAME=MYSQL_MCP_CONNECTION_NAME
@@ -105,8 +109,8 @@ function write_e2e_env(){
     POSTGRES_CAS_PASS=POSTGRES_CAS_PASS
     POSTGRES_CUSTOMER_CAS_CONNECTION_NAME=POSTGRES_CUSTOMER_CAS_CONNECTION_NAME
     POSTGRES_CUSTOMER_CAS_PASS=POSTGRES_CUSTOMER_CAS_PASS
-    POSTGRES_CUSTOMER_CAS_DOMAIN_NAME=POSTGRES_CUSTOMER_CAS_DOMAIN_NAME
-    POSTGRES_CUSTOMER_CAS_INVALID_DOMAIN_NAME=POSTGRES_CUSTOMER_CAS_INVALID_DOMAIN_NAME
+    POSTGRES_CUSTOMER_CAS_PASS_VALID_DOMAIN_NAME=POSTGRES_CUSTOMER_CAS_DOMAIN_NAME
+    POSTGRES_CUSTOMER_CAS_PASS_INVALID_DOMAIN_NAME=POSTGRES_CUSTOMER_CAS_INVALID_DOMAIN_NAME
     POSTGRES_MCP_CONNECTION_NAME=POSTGRES_MCP_CONNECTION_NAME
     POSTGRES_MCP_PASS=POSTGRES_MCP_PASS
     SQLSERVER_CONNECTION_NAME=SQLSERVER_CONNECTION_NAME
@@ -131,6 +135,10 @@ function write_e2e_env(){
     val=$(gcloud secrets versions access latest --project "$TEST_PROJECT" --secret="$secret_name")
     echo "export $env_var_name='$val'"
   done
+
+  echo "export MYSQL_IAM_USER='$(whoami)'"
+  echo "export POSTGRES_IAM_USER='$(whoami)@google.com'"
+
   } > "$outfile"
 
 }

core/src/main/java/com/google/cloud/sql/core/Connector.java

@@ -24,9 +24,12 @@
 import com.google.common.util.concurrent.ListeningScheduledExecutorService;
 import java.io.File;
 import java.io.IOException;
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.net.Socket;
+import java.net.UnknownHostException;
 import java.security.KeyPair;
+import java.util.List;
 import java.util.Timer;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutionException;
@@ -52,6 +55,7 @@ class Connector {
   private final ConnectorConfig config;
 
   private final InstanceConnectionNameResolver instanceNameResolver;
+  private final DnsResolver dnsResolver;
   private final Timer instanceNameResolverTimer;
   private final ProtocolHandler mdxProtocolHandler;
 
@@ -65,9 +69,9 @@ class Connector {
       long refreshTimeoutMs,
       int serverProxyPort,
       InstanceConnectionNameResolver instanceNameResolver,
+      DnsResolver dnsResolver,
       ProtocolHandler mdxProtocolHandler) {
     this.config = config;
-
     this.adminApi =
         connectionInfoRepositoryFactory.create(instanceCredentialFactory.create(), config);
     this.instanceCredentialFactory = instanceCredentialFactory;
@@ -76,6 +80,7 @@ class Connector {
     this.minRefreshDelayMs = minRefreshDelayMs;
     this.serverProxyPort = serverProxyPort;
     this.instanceNameResolver = instanceNameResolver;
+    this.dnsResolver = dnsResolver;
     this.instanceNameResolverTimer = new Timer("InstanceNameResolverTimer", true);
     this.mdxProtocolHandler = mdxProtocolHandler;
   }
@@ -125,6 +130,40 @@ Socket connect(ConnectionConfig config, long timeoutMs) throws IOException {
     try {
       ConnectionMetadata metadata = instance.getConnectionMetadata(timeoutMs);
       String instanceIp = metadata.getPreferredIpAddress();
+
+      // If a domain name was used to connect, resolve it to an IP address
+      if (!Strings.isNullOrEmpty(instance.getConfig().getDomainName())) {
+        try {
+          List<InetAddress> addrs = dnsResolver.resolveHost(instance.getConfig().getDomainName());
+          if (addrs != null && !addrs.isEmpty()) {
+            logger.debug(
+                String.format(
+                    "[%s] custom DNS name %s resolved to %s, using it to connect",
+                    instance.getConfig().getCloudSqlInstance(),
+                    instance.getConfig().getDomainName(),
+                    addrs.get(0).getHostAddress()));
+            instanceIp = addrs.get(0).getHostAddress();
+          } else {
+            logger.debug(
+                String.format(
+                    "[%s] custom DNS name %s resolved but returned no entries, using %s from"
+                        + " instance metadata",
+                    instance.getConfig().getCloudSqlInstance(),
+                    instance.getConfig().getDomainName(),
+                    instanceIp));
+          }
+        } catch (UnknownHostException e) {
+          logger.debug(
+              String.format(
+                  "[%s] custom DNS name %s did not resolve to an IP address: %s, using %s from"
+                      + " instance metadata",
+                  instance.getConfig().getCloudSqlInstance(),
+                  instance.getConfig().getDomainName(),
+                  e.getMessage(),
+                  instanceIp));
+        }
+      }
+
       logger.debug(String.format("[%s] Connecting to instance.", instanceIp));
 
