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 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         return dst_output(skb);
  71 }
  72
  73 int ip_forward(struct sk_buff *skb)
  74 {
  75         u32 mtu;
  76         struct iphdr *iph;      /* Our header */
  77         struct rtable *rt;      /* Route we use */
  78         struct ip_options *opt  = &(IPCB(skb)->opt);
  79
  80         /* that should never happen */
  81         if (skb->pkt_type != PACKET_HOST)
  82                 goto drop;
  83
  84         if (skb_warn_if_lro(skb))
  85                 goto drop;
  86
  87         if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
  88                 goto drop;
  89
  90         if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  91                 return NET_RX_SUCCESS;
  92
  93         skb_forward_csum(skb);
  94
  95         /*
  96          *      According to the RFC, we must first decrease the TTL field. If
  97          *      that reaches zero, we must reply an ICMP control message telling
  98          *      that the packet's lifetime expired.
  99          */
 100         if (ip_hdr(skb)->ttl <= 1)
 101                 goto too_many_hops;
 102
 103         if (!xfrm4_route_forward(skb))
 104                 goto drop;
 105
 106         rt = skb_rtable(skb);
 107
 108         if (opt->is_strictroute && rt->rt_uses_gateway)
 109                 goto sr_failed;
 110
 111         IPCB(skb)->flags |= IPSKB_FORWARDED;
 112         mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
 113         if (!ip_may_fragment(skb) && ip_exceeds_mtu(skb, mtu)) {
 114                 IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
 115                 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
 116                           htonl(mtu));
 117                 goto drop;
 118         }
 119
 120         /* We are about to mangle packet. Copy it! */
 121         if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
 122                 goto drop;
 123         iph = ip_hdr(skb);
 124
 125         /* Decrease ttl after skb cow done */
 126         ip_decrease_ttl(iph);
 127
 128         /*
 129          *      We now generate an ICMP HOST REDIRECT giving the route
 130          *      we calculated.
 131          */
 132         if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))
 133                 ip_rt_send_redirect(skb);
 134
 135         skb->priority = rt_tos2priority(iph->tos);
 136
 137         return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, skb, skb->dev,
 138                        rt->dst.dev, ip_forward_finish);
 139
 140 sr_failed:
 141         /*
 142          *      Strict routing permits no gatewaying
 143          */
 144          icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 145          goto drop;
 146
 147 too_many_hops:
 148         /* Tell the sender its packet died... */
 149         IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_INHDRERRORS);
 150         icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 151 drop:
 152         kfree_skb(skb);
 153         return NET_RX_DROP;
 154 }