SplotyCode/netty-dns-support

Enhances Netty's DNS support with the ability to parse and programmatically manipulate DNS records with a clean API

Netty DNS Support

This is an additional library, which hopefully can be contributed to Netty, which makes it easy to work with DNS records programmatically.

It builds on the patches in this fork of Netty (pull request pending) - diff here, which refactors a few things, adds mDNS support (Rendezvous/Avahi) and flexible writing of DNS names supporting pointer compression, punycode and mDNS's UTF-8 encoding.

In particular, it adds a TypedDnsRecordDecoder and TypedDnsRecordEncoder which encode and decode records using a CodecRegistry which can be added to at runtime. So records can be decoded into Java objects of whatever type you'd like; there is built-in support for the record types A, AAAA, NS, PTR, CNAME, DNAME, SOA, TXT, LOC, OPT, SRV, MX, NSEC and URI.

Additionally, a similar codec registry is used for the sub-records in OPT pseudo-records, and DnsRecordCodec instances are used to encode and decode those (ECS and COOKIE are supported out-of-the-box).

Adding a DnsRecordCodec

Record codecs are simple (where what they encode and decode is, anyway):

final class MailExchangerRecordCodec extends DnsRecordCodec<MailExchanger> {

    MailExchangerRecordCodec() {
        super(MailExchanger.class);
    }

    @Override
    public MailExchanger read(ByteBuf buf, NameCodec forReadingNames, int length) throws DnsDecoderException,
            UnmappableCharacterException {
        int pref = buf.readShort();
        CharSequence mx = forReadingNames.readName(buf);
        return new MailExchanger(pref, mx);
    }

    @Override
    public void write(MailExchanger value, NameCodec names, ByteBuf into) throws IOException {
        into.writeShort(value.pref());
        names.writeName(value.mx(), into);
    }
}

Using a Record Codec

Creating a server or client starts with a message encoder and decoder. Using the codec above could be as simple as:

        DnsMessageEncoder x = DnsMessageEncoder.builder()
                .withRecordEncoder(new TypedDnsRecordEncoder(DnsRecordCodecRegistry
                        .builderWithDefaultCodecs()
                        .add(new MailExchangerRecordCodec(), DnsRecordType.MX)))
                .mDNS()
                .withIllegalRecordPolicy(THROW)
                .withNameFeatures(MDNS_UTF_8, COMPRESSION, WRITE_TRAILING_DOT)
                .build();

Then decoded MX records will have a payload of this type. The code is nearly identical for decoding.

Building

The Netty fork has its own version number, 4.1-dns-dev which this code depends on, so for now you will need to build it.

Example

For a brief example that does something useful, here is a program that lists Avahi/Rendevous mDNS service messages going by on the local network, after issuing a query to all hosts on that network to enumerate their services:

public class MdnsTest {
    private final EventLoopGroup group;
    private final DnsClientHandler clientHandler;
    private final ChannelFuture closeFuture;
    private MdnsTest(NetworkInterface iface) throws Exception {
        this.group = new NioEventLoopGroup();
        this.clientHandler = new DnsClientHandler();
        Bootstrap b = new Bootstrap();
        b.group(group)
            .channel(NioDatagramChannel.class)
            .option(ChannelOption.IP_MULTICAST_IF, iface)
            .option(ChannelOption.SO_REUSEADDR, true)
            .option(ChannelOption.IP_MULTICAST_LOOP_DISABLED, false)
            .option(ChannelOption.ALLOCATOR, PooledByteBufAllocator.DEFAULT)
            .handler(DnsMessageDecoder.builder()
                    .withRecordDecoder(
                            new TypedDnsRecordDecoder(true))
                    .mDNS()
                    .buildUdpQueryAndResponseDecoder());

        Channel channel = b.bind(5353)
                .sync()
                .channel();
        ChannelPipeline pipeline = channel.pipeline();
        pipeline.addLast("responseDecoder", clientHandler);
        pipeline.addLast("queryEncoder", DnsMessageEncoder
                .builder().mDNS().buildUdpQueryEncoder());
        this.closeFuture = channel.closeFuture();

        // Ask all mDNS devices on the local network to enumerate their services
        InetSocketAddress localMulticast6
          = new InetSocketAddress(Inet6Address.getByName("FF02::FB"), 5353);

        InetSocketAddress localMulticast4
          = new InetSocketAddress(Inet4Address.getByName("224.0.0.251"), 5353);

        DatagramDnsQuery query = new DatagramDnsQuery(null, localMulticast6, 1);
        DefaultDnsQuestion question = new DefaultDnsQuestion(
                "_services._dns-sd._udp.local", DnsRecordType.PTR);

        query.addRecord(DnsSection.QUESTION, question);
        channel.writeAndFlush(query);

        query = new DatagramDnsQuery(null, localMulticast4, 1);
        query.addRecord(DnsSection.QUESTION, question);
        channel.writeAndFlush(query);
    }

    class DnsClientHandler extends SimpleChannelInboundHandler<DnsResponse> {
        DnsClientHandler() {
            super(DnsResponse.class);
        }
        @Override
        protected void channelRead0(ChannelHandlerContext chc,
                DnsResponse msg) throws Exception {
            
            System.out.println("\n**************************\n");
            System.out.println(msg);
        }
    }

    public static void main(String[] args) throws Exception {
        NetworkInterface iface = null;
        Enumeration<NetworkInterface> en = NetworkInterface.getNetworkInterfaces();
        while (en.hasMoreElements()) {
            NetworkInterface in = en.nextElement();
            if (in.isUp() && !in.isLoopback() && !in.isVirtual()) {
                iface = in;
                break;
            }
        }
        if (iface == null) {
            throw new Error("Could not find a network interface");
        }
        System.out.println("Using interface " + iface.getDisplayName());
        MdnsTest test = new MdnsTest(iface);
        test.closeFuture.await();
    }
}