       SSLSocket socket = (SSLSocket) metadata.getSslContext().getSocketFactory().createSocket();

core/src/main/java/com/google/cloud/sql/core/DnsJavaResolver.java

@@ -16,11 +16,15 @@
 
 package com.google.cloud.sql.core;
 
+import java.net.InetAddress;
 import java.net.UnknownHostException;
 import java.util.Arrays;
 import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
 import java.util.stream.Collectors;
 import javax.naming.NameNotFoundException;
+import org.xbill.DNS.ARecord;
 import org.xbill.DNS.Lookup;
 import org.xbill.DNS.Record;
 import org.xbill.DNS.SimpleResolver;
@@ -105,4 +109,43 @@ public Collection<String> resolveTxt(String domainName) throws NameNotFoundExcep
       throw new RuntimeException("Invalid domain name format: " + domainName, e);
     }
   }
+
+  /**
+   * Resolve an A record.
+   * @param hostName the hostname to look up
+   * @return the resolved IP addresses
+   * @throws UnknownHostException if no records are found.
+   */
+  @Override
+  public List<InetAddress> resolveHost(String hostName) throws UnknownHostException {
+    try {
+      Lookup lookup = new Lookup(hostName, Type.A);
+      if (this.resolver != null) {
+        lookup.setResolver(this.resolver);
+      }
+      lookup.run();
+
+      int resultCode = lookup.getResult();
+      if (resultCode == Lookup.HOST_NOT_FOUND) {
+        throw new UnknownHostException("DNS record not found for " + hostName);
+      }
+      if (resultCode != Lookup.SUCCESSFUL) {
+        throw new UnknownHostException(
+            "DNS lookup failed for " + hostName + ": " + lookup.getErrorString());
+      }
+
+      Record[] records = lookup.getAnswers();
+      if (records == null || records.length == 0) {
+        return Collections.emptyList();
+      }
+
+      return Arrays.stream(records)
+          .map(r -> (ARecord) r)
+          .map(ARecord::getAddress)
+          .collect(Collectors.toList());
+
+    } catch (TextParseException e) {
+      throw new UnknownHostException("Invalid domain name format: " + hostName);
+    }
+  }
 }

core/src/main/java/com/google/cloud/sql/core/DnsResolver.java

@@ -16,10 +16,15 @@
 
 package com.google.cloud.sql.core;
 
+import java.net.InetAddress;
+import java.net.UnknownHostException;
 import java.util.Collection;
+import java.util.List;
 import javax.naming.NameNotFoundException;
 
 /** Wraps the Java DNS API. */
 interface DnsResolver {
   Collection<String> resolveTxt(String domainName) throws NameNotFoundException;
+
+  List<InetAddress> resolveHost(String hostName) throws UnknownHostException;
 }

core/src/main/java/com/google/cloud/sql/core/InternalConnectorRegistry.java

@@ -339,6 +339,7 @@ private Connector createConnector(ConnectorConfig config) {
         connectTimeoutMs,
         serverProxyPort,
         new DnsInstanceConnectionNameResolver(new DnsJavaResolver()),
+        new DnsJavaResolver(),
         this.mdxProtocolHandler);
   }