Adds a DomainNameSystem utility class

This class uses JNDI to look up resource record values from the Domain
Name System using the system configured resolver.

See: #18342

src/main/java/org/torproject/onionoo/util/DomainNameSystem.java

+/* Copyright 2018 The Tor Project
+ * See LICENSE for licensing information */
+
+package org.torproject.onionoo.util;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.util.Hashtable;
+import java.util.Set;
+import java.util.TreeSet;
+
+import javax.naming.Context;
+import javax.naming.NamingEnumeration;
+import javax.naming.NamingException;
+import javax.naming.directory.Attribute;
+import javax.naming.directory.Attributes;
+import javax.naming.directory.DirContext;
+import javax.naming.directory.InitialDirContext;
+
+public class DomainNameSystem {
+
+  private DirContext dnsContext;
+  private Logger log;
+
+  /** Creates a new instance. */
+  public DomainNameSystem() {
+    log = LoggerFactory.getLogger(DomainNameSystem.class);
+    Hashtable<String, String> envProps = new Hashtable<>();
+    envProps.put(Context.INITIAL_CONTEXT_FACTORY,
+        "com.sun.jndi.dns.DnsContextFactory");
+    try {
+      dnsContext = new InitialDirContext(envProps);
+    } catch (NamingException e) {
+      log.error(
+          "Unable to create directory context. "
+          + "Host name lookup will be disabled!");
+    }
+  }
+
+  /**
+   * Returns all the values of DNS resource records found for a given host name
+   * and type.
+   */
+  public String[] getRecords(String hostName, String type)
+          throws NamingException {
+    if (dnsContext == null) {
+      /* Initial setup failed, so all lookups will fail. */
+      throw new NamingException();
+    }
+    Set<String> results = new TreeSet<String>();
+    Attributes dnsEntries =
+        dnsContext.getAttributes(hostName, new String[] { type });
+    if (dnsEntries != null) {
+      Attribute dnsAttribute = dnsEntries.get(type);
+      if (dnsAttribute != null) {
+        NamingEnumeration<?> dnsEntryIterator = dnsEntries.get(type).getAll();
+        while (dnsEntryIterator.hasMoreElements()) {
+          results.add(dnsEntryIterator.next().toString());
+        }
+      }
+    }
+    return results.toArray(new String[results.size()]);
+  }
+
+}