如何为DNS查询指定本地/绑定地址？

Great question! You’re spot-on about the behavior you’re observing: your code correctly sets the source IP 1.2.3.4 for the TCP connection, but the DNS lookup that resolves mydomain.com is handled separately by the JVM’s default resolver. This resolver doesn’t automatically inherit the bound address from your Socket, which is why the DNS traffic uses your system’s default network interface instead.

To fix this, you need to take manual control of the DNS resolution process so you can force it to use your specified local address. Here are two reliable methods:

方法一：手动发送DNS查询（无需第三方依赖） #

You can use a DatagramSocket bound to your target local address to send a raw DNS query to your preferred DNS server, parse the response to get the domain’s IP, then use that IP to establish your TCP connection. This gives you full control over the source address used for DNS traffic.

Here’s a working example (simplified for A-record queries):

import java.net.*;
import java.io.*;
import java.nio.charset.StandardCharsets;

public class BoundDnsLookup {
    public static InetAddress resolveDomainWithSourceIp(String domain, String sourceIp, String dnsServer) throws IOException {
        // Bind a UDP socket to the specified local address
        try (DatagramSocket dnsSocket = new DatagramSocket(new InetSocketAddress(sourceIp, 0))) {
            // Build a basic DNS query packet (A-record lookup)
            ByteArrayOutputStream queryStream = new ByteArrayOutputStream();
            // DNS header: ID=0x0100, standard query, 1 question
            queryStream.write(new byte[]{0x01, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00});
            
            // Split domain into labels and write to query
            for (String label : domain.split("\\.")) {
                queryStream.write(label.length());
                queryStream.write(label.getBytes(StandardCharsets.US_ASCII));
            }
            // End of domain, specify A-record (0x0001) and IN class (0x0001)
            queryStream.write(new byte[]{0x00, 0x00, 0x01, 0x00, 0x01});

            byte[] queryBytes = queryStream.toByteArray();
            DatagramPacket request = new DatagramPacket(queryBytes, queryBytes.length, InetAddress.getByName(dnsServer), 53);
            dnsSocket.send(request);

            // Receive and parse the DNS response
            byte[] responseBuffer = new byte[512];
            DatagramPacket response = new DatagramPacket(responseBuffer, responseBuffer.length);
            dnsSocket.receive(response);

            byte[] responseData = response.getData();
            int offset = 12; // Skip DNS header (12 bytes)
            
            // Skip the question section of the response
            while (responseData[offset] != 0) {
                offset += responseData[offset] + 1;
            }
            offset += 4; // Skip null terminator, type, and class fields

            // Extract the first A-record IP
            if (responseData[offset + 2] == 0x01 && responseData[offset + 3] == 0x01) { // Verify it's an A-record
                int ipByteLength = ((responseData[offset + 4] & 0xFF) << 8) | (responseData[offset + 5] & 0xFF);
                if (ipByteLength == 4) {
                    byte[] ipBytes = new byte[4];
                    System.arraycopy(responseData, offset + 10, ipBytes, 0, 4);
                    return InetAddress.getByAddress(ipBytes);
                }
            }
            throw new IOException("Failed to resolve A-record for: " + domain);
        }
    }

    public static void main(String[] args) throws IOException {
        String sourceIp = "1.2.3.4";
        String targetDomain = "mydomain.com";
        String dnsServer = "8.8.8.8"; // Use your preferred DNS server

        // Resolve the domain using our bounded socket
        InetAddress resolvedIp = resolveDomainWithSourceIp(targetDomain, sourceIp, dnsServer);

        // Establish TCP connection with the same source address
        Socket tcpSocket = new Socket();
        tcpSocket.bind(new InetSocketAddress(sourceIp, 0));
        tcpSocket.connect(new InetSocketAddress(resolvedIp, 1234));

        // Do your work here...

        tcpSocket.close();
    }
}

方法二：使用第三方DNS库（简化开发） #

If you don’t want to mess with raw DNS protocol parsing, use a mature library like dnsjava which supports specifying the source address for DNS lookouts out of the box.

First, add the library dependency (for Maven):

<dependency>
    <groupId>org.xbill.DNS</groupId>
    <artifactId>dnsjava</artifactId>
    <version>3.5.3</version>
</dependency>

Then use this code to resolve the domain with your specified source address:

import org.xbill.DNS.*;
import java.net.*;

public class DnsJavaBoundLookup {
    public static InetAddress resolveWithSourceIp(String domain, String sourceIp) throws Exception {
        Lookup dnsLookup = new Lookup(domain, Type.A);
        // Set the local address for the DNS query
        dnsLookup.setLocalAddress(InetAddress.getByName(sourceIp));
        
        Record[] records = dnsLookup.run();
        if (records != null && records.length > 0) {
            ARecord aRecord = (ARecord) records[0];
            return aRecord.getAddress();
        }
        throw new UnknownHostException("Could not resolve domain: " + domain);
    }

    public static void main(String[] args) throws Exception {
        String sourceIp = "1.2.3.4";
        String targetDomain = "mydomain.com";

        InetAddress resolvedIp = resolveWithSourceIp(targetDomain, sourceIp);

        Socket tcpSocket = new Socket();
        tcpSocket.bind(new InetSocketAddress(sourceIp, 0));
        tcpSocket.connect(new InetSocketAddress(resolvedIp, 1234));

        // Do your work here...

        tcpSocket.close();
    }
}

Key Notes #

• The JVM’s default InetAddress.getByName() method relies on your system’s native DNS resolver, which picks the default network interface for DNS traffic—this is why it ignores your Socket’s bound address.
• Both methods above ensure the DNS query’s UDP packets originate from 1.2.3.4, matching the source address you use for the TCP connection.

内容的提问来源于stack exchange，提问作者Ben Barkay