c - 使用 C 程序在 Linux 中捕获 UDP 数据时接收相同的源地址和目标 IP 地址

Question

我试图在从端口 6343 捕获 UDP 数据时显示源和目标 IP 地址。我观察到的是我为这两个字段显示了相同的 IP 地址。以下是我正在运行的代码，

#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
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 *)&saddr,0,sizeof(saddr));

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

我收到的输出是:

Source Port : 55600 , Destination Port : 9162, UDP Length : 7173, Source IP : 147.188.195.6, Destination IP : 147.188.195.6, Protocol : 188, total length : 5

我是在显示 IP 地址时出错了，还是没有指向正确的位置来获取地址？

Answer 1

inet_ntoa() 是一个不可重入函数。它会在您每次调用它时返回指向同一缓冲区的指针，因此当您在一条语句中调用它两次时，其中一个调用会覆盖另一个调用生成的缓冲区的内容。

而是分几个步骤来做:

printf("\t Source Port : %d , Destination Port : %d, UDP Length :%d, ",
       ntohs(udph->source), ntohs(udph->dest), ntohs(data_size));  
printf("Source IP  : %s, ", inet_ntoa(saddr.sin_addr));
printf("Destination IP  : %s, ", inet_ntoa(daddr.sin_addr));
printf("Protocol : %d, total length : %d \n", 
      (unsigned int)iph->protocol, ntohs(iph->tot_len));