socket结构体：内核在创建socket时会初始化sock结构体，这个结构体包含了一系列操作函数的指针。这些函数用于处理socket的连接、接收和发送等操作。

IP头填充：在RAW协议中，IP头的填充需要由用户代码完成。内核只负责填充MAC头和源IP字段，而IP头的其他字段（如目标IP、上层协议等）需要通过用户提供的数据进行填充。

示例代码分析：

• 发送数据：

  #include 
        
         
  #include 
         
          
  #include 
          
           
  #include 
           
            
  #include 
            
             
  #include 
             
               int main() { int s; struct sockaddr_in daddr; char packet[50]; struct iphdr *ip = (struct iphdr *)packet; if ((s = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) { perror("socket error"); exit(EXIT_FAILURE); } daddr.sin_family = AF_INET; daddr.sin_port = 0; inet_pton(AF_INET, DEST, (struct in_addr *)&daddr.sin_addr.s_addr); memset(daddr.sin_zero, 0, sizeof(daddr.sin_zero)); memset(packet, 'A', sizeof(packet)); ip->ihl = 5; ip->version = 4; ip->tos = 0; ip->tot_len = htons(40); ip->frag_off = 0; ip->ttl = 64; ip->protocol = IPPROTO_RAW; ip->check = 0; ip->saddr = daddr.sin_addr.s_addr; ip->daddr = daddr.sin_addr.s_addr; if (sendto(s, (char *)packet, sizeof(packet), 0, (struct sockaddr *)&daddr, sizeof(daddr)) < 0) { perror("sendto error"); exit(EXIT_FAILURE); } }
             
            
           
          
         
        

  该代码展示了如何通过RAWsocket发送IP数据包。用户需要手动填充IP头字段，并使用sendto函数将数据包发送到目标地址。

• 接收数据：

  #include 
        
         
  #include 
         
          
  #include 
          
           
  #include 
           
            
  #include 
            
             
  #include 
             
               int main() { int s; struct sockaddr_in saddr; char packet[50]; if ((s = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0) { perror("socket error"); exit(EXIT_FAILURE); } memset(packet, 0, sizeof(packet)); socklen_t *len = (socklen_t *)sizeof(saddr); int fromlen = sizeof(saddr); while (1) { if (recvfrom(s, (char *)&packet, sizeof(packet), 0, (struct sockaddr *)&saddr, &fromlen) < 0) { perror("packet receive error"); break; } int i = sizeof(struct iphdr); while (i < sizeof(packet)) { fprintf(stderr, "%c", packet[i]); i++; } printf("\n"); } }
             
            
           
          
         
        

  该代码展示了如何通过RAWsocket接收IP数据包。recvfrom函数用于接收数据包，并将接收到的数据解析并显示。

raw_rcv函数：

int raw_rcv(struct sock *sk, struct sk_buff *skb, struct device *dev, long saddr, long daddr) {
    skb->sk = sk;
    skb->len = ntohs(skb->ip_hdr->tot_len);
    skb->h.raw = (unsigned char *)skb->ip_hdr;
    skb->dev = dev;
    skb->saddr = daddr;
    skb->daddr = saddr;
    if (sock_queue_rcv_skb(sk, skb) < 0) {
        ip_statistics.IpInDiscards++;
        skb->sk = NULL;
        kfree_skb(skb, FREE_READ);
        return(0);
    }
    ip_statistics.IpInDelivers++;
    release_sock(sk);
    return(0);
}

该函数负责将接收到的数据包拆分并挂载到socket的接收队列中。

raw_sendto函数：

static int raw_sendto(struct sock *sk, unsigned char *from, int len, int noblock, unsigned flags, struct sockaddr_in *usin, int addr_len) {
    struct sk_buff *skb;
    struct device *dev = NULL;
    struct sockaddr_in sin;
    int tmp;
    int err;
    if (flags & MSG_OOB) {
        return -EOPNOTSUPP;
    }
    if (flags & ~MSG_DONTROUTE) {
        return -EINVAL;
    }
    if (usin) {
        if (addr_len < sizeof(sin)) {
            return -EINVAL;
        }
        memcpy(&sin, usin, sizeof(sin));
        if (sin.sin_family != AF_INET) {
            return -EINVAL;
        }
    } else {
        sin.sin_family = AF_INET;
        sin.sin_port = sk->protocol;
        sin.sin_addr.s_addr = sk->daddr;
    }
    if (sin.sin_port == 0) {
        sin.sin_port = sk->protocol;
    }
    if (sin.sin_addr.s_addr == INADDR_ANY) {
        sin.sin_addr.s_addr = ip_my_addr();
    }
    if (sk->broadcast == 0 && ip_chk_addr(sin.sin_addr.s_addr) == IS_BROADCAST) {
        return -EACCES;
    }
    skb = sock_alloc_send_skb(sk, len + sk->prot->max_header, noblock, &err);
    if (skb == NULL) {
        return err;
    }
    skb->sk = sk;
    skb->free = 1;
    skb->localroute = sk->localroute | (flags & MSG_DONTROUTE);
    tmp = sk->prot->build_header(skb, sk->saddr, sin.sin_addr.s_addr, &dev, sk->protocol, sk->opt, skb->mem_len, sk->ip_tos, sk->ip_ttl);
    if (tmp < 0) {
        kfree_skb(skb, FREE_WRITE);
        release_sock(sk);
        return tmp;
    }
    memcpy_fromfs(skb->data + tmp, from, len);
    if (sk->protocol == IPPROTO_RAW) {
        unsigned char *buff;
        struct iphdr *iph;
        buff = skb->data;
        buff += tmp;
        iph = (struct iphdr *)buff;
        iph->saddr = sk->saddr;
    }
    skb->len = tmp + len;
    if (sk->prot->queue_xmit(sk, dev, skb, 1)) {
        release_sock(sk);
    }
    return len;
}

该函数负责将数据包构建并发送到目标地址。它首先构建IP头，然后将应用程序提供的数据添加到数据部分，并通过sendto函数发送数据包。