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