Code a simple blackhole

Source Code:

 #include <linux/kernel.h>
 #include <linux/module.h>
 #include <linux/netfilter.h>
 #include <linux/netfilter_ipv4.h>
 #include <linux/skbuff.h>
 #include <linux/tcp.h>
 #include <linux/udp.h>
 #include <linux/ip.h>

static struct nf_hook_ops nfho;
struct tcphdr *tcp_header;
struct iphdr *ip_header;

unsigned int hook_func(
  const struct nf_hook_ops *ops, 
  struct sk_buff *skb, 
  const struct net_device *in, 
  const struct net_device *out, 
  int (*okfn)(struct sk_buff *)) {

  ip_header = (struct iphdr *)skb_network_header(skb);
  if (ip_header->protocol == /* TCP */ 6) {
    tcp_header = tcp_hdr(skb);
    u32 saddr, daddr;
    u16 sport, dport;
    saddr = ntohl(ip_header->saddr);
    daddr = ntohl(ip_header->daddr);
    sport = ntohs(tcp_header->source);
    dport = ntohs(tcp_header->dest);
// if packet from local port 7000, drop it.
    if (sport == 7000) {
      printk(KERN_INFO "got tcp packet at 7000 port.\n");
      return NF_DROP;
    }
  }
  return NF_ACCEPT;
}

int init_module() {
  nfho.hook = hook_func;
  nfho.hooknum = NF_INET_LOCAL_OUT;
  nfho.pf = PF_INET;
  nfho.priority = NF_IP_PRI_FIRST;
  nf_register_hook(&nfho);
  return 0;
}

void cleanup_module() {
  nf_unregister_hook(&nfho);
}