Question

I am trying to display the source and destination IP address while capturing UDP data from port 6343. What I observe is I am displaying a same IP address for both the fields. The following is the code I am running,

#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;
}

The output I am receiving is :

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

Am I going wrong while displaying the IP address or not pointing the right location to fetch the address??

Answer 1

inet_ntoa() is a non-reentrant function. It returns a pointer to the same buffer every time you call it, so when you call it twice in a statement, one of the call overwrites the content of the buffer that the other call produced.

Do it in several steps rather:

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));

Answer 2

在必须保存源IP地址和目标IP地址的情况下，最好使用strcpy函数。例如，

strcpy（src_ipaddr，inet_ntoa（saddr.sin_addr））; strcpy（dst_ipaddr，inet_ntoa（daddr.sin_addr））;

其中src_ipaddr和dst_ipaddr是char *。

Receiving same address for Source and Destination IP address while capturing UDP data in Linux using C program

2 个答案: