LDAP Authentication & Authorization Dissected and Digested

LDAP is one of those things that I’ve integrated with a few times but never put enough energy into to really get the details or understand it much.  There’s always been someone I can bounce questions off of and thankfully those people were available again as I started working out the details of performing LDAP authentication.
The steps below are general enough to be used by anyone and will hopefully shed some light into the steps performed in LDAP authentication.  The process below also includes some steps for authorization.

Authentication

1.  Get a connection to the LDAP server.
With the host and port for the LDAP server, create a connection to it.  This can be  a simple direct connection or a pooled connection.  If more than a basic test, it is best to use a pooled connection.  Log and fail if a connection cannot be created.

2.  Bind as the application user.
Bind the connection to the application user.  This user should have enough permissions to search the area of LDAP where users are located.  This user may also have permissions to search for things outside of the users area (groups, authorization).  Log and fail if the application user cannot bind.
3.  Search for the DN (distinguished name) of the user to be authenticated.
This is where we verify the username is valid.  This does not authenticate the user but simply makes sure the requested username exists in the system.  Log and fail if the user’s DN is not found.
4.  Bind as user to be authenticated using DN from step 3.
Now for the moment of truth.  Bind to the connection using the DN found in step 3 and the password supplied by the user.  Log and fail if unable to bind using the user’s DN and password.

Authorization

5.  Re-bind as application user.
To check the authorization of a user, we need to read attributes from the user’s account. To do this, we need to re-bind as the application user.
6.  Search for user and require attributes.
A filter is used to search for the user like was done in step 3 but we’ll add an extra check to the query to look for the attributes that show we’re authorized.

Example Code

The code shown here is using the JLDAP library from Novell.  Interesting includes are noted at the top. The utility class used for escaping the search filter can be found in the Sakai Project’s Nakamura codebase.

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001	import com.novell.ldap.LDAPAttribute;

002	import com.novell.ldap.LDAPConnection;

003	import com.novell.ldap.LDAPEntry;

004	import com.novell.ldap.LDAPException;

005	import com.novell.ldap.LDAPSearchResults;

006

// ...

007	String baseDn = "ou=People,o=MyOrg";

008	String userFilter = "uid = {}";

009	String authzFilter = "authzAttr=special:Entitlement:value";

010

// ...

011	public boolean authenticate(Credentials credentials) throws RepositoryException {

012	boolean auth = false;

013	if (credentials instanceof SimpleCredentials) {

014	// get application user credentials

015	String appUser = connMgr.getConfig().getLdapUser();

016	String appPass = connMgr.getConfig().getLdapPassword();

017

018	// get user credentials

019	SimpleCredentials sc = (SimpleCredentials) credentials;

020

021	long timeStart = System.currentTimeMillis();

022

023	String userDn = LdapUtil.escapeLDAPSearchFilter(userFilter.replace("{}",

024	sc.getUserID()));

025	String userPass = new String(sc.getPassword());

026

027	LDAPConnection conn = null;

028

try {

029	// 1) Get a connection to the server

030

try {

031	conn = connMgr.getConnection();

032	log.debug("Connected to LDAP server");

033	} catch (LDAPException e) {

034	throw new IllegalStateException("Unable to connect to LDAP server ["

035	+ connMgr.getConfig().getLdapHost() + "]");

036

}

037

038	// 2) Bind as app user

039

try {

040	conn.bind(LDAPConnection.LDAP_V3, appUser, appPass.getBytes(UTF8));

041	log.debug("Bound as application user");

042	} catch (LDAPException e) {

043	throw new IllegalArgumentException("Can't bind application user [" + appUser

044	+ "]", e);

045

}

046

047	// 3) Search for username (not authz).

048	// If search fails, log/report invalid username or password.

049	LDAPSearchResults results = conn.search(baseDn, LDAPConnection.SCOPE_SUB, userDn,

050	null, true);

051	if (results.hasMore()) {

052	log.debug("Found user via search");

053

} else {

054	throw new IllegalArgumentException("Can't find user [" + userDn + "]");

055

}

056

057	// 4) Bind as user.

058	// If bind fails, log/report invalid username or password.

059

060	// value is set below. define here for use in authz check.

061	String userEntryDn = null;

062

try {

063	LDAPEntry userEntry = results.next();

064	LDAPAttribute objectClass = userEntry.getAttribute("objectClass");

065

066	if ("aliasObject".equals(objectClass.getStringValue())) {

067	LDAPAttribute aliasDN = userEntry.getAttribute("aliasedObjectName");

068	userEntryDn = aliasDN.getStringValue();

069

} else {

070	userEntryDn = userEntry.getDN();

071

}

072

073	conn.bind(LDAPConnection.LDAP_V3, userEntryDn, userPass.getBytes(UTF8));

074	log.debug("Bound as user");

075	} catch (LDAPException e) {

076	log.warn("Can't bind user [{}]", userDn);

077

throw e;

078

}

079

080	if (authzFilter.length() > 0) {

081	// 5) Return to app user

082

try {

083	conn.bind(LDAPConnection.LDAP_V3, appUser, appPass.getBytes(UTF8));

084	log.debug("Rebound as application user");

085	} catch (LDAPException e) {

086	throw new IllegalArgumentException("Can't bind application user [" + appUser

087

+ "]");

088

}

089

090	// 6) Search user DN with authz filter

091	// If search fails, log/report that user is not authorized

092	String userAuthzFilter = "(&(" + userEntryDn + ")(" + authzFilter + "))";

093	results = conn.search(baseDn, LDAPConnection.SCOPE_SUB, userAuthzFilter, null,

094

true);

095	if (results.hasMore()) {

096	log.debug("Found user + authz filter via search");

097

} else {

098	throw new IllegalArgumentException("User not authorized [" + userDn + "]");

099

}

100

}

101

102	// FINALLY!

103	auth = true;

104	log.info("User [{}] authenticated with LDAP in {}ms", userDn,

105	System.currentTimeMillis() - timeStart);

106	} catch (Exception e) {

107	log.warn(e.getMessage(), e);

108	} finally {

109	connMgr.returnConnection(conn);

110

}

111

}

112	return auth;

113

}

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Courtesy:http://thecarlhall.wordpress.com/2011/01/04/ldap-authentication-authorization-dissected-and-digested/