1// SPDX-License-Identifier: GPL-2.0
2/*
3 * INET An implementation of the TCP/IP protocol suite for the LINUX
4 * operating system. INET is implemented using the BSD Socket
5 * interface as the means of communication with the user level.
6 *
7 * The IP forwarding functionality.
8 *
9 * Authors: see ip.c
10 *
11 * Fixes:
12 * Many : Split from ip.c , see ip_input.c for
13 * history.
14 * Dave Gregorich : NULL ip_rt_put fix for multicast
15 * routing.
16 * Jos Vos : Add call_out_firewall before sending,
17 * use output device for accounting.
18 * Jos Vos : Call forward firewall after routing
19 * (always use output device).
20 * Mike McLagan : Routing by source
21 */
22
23#include <linux/types.h>
24#include <linux/mm.h>
25#include <linux/skbuff.h>
26#include <linux/ip.h>
27#include <linux/icmp.h>
28#include <linux/netdevice.h>
29#include <linux/slab.h>
30#include <net/sock.h>
31#include <net/ip.h>
32#include <net/tcp.h>
33#include <net/udp.h>
34#include <net/icmp.h>
35#include <linux/tcp.h>
36#include <linux/udp.h>
37#include <linux/netfilter_ipv4.h>
38#include <net/checksum.h>
39#include <linux/route.h>
40#include <net/route.h>
41#include <net/xfrm.h>
42
43static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
44{
45 if (skb->len <= mtu)
46 return false;
47
48 if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0))
49 return false;
50
51 /* original fragment exceeds mtu and DF is set */
52 if (unlikely(IPCB(skb)->frag_max_size > mtu))
53 return true;
54
55 if (skb->ignore_df)
56 return false;
57
58 if (skb_is_gso(skb) && skb_gso_validate_network_len(skb, mtu))
59 return false;
60
61 return true;
62}
63
64
65static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
66{
67 struct ip_options *opt = &(IPCB(skb)->opt);
68
69 __IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
70 __IP_ADD_STATS(net, IPSTATS_MIB_OUTOCTETS, skb->len);
71
72#ifdef CONFIG_NET_SWITCHDEV
73 if (skb->offload_l3_fwd_mark) {
74 consume_skb(skb);
75 return 0;
76 }
77#endif
78
79 if (unlikely(opt->optlen))
80 ip_forward_options(skb);
81
82 skb->tstamp = 0;
83 return dst_output(net, sk, skb);
84}
85
86int ip_forward(struct sk_buff *skb)
87{
88 u32 mtu;
89 struct iphdr *iph; /* Our header */
90 struct rtable *rt; /* Route we use */
91 struct ip_options *opt = &(IPCB(skb)->opt);
92 struct net *net;
93
94 /* that should never happen */
95 if (skb->pkt_type != PACKET_HOST)
96 goto drop;
97
98 if (unlikely(skb->sk))
99 goto drop;
100
101 if (skb_warn_if_lro(skb))
102 goto drop;
103
104 if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
105 goto drop;
106
107 if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
108 return NET_RX_SUCCESS;
109
110 skb_forward_csum(skb);
111 net = dev_net(skb->dev);
112
113 /*
114 * According to the RFC, we must first decrease the TTL field. If
115 * that reaches zero, we must reply an ICMP control message telling
116 * that the packet's lifetime expired.
117 */
118 if (ip_hdr(skb)->ttl <= 1)
119 goto too_many_hops;
120
121 if (!xfrm4_route_forward(skb))
122 goto drop;
123
124 rt = skb_rtable(skb);
125
126 if (opt->is_strictroute && rt->rt_uses_gateway)
127 goto sr_failed;
128
129 IPCB(skb)->flags |= IPSKB_FORWARDED;
130 mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
131 if (ip_exceeds_mtu(skb, mtu)) {
132 IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
133 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
134 htonl(mtu));
135 goto drop;
136 }
137
138 /* We are about to mangle packet. Copy it! */
139 if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
140 goto drop;
141 iph = ip_hdr(skb);
142
143 /* Decrease ttl after skb cow done */
144 ip_decrease_ttl(iph);
145
146 /*
147 * We now generate an ICMP HOST REDIRECT giving the route
148 * we calculated.
149 */
150 if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
151 !skb_sec_path(skb))
152 ip_rt_send_redirect(skb);
153
154 if (net->ipv4.sysctl_ip_fwd_update_priority)
155 skb->priority = rt_tos2priority(iph->tos);
156
157 return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
158 net, NULL, skb, skb->dev, rt->dst.dev,
159 ip_forward_finish);
160
161sr_failed:
162 /*
163 * Strict routing permits no gatewaying
164 */
165 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
166 goto drop;
167
168too_many_hops:
169 /* Tell the sender its packet died... */
170 __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
171 icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
172drop:
173 kfree_skb(skb);
174 return NET_RX_DROP;
175}
176