/projects/james-2.2.0/src/java/org/apache/james/dnsserver/DNSServer.java

/***********************************************************************
 * Copyright (c) 2000-2004 The Apache Software Foundation.             *
 * All rights reserved.                                                *
 * ------------------------------------------------------------------- *
 * Licensed 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:                                *
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 *     http://www.apache.org/licenses/LICENSE-2.0                      *
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 * distributed under the License is distributed on an "AS IS" BASIS,   *
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 * implied.  See the License for the specific language governing       *
 * permissions and limitations under the License.                      *
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package org.apache.james.dnsserver;

import org.apache.avalon.framework.activity.Initializable;
import org.apache.avalon.framework.activity.Disposable;
import org.apache.avalon.framework.configuration.Configurable;
import org.apache.avalon.framework.configuration.Configuration;
import org.apache.avalon.framework.configuration.ConfigurationException;
import org.apache.avalon.framework.logger.AbstractLogEnabled;
import org.xbill.DNS.Cache;
import org.xbill.DNS.Credibility;
import org.xbill.DNS.DClass;
import org.xbill.DNS.ExtendedResolver;
import org.xbill.DNS.FindServer;
import org.xbill.DNS.Lookup;
import org.xbill.DNS.Message;
import org.xbill.DNS.MXRecord;
import org.xbill.DNS.ARecord;
import org.xbill.DNS.Name;
import org.xbill.DNS.Rcode;
import org.xbill.DNS.Record;
import org.xbill.DNS.Resolver;
import org.xbill.DNS.RRset;
import org.xbill.DNS.SetResponse;
import org.xbill.DNS.TextParseException;
import org.xbill.DNS.Type;

import java.net.InetAddress;
import java.net.UnknownHostException;
import java.util.*;

/**
 * Provides DNS client functionality to services running
 * inside James
 */
public class DNSServer
    extends AbstractLogEnabled
    implements Configurable, Initializable, Disposable,
    org.apache.james.services.DNSServer, DNSServerMBean {

    /**
     * A resolver instance used to retrieve DNS records.  This
     * is a reference to a third party library object.
     */
    private Resolver resolver;

    /**
     * A TTL cache of results received from the DNS server.  This
     * is a reference to a third party library object.
     */
    private Cache cache;

    /**
     * Whether the DNS response is required to be authoritative
     */
    private int dnsCredibility;

    /**
     * The DNS servers to be used by this service
     */
    private List dnsServers = new ArrayList();

    /**
     * The MX Comparator used in the MX sort.
     */
    private Comparator mxComparator = new MXRecordComparator();

    /**
     * @see org.apache.avalon.framework.configuration.Configurable#configure(Configuration)
     */
    public void configure( final Configuration configuration )
        throws ConfigurationException {

        final boolean autodiscover =
            configuration.getChild( "autodiscover" ).getValueAsBoolean( true );

        if (autodiscover) {
            getLogger().info("Autodiscovery is enabled - trying to discover your system's DNS Servers");
            String[] serversArray = FindServer.servers();
            if (serversArray != null) {
                for ( int i = 0; i < serversArray.length; i++ ) {
                    dnsServers.add(serversArray[ i ]);
                    getLogger().info("Adding autodiscovered server " + serversArray[i]);
                }
            }
        }

        // Get the DNS servers that this service will use for lookups
        final Configuration serversConfiguration = configuration.getChild( "servers" );
        final Configuration[] serverConfigurations =
            serversConfiguration.getChildren( "server" );

        for ( int i = 0; i < serverConfigurations.length; i++ ) {
            dnsServers.add( serverConfigurations[ i ].getValue() );
        }

        if (dnsServers.isEmpty()) {
            getLogger().info("No DNS servers have been specified or found by autodiscovery - adding 127.0.0.1");
            dnsServers.add("127.0.0.1");
        }

        final boolean authoritative =
            configuration.getChild( "authoritative" ).getValueAsBoolean( false );
        // TODO: Check to see if the credibility field is being used correctly.  From the
        //       docs I don't think so
        dnsCredibility = authoritative ? Credibility.AUTH_ANSWER : Credibility.NONAUTH_ANSWER;
    }

