/*
       * INET		An implementation of the TCP/IP protocol suite for the LINUX
       *		operating system.  INET is implemented using the  BSD Socket
       *		interface as the means of communication with the user level.
       *
       *		The IP forwarding functionality.
       *		
       * Version:	$Id: ip_forward.c,v 1.47 2000/10/24 22:54:26 davem Exp $
       *
       * Authors:	see ip.c
       *
       * Fixes:
       *		Many		:	Split from ip.c , see ip_input.c for 
       *					history.
       *		Dave Gregorich	:	NULL ip_rt_put fix for multicast 
       *					routing.
       *		Jos Vos		:	Add call_out_firewall before sending,
       *					use output device for accounting.
       *		Jos Vos		:	Call forward firewall after routing
       *					(always use output device).
       *		Mike McLagan	:	Routing by source
       */
      
      #include <linux/config.h>
      #include <linux/types.h>
      #include <linux/mm.h>
      #include <linux/sched.h>
      #include <linux/skbuff.h>
      #include <linux/ip.h>
      #include <linux/icmp.h>
      #include <linux/netdevice.h>
      #include <net/sock.h>
      #include <net/ip.h>
      #include <net/tcp.h>
      #include <net/udp.h>
      #include <net/icmp.h>
      #include <linux/tcp.h>
      #include <linux/udp.h>
      #include <linux/netfilter_ipv4.h>
      #include <net/checksum.h>
      #include <linux/route.h>
      #include <net/route.h>
      
  44  static inline int ip_forward_finish(struct sk_buff *skb)
      {
      	struct ip_options * opt	= &(IPCB(skb)->opt);
      
      	IP_INC_STATS_BH(IpForwDatagrams);
      
  50  	if (opt->optlen == 0) {
      #ifdef CONFIG_NET_FASTROUTE
      		struct rtable *rt = (struct rtable*)skb->dst;
      
      		if (rt->rt_flags&RTCF_FAST && !netdev_fastroute_obstacles) {
      			struct dst_entry *old_dst;
      			unsigned h = ((*(u8*)&rt->key.dst)^(*(u8*)&rt->key.src))&NETDEV_FASTROUTE_HMASK;
      
      			write_lock_irq(&skb->dev->fastpath_lock);
      			old_dst = skb->dev->fastpath[h];
      			skb->dev->fastpath[h] = dst_clone(&rt->u.dst);
      			write_unlock_irq(&skb->dev->fastpath_lock);
      
      			dst_release(old_dst);
      		}
      #endif
  66  		return (ip_send(skb));
      	}
      
      	ip_forward_options(skb);
  70  	return (ip_send(skb));
      }
      
  73  int ip_forward(struct sk_buff *skb)
      {
      	struct net_device *dev2;	/* Output device */
      	struct iphdr *iph;	/* Our header */
      	struct rtable *rt;	/* Route we use */
      	struct ip_options * opt	= &(IPCB(skb)->opt);
      	unsigned short mtu;
      
  81  	if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  82  		return NET_RX_SUCCESS;
      
  84  	if (skb->pkt_type != PACKET_HOST)
  85  		goto drop;
      	
      	/*
      	 *	According to the RFC, we must first decrease the TTL field. If
      	 *	that reaches zero, we must reply an ICMP control message telling
      	 *	that the packet's lifetime expired.
      	 */
      
      	iph = skb->nh.iph;
      	rt = (struct rtable*)skb->dst;
      
  96  	if (iph->ttl <= 1)
  97                  goto too_many_hops;
      
  99  	if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
 100                  goto sr_failed;
      
      	/*
      	 *	Having picked a route we can now send the frame out
      	 *	after asking the firewall permission to do so.
      	 */
      
      	skb->priority = rt_tos2priority(iph->tos);
      	dev2 = rt->u.dst.dev;
      	mtu = rt->u.dst.pmtu;
      
      	/*
      	 *	We now generate an ICMP HOST REDIRECT giving the route
      	 *	we calculated.
      	 */
 115  	if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr)
      		ip_rt_send_redirect(skb);
      
      	/* We are about to mangle packet. Copy it! */
 119  	if ((skb = skb_cow(skb, dev2->hard_header_len)) == NULL)
 120  		return NET_RX_DROP;
      	iph = skb->nh.iph;
      	opt = &(IPCB(skb)->opt);
      
      	/* Decrease ttl after skb cow done */
      	ip_decrease_ttl(iph);
      
      	/*
      	 * We now may allocate a new buffer, and copy the datagram into it.
      	 * If the indicated interface is up and running, kick it.
      	 */
      
 132  	if (skb->len > mtu && (ntohs(iph->frag_off) & IP_DF))
 133  		goto frag_needed;
      
      #ifdef CONFIG_IP_ROUTE_NAT
      	if (rt->rt_flags & RTCF_NAT) {
      		if (ip_do_nat(skb)) {
      			kfree_skb(skb);
      			return NET_RX_BAD;
      		}
      	}
      #endif
      
      	return NF_HOOK(PF_INET, NF_IP_FORWARD, skb, skb->dev, dev2,
 145  		       ip_forward_finish);
      
      frag_needed:
      	IP_INC_STATS_BH(IpFragFails);
      	icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(mtu));
 150          goto drop;
      
      sr_failed:
              /*
      	 *	Strict routing permits no gatewaying
      	 */
               icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 157           goto drop;
      
      too_many_hops:
              /* Tell the sender its packet died... */
              icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
      drop:
      	kfree_skb(skb);
 164  	return NET_RX_DROP;
      }