为什么我得到负的目的地址从数据包

Question

我试图解析数据包以获得目的IP地址并将其转换为主机名。问题是我得到了一个负的目标地址，像这样:

• IP版本:4
• 头长度:20
• 总长度:60
• 协议:6
• 源IP: 10.0.2.0
• 目标IP: -83. 4.102.16
<
• 未知主机/gh>

---

• IP版本:4
• 头长度:20
• 总长度:92
• 协议:6
• 源IP: -64. 88.1.5
• 目标IP: 54. -9. -67. -15

---

int buffer = packet.get();
    int version;
    int headerlength;
    version = buffer >> 4;
    headerlength = buffer & 0x0F;
    headerlength *= 4;
    Log.d(TAG, "IP Version:"+version);
    Log.d(TAG, "Header Length:"+headerlength);
    String status = "";
    status += "Header Length:"+headerlength;
    buffer = packet.get();      //DSCP + EN
    buffer = packet.getChar();  //Total Length
    Log.d(TAG, "Total Length:"+buffer);
    buffer = packet.getChar();  //Identification
    buffer = packet.getChar();  //Flags + Fragment Offset
    buffer = packet.get();      //Time to Live
    buffer = packet.get();      //Protocol
    Log.d(TAG, "Protocol:"+buffer);
    status += "  Protocol:"+buffer;
    buffer = packet.getChar();  //Header checksum
    String sourceIP  = "";
    buffer = packet.get();  //Source IP 1st Octet
    sourceIP += buffer;
    sourceIP += ".";
    buffer = packet.get();  //Source IP 2nd Octet
    sourceIP += buffer;
    sourceIP += ".";
    buffer = packet.get();  //Source IP 3rd Octet
    sourceIP += buffer;
    sourceIP += ".";
    buffer = packet.get();  //Source IP 4th Octet
    sourceIP += buffer;
    Log.d(TAG, "Source IP:"+sourceIP);
    status += "   Source IP:"+sourceIP;
    String destIP  = "";
    buffer = packet.get();  //Destination IP 1st Octet
    destIP += buffer;
    destIP += ".";
    buffer = packet.get();  //Destination IP 2nd Octet
    destIP += buffer;
    destIP += ".";
    buffer = packet.get();  //Destination IP 3rd Octet
    destIP += buffer;
    destIP += ".";
    buffer = packet.get();  //Destination IP 4th Octet
    destIP += buffer;
    Log.d(TAG, "Destination IP:" + destIP);
    status += "   Destination IP:"+destIP;
    try {
        InetAddress addr = InetAddress.getByName(destIP);
        String host = addr.getHostName();
        Log.d(TAG, "Hostname:" + host);
    }
    catch (UnknownHostException e) {
        Log.d(TAG,"Unknown host");
    }

我做错了什么吗?我该怎么做才对呢?

Answer 1

packet.get()返回byte。在java中，byte是有符号的，但是您期望一个无符号值。使用byte，您可以获得任何字节的负值，应该大于127

例子

byte b = 0xFF; // 255
System.out.println(b == -1); // prints true

---

"convert"将byte转换为正确的无符号值，您必须使用使用超过1字节的原始整型，并使用一些小技巧来获得无符号值:

byte b = // your value goes here
int printInt = ((int) b) & 0xFF;
// printInt contains the unsigned value of the byte.