mon-compte-github/DNSClient.java

## README.md

      
    Raw
  

              README.md
            
          
    DNSClient

Quick and dirty implementation of a simple DNSClient. Uncomment DNS_SERVER_ADDRESS to change the DNS server (root, domain or recursive).
Run

When querying a standard DNS server (1.1.1.1 for example).
% javac DNSClient.java && java DNSClient www.heroku.com
Found 3 answer(s)
 * www.heroku.com. - CNAME : mysterious-spire-9566.polar-eyrie-3327.herokuspace.com.
 * mysterious-spire-9566.polar-eyrie-3327.herokuspace.com. - IP : 54.156.19.143
 * mysterious-spire-9566.polar-eyrie-3327.herokuspace.com. - IP : 34.197.145.217

% java DNSClient www.mit.edu   
Found 3 answer(s)
 * www.mit.edu. - CNAME : www.mit.edu.edgekey.net.
 * www.mit.edu.edgekey.net. - CNAME : e9566.dscb.akamaiedge.net.
 * e9566.dscb.akamaiedge.net. - IP : 23.43.211.181

When querying a root DNS server :
$ java DNSClient com           
Found 13 authoritative nameserver(s)
 * com. - NS : a.gtld-servers.net.
 * com. - NS : b.gtld-servers.net.
 * com. - NS : c.gtld-servers.net.
 * com. - NS : d.gtld-servers.net.
 * com. - NS : e.gtld-servers.net.
 * com. - NS : f.gtld-servers.net.
 * com. - NS : g.gtld-servers.net.
 * com. - NS : h.gtld-servers.net.
 * com. - NS : i.gtld-servers.net.
 * com. - NS : j.gtld-servers.net.
 * com. - NS : k.gtld-servers.net.
 * com. - NS : l.gtld-servers.net.
 * com. - NS : m.gtld-servers.net.
Found 15 additional record(s)
 * a.gtld-servers.net. - IP : 192.5.6.30
 * b.gtld-servers.net. - IP : 192.33.14.30
 * c.gtld-servers.net. - IP : 192.26.92.30
 * d.gtld-servers.net. - IP : 192.31.80.30
 * e.gtld-servers.net. - IP : 192.12.94.30
 * f.gtld-servers.net. - IP : 192.35.51.30
 * g.gtld-servers.net. - IP : 192.42.93.30
 * h.gtld-servers.net. - IP : 192.54.112.30
 * i.gtld-servers.net. - IP : 192.43.172.30
 * j.gtld-servers.net. - IP : 192.48.79.30
 * k.gtld-servers.net. - IP : 192.52.178.30
 * l.gtld-servers.net. - IP : 192.41.162.30
 * m.gtld-servers.net. - IP : 192.55.83.30
 * a.gtld-servers.net. - AAAA : 2001:503:a83e:0:0:0:2:30
 * b.gtld-servers.net. - AAAA : 2001:503:231d:0:0:0:2:30
Protocol

DNS protocol is very simple, the only tricky part is the (de)compression algorithm.
Implementation provided here is reaaaally ugly ... but runs properly ;-).
See https://www2.cs.duke.edu/courses/fall16/compsci356/DNS/DNS-primer.pdf for explanations.

  
## DNSClient.java
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.nio.charset.StandardCharsets;

public class DNSClient {

    private static final int A_TYPE = 1;
    private static final int NS_TYPE = 2;
    private static final int CNAME_TYPE = 5;
    private static final int SOA_TYPE = 6;
    private static final int MX_TYPE = 15;
    private static final int TXT_TYPE = 16;
    private static final int AAAA_TYPE = 28;

    // cloudflare
    private static final String DNS_SERVER_ADDRESS = "1.1.1.1";

    // sfr
    //private static final String DNS_SERVER_ADDRESS = "89.2.0.1";

    // root server - when querying a root server with a fqdn, only the tld is taken into account
    //private static final String DNS_SERVER_ADDRESS = "198.41.0.4";

    // e.nic.fr - when querying a tld server with a fqdn, only the tld and domain are taken into account
    //private static final String DNS_SERVER_ADDRESS = "193.176.144.22";

    private static final int DNS_SERVER_PORT = 53;

