如何使用 C 程序读取来自端口的 UDP 数据负载

Question

我正在尝试从端口 6343 捕获 UDP 数据包。在捕获它的同时，我还必须捕获有效负载。我正在获取 ASCII 格式的有效负载。我必须阅读有效载荷的内容。以下是我的代码：

#include<stdio.h> //For standard things
#include<stdlib.h>    //malloc
#include<string.h>    //memset
#include<netinet/ip_icmp.h>   //Provides declarations for icmp header
#include<netinet/udp.h>   //Provides declarations for udp header
#include<netinet/tcp.h>   //Provides declarations for tcp header
#include<netinet/ip.h>    //Provides declarations for ip header
#include<sys/socket.h>
#include<arpa/inet.h>
#define PORT 6343


void print_udp_packet(unsigned char*, int);
void ProcessPacket(unsigned char*, int);
void PrintData (unsigned char* , int);

int sockt;
int i,j;

struct sockaddr_in source,dest; 

int main()
{
    int saddr_size,data_size;
    struct sockaddr_in saddr;
    struct sockaddr_in daddr;

    //struct in_addr in;
    unsigned char *buffer = (unsigned char *)malloc(65536); // Its Big ! Malloc allocates a block of size bytes of memory,returning a pointer to the begining of the block

    struct udphdr *udph = (struct udphdr*)(buffer + sizeof(struct iphdr));
    struct iphdr *iph = (struct iphdr *)buffer;
    memset(&source,0,sizeof(source));
    source.sin_addr.s_addr = iph ->saddr;
    memset(&dest,0,sizeof(dest));
    dest.sin_addr.s_addr = iph->daddr;
    unsigned short iphdrlen = iph->ihl*4;


    printf("Starting...\n");
    //Create a socket
    sockt = socket(AF_INET ,SOCK_DGRAM ,0);
    if(sockt < 0)
    {
        printf("Socket Error\n");
        return 1;
    }
    memset((char *)&daddr,0,sizeof(daddr));

    //prepare the sockaddr_in structure
    daddr.sin_family = AF_INET;
    daddr.sin_addr.s_addr = htonl(INADDR_ANY);
    daddr.sin_port = htons(PORT);

    //Bind
    if(bind(sockt,(struct sockaddr *)&daddr, sizeof(daddr))<0)
    {
      printf("bind failed");
      return 1;
    }
    printf("bind done");

    while(1)
    {
    saddr_size = sizeof saddr;
    printf("waiting for data...");

    //Receive a packet
    data_size = recvfrom(sockt , buffer ,65536 , 0 , (struct sockaddr*) &saddr , (socklen_t*)&saddr_size);
    if(data_size <0)
    {
      printf("Packets not recieved \n");
      return 1;
    }

    printf("Packets arrived from %d \n",ntohs(daddr.sin_port));
    printf("\t Source Port : %d , Destination Port : %d, UDP Length : %d,Protocol : %d, total length : %d \n", ntohs(udph->source), ntohs(udph->dest), ntohs(data_size), (unsigned int)iph->protocol, ntohs(iph->tot_len)); 
    printf("Source IP        : %s\n",inet_ntoa(saddr.sin_addr));
    printf("Destination IP   : %s\n",inet_ntoa(daddr.sin_addr));
    ProcessPacket(buffer,data_size);
    }
    close(sockt);
    printf("Finished");
    return 0;
}

void ProcessPacket(unsigned char* buffer, int size)
{

    print_udp_packet(buffer ,size);

}

void print_udp_packet(unsigned char *buffer , int size)
{

    unsigned short iphdrlen;

    struct iphdr *iph = (struct iphdr *)buffer;
    iphdrlen = iph->ihl*4;

    struct udphdr *udph = (struct udphdr*)(buffer + iphdrlen);

    printf("Data Payload\n");  
    PrintData(buffer + iphdrlen + sizeof udph ,( size - sizeof udph - iph->ihl * 4 ));

    printf("\n###########################################################");
}

void PrintData (unsigned char* data , int size)
{

    for(i=0 ; i < size ; i++)
    {
        if( i!=0 && i%16==0)   //if one line of hex printing is complete...
        {
            printf("         ");
            for(j=i-16 ; j<i ; j++)
            {
                if(data[j]>=32 && data[j]<=128)
                    printf("%c",(unsigned char)data[j]); //if its a number or alphabet

                else printf("."); //otherwise print a dot
            }
            printf("\n");
        } 

        if(i%16==0) printf("   ");
            printf(" %02X",(unsigned int)data[i]);

        if( i==size-1)  //print the last spaces
        {
            for(j=0;j<15-i%16;j++) printf("   "); //extra spaces

            printf("   ");

            for(j=i-i%16 ; j<=i ; j++)
            {
                if(data[j]>=32 && data[j]<=128) printf("%c",(unsigned char)data[j]);
                else printf(".");
            }
            printf("\n");
        }
    }
}

