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 bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu) 43 { 44 if (skb->len <= mtu) 45 return false; 46 47 if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0)) 48 return false; 49 50 /* original fragment exceeds mtu and DF is set */ 51 if (unlikely(IPCB(skb)->frag_max_size > mtu)) 52 return true; 53 54 if (skb->ignore_df) 55 return false; 56 57 if (skb_is_gso(skb) && skb_gso_network_seglen(skb) <= mtu) 58 return false; 59 60 return true; 61 } 62 63 64 static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb) 65 { 66 struct ip_options *opt = &(IPCB(skb)->opt); 67 68 IP_INC_STATS_BH(net, IPSTATS_MIB_OUTFORWDATAGRAMS); 69 IP_ADD_STATS_BH(net, IPSTATS_MIB_OUTOCTETS, skb->len); 70 71 if (unlikely(opt->optlen)) 72 ip_forward_options(skb); 73 74 return dst_output(net, sk, skb); 75 } 76 77 int ip_forward(struct sk_buff *skb) 78 { 79 u32 mtu; 80 struct iphdr *iph; /* Our header */ 81 struct rtable *rt; /* Route we use */ 82 struct ip_options *opt = &(IPCB(skb)->opt); 83 struct net *net; 84 85 /* that should never happen */ 86 if (skb->pkt_type != PACKET_HOST) 87 goto drop; 88 89 if (unlikely(skb->sk)) 90 goto drop; 91 92 if (skb_warn_if_lro(skb)) 93 goto drop; 94 95 if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb)) 96 goto drop; 97 98 if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb)) 99 return NET_RX_SUCCESS; 100 101 skb_forward_csum(skb); 102 net = dev_net(skb->dev); 103 104 /* 105 * According to the RFC, we must first decrease the TTL field. If 106 * that reaches zero, we must reply an ICMP control message telling 107 * that the packet's lifetime expired. 108 */ 109 if (ip_hdr(skb)->ttl <= 1) 110 goto too_many_hops; 111 112 if (!xfrm4_route_forward(skb)) 113 goto drop; 114 115 rt = skb_rtable(skb); 116 117 if (opt->is_strictroute && rt->rt_uses_gateway) 118 goto sr_failed; 119 120 IPCB(skb)->flags |= IPSKB_FORWARDED; 121 mtu = ip_dst_mtu_maybe_forward(&rt->dst, true); 122 if (ip_exceeds_mtu(skb, mtu)) { 123 IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS); 124 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, 125 htonl(mtu)); 126 goto drop; 127 } 128 129 /* We are about to mangle packet. Copy it! */ 130 if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len)) 131 goto drop; 132 iph = ip_hdr(skb); 133 134 /* Decrease ttl after skb cow done */ 135 ip_decrease_ttl(iph); 136 137 /* 138 * We now generate an ICMP HOST REDIRECT giving the route 139 * we calculated. 140 */ 141 if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr && 142 !skb_sec_path(skb)) 143 ip_rt_send_redirect(skb); 144 145 skb->priority = rt_tos2priority(iph->tos); 146 147 return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, 148 net, NULL, skb, skb->dev, rt->dst.dev, 149 ip_forward_finish); 150 151 sr_failed: 152 /* 153 * Strict routing permits no gatewaying 154 */ 155 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0); 156 goto drop; 157 158 too_many_hops: 159 /* Tell the sender its packet died... */ 160 IP_INC_STATS_BH(net, IPSTATS_MIB_INHDRERRORS); 161 icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0); 162 drop: 163 kfree_skb(skb); 164 return NET_RX_DROP; 165 } 166