    //
    // Helpers
    //

    /**
     * Reads string from buffer, starting at offset.
     * Last character can be '\0' or a pointer.
     * @returns The whole string.
     */
    private static String readStringAt(byte[] buffer, int offset) {
        String result = "";

        byte b = buffer[offset++];
        while(b != 0) {
            if((b & 0xC0) == 0xC0) {
                // it's a pointer => inception !
                offset = (b & 0x3F) << 8 | buffer[offset++];
            } else {
                // immediate value, b is string length
                result += new String(buffer, offset, b, StandardCharsets.UTF_8) + ".";
                offset += b;
            }
            b = buffer[offset++];
        }
        return result;
    }

    /**
     * Reads a string from stream. Can be a literal value,
     * a pointer or a literal ending with a pointer.
     */
    private static String readStringFrom(DataInputStream din, byte[] buffer) throws IOException {
        String result = "";

        byte b = din.readByte();
        while(b != 0) {
            if((b & 0xC0) == 0xC0) {
                // it's a pointer !
                int offset = ((b & 0x3F) << 8 | (0xFF & din.readByte()));
                result += readStringAt(buffer, offset);
                b = 0; // break
            } else {
                byte[] temp = new byte[b];
                din.read(temp);
                result += new String(temp, StandardCharsets.UTF_8) + ".";
                b = din.readByte();
            }
        }
        return result;
    }

    //
    // main
    //

    public static void main(String[] args) throws IOException {

        if(args.length == 0) {
            System.err.println("usage: " + DNSClient.class.getName() + " domain");
            return;
        }

        final InetAddress ipAddress = InetAddress.getByName(DNS_SERVER_ADDRESS);
        final ByteArrayOutputStream baos = new ByteArrayOutputStream();
        final DataOutputStream dos = new DataOutputStream(baos);

        // DataOutputStream est Big Endian, ça tombe bien
        // puisque c'est en network byte order ^^

        // l'identifiant va permettre de relier la réponse à la requête
        // ici on peut utiliser une valeur en dur puisqu'une seule requête
        final short uniqId = 1337;

        // ID
        dos.writeShort(uniqId);

        // FLAGS
        short flags = 0;
        // QR 0 = query, 1 = response
        flags |= (0 << 15);
        // OP 0 = std query
        flags |= (0 << 11);
        // AA (reponse only)
        // TR (truncated)
        // RD 1 = recursion desired
        flags |= (1 << 8);
        // RA (response only)
        // Z  (reserved)
        // RCODE (response only)
        dos.writeShort(flags);

        // QDCOUNT
        dos.writeShort(0x01);

        // ANCOUNT
        dos.writeShort(0x00);

        // NSCOUNT
        dos.writeShort(0x00);

        // ARCOUNT
        dos.writeShort(0x00);

        // QUESTION

        // we should use compression here :-S

        // QNAME
        final String[] parts = args[0].split("\\.");
        for(String part : parts) {
            dos.writeByte(part.length());
            final byte[] str = part.getBytes(StandardCharsets.UTF_8);
            dos.write(str, 0, str.length);
        }
        dos.writeByte(0x00);

        // QTYPE
        dos.writeShort(A_TYPE);

        // QCLASS (1=Internet)
        dos.writeShort(0x01);

        final byte[] dnsFrame = baos.toByteArray();

        // --- SEND --- >
        final DatagramSocket socket = new DatagramSocket();
        final DatagramPacket dnsReqPacket = new DatagramPacket(dnsFrame, dnsFrame.length, ipAddress, DNS_SERVER_PORT);
        socket.send(dnsReqPacket);

        //                          (><) --------- (><)
        // ->---- (><)               /               \
        //          \               /                 \
        //          (><) -------- (><)                 \
        //                                            (><)------->--

        // <--- RECV ---

        // https://serverfault.com/questions/587625/why-dns-through-udp-has-a-512-bytes-limit
        // Why DNS through UDP has a 512 bytes limit?
        final byte[] buf = new byte[512];
        final DatagramPacket packet = new DatagramPacket(buf, buf.length);
        socket.receive(packet);

        final DataInputStream din = new DataInputStream(new ByteArrayInputStream(buf));
        if(din.readShort() != uniqId) { /* ... */ }

        final short respFlags = din.readShort();

