Question

我在C中实现原始套接字。在下面的代码中，我正在解析从发送方收到的IP头。

a.  I will send back the ack as well so storing IP address received in a buffer(ret_ip). 
b.  I don't have another computer so using lo (local loop back) as my interface.

//first two printf statements are printing the right address, 10.100.207.74
//daddr SENT = 32.112.247.9saddr SENT = 36.112.247.9

我怎样才能正确理解？我认为这个问题是由于memcpy的第一个参数指向unsigned char而第二个参数指向_be32。

我在程序中实际想做的是：ret_ip的前4个字节应包含目标地址，接下来4个包含源地址。然后我将创建IP头并使dest addr = source addr和source-addr = dest-addr。并向发送者发送ACK。

char* ParseIPHeader(unsigned char *packet,int len)
{
struct ethhdr *ethernet_header;
struct iphdr *ip_header;
char *ret_ip;   
ethernet_header=(struct ethhdr *)packet;

if(ntohs(ethernet_header->h_proto)==ETH_P_IP)
{
    if(len>=(sizeof(struct ethhdr)+sizeof(struct iphdr)))
    {
        ip_header=(struct iphdr*)(packet+sizeof(struct ethhdr));
        ret_ip=malloc(2*(sizeof(ip_header->daddr)));
        printf("Dest IP address: %s\n",inet_ntoa(ip_header->daddr));  
        printf("Source IP address: %s\n",inet_ntoa(ip_header->saddr));
        memcpy(ret_ip,&(ip_header->daddr),sizeof(ip_header->daddr));
        memcpy(ret_ip+4,&(ip_header->saddr),4);
        printf("daddr SENT = %s",inet_ntoa(ret_ip));  
        printf("saddr SENT = %s",inet_ntoa(ret_ip+4));
    }
    else
    printf("IP packet does not have full header\n");
}
else
{ 
//not an IP packet
}
return ret_ip;
}

谢谢:)

Answer 1

第一个问题是你的记忆分配

ret_ip=malloc(2*(ip_header->daddr));

应该是

ret_ip=malloc(2*(sizeof(ip_header->daddr)));

但为什么你不再使用ip_hdr结构？例如

struct iphdr *ret_ip = malloc(sizeof(iphdr));
ret_ip->daddr = ip_header->saddr;
ret_ip->saddr = ip_header->daddr;

我建议这个解决方案更容易;）

将IP地址分配给缓冲区并打印缓冲区内容不匹配

1 个答案: