xref: /linux/net/ipv4/ip_forward.c (revision 26b0d14106954ae46d2f4f7eec3481828a210f7d)
1 /*
2  * INET		An implementation of the TCP/IP protocol suite for the LINUX
3  *		operating system.  INET is implemented using the  BSD Socket
4  *		interface as the means of communication with the user level.
5  *
6  *		The IP forwarding functionality.
7  *
8  * Authors:	see ip.c
9  *
10  * Fixes:
11  *		Many		:	Split from ip.c , see ip_input.c for
12  *					history.
13  *		Dave Gregorich	:	NULL ip_rt_put fix for multicast
14  *					routing.
15  *		Jos Vos		:	Add call_out_firewall before sending,
16  *					use output device for accounting.
17  *		Jos Vos		:	Call forward firewall after routing
18  *					(always use output device).
19  *		Mike McLagan	:	Routing by source
20  */
21 
22 #include <linux/types.h>
23 #include <linux/mm.h>
24 #include <linux/skbuff.h>
25 #include <linux/ip.h>
26 #include <linux/icmp.h>
27 #include <linux/netdevice.h>
28 #include <linux/slab.h>
29 #include <net/sock.h>
30 #include <net/ip.h>
31 #include <net/tcp.h>
32 #include <net/udp.h>
33 #include <net/icmp.h>
34 #include <linux/tcp.h>
35 #include <linux/udp.h>
36 #include <linux/netfilter_ipv4.h>
37 #include <net/checksum.h>
38 #include <linux/route.h>
39 #include <net/route.h>
40 #include <net/xfrm.h>
41 
42 static int ip_forward_finish(struct sk_buff *skb)
43 {
44 	struct ip_options *opt	= &(IPCB(skb)->opt);
45 
46 	IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);
47 	IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len);
48 
49 	if (unlikely(opt->optlen))
50 		ip_forward_options(skb);
51 
52 	return dst_output(skb);
53 }
54 
55 int ip_forward(struct sk_buff *skb)
56 {
57 	struct iphdr *iph;	/* Our header */
58 	struct rtable *rt;	/* Route we use */
59 	struct ip_options *opt	= &(IPCB(skb)->opt);
60 
61 	if (skb_warn_if_lro(skb))
62 		goto drop;
63 
64 	if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
65 		goto drop;
66 
67 	if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
68 		return NET_RX_SUCCESS;
69 
70 	if (skb->pkt_type != PACKET_HOST)
71 		goto drop;
72 
73 	skb_forward_csum(skb);
74 
75 	/*
76 	 *	According to the RFC, we must first decrease the TTL field. If
77 	 *	that reaches zero, we must reply an ICMP control message telling
78 	 *	that the packet's lifetime expired.
79 	 */
80 	if (ip_hdr(skb)->ttl <= 1)
81 		goto too_many_hops;
82 
83 	if (!xfrm4_route_forward(skb))
84 		goto drop;
85 
86 	rt = skb_rtable(skb);
87 
88 	if (opt->is_strictroute && opt->nexthop != rt->rt_gateway)
89 		goto sr_failed;
90 
91 	if (unlikely(skb->len > dst_mtu(&rt->dst) && !skb_is_gso(skb) &&
92 		     (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) {
93 		IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
94 		icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
95 			  htonl(dst_mtu(&rt->dst)));
96 		goto drop;
97 	}
98 
99 	/* We are about to mangle packet. Copy it! */
100 	if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
101 		goto drop;
102 	iph = ip_hdr(skb);
103 
104 	/* Decrease ttl after skb cow done */
105 	ip_decrease_ttl(iph);
106 
107 	/*
108 	 *	We now generate an ICMP HOST REDIRECT giving the route
109 	 *	we calculated.
110 	 */
111 	if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))
112 		ip_rt_send_redirect(skb);
113 
114 	skb->priority = rt_tos2priority(iph->tos);
115 
116 	return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, skb, skb->dev,
117 		       rt->dst.dev, ip_forward_finish);
118 
119 sr_failed:
120 	/*
121 	 *	Strict routing permits no gatewaying
122 	 */
123 	 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
124 	 goto drop;
125 
126 too_many_hops:
127 	/* Tell the sender its packet died... */
128 	IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_INHDRERRORS);
129 	icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
130 drop:
131 	kfree_skb(skb);
132 	return NET_RX_DROP;
133 }
134