        // maybe we should check the truncated flag ¯\_(ツ)_/¯

        if((respFlags & 0x07) != 0) {
            System.err.println("Erreur " + (respFlags & 0x07) + " ?!? ");

        } else {

            final int qdCount = din.readShort();
            final int anCount = din.readShort();
            final int nsCount = din.readShort();
            final int arCount = din.readShort();

            // skip queries
            for(int k=0; k<qdCount; k++) {
                // skip queries (qname, qtype, qclass)
                String dummy = readStringFrom(din, buf);
                din.readShort();
                din.readShort();
            }

            // record format : [ NAME | TYPE | CLASS | TTL | RDLENGTH | RDATA ]

            // The compression scheme allows a domain name in a message to be represented as either:
            // • a sequence of labels ending in a zero octet
            // • a pointer
            // • a sequence of labels ending with a pointer

            for(int k=0; k<(anCount + nsCount + arCount); k++) {

                if(anCount != 0 && k == 0) {
                    System.out.println("Found " + anCount + " answer(s)");
                } else if(nsCount != 0 && k == anCount) {
                    System.out.println("Found " + nsCount + " authoritative nameserver(s)");
                } else if(arCount != 0 && k == (anCount + nsCount)) {
                    System.out.println("Found " + arCount + " additional record(s)");
                }

                String domain = readStringFrom(din, buf);

                short recordType = din.readShort();

                // class is always 1 (internet)
                short clazz = din.readShort();

                int ttl = din.readInt();

                short dataLen = din.readShort();

                // rdata depends on type
                switch(recordType) {
                    case A_TYPE:
                        final int ip = din.readInt();
                        System.out.println(" * " + domain + " - IP : " + String.format("%d.%d.%d.%d",
                            (ip >> 24 & 0xff), (ip >> 16 & 0xff), (ip >> 8 & 0xff), (ip >> 0 & 0xff)));
                        break;

                    case NS_TYPE:
                        final String hostname = readStringFrom(din, buf);
                        System.out.println(" * " + domain + " - NS : " + hostname);
                        break;

                    case CNAME_TYPE:
                        final String cname = readStringFrom(din, buf);
                        System.out.println(" * " + domain + " - CNAME : " + cname);
                        break;

                    case SOA_TYPE:
                        final String primaryNameServer = readStringFrom(din, buf);
                        final String mailbox = readStringFrom(din, buf);
                        din.readInt(); // serial number
                        din.readInt(); // refresh interval
                        din.readInt(); // retry interval
                        din.readInt(); // expire limit
                        din.readInt(); // minimum ttl
                        System.out.println(" * " + domain + " - SOA : " + primaryNameServer + "; " + mailbox);
                        break;

                    case MX_TYPE:
                        final short preference = din.readShort();
                        final String mailServer = readStringFrom(din, buf);
                        System.out.println(" * " + domain + " - MX : " + mailServer);
                        break;

                    case TXT_TYPE:
                        final String text = readStringFrom(din, buf);
                        System.out.println(" * " + domain + " - TXT : " + text);
                        break;

                    case AAAA_TYPE:
                        System.out.print(" * " + domain + " - AAAA : ");
                        // how are multiple IPs stored ? concatenated ?
                        for(int m=0; m<(dataLen>>1); m++) {
                            System.out.print(String.format("%x", din.readShort()));
                            if(m % 8 != 7) System.out.print(":");
                            if(m > 0 && m % 8 == 0) {
                                System.out.print(" | ");
                            }
                        }
                        System.out.println();
                        break;

                    default:
                        System.err.println("Type #" + String.format("%x", recordType) + " not yet supported");
                        din.skipBytes(dataLen);
                        break;
                }

            }

        }

    }

}
	import java.io.ByteArrayInputStream;
	import java.io.ByteArrayOutputStream;
	import java.io.DataInputStream;
	import java.io.DataOutputStream;
	import java.io.IOException;
	import java.net.DatagramPacket;
	import java.net.DatagramSocket;
	import java.net.InetAddress;
	import java.nio.charset.StandardCharsets;

	public class DNSClient {

	private static final int A_TYPE = 1;
	private static final int NS_TYPE = 2;
	private static final int CNAME_TYPE = 5;
	private static final int SOA_TYPE = 6;
	private static final int MX_TYPE = 15;
	private static final int TXT_TYPE = 16;
	private static final int AAAA_TYPE = 28;

