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当我收到带有我的 Android VpnService的 IP 数据包时，我首先读取标头（如此处），然后我尝试按如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")));
}

这导致输出如下：

数据包数据：����5��(��������������������www�google.com������

或者来自未加密网页的另一个示例：

数据包：IP Version=4，Header-Length=20，Total-Length=60，Destination-IP=xx.xx.xxx.xx，Hostname=yyyyy.zzz，Source-IP=10.1.10.1，Protocol=17，Data -剩余=40

数据包数据：N����5��(@�F������������������m�posttimees�ee������

我尝试了几种不同的字符串编码类型，我认为这不是问题。

我错过了什么步骤？

4

1 回答 1

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我自己解决了这个问题。除了这里讨论的 UDP 标头之外，我需要的答案是在这篇关于手写 DNS 数据包和UDP 协议的文章中。

问题是上面解析的数据仍然包含 UDP 和 DNS 头，我们只解析了应用层头

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：

标头：
IP 版本=4
标头长度=20
总长度=59
目标=88.88.888.88 / dns.mydns.net
源IP=10.1.10.1
协议=17
源端口=44444目标端口
Udp=53
QName=www.google .com
Remaining-Data: null //为了理智

于 2018-08-16T11:58:58.240 回答