    /**
     * @see org.apache.avalon.framework.activity.Initializable#initialize()
     */
    public void initialize()
        throws Exception {

        getLogger().debug("DNSServer init...");

        // If no DNS servers were configured, default to local host
        if (dnsServers.isEmpty()) {
            try {
                dnsServers.add( InetAddress.getLocalHost().getHostName() );
            } catch ( UnknownHostException ue ) {
                dnsServers.add( "127.0.0.1" );
            }
        }

        //Create the extended resolver...
        final String[] serversArray = (String[])dnsServers.toArray(new String[0]);

        if (getLogger().isInfoEnabled()) {
            for(int c = 0; c < serversArray.length; c++) {
                getLogger().info("DNS Server is: " + serversArray[c]);
            }
        }

        try {
            resolver = new ExtendedResolver( serversArray );
            Lookup.setDefaultResolver(resolver);
        } catch (UnknownHostException uhe) {
            getLogger().fatalError("DNS service could not be initialized.  The DNS servers specified are not recognized hosts.", uhe);
            throw uhe;
        }

        cache = new Cache (DClass.IN);

        getLogger().debug("DNSServer ...init end");
    }

    /**
     * <p>Return the list of DNS servers in use by this service</p>
     *
     * @return an array of DNS server names
     */
    public String[] getDNSServers() {
        return (String[])dnsServers.toArray(new String[0]);
    }
    
    /**
     * <p>Return a prioritized unmodifiable list of MX records
     * obtained from the server.</p>
     *
     * @param hostname domain name to look up
     *
     * @return a unmodifiable list of MX records corresponding to
     *         this mail domain name
     */
    public Collection findMXRecords(String hostname) {
        Record answers[] = lookup(hostname, Type.MX);
        List servers = new ArrayList();
        try {
            if (answers == null) {
                return servers;
            }

            MXRecord mxAnswers[] = new MXRecord[answers.length];
            for (int i = 0; i < answers.length; i++) {
                mxAnswers[i] = (MXRecord)answers[i];
            }

            Arrays.sort(mxAnswers, mxComparator);

            for (int i = 0; i < mxAnswers.length; i++) {
                servers.add(mxAnswers[i].getTarget ().toString ());
                getLogger().debug(new StringBuffer("Found MX record ").append(mxAnswers[i].getTarget ().toString ()).toString());
            }
            return Collections.unmodifiableCollection(servers);
        } finally {
            //If we found no results, we'll add the original domain name if
            //it's a valid DNS entry
            if (servers.size () == 0) {
                StringBuffer logBuffer =
                    new StringBuffer(128)
                            .append("Couldn't resolve MX records for domain ")
                            .append(hostname)
                            .append(".");
                getLogger().info(logBuffer.toString());
                try {
                    getByName(hostname);
                    servers.add(hostname);
                } catch (UnknownHostException uhe) {
                    // The original domain name is not a valid host,
                    // so we can't add it to the server list.  In this
                    // case we return an empty list of servers
                    logBuffer = new StringBuffer(128)
                                .append("Couldn't resolve IP address for host ")
                                .append(hostname)
                                .append(".");
                    getLogger().error(logBuffer.toString());
                }
            }
        }
    }

    /**
     * Looks up DNS records of the specified type for the specified name.
     *
     * This method is a public wrapper for the private implementation
     * method
     *
     * @param name the name of the host to be looked up
     * @param type the type of record desired
     */
    public Record[] lookup(String name, int type) {
        return rawDNSLookup(name,false,type);
    }

    /**
     * Looks up DNS records of the specified type for the specified name
     *
     * @param namestr the name of the host to be looked up
     * @param querysent whether the query has already been sent to the DNS servers
     * @param type the type of record desired
     */
    private Record[] rawDNSLookup(String namestr, boolean querysent, int type) {
        Name name = null;
        try {
            name = Name.fromString(namestr, Name.root);
        } catch (TextParseException tpe) {
            // TODO: Figure out how to handle this correctly.
            getLogger().error("Couldn't parse name " + namestr, tpe);
            return null;
        }
        int dclass = DClass.IN;