	// cloudflare
	private static final String DNS_SERVER_ADDRESS = "1.1.1.1";

	// sfr
	//private static final String DNS_SERVER_ADDRESS = "89.2.0.1";

	// root server - when querying a root server with a fqdn, only the tld is taken into account
	//private static final String DNS_SERVER_ADDRESS = "198.41.0.4";

	// e.nic.fr - when querying a tld server with a fqdn, only the tld and domain are taken into account
	//private static final String DNS_SERVER_ADDRESS = "193.176.144.22";

	private static final int DNS_SERVER_PORT = 53;

	//
	// Helpers
	//

	/**
	* Reads string from buffer, starting at offset.
	* Last character can be '\0' or a pointer.
	* @returns The whole string.
	*/
	private static String readStringAt(byte[] buffer, int offset) {
	String result = "";

	byte b = buffer[offset++];
	while(b != 0) {
	if((b & 0xC0) == 0xC0) {
	// it's a pointer => inception !
	offset = (b & 0x3F) << 8 \| buffer[offset++];
	} else {
	// immediate value, b is string length
	result += new String(buffer, offset, b, StandardCharsets.UTF_8) + ".";
	offset += b;
	}
	b = buffer[offset++];
	}
	return result;
	}

	/**
	* Reads a string from stream. Can be a literal value,
	* a pointer or a literal ending with a pointer.
	*/
	private static String readStringFrom(DataInputStream din, byte[] buffer) throws IOException {
	String result = "";

	byte b = din.readByte();
	while(b != 0) {
	if((b & 0xC0) == 0xC0) {
	// it's a pointer !
	int offset = ((b & 0x3F) << 8 \| (0xFF & din.readByte()));
	result += readStringAt(buffer, offset);
	b = 0; // break
	} else {
	byte[] temp = new byte[b];
	din.read(temp);
	result += new String(temp, StandardCharsets.UTF_8) + ".";
	b = din.readByte();
	}
	}
	return result;
	}

	//
	// main
	//

	public static void main(String[] args) throws IOException {

	if(args.length == 0) {
	System.err.println("usage: " + DNSClient.class.getName() + " domain");
	return;
	}

	final InetAddress ipAddress = InetAddress.getByName(DNS_SERVER_ADDRESS);
	final ByteArrayOutputStream baos = new ByteArrayOutputStream();
	final DataOutputStream dos = new DataOutputStream(baos);

	// DataOutputStream est Big Endian, ça tombe bien
	// puisque c'est en network byte order ^^

	// l'identifiant va permettre de relier la réponse à la requête
	// ici on peut utiliser une valeur en dur puisqu'une seule requête
	final short uniqId = 1337;

	// ID
	dos.writeShort(uniqId);

	// FLAGS
	short flags = 0;
	// QR 0 = query, 1 = response
	flags \|= (0 << 15);
	// OP 0 = std query
	flags \|= (0 << 11);
	// AA (reponse only)
	// TR (truncated)
	// RD 1 = recursion desired
	flags \|= (1 << 8);
	// RA (response only)
	// Z (reserved)
	// RCODE (response only)
	dos.writeShort(flags);

	// QDCOUNT
	dos.writeShort(0x01);

	// ANCOUNT
	dos.writeShort(0x00);

	// NSCOUNT
	dos.writeShort(0x00);

	// ARCOUNT
	dos.writeShort(0x00);

	// QUESTION

	// we should use compression here :-S

	// QNAME
	final String[] parts = args[0].split("\\.");
	for(String part : parts) {
	dos.writeByte(part.length());
	final byte[] str = part.getBytes(StandardCharsets.UTF_8);
	dos.write(str, 0, str.length);
	}
	dos.writeByte(0x00);

	// QTYPE
	dos.writeShort(A_TYPE);

	// QCLASS (1=Internet)
	dos.writeShort(0x01);

	final byte[] dnsFrame = baos.toByteArray();

	// --- SEND --- >
	final DatagramSocket socket = new DatagramSocket();
	final DatagramPacket dnsReqPacket = new DatagramPacket(dnsFrame, dnsFrame.length, ipAddress, DNS_SERVER_PORT);
	socket.send(dnsReqPacket);

