When you view any page in Guacamole, whether that be the login screen or the client interface, the page makes an authentication attempt with the web application, sending all available credentials. After entering your username and password, the exact same process occurs, except the web application receives the username and password as well.

The web application handles this authentication attempt by collecting all credentials available and passing them to designated classes called "authentication providers". Given the set of credentials, authentication providers return a context object that provides restricted access to other users and connections, if any.

For simplicity's sake, and because this is how things are done upstream in the Guacamole project, we will use Maven to build our extension.

The bare minimum required for a Guacamole authentication extension is a pom.xml file listing guacamole-ext as a dependency, a single .java file implementing our stub of an authentication provider, and a guac-manifest.json file describing the extension and pointing to our authentication provider class.

In our stub, we won't actually do any authentication yet; we'll just universally reject all authentication attempts by returning null for any credentials given. You can verify that this is what happens by checking the server logs.

<project xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
                        http://maven.apache.org/maven-v4_0_0.xsd">

    <modelVersion>4.0.0</modelVersion>
    <groupId>org.apache.guacamole</groupId>
    <artifactId>guacamole-auth-tutorial</artifactId>
    <packaging>jar</packaging>
    <version>0.9.13-incubating</version>
    <name>guacamole-auth-tutorial</name>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <build>
        <plugins>

            <!-- Written for 1.6 -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.3</version>
                <configuration>
                    <source>1.6</source>
                    <target>1.6</target>
                </configuration>
            </plugin>

        </plugins>
    </build>

    <dependencies>

        <!-- Guacamole Extension API -->
        <dependency>
            <groupId>org.apache.guacamole</groupId>
            <artifactId>guacamole-ext</artifactId>
            <version>0.9.13-incubating</version>
            <scope>provided</scope>
        </dependency>

    </dependencies>

</project>

We won't need to update this pom.xml throughout the rest of the tutorial. Even after adding new files, Maven will just find them and compile as necessary.

Naturally, we need the actual authentication extension skeleton code. While you can put this in whatever file and package you want, for the sake of this tutorial, we will assume you are using org.apache.guacamole.auth.TutorialAuthenticationProvider.

package org.apache.guacamole.auth;

import java.util.Map;
import org.apache.guacamole.GuacamoleException;
import org.apache.guacamole.net.auth.simple.SimpleAuthenticationProvider;
import org.apache.guacamole.net.auth.Credentials;
import org.apache.guacamole.protocol.GuacamoleConfiguration;

/**
 * Authentication provider implementation intended to demonstrate basic use
 * of Guacamole's extension API. The credentials and connection information for
 * a single user are stored directly in guacamole.properties.
 */
public class TutorialAuthenticationProvider extends SimpleAuthenticationProvider {

    @Override
    public String getIdentifier() {
        return "tutorial";
    }

    @Override
    public Map<String, GuacamoleConfiguration>
        getAuthorizedConfigurations(Credentials credentials)
        throws GuacamoleException {

        // Do nothing ... yet
        return null;        

    }

}

To conform with Maven, this skeleton file must be placed within src/main/java/org/apache/guacamole/auth as TutorialAuthenticationProvider.java.

Notice how simple the authentication provider is. The SimpleAuthenticationProvider base class simplifies the AuthenticationProvider interface, requiring nothing more than a unique identifier (we will use "tutorial") and a single getAuthorizedConfigurations() implementation, which must return a Map of GuacamoleConfiguration each associated with some arbitrary unique ID. This unique ID will be presented to the user in the connection list after they log in.

For now, getAuthorizedConfigurations() will just return null. This will cause Guacamole to report an invalid login for every attempt. Note that there is a difference in semantics between returning an empty map and returning null, as the former indicates the credentials are authorized but simply have no associated configurations, while the latter indicates the credentials are not authorized at all.

