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Linux/net/ipv4/ip_forward.c

  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_validate_mtu(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(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
 69         __IP_ADD_STATS(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 | IPSKB_FRAG_SEGS;
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(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 

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