/* * 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. * * 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/types.h>#include <linux/mm.h>#include <linux/skbuff.h>#include <linux/ip.h>#include <linux/icmp.h>#include <linux/netdevice.h>#include <linux/slab.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>#include <net/xfrm.h> static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu){	if (skb->len <= mtu)		return false; 	if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0))		return false; 	/* original fragment exceeds mtu and DF is set */	if (unlikely(IPCB(skb)->frag_max_size > mtu))		return true; 	if (skb->ignore_df)		return false; 	if (skb_is_gso(skb) && skb_gso_network_seglen(skb) <= mtu)		return false; 	return true;}  static int ip_forward_finish(struct sock *sk, struct sk_buff *skb){	struct ip_options *opt	= &(IPCB(skb)->opt); 	IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);	IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len); 	if (unlikely(opt->optlen))		ip_forward_options(skb); 	skb_sender_cpu_clear(skb);	return dst_output_sk(sk, skb);} int ip_forward(struct sk_buff *skb){	u32 mtu;	struct iphdr *iph;	/* Our header */	struct rtable *rt;	/* Route we use */	struct ip_options *opt	= &(IPCB(skb)->opt); 	/* that should never happen */	if (skb->pkt_type != PACKET_HOST)		goto drop; 	if (unlikely(skb->sk))		goto drop; 	if (skb_warn_if_lro(skb))		goto drop; 	if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))		goto drop; 	if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))		return NET_RX_SUCCESS; 	skb_forward_csum(skb); 	/*	 *	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.	 */	if (ip_hdr(skb)->ttl <= 1)		goto too_many_hops; 	if (!xfrm4_route_forward(skb))		goto drop; 	rt = skb_rtable(skb); 	if (opt->is_strictroute && rt->rt_uses_gateway)		goto sr_failed; 	IPCB(skb)->flags |= IPSKB_FORWARDED;	mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);	if (ip_exceeds_mtu(skb, mtu)) {		IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,			  htonl(mtu));		goto drop;	} 	/* We are about to mangle packet. Copy it! */	if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))		goto drop;	iph = ip_hdr(skb); 	/* Decrease ttl after skb cow done */	ip_decrease_ttl(iph); 	/*	 *	We now generate an ICMP HOST REDIRECT giving the route	 *	we calculated.	 */	if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&	    !skb_sec_path(skb))		ip_rt_send_redirect(skb); 	skb->priority = rt_tos2priority(iph->tos); 	return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, NULL, skb,		       skb->dev, rt->dst.dev, ip_forward_finish); sr_failed:	/*	 *	Strict routing permits no gatewaying	 */	 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);	 goto drop; too_many_hops:	/* Tell the sender its packet died... */	IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_INHDRERRORS);	icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);drop:	kfree_skb(skb);	return NET_RX_DROP;}