Linux内核调用ip_output发送SYN包

void send_syn(void)
{
 struct sk_buff *skb;
 struct iphdr *iph;
 struct tcphdr *tcph;
 struct rtable *rt =NULL;
 u8 mac[12];
 u32 i;
 int ret;

 //查路由，主要是获取出接口然后绑定邻居表项
 struct flowi fl =
 {
  .fl4_dst = 169741057,//syn包的目的地址地址
  .fl_ip_dport = 8080,//目的端口
  .fl_ip_sport = 0,
  .mark = 0,
  .iif = 0,
  .proto = IPPROTO_TCP,//协议
 };

//生成一个skb，这个skb就是syn包
 skb = __alloc_skb(128, GFP_ATOMIC, 0, -1);

 /*mac*/
 skb->mac.raw = skb->data;

 /*ip*/
 iph = skb->nh.raw = skb->mac.raw + 14;//mac大小14，所以偏过这么长的长度，填写ip头，tcp头等，mac头由邻居子系统填写
 iph->version = 4;//v4
 iph->ihl = 5;//ip头长度
 iph->tos = 0;
 iph->frag_off = 0;
 iph->ttl = 127;
 iph->protocol = IPPROTO_TCP;
 iph->saddr = 169741309;
 iph->daddr = 169741057;
 iph->tot_len = 20 + 20;//ip头+tcp头+tcp数据负载

 /*tcp*/
 tcph = skb->h.th = (struct tcphdr *)((u8*)iph + sizeof(struct iphdr));//获取tcp头
 memset(tcph,0x0,sizeof(struct tcphdr));
 tcph->source = 12345;
 tcph->dest = 8080;
 tcph->seq = 1;
 tcph->ack_seq = 0x0;
 tcph->window = 0xffff;
 tcph->syn = 1;
 tcph->ack = 0;
 tcph->psh = 0;
 tcph->doff = sizeof(struct tcphdr)/4;tcp部分的长度

 /*checksum*/
 tcph->check = csum_tcpudp_magic(iph->saddr, iph->daddr, tcph->doff<<2, IPPROTO_TCP,
     csum_partial((u8*)tcph, tcph->doff<<2, 0));
 iph->check = ip_fast_csum((u8*)iph, iph->ihl);

 /*skb info*/
 skb->len = 14 + 20 + 20;/*mac + iph + tcph*/
 skb->protocol = ETH_P_IP;

 if(ip_route_output_key(&net, &rt, &fl))
 {
  printk("route fail\n");
  kfree_skb(skb);
  return;
 }

 printk("route sucess\n");

 if (unlikely(rt == NULL)){
  printk("rt.is null\n");
  return;
 }

 skb->dst = &rt->u.dst;

 ret = ip_output(skb);
 printk("ret:%d\n",ret);
}