Question

当我用Android VpnService收到IP数据包时，我首先读取了标头（as here），然后尝试按以下方式打印接收到的数据：

int lengthRemaining = packet.remaining();
if (lengthRemaining > 0) {
    byte[] data = new byte[lengthRemaining];
    packet.get(data, packet.arrayOffset(), lengthRemaining);
    Log.d(TAG, "Packet-Data: " + new String(data, Charset.forName("UTF-8")));
}

结果如下所示：

数据包数据：��5��（��wwwwwwgoogle�com��

或未加密网页中的另一个示例：

数据包：IP版本= 4，报头长度= 20，总长度= 60，目标IP = xx.xx.xxx.xx，主机名= yyyyy.zzz，源IP = 10.1.10.1，协议= 17，Data-Remaining = 40

数据包数据：N��5��（@�F��m�postimees�ee��

我尝试了几种不同的String编码类型，我认为这不是问题。

我错过了哪一步？

Answer 1

我自己解决了这个问题。除了UDP标头this article之外，我还需要UDP protocol上有关手写DNS数据包和as discussed here的答案。

问题是上面解析的数据仍然包含UDP和DNS标头，我们只分析了应用层标头

以下代码将从DNS数据包中读取DNS域

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 + EN
int totalLength = packet.getChar(); //Total Length
packet.getChar();                   //Identification
packet.getChar();                   //Flags + Fragment Offset
packet.get();                       //Time to Live
int 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 = new byte[0];
byte[] qnameSectionBytes;
int oldLength;
while (qnameSectionByteCount > 0 && qnameSectionByteCount <= packet.remaining()) {
    qnameSectionBytes = new byte[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 iteration
    if (qnameSectionByteCount > 0) {
        //add a dot
        byte[] 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 = new byte[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);

并产生如下输出：

标题：
  IP版本= 4
  标题长度= 20
  总长度= 59
  目的地= 88.88.888.88 / dns.mydns.net
  源IP = 10.1.10.1
  协议= 17
  源端口= 44444
  DestPortUdp = 53
  QName = www.google.com
  剩余数据：null //出于理智

将我的IP数据包数据转换为字符串的正确方法是什么？

1 个答案: