Apache ZooKeeper is an open-source server which enables highly reliable distributed coordination.

Apache ZooKeeper Server Source Code files are provided in the source packge (apache-zookeeper-3.7.0.tar.gz). You can download it at Apache ZooKeeper Website.

You can also browse Apache ZooKeeper Server Source Code below:

⏎ org/apache/zookeeper/server/quorum/auth/SaslQuorumServerCallbackHandler.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.zookeeper.server.quorum.auth;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import javax.security.auth.callback.Callback;
import javax.security.auth.callback.CallbackHandler;
import javax.security.auth.callback.NameCallback;
import javax.security.auth.callback.PasswordCallback;
import javax.security.auth.callback.UnsupportedCallbackException;
import javax.security.auth.login.AppConfigurationEntry;
import javax.security.auth.login.Configuration;
import javax.security.sasl.AuthorizeCallback;
import javax.security.sasl.RealmCallback;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * This is used by the SASL mechanisms to get further information to complete
 * the authentication. For example, a SASL mechanism might use this callback
 * handler to do verification operation. This is used by the QuorumServer to
 * perform the mutual quorum peer authentication.
 */
public class SaslQuorumServerCallbackHandler implements CallbackHandler {

    private static final String USER_PREFIX = "user_";
    private static final Logger LOG = LoggerFactory.getLogger(SaslQuorumServerCallbackHandler.class);

    private String userName;
    private final Map<String, String> credentials = new HashMap<>();
    private final Set<String> authzHosts;

    public SaslQuorumServerCallbackHandler(
        Configuration configuration,
        String serverSection,
        Set<String> authzHosts) throws IOException {
        AppConfigurationEntry[] configurationEntries = configuration.getAppConfigurationEntry(serverSection);

        if (configurationEntries == null) {
            String errorMessage = "Could not find a '" + serverSection + "' entry in this configuration: Server cannot start.";
            LOG.error(errorMessage);
            throw new IOException(errorMessage);
        }
        credentials.clear();
        for (AppConfigurationEntry entry : configurationEntries) {
            Map<String, ?> options = entry.getOptions();
            // Populate DIGEST-MD5 user -> password map with JAAS configuration entries from the "QuorumServer" section.
            // Usernames are distinguished from other options by prefixing the username with a "user_" prefix.
            for (Map.Entry<String, ?> pair : options.entrySet()) {
                String key = pair.getKey();
                if (key.startsWith(USER_PREFIX)) {
                    String userName = key.substring(USER_PREFIX.length());
                    credentials.put(userName, (String) pair.getValue());
                }
            }
        }

        // authorized host lists
        this.authzHosts = authzHosts;
    }

    public void handle(Callback[] callbacks) throws UnsupportedCallbackException {
        for (Callback callback : callbacks) {
            if (callback instanceof NameCallback) {
                handleNameCallback((NameCallback) callback);
            } else if (callback instanceof PasswordCallback) {
                handlePasswordCallback((PasswordCallback) callback);
            } else if (callback instanceof RealmCallback) {
                handleRealmCallback((RealmCallback) callback);
            } else if (callback instanceof AuthorizeCallback) {
                handleAuthorizeCallback((AuthorizeCallback) callback);
            }
        }
    }

    private void handleNameCallback(NameCallback nc) {
        // check to see if this user is in the user password database.
        if (credentials.get(nc.getDefaultName()) == null) {
            LOG.warn("User '{}' not found in list of DIGEST-MD5 authenticateable users.", nc.getDefaultName());
            return;
        }
        nc.setName(nc.getDefaultName());
        userName = nc.getDefaultName();
    }

    private void handlePasswordCallback(PasswordCallback pc) {
        if (credentials.containsKey(userName)) {
            pc.setPassword(credentials.get(userName).toCharArray());
        } else {
            LOG.warn("No password found for user: {}", userName);
        }
    }

    private void handleRealmCallback(RealmCallback rc) {
        LOG.debug("QuorumLearner supplied realm: {}", rc.getDefaultText());
        rc.setText(rc.getDefaultText());
    }

    private void handleAuthorizeCallback(AuthorizeCallback ac) {
        String authenticationID = ac.getAuthenticationID();
        String authorizationID = ac.getAuthorizationID();

        boolean authzFlag = false;
        // 1. Matches authenticationID and authorizationID
        authzFlag = authenticationID.equals(authorizationID);

        // 2. Verify whether the connecting host is present in authorized hosts.
        // If not exists, then connecting peer is not authorized to join the
        // ensemble and will reject it.
        if (authzFlag) {
            String[] components = authorizationID.split("[/@]");
            if (components.length == 3) {
                authzFlag = authzHosts.contains(components[1]);
            }
            if (!authzFlag) {
                LOG.error("SASL authorization completed, {} is not authorized to connect", components[1]);
            }
        }

        // Sets authorization flag
        ac.setAuthorized(authzFlag);
        if (ac.isAuthorized()) {
            ac.setAuthorizedID(authorizationID);
            LOG.info("Successfully authenticated learner: authenticationID={};  authorizationID={}.",
                     authenticationID, authorizationID);
        }
        LOG.debug("SASL authorization completed, authorized flag set to {}", ac.isAuthorized());
    }

}

⏎ org/apache/zookeeper/server/quorum/auth/SaslQuorumServerCallbackHandler.java

Or download all of them as a single archive file:

File name: zookeeper-server-3.7.0-fyi.zip
File size: 871011 bytes
Release date: 2021-05-17
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