The only remaining piece for the overall skeleton to be complete is a guac-manifest.json file. This file is absolutely required for all Guacamole extensions. The guac-manifest.json format is described in more detail in Chapter 18, guacamole-ext. It provides for quite a few properties, but for our authentication extension we are mainly interested in the Guacamole version sanity check (to make sure an extension built for the API of Guacamole version X is not accidentally used against version Y) and telling Guacamole where to find our authentication provider class.

The Guacamole extension format requires that guac-manifest.json be placed in the root directory of the extension .jar file. To accomplish this with Maven, we place it within the src/main/resources directory. Maven will automatically pick it up during the build and include it within the .jar.

{

    "guacamoleVersion" : "0.9.13-incubating",

    "name"      : "Tutorial Authentication Extension",
    "namespace" : "guac-auth-tutorial",

    "authProviders" : [
        "org.apache.guacamole.auth.TutorialAuthenticationProvider"
    ]

}

Once all three of the above files are in place, the extension will build, and can even be installed within Guacamole (see the section called “Installing the extension” at the end of this chapter), even though it is just a skeleton at this point. It won't do anything yet other than reject all authentication attempts, but it's good to at least try building the extension to make sure nothing is missing and that all steps have been followed correctly so far:

$ mvn package
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Building guacamole-auth-tutorial 0.9.13-incubating
[INFO] ------------------------------------------------------------------------
...
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.345 s
[INFO] Finished at: 2015-12-16T13:39:00-08:00
[INFO] Final Memory: 14M/138M
[INFO] ------------------------------------------------------------------------
$

Assuming you see the "BUILD SUCCESS" message when you build the extension, there will be a new file, target/guacamole-auth-tutorial-0.9.13-incubating.jar, which can be installed within Guacamole and tested. If you changed the name or version of the project in the pom.xml file, the name of this new .jar file will be different, but it can still be found within target/.

Once we receive credentials, we need to validate those credentials against the associated properties in guacamole.properties (our source of authentication information for the sake of this tutorial).

We will define four properties:

If the username and password match what is stored in the file, we read the configuration information, store it in a GuacamoleConfiguration, and return the configuration within a set, telling Guacamole that this user is authorized but only to access the configurations returned.

Upstream, we always place the properties of authentication providers in their own class, and so we will also do that here in this tutorial, as it keeps things organized.

package org.apache.guacamole.auth;

import org.apache.guacamole.properties.StringGuacamoleProperty;

/**
 * Utility class containing all properties used by the custom authentication
 * tutorial. The properties defined here must be specified within
 * guacamole.properties to configure the tutorial authentication provider.
 */
public class TutorialGuacamoleProperties {

    /**
     * This class should not be instantiated.
     */
    private TutorialGuacamoleProperties() {}

    /**
     * The only user to allow.
     */
    public static final StringGuacamoleProperty TUTORIAL_USER = 
        new StringGuacamoleProperty() {

        @Override
        public String getName() { return "tutorial-user"; }

    };

    /**
     * The password required for the specified user.
     */
    public static final StringGuacamoleProperty TUTORIAL_PASSWORD = 
        new StringGuacamoleProperty() {

        @Override
        public String getName() { return "tutorial-password"; }

    };


    /**
     * The protocol to use when connecting.
     */
    public static final StringGuacamoleProperty TUTORIAL_PROTOCOL = 
        new StringGuacamoleProperty() {

        @Override
        public String getName() { return "tutorial-protocol"; }

    };


    /**
     * All parameters associated with the connection, as a comma-delimited
     * list of name="value" 
     */
    public static final StringGuacamoleProperty TUTORIAL_PARAMETERS = 
        new StringGuacamoleProperty() {

        @Override
        public String getName() { return "tutorial-parameters"; }

    };

}

Normally, we would define a new type of GuacamoleProperty to handle the parsing of the parameters required by TUTORIAL_PARAMETERS, but for the sake of simplicity, parsing of this parameter will be embedded in the authentication function later.

You will need to modify your existing guacamole.properties file, adding each of the above properties to describe one of your available connections.

