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

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