What Is The Correct Way To Convert My Ip Packet Data Into A String?

When I receive an IP Packet with my Android VpnService I read the headers first (as here), then I try to print the received data as follows: int lengthRemaining = packet.remaining(

Solution 1:

I've solved this myself. The answers I needed were on this article about handwriting DNS packets and the UDP protocol in addition to the UDP headers as discussed here.

The problem was that the data as parsed above still contained UDP and DNS headers, we had only parsed the application layer headers

The following code will read DNS Domain from a DNS packet

int buffer = packet.get();
int ipVersion = buffer >> 4;
int headerLength = buffer & 0x0F;   //the number of 32 bit words in the header
headerLength *= 4;
packet.get();                       //DSCP + ENint totalLength = packet.getChar(); //Total Length
packet.getChar();                   //Identification
packet.getChar();                   //Flags + Fragment Offset
packet.get();                       //Time to Liveint protocol = packet.get();        //Protocol
packet.getChar();                   //Header checksum

String sourceIP  = "";
sourceIP += packet.get() & 0xFF; //Source IP 1st Octet
sourceIP += ".";
sourceIP += packet.get() & 0xFF; //Source IP 2nd Octet
sourceIP += ".";
sourceIP += packet.get() & 0xFF; //Source IP 3rd Octet
sourceIP += ".";
sourceIP += packet.get() & 0xFF; //Source IP 4th Octet

String destIP  = "";
destIP += packet.get() & 0xFF; //Destination IP 1st Octet
destIP += ".";
destIP += packet.get() & 0xFF; //Destination IP 2nd Octet
destIP += ".";
destIP += packet.get() & 0xFF; //Destination IP 3rd Octet
destIP += ".";
destIP += packet.get() & 0xFF; //Destination IP 4th Octet//NOTE: RFC diagram showed a byte of zeroes here, but it doesn't appear to match the implementation//int supposedZeroes = packet.get();//Log.d(TAG, "Supposed Zeroes: " + supposedZeroes);int sourcePortUdp = packet.getChar();
int destPortUdp = packet.getChar();
packet.getChar(); //UDP Data Length
packet.getChar(); //UDP Checksum//NOTE: DNS HEADERS INSIDE UDP DATA - https://routley.io/tech/2017/12/28/hand-writing-dns-messages.html
packet.getChar(); //DNS ID
packet.get(); //OPTIONS: QR + OPCODE + AA + TC + RD
packet.get(); //OPTIONS: Z + RCODE
packet.getChar(); //DNS QDCOUNT //number of entities/questions
packet.getChar(); //DNS ANCOUNT //num answers
packet.getChar(); //DNS NSCOUNT //num auth records
packet.getChar(); //DNS ARCOUNT //num additional records//NOTE: QNAME is url encoded, in several separated sections, each preceded by an int saying the number of bytes//NOTE: The QNAME section is terminated with a zero byte (00).int qnameSectionByteCount = packet.get();
byte[] qnameBytes = newbyte[0];
byte[] qnameSectionBytes;
int oldLength;
while (qnameSectionByteCount > 0 && qnameSectionByteCount <= packet.remaining()) {
    qnameSectionBytes = newbyte[qnameSectionByteCount];
    packet.get(qnameSectionBytes);
    //insert the bytes from the new section
    oldLength = qnameBytes.length;
    qnameBytes = Arrays.copyOf(qnameBytes, oldLength + qnameSectionBytes.length);
    System.arraycopy(qnameSectionBytes, 0, qnameBytes, oldLength, qnameSectionBytes.length);
    //get the byte that determines if there is another loop iteration
    qnameSectionByteCount = packet.get();
    //add a connecting dot if there will be another loop iterationif (qnameSectionByteCount > 0) {
        //add a dotbyte[] dot = ".".getBytes();
        oldLength = qnameBytes.length;
        qnameBytes = Arrays.copyOf(qnameBytes, oldLength + dot.length);
        System.arraycopy(dot, 0, qnameBytes, oldLength, dot.length);
    }
}
packet.getChar(); //QCLASS
packet.getChar(); //QCLASS

String destHostName;
try {
    InetAddress addr = InetAddress.getByName(destIP);
    destHostName = addr.getHostName();
} catch (UnknownHostException e) {
    destHostName = "Unresolved";
}

int orphanDataLength = packet.remaining();
String dataStr = null;
if (orphanDataLength > 0) {
    byte[] data = newbyte[orphanDataLength];
    packet.get(data, packet.arrayOffset(), orphanDataLength);
    dataStr = new String(data, Charset.forName("UTF-8"));
}

Log.v(TAG, "---\nHeaders:\nIP Version=" + ipVersion + "\nHeader-Length=" + headerLength
        + "\nTotal-Length=" + totalLength + "\nDestination=" + destIP + " / "
        + destHostName + "\nSource-IP=" + sourceIP + "\nProtocol=" + protocol
        + "\nSource-Port=" + sourcePortUdp + "\nDestPortUdp=" + destPortUdp + "\nQname="
        + new String(qnameBytes, Charset.forName("UTF-8")) + "\nRemaining-Data: " + dataStr);

And produce output something like this:

Headers: IP Version=4 Header-Length=20 Total-Length=59 Destination=88.88.888.88 / dns.mydns.net Source-IP=10.1.10.1 Protocol=17 Source-Port=44444 DestPortUdp=53 QName=www.google.com Remaining-Data: null //for sanity