        SetResponse cached = cache.lookupRecords(name, type, dnsCredibility);
        if (cached.isSuccessful()) {
            getLogger().debug(new StringBuffer(256)
                             .append("Retrieving MX record for ")
                             .append(name).append(" from cache")
                             .toString());

            return processSetResponse(cached);
        }
        else if (cached.isNXDOMAIN() || cached.isNXRRSET()) {
            return null;
        }
        else if (querysent) {
            return null;
        }
        else {
            getLogger().debug(new StringBuffer(256)
                             .append("Looking up MX record for ")
                             .append(name)
                             .toString());
            Record question = Record.newRecord(name, type, dclass);
            Message query = Message.newQuery(question);
            Message response = null;

            try {
                response = resolver.send(query);
            }
            catch (Exception ex) {
                getLogger().warn("Query error!", ex);
                return null;
            }

            int rcode = response.getHeader().getRcode();
            if (rcode == Rcode.NOERROR || rcode == Rcode.NXDOMAIN) {
                cached = cache.addMessage(response);
                if (cached != null && cached.isSuccessful()) {
                    return processSetResponse(cached);
                }
            }

            if (rcode != Rcode.NOERROR) {
                return null;
            }

            return rawDNSLookup(namestr, true, type);
        }
    }
    
    private Record[] processSetResponse(SetResponse sr) {
        Record [] answers;
        int answerCount = 0, n = 0;

        RRset [] rrsets = sr.answers();
        answerCount = 0;
        for (int i = 0; i < rrsets.length; i++) {
            answerCount += rrsets[i].size();
        }

        answers = new Record[answerCount];

        for (int i = 0; i < rrsets.length; i++) {
            Iterator iter = rrsets[i].rrs();
            while (iter.hasNext()) {
                Record r = (Record)iter.next();
                answers[n++] = r;
            }
        }
        return answers;
    }

    /* RFC 2821 section 5 requires that we sort the MX records by their
     * preference, and introduce a randomization.  This Comparator does
     * comparisons as normal unless the values are equal, in which case
     * it "tosses a coin", randomly speaking.
     *
     * This way MX record w/preference 0 appears before MX record
     * w/preference 1, but a bunch of MX records with the same preference
     * would appear in different orders each time.
     *
     * Reminder for maintainers: the return value on a Comparator can
     * be counter-intuitive for those who aren't used to the old C
     * strcmp function:
     *
     * < 0 ==> a < b
     * = 0 ==> a = b
     * > 0 ==> a > b
     */
    private static class MXRecordComparator implements Comparator {
        private final static Random random = new Random();
        public int compare (Object a, Object b) {
            int pa = ((MXRecord)a).getPriority();
            int pb = ((MXRecord)b).getPriority();
            return (pa == pb) ? (512 - random.nextInt(1024)) : pa - pb;
        }
    }

    /*
     * Returns an Iterator over org.apache.mailet.HostAddress, a
     * specialized subclass of javax.mail.URLName, which provides
     * location information for servers that are specified as mail
     * handlers for the given hostname.  This is done using MX records,
     * and the HostAddress instances are returned sorted by MX priority.
     * If no host is found for domainName, the Iterator returned will be
     * empty and the first call to hasNext() will return false.  The
     * Iterator is a nested iterator: the outer iteration is over the
     * results of the MX record lookup, and the inner iteration is over
     * potentially multiple A records for each MX record.  DNS lookups
     * are deferred until actually needed.
     *
     * @since v2.2.0a16-unstable
     * @param domainName - the domain for which to find mail servers
     * @return an Iterator over HostAddress instances, sorted by priority
     */
    public Iterator getSMTPHostAddresses(final String domainName) {
        return new Iterator() {
            private Iterator mxHosts = findMXRecords(domainName).iterator();
            private Iterator addresses = null;