# Username and password
tutorial-user:     tutorial
tutorial-password: password

# Connection information
tutorial-protocol:   vnc
tutorial-parameters: hostname=localhost, port=5900

Once these properties and their accessor class are in place, it's simple enough to read the properties within getAuthorizedConfigurations() and authenticate the user based on their username and password.

@Override
public Map<String, GuacamoleConfiguration>
    getAuthorizedConfigurations(Credentials credentials)
    throws GuacamoleException {

    // Get the Guacamole server environment
    Environment environment = new LocalEnvironment();

    // Get username from guacamole.properties
    String username = environment.getRequiredProperty(
        TutorialGuacamoleProperties.TUTORIAL_USER
    );      

    // If wrong username, fail
    if (!username.equals(credentials.getUsername()))
        return null;

    // Get password from guacamole.properties
    String password = environment.getRequiredProperty(
        TutorialGuacamoleProperties.TUTORIAL_PASSWORD
    );      

    // If wrong password, fail
    if (!password.equals(credentials.getPassword()))
        return null;

    // Successful login. Return configurations (STUB)
    return new HashMap<String, GuacamoleConfiguration>();

}

As is, the authentication provider will work in its current state in that the correct username and password will authenticate the user, while an incorrect username or password will not, but we still aren't returning an actual map of configurations. We need to construct the configuration based on the properties in the guacamole.properties file after the user has been authenticated, and return that configuration to the web application.

The only remaining task before we have a fully-functioning authentication provider is to actually parse the configuration from the guacamole.properties file.

@Override
public Map<String, GuacamoleConfiguration>
    getAuthorizedConfigurations(Credentials credentials)
    throws GuacamoleException {

    // Get the Guacamole server environment
    Environment environment = new LocalEnvironment();

    // Get username from guacamole.properties
    String username = environment.getRequiredProperty(
        TutorialGuacamoleProperties.TUTORIAL_USER
    );      

    // If wrong username, fail
    if (!username.equals(credentials.getUsername()))
        return null;

    // Get password from guacamole.properties
    String password = environment.getRequiredProperty(
        TutorialGuacamoleProperties.TUTORIAL_PASSWORD
    );      

    // If wrong password, fail
    if (!password.equals(credentials.getPassword()))
        return null;

    // Successful login. Return configurations.
    Map<String, GuacamoleConfiguration> configs = 
        new HashMap<String, GuacamoleConfiguration>();

    // Create new configuration
    GuacamoleConfiguration config = new GuacamoleConfiguration();

    // Set protocol specified in properties
    config.setProtocol(environment.getRequiredProperty(
        TutorialGuacamoleProperties.TUTORIAL_PROTOCOL
    ));

    // Set all parameters, splitting at commas
    for (String parameterValue : environment.getRequiredProperty(
        TutorialGuacamoleProperties.TUTORIAL_PARAMETERS
    ).split(",\\s*")) {

        // Find the equals sign
        int equals = parameterValue.indexOf('=');
        if (equals == -1)
            throw new GuacamoleServerException("Required equals sign missing");

        // Get name and value from parameter string
        String name = parameterValue.substring(0, equals);
        String value = parameterValue.substring(equals+1);

        // Set parameter as specified
        config.setParameter(name, value);

    }

    configs.put("Tutorial Connection", config);
    return configs;

}

The extension is now complete and can be built as described earlier in the section called “Building the extension”.

Guacamole extensions are self-contained .jar files which are installed by being placed within GUACAMOLE_HOME/extensions, and this extension is no different. As described in Chapter 5, Configuring Guacamole, GUACAMOLE_HOME is a placeholder used to refer to the directory that Guacamole uses to locate its configuration files and extensions. Typically, this will be the .guacamole directory within the home directory of the user running Tomcat.

To install your extension, ensure that the required properties have been added to your guacamole.properties, copy the target/guacamole-auth-tutorial-0.9.13-incubating.jar file into GUACAMOLE_HOME/extensions and restart Tomcat. Guacamole will automatically load your extension, logging an informative message that it has done so:

Extension "Tutorial Authentication Extension" loaded.