LDAPAuthentication with Active Directory

Here’s something I had to piece together myself from various code fragments. Its a class to authenticate a user in Windows Active Directory environment using LDAP. It first locates the domain controllers (DNS lookup of SRV records for _ldap._tcp.domain), parses out the server part and then tries to authenticate the user against a domain controller.

[sourcecode language=”java”]
import java.util.ArrayList;
import java.util.Hashtable;
import java.util.List;

import javax.naming.AuthenticationException;
import javax.naming.CommunicationException;
import javax.naming.Context;
import javax.naming.NamingException;
import javax.naming.directory.Attribute;
import javax.naming.directory.Attributes;
import javax.naming.directory.DirContext;
import javax.naming.directory.InitialDirContext;

import com.sun.jndi.ldap.LdapCtxFactory;

* LDAPAuthentication class for authenticating Microsoft Active Directory users
* If the user or password is wrong, you’ll get an AuthenticationException If
* none of the domain controllers are reachable, you’ll get a
* CommunicationException. If a domain controller cannot be located (via DNS)
* you’ll get a NamingException.
* @author Roger Armstrong, Armstrong Consulting GmbH
public class LDAPAuthentication {
public static void authenticateUser(String user, String password, String domain) throws AuthenticationException, NamingException {
List ldapServers = findLDAPServersInWindowsDomain(domain);
if (ldapServers.isEmpty())
throw new NamingException(“Can’t locate an LDAP server (try nslookup type=SRV _ldap._tcp.” + domain + “)”);

Hashtable props = new Hashtable();
String principalName = user + “@” + domain;
props.put(Context.SECURITY_PRINCIPAL, principalName);
props.put(Context.SECURITY_CREDENTIALS, password);
Integer count = 0;
for (String ldapServer : ldapServers) {
try {
LdapCtxFactory.getLdapCtxInstance(“ldap://” + ldapServer, props);
} catch (CommunicationException e) { // this is what’ll happen if one of the domain controllers is unreachable
if (count.equals(ldapServers.size())) {
// we’ve got no more servers to try, so throw the CommunicationException to indicate that we failed to reach an LDAP server
throw e;

private static List findLDAPServersInWindowsDomain(String domain) throws NamingException {
List servers = new ArrayList();
Hashtable env = new Hashtable();
env.put(Context.INITIAL_CONTEXT_FACTORY, “com.sun.jndi.dns.DnsContextFactory”);
env.put(“java.naming.provider.url”, “dns:”);
DirContext ctx = new InitialDirContext(env);
Attributes attributes = ctx.getAttributes(“_ldap._tcp.” + domain, new String[] { “SRV” }); // that’s how Windows domain controllers are registered in DNS
Attribute a = attributes.get(“SRV”);
for (int i = 0; i < a.size(); i++) { String srvRecord = a.get(i).toString(); // each SRV record is in the format "0 100 389 dc1.company.com." // priority weight port server (space separated) servers.add(srvRecord.split(" ")[3]); } ctx.close(); return servers; } } [/sourcecode]

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