            public boolean hasNext() {
                /* Make sure that when next() is called, that we can
                 * provide a HostAddress.  This means that we need to
                 * have an inner iterator, and verify that it has
                 * addresses.  We could, for example, run into a
                 * situation where the next mxHost didn't have any valid
                 * addresses.
                 */
                if ((addresses == null || !addresses.hasNext()) && mxHosts.hasNext()) do {
                    final String nextHostname = (String)mxHosts.next();
                    InetAddress[] addrs = null;
                    try {
                        addrs = getAllByName(nextHostname);
                    } catch (UnknownHostException uhe) {
                        // this should never happen, since we just got
                        // this host from mxHosts, which should have
                        // already done this check.
                        StringBuffer logBuffer = new StringBuffer(128)
                                                 .append("Couldn't resolve IP address for discovered host ")
                                                 .append(nextHostname)
                                                 .append(".");
                        getLogger().error(logBuffer.toString());
                    }
                    final InetAddress[] ipAddresses = addrs;

                    addresses = new Iterator() {
                        int i = 0;

                        public boolean hasNext() {
                            return ipAddresses != null && i < ipAddresses.length;
                        }

                        public Object next() {
                            return new org.apache.mailet.HostAddress(nextHostname, "smtp://" + ipAddresses[i++].getHostAddress());
                        }

                        public void remove() {
                            throw new UnsupportedOperationException ("remove not supported by this iterator");
                        }
                    };
                } while (!addresses.hasNext() && mxHosts.hasNext());

                return addresses != null && addresses.hasNext();
            }

            public Object next() {
                return addresses != null ? addresses.next() : null;
            }

            public void remove() {
                throw new UnsupportedOperationException ("remove not supported by this iterator");
            }
        };
    }

    /* java.net.InetAddress.get[All]ByName(String) allows an IP literal
     * to be passed, and will recognize it even with a trailing '.'.
     * However, org.xbill.DNS.Address does not recognize an IP literal
     * with a trailing '.' character.  The problem is that when we
     * lookup an MX record for some domains, we may find an IP address,
     * which will have had the trailing '.' appended by the time we get
     * it back from dnsjava.  An MX record is not allowed to have an IP
     * address as the right-hand-side, but there are still plenty of
     * such records on the Internet.  Since java.net.InetAddress can
     * handle them, for the time being we've decided to support them.
     *
     * These methods are NOT intended for use outside of James, and are
     * NOT declared by the org.apache.james.services.DNSServer.  This is
     * currently a stopgap measure to be revisited for the next release.
     */

    private static String allowIPLiteral(String host) {
        if ((host.charAt(host.length() - 1) == '.')) {
            String possible_ip_literal = host.substring(0, host.length() - 1);
            if (org.xbill.DNS.Address.isDottedQuad(possible_ip_literal)) {
                host = possible_ip_literal;
            }
        }
        return host;
    }

    /**
     * @see java.net.InetAddress#getByName(String)
     */
    public static InetAddress getByName(String host) throws UnknownHostException {
        return org.xbill.DNS.Address.getByName(allowIPLiteral(host));
    }

    /**
     * @see java.net.InetAddress#getByAllName(String)
     */
    public static InetAddress[] getAllByName(String host) throws UnknownHostException {
        return org.xbill.DNS.Address.getAllByName(allowIPLiteral(host));
    }

    /**
     * A way to get mail hosts to try.  If any MX hosts are found for the
     * domain name with which this is constructed, then these MX hostnames
     * are returned in priority sorted order, lowest priority numbers coming
     * first.  And, whenever multiple hosts have the same priority then these
     * are returned in a randomized order within that priority group, as
     * specified in RFC 2821, Section 5.
     *
     * If no MX hosts are found for the domain name, then a DNS search is
     * performed for an A record.  If an A record is found then domainName itself
     * will be returned by the Iterator, and it will be the only object in
     * the Iterator.  If however no A record is found (in addition to no MX
     * record) then the Iterator constructed will be empty; the first call to
     * its hasNext() will return false.
     *
     * This behavior attempts to satisfy the requirements of RFC 2821, Section 5.
     * @since v2.2.0a16-unstable
     */

    /**** THIS CODE IS BROKEN AND UNUSED ****/

    /* this code was used in getSMTPHostAddresses as:
       private Iterator mxHosts = new MxSorter(domainName);

