The Apache Tomcat Servlet/JSP Container

Apache Tomcat 7

Version 7.0.70, Jun 15 2016
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User Guide


Apache Tomcat Development

Realm Configuration HOW-TO

Table of Contents
Quick Start

This document describes how to configure Tomcat to support container managed security, by connecting to an existing "database" of usernames, passwords, and user roles. You only need to care about this if you are using a web application that includes one or more <security-constraint> elements, and a <login-config> element defining how users are required to authenticate themselves. If you are not utilizing these features, you can safely skip this document.

For fundamental background information about container managed security, see the Servlet Specification (Version 2.4), Section 12.

For information about utilizing the Single Sign On feature of Tomcat (allowing a user to authenticate themselves once across the entire set of web applications associated with a virtual host), see here.

What is a Realm?

A Realm is a "database" of usernames and passwords that identify valid users of a web application (or set of web applications), plus an enumeration of the list of roles associated with each valid user. You can think of roles as similar to groups in Unix-like operating systems, because access to specific web application resources is granted to all users possessing a particular role (rather than enumerating the list of associated usernames). A particular user can have any number of roles associated with their username.

Although the Servlet Specification describes a portable mechanism for applications to declare their security requirements (in the web.xml deployment descriptor), there is no portable API defining the interface between a servlet container and the associated user and role information. In many cases, however, it is desirable to "connect" a servlet container to some existing authentication database or mechanism that already exists in the production environment. Therefore, Tomcat defines a Java interface (org.apache.catalina.Realm) that can be implemented by "plug in" components to establish this connection. Six standard plug-ins are provided, supporting connections to various sources of authentication information:

  • JDBCRealm - Accesses authentication information stored in a relational database, accessed via a JDBC driver.
  • DataSourceRealm - Accesses authentication information stored in a relational database, accessed via a named JNDI JDBC DataSource.
  • JNDIRealm - Accesses authentication information stored in an LDAP based directory server, accessed via a JNDI provider.
  • UserDatabaseRealm - Accesses authentication information stored in an UserDatabase JNDI resource, which is typically backed by an XML document (conf/tomcat-users.xml).
  • MemoryRealm - Accesses authentication information stored in an in-memory object collection, which is initialized from an XML document (conf/tomcat-users.xml).
  • JAASRealm - Accesses authentication information through the Java Authentication & Authorization Service (JAAS) framework.

It is also possible to write your own Realm implementation, and integrate it with Tomcat. To do so, you need to:

  • Implement org.apache.catalina.Realm,
  • Place your compiled realm in $CATALINA_HOME/lib,
  • Declare your realm as described in the "Configuring a Realm" section below,
  • Declare your realm to the MBeans Descriptor.

Configuring a Realm

Before getting into the details of the standard Realm implementations, it is important to understand, in general terms, how a Realm is configured. In general, you will be adding an XML element to your conf/server.xml configuration file, that looks something like this:

<Realm className="... class name for this implementation"
       ... other attributes for this implementation .../>

The <Realm> element can be nested inside any one of of the following Container elements. The location of the Realm element has a direct impact on the "scope" of that Realm (i.e. which web applications will share the same authentication information):

  • Inside an <Engine> element - This Realm will be shared across ALL web applications on ALL virtual hosts, UNLESS it is overridden by a Realm element nested inside a subordinate <Host> or <Context> element.
  • Inside a <Host> element - This Realm will be shared across ALL web applications for THIS virtual host, UNLESS it is overridden by a Realm element nested inside a subordinate <Context> element.
  • Inside a <Context> element - This Realm will be used ONLY for THIS web application.
Common Features
Digested Passwords

For each of the standard Realm implementations, the user's password (by default) is stored in clear text. In many environments, this is undesirable because casual observers of the authentication data can collect enough information to log on successfully, and impersonate other users. To avoid this problem, the standard implementations support the concept of digesting user passwords. This allows the stored version of the passwords to be encoded (in a form that is not easily reversible), but that the Realm implementation can still utilize for authentication.

When a standard realm authenticates by retrieving the stored password and comparing it with the value presented by the user, you can select digested passwords by specifying the digest attribute on your <Realm> element. The value for this attribute must be one of the digest algorithms supported by the class (SHA, MD2, or MD5). When you select this option, the contents of the password that is stored in the Realm must be the cleartext version of the password, as digested by the specified algorithm.

When the authenticate() method of the Realm is called, the (cleartext) password specified by the user is itself digested by the same algorithm, and the result is compared with the value returned by the Realm. An equal match implies that the cleartext version of the original password is the same as the one presented by the user, so that this user should be authorized.

To calculate the digested value of a cleartext password, two convenience techniques are supported:

  • If you are writing an application that needs to calculate digested passwords dynamically, call the static Digest() method of the org.apache.catalina.realm.RealmBase class, passing the cleartext password and the digest algorithm name as arguments. This method will return the digested password.
  • If you want to execute a command line utility to calculate the digested password, simply execute
    CATALINA_HOME/bin/digest.[bat|sh] -a {algorithm} {cleartext-password}
    and the digested version of this cleartext password will be returned to standard output.

If using digested passwords with DIGEST authentication, the cleartext used to generate the digest is different and the digest must use the MD5 algorithm. In the examples above {cleartext-password} must be replaced with {username}:{realm}:{cleartext-password}. For example, in a development environment this might take the form testUser:Authentication required:testPassword. The value for {realm} is taken from the <realm-name> element of the web application's <login-config>. If not specified in web.xml, the default value of Authentication required is used.

Non-ASCII usernames and/or passwords are supported using

CATALINA_HOME/bin/digest.[bat|sh] -a {algorithm} -e {encoding} {input}
but care is required to ensure that the non-ASCII input is correctly passed to the digester. The digester returns {input}:{digest}. If the input appears corrupted in the return, the digest will be invalid.

Example Application

The example application shipped with Tomcat includes an area that is protected by a security constraint, utilizing form-based login. To access it, point your browser at http://localhost:8080/examples/jsp/security/protected/ and log on with one of the usernames and passwords described for the default UserDatabaseRealm.