我收到的输出如下：

Source Port : 55784 , Destination Port : 59568, UDP Length : 5122,Protocol : 188, total length : 5122 
Source IP        : 147.188.195.6
Destination IP   : 0.0.0.0
Data Payload
    93 BC C0 06 00 00 00 00 00 1F 39 D4 D9 E8 E8 B0         ..........9.....
    00 00 00 03 00 00 00 01 00 00 00 9C 00 B1 C9 4E         ...............N
    00 00 00 1D 00 00 01 00 78 F9 F5 6B 00 10 67 8F         ........x..k..g.
    00 00 00 1D 00 00 00 28 00 00 00 02 00 00 00 01         .......(........
    00 00 00 5C 00 00 00 01 00 00 00 4E 00 00 00 04         ...\.......N....
    00 00 00 4C F0 92 1C 48 C2 00 00 0E 0C 30 C7 C7         ...L...H.....0..
    08 00 45 00 00 3C 28 4D 40 00 25 06 1C 7E 4C 5C         ..E..<(M@.%..~L\
    70 AE 93 BC C0 2A B4 FE 00 50 59 41 29 30 00 00         p....*...PYA)0..
    00 00 A0 02 FF FF FD BF 00 00 02 04 05 B4 04 02         ................
    08 0A 00 13 01 7D 00 00 00 00 01 03 03 06 00 00         .....}..........
    00 00 03 E9 00 00 00 10 00 00 00 03 00 00 00 02         ................
    00 00 00 02 FF FF FF FF 00 00 00 01 00 00 00 A8         ................
    00 B1 C9 4F 00 00 00 1D 00 00 01 00 78 F9 F6 82         ...O........x...
    00 10 67 8F 00 00 00 1D 00 00 00 00 00 00 00 02         ..g.............
    00 00 00 01 00 00 00 68 00 00 00 01 00 00 00 59         .......h.......Y
    00 00 00 04 00 00 00 58 F0 92 1C 48 C2 00 00 0E         .......X...H....
    0C 30 C7 C7 08 00 45 00 00 47 D8 94 00 00 2F 11         .0....E..G..../.
    2F 0A 46 27 EA 1F 93 BC C0 04 17 72 00 35 00 33         /.F'.......r.5.3
    BF 19 43 B9 00 10 00 01 00 00 00 00 00 01 03 6E         ..C............n
    73 32 04 73 75 73 78 02 61 63 02 75 6B 00 00 01         s2.susx.ac.uk...
    00 01 00 00 29 10 00 00 00 80 00 00 00 00 00 00         ....)....�......
    00 00 03 E9 00 00 00 10 00 00 00 03 00 00 00 02         ................
    00 00 00 02 FF FF FF FF 00 00 00 01 00 00 00 9C         ................
    00 B1 C9 50 00 00 00 1D 00 00 01 00 78 F9 F6 F0         ...P........x...
    00 10 67 8F 00 00 00 00 00 00 00 1D 00 00 00 02         ..g.............
    00 00 00 01 00 00 00 5C 00 00 00 01 00 00 00 50         .......\.......P
    00 00 00 04 00 00 00 4C 00 0E 0C 30 C7 C7 F0 92         .......L...0....
    1C 48 C2 00 08 00 45 00 00 3E BC F6 00 00 3F 11         .H....E..>....?.
    38 9F 93 BC C0 04 C2 53 70 05 00 35 BE FE 00 2A         8......Sp..5...*
    28 53 82 D8 81 05 00 01 00 00 00 00 00 00 03 77         (S.............w
    77 77 06 67 6F 6F 67 6C 65 02 63 6F 02 75 6B 00         ww.google.co.uk.
    00 01 00 01 00 00 03 E9 00 00 00 10 FF FF FF FF         ................
    00 00 00 00 00 00 00 03 FF FF FF FF               ............

我很困惑如何把这些数据变成可读的形式来知道payload的内容？

Answer 1

首先，您打印的端口号、协议和数据长度不正确。正如我在my answer to your previous question 中提到的，IP 和UDP 标头不包含在recvfrom 填充的缓冲区中。源端口和目的端口分别在saddr.sin_port和daddr.sin_port（打印前一定要在那些字段上调用ntohs），UDP有效载荷长度是recvfrom的返回值，即data_size。

至于数据负载的格式，你需要确切地知道你收到的数据是什么格式的。基于此，您可以提取相关字段并以可读格式打印它们。对于初学者，您可以从main 调用PrintData 而不是ProcessPacket，因为您无需担心IP 和UDP 标头。