       This class effectively replaces findMXRecords.  If
       it is to be kept, it should replace the body of that
       method.  The fixes would be to either implement a
       more robust DNS lookup, or to replace the Type.A
       lookup with InetAddress.getByName(), which is what
       findMXRecords uses.  */

    private class MxSorter implements Iterator {
        private int priorListPriority = Integer.MIN_VALUE;
        private ArrayList equiPriorityList = new ArrayList();
        private Record[] mxRecords;
        private Random rnd = new Random ();

        /* The implementation of this class attempts to achieve efficiency by
         * performing no more sorting of the rawMxRecords than necessary. In the
         * large majority of cases the first attempt, made by a client of this class
         * to connect to an SMTP server for a given domain, will succeed. As such,
         * in most cases only one call will be made to this Iterator's
         * next(), and in that majority of cases there will have been no need
         * to sort the array of MX Records.  This implementation would, however, be
         * relatively inefficient in the case where all hosts fail, when every
         * Object is called out of a long Iterator.
         */

        private MxSorter(String domainName) {
            mxRecords =  lookup(domainName, Type.MX);
            if (mxRecords == null || mxRecords.length == 0) {
                //no MX records were found, so try to use the domainName
                Record[] aRecords = lookup(domainName, Type.A);
                if(aRecords != null && aRecords.length > 0) {
                    equiPriorityList.add(domainName);
                }
            }
        }

        /**
         * Sets presentPriorityList to contain all hosts
         * which have the least priority greater than pastPriority.
         * When this is called, both (rawMxRecords.length > 0) and
         * (presentPriorityList.size() == 0), by contract.
         * In the case where this is called repeatedly, so that priorListPriority
         * has already become the highest of the priorities in the rawMxRecords,
         * then this returns without having added any elements to
         * presentPriorityList; presentPriorityList.size remains zero.
         */
        private void createPriorityList(){
            int leastPriorityFound = Integer.MAX_VALUE;
            /* We loop once through the rawMxRecords, finding the lowest priority
             * greater than priorListPriority, and collecting all the hostnames
             * with that priority into equiPriorityList.
             */
            for (int i = 0; i < mxRecords.length; i++) {
                MXRecord thisRecord = (MXRecord)mxRecords[i];
                int thisRecordPriority = thisRecord.getPriority();
                if (thisRecordPriority > priorListPriority) {
                    if (thisRecordPriority < leastPriorityFound) {
                        equiPriorityList.clear();
                        leastPriorityFound = thisRecordPriority;
                        equiPriorityList.add(thisRecord.getTarget().toString());
                    } else if (thisRecordPriority == leastPriorityFound) {
                        equiPriorityList.add(thisRecord.getTarget().toString());
                    }
                }
            }
            priorListPriority = leastPriorityFound;
        }

        public boolean hasNext(){
            if (equiPriorityList.size() > 0){
                return true;
            }else if (mxRecords != null && mxRecords.length > 0){
                createPriorityList();
                return equiPriorityList.size() > 0;
            } else{
                return false;
            }
        }

        public Object next(){
            if (hasNext()){
                /* this randomization is done to comply with RFC-2821 */
                /* Note: java.util.Random.nextInt(limit) is about twice as fast as (int)(Math.random()*limit) */
                int getIndex = rnd.nextInt(equiPriorityList.size());
                Object returnElement = equiPriorityList.get(getIndex);
                equiPriorityList.remove(getIndex);
                return returnElement;
            }else{
                throw new NoSuchElementException();
            }
        }

        public void remove () {
            throw new UnsupportedOperationException ("remove not supported by this iterator");
        }
    }

    
    /**
     * The dispose operation is called at the end of a components lifecycle.
     * Instances of this class use this method to release and destroy any
     * resources that they own.
     *
     * This implementation shuts down org.xbill.DNS.Cache
     *
     * @throws Exception if an error is encountered during shutdown
     */
    public void dispose()
    {
        //setting the clean interval to a negative value, will terminate
        //the Cache cleaner thread.
        cache.setCleanInterval (-1);
    } 
}