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 int ip_forward_finish(struct sk_buff *skb)
  43 {
  44         struct ip_options *opt  = &(IPCB(skb)->opt);
  45
  46         IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);
  47         IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len);
  48
  49         if (unlikely(opt->optlen))
  50                 ip_forward_options(skb);
  51
  52         return dst_output(skb);
  53 }
  54
  55 int ip_forward(struct sk_buff *skb)
  56 {
  57         u32 mtu;
  58         struct iphdr *iph;      /* Our header */
  59         struct rtable *rt;      /* Route we use */
  60         struct ip_options *opt  = &(IPCB(skb)->opt);
  61
  62         if (skb_warn_if_lro(skb))
  63                 goto drop;
  64
  65         if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
  66                 goto drop;
  67
  68         if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  69                 return NET_RX_SUCCESS;
  70
  71         if (skb->pkt_type != PACKET_HOST)
  72                 goto drop;
  73
  74         skb_forward_csum(skb);
  75
  76         /*
  77          *      According to the RFC, we must first decrease the TTL field. If
  78          *      that reaches zero, we must reply an ICMP control message telling
  79          *      that the packet's lifetime expired.
  80          */
  81         if (ip_hdr(skb)->ttl <= 1)
  82                 goto too_many_hops;
  83
  84         if (!xfrm4_route_forward(skb))
  85                 goto drop;
  86
  87         rt = skb_rtable(skb);
  88
  89         if (opt->is_strictroute && rt->rt_uses_gateway)
  90                 goto sr_failed;
  91
  92         IPCB(skb)->flags |= IPSKB_FORWARDED;
  93         mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
  94         if (unlikely(skb->len > mtu && !skb_is_gso(skb) &&
  95                      (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) {
  96                 IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
  97                 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
  98                           htonl(mtu));
  99                 goto drop;
 100         }
 101
 102         /* We are about to mangle packet. Copy it! */
 103         if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
 104                 goto drop;
 105         iph = ip_hdr(skb);
 106
 107         /* Decrease ttl after skb cow done */
 108         ip_decrease_ttl(iph);
 109
 110         /*
 111          *      We now generate an ICMP HOST REDIRECT giving the route
 112          *      we calculated.
 113          */
 114         if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))
 115                 ip_rt_send_redirect(skb);
 116
 117         skb->priority = rt_tos2priority(iph->tos);
 118
 119         return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, skb, skb->dev,
 120                        rt->dst.dev, ip_forward_finish);
 121
 122 sr_failed:
 123         /*
 124          *      Strict routing permits no gatewaying
 125          */
 126          icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 127          goto drop;
 128
 129 too_many_hops:
 130         /* Tell the sender its packet died... */
 131         IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_INHDRERRORS);
 132         icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 133 drop:
 134         kfree_skb(skb);
 135         return NET_RX_DROP;
 136 }