chore: Create a new DnsResolver class that wraps the Java DNS API

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

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2024 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
+ *
+ * http://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.
+ */
+
+package com.google.cloud.sql.core;
+
+import java.util.Collection;
+import javax.naming.NameNotFoundException;
+
+interface DnsResolver {
+  Collection<String> resolveTxt(String domainName) throws NameNotFoundException;
+}

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

@@ -0,0 +1,74 @@
+/*
+ * Copyright 2024 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
+ *
+ * http://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.
+ */
+
+package com.google.cloud.sql.core;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.stream.Collectors;
+import javax.naming.NameNotFoundException;
+import javax.naming.NamingException;
+import javax.naming.directory.Attribute;
+import javax.naming.directory.InitialDirContext;
+
+/** Implements DnsResolver using the Java JNDI built-in DNS directory. */
+class JndiDnsResolver implements DnsResolver {
+  private final String jndiPrefix;
+
+  /** Creates a resolver using the system DNS settings. */
+  JndiDnsResolver() {
+    this.jndiPrefix = "dns:";
+  }
+
+  /**
+   * Creates a DNS resolver that uses a specific DNS server.
+   *
+   * @param dnsServer the DNS server hostname
+   * @param port the DNS server port (DNS servers usually use port 53)
+   */
+  JndiDnsResolver(String dnsServer, int port) {
+    this.jndiPrefix = "dns://" + dnsServer + ":" + port + "/";
+  }
+
+  /**
+   * Returns DNS records for a domain name, sorted alphabetically.
+   *
+   * @param domainName the domain name to lookup
+   * @return the list of record
+   * @throws javax.naming.NameNotFoundException when the domain name did not resolve.
+   */
+  @Override
+  public Collection<String> resolveTxt(String domainName)
+      throws javax.naming.NameNotFoundException {
+    try {
+      // Notice: This is old Java 1.2 style code. It uses the ancient JNDI DNS Provider api.
+      // See https://docs.oracle.com/javase/7/docs/technotes/guides/jndi/jndi-dns.html
+      Attribute attr =
+          new InitialDirContext()
+              .getAttributes(jndiPrefix + domainName, new String[] {"TXT"})
+              .get("TXT");
+      // attr.getAll() returns a Vector containing strings, one for each record returned by dns.
+      return Collections.list(attr.getAll()).stream()
+          .map((Object v) -> (String) v)
+          .sorted() // sort multiple records alphabetically
+          .collect(Collectors.toList());
+    } catch (NameNotFoundException e) {
+      throw e;
+    } catch (NamingException e) {
+      throw new RuntimeException("Unable to look up domain name " + domainName, e);
+    }
+  }
+}