	// (><) --------- (><)
	// ->---- (><) / \
	// \ / \
	// (><) -------- (><) \
	// (><)------->--

	// <--- RECV ---

	// https://serverfault.com/questions/587625/why-dns-through-udp-has-a-512-bytes-limit
	// Why DNS through UDP has a 512 bytes limit?
	final byte[] buf = new byte[512];
	final DatagramPacket packet = new DatagramPacket(buf, buf.length);
	socket.receive(packet);

	final DataInputStream din = new DataInputStream(new ByteArrayInputStream(buf));
	if(din.readShort() != uniqId) { /* ... */ }

	final short respFlags = din.readShort();

	// maybe we should check the truncated flag ¯\_(ツ)_/¯

	if((respFlags & 0x07) != 0) {
	System.err.println("Erreur " + (respFlags & 0x07) + " ?!? ");

	} else {

	final int qdCount = din.readShort();
	final int anCount = din.readShort();
	final int nsCount = din.readShort();
	final int arCount = din.readShort();

	// skip queries
	for(int k=0; k<qdCount; k++) {
	// skip queries (qname, qtype, qclass)
	String dummy = readStringFrom(din, buf);
	din.readShort();
	din.readShort();
	}

	// record format : [ NAME \| TYPE \| CLASS \| TTL \| RDLENGTH \| RDATA ]

	// The compression scheme allows a domain name in a message to be represented as either:
	// • a sequence of labels ending in a zero octet
	// • a pointer
	// • a sequence of labels ending with a pointer

	for(int k=0; k<(anCount + nsCount + arCount); k++) {

	if(anCount != 0 && k == 0) {
	System.out.println("Found " + anCount + " answer(s)");
	} else if(nsCount != 0 && k == anCount) {
	System.out.println("Found " + nsCount + " authoritative nameserver(s)");
	} else if(arCount != 0 && k == (anCount + nsCount)) {
	System.out.println("Found " + arCount + " additional record(s)");
	}

	String domain = readStringFrom(din, buf);

	short recordType = din.readShort();

	// class is always 1 (internet)
	short clazz = din.readShort();

	int ttl = din.readInt();

	short dataLen = din.readShort();

	// rdata depends on type
	switch(recordType) {
	case A_TYPE:
	final int ip = din.readInt();
	System.out.println(" * " + domain + " - IP : " + String.format("%d.%d.%d.%d",
	(ip >> 24 & 0xff), (ip >> 16 & 0xff), (ip >> 8 & 0xff), (ip >> 0 & 0xff)));
	break;

	case NS_TYPE:
	final String hostname = readStringFrom(din, buf);
	System.out.println(" * " + domain + " - NS : " + hostname);
	break;

	case CNAME_TYPE:
	final String cname = readStringFrom(din, buf);
	System.out.println(" * " + domain + " - CNAME : " + cname);
	break;

	case SOA_TYPE:
	final String primaryNameServer = readStringFrom(din, buf);
	final String mailbox = readStringFrom(din, buf);
	din.readInt(); // serial number
	din.readInt(); // refresh interval
	din.readInt(); // retry interval
	din.readInt(); // expire limit
	din.readInt(); // minimum ttl
	System.out.println(" * " + domain + " - SOA : " + primaryNameServer + "; " + mailbox);
	break;

	case MX_TYPE:
	final short preference = din.readShort();
	final String mailServer = readStringFrom(din, buf);
	System.out.println(" * " + domain + " - MX : " + mailServer);
	break;

	case TXT_TYPE:
	final String text = readStringFrom(din, buf);
	System.out.println(" * " + domain + " - TXT : " + text);
	break;

	case AAAA_TYPE:
	System.out.print(" * " + domain + " - AAAA : ");
	// how are multiple IPs stored ? concatenated ?
	for(int m=0; m<(dataLen>>1); m++) {
	System.out.print(String.format("%x", din.readShort()));
	if(m % 8 != 7) System.out.print(":");
	if(m > 0 && m % 8 == 0) {
	System.out.print(" \| ");
	}
	}
	System.out.println();
	break;

	default:
	System.err.println("Type #" + String.format("%x", recordType) + " not yet supported");
	din.skipBytes(dataLen);
	break;
	}

	}

	}

	}

	}