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 <net/sock.h>
  29 #include <net/ip.h>
  30 #include <net/tcp.h>
  31 #include <net/udp.h>
  32 #include <net/icmp.h>
  33 #include <linux/tcp.h>
  34 #include <linux/udp.h>
  35 #include <linux/netfilter_ipv4.h>
  36 #include <net/checksum.h>
  37 #include <linux/route.h>
  38 #include <net/route.h>
  39 #include <net/xfrm.h>
  40
  41 static int ip_forward_finish(struct sk_buff *skb)
  42 {
  43         struct ip_options * opt = &(IPCB(skb)->opt);
  44
  45         IP_INC_STATS_BH(IPSTATS_MIB_OUTFORWDATAGRAMS);
  46
  47         if (unlikely(opt->optlen))
  48                 ip_forward_options(skb);
  49
  50         return dst_output(skb);
  51 }
  52
  53 int ip_forward(struct sk_buff *skb)
  54 {
  55         struct iphdr *iph;      /* Our header */
  56         struct rtable *rt;      /* Route we use */
  57         struct ip_options * opt = &(IPCB(skb)->opt);
  58
  59         if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
  60                 goto drop;
  61
  62         if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  63                 return NET_RX_SUCCESS;
  64
  65         if (skb->pkt_type != PACKET_HOST)
  66                 goto drop;
  67
  68         skb_forward_csum(skb);
  69
  70         /*
  71          *      According to the RFC, we must first decrease the TTL field. If
  72          *      that reaches zero, we must reply an ICMP control message telling
  73          *      that the packet's lifetime expired.
  74          */
  75         if (ip_hdr(skb)->ttl <= 1)
  76                 goto too_many_hops;
  77
  78         if (!xfrm4_route_forward(skb))
  79                 goto drop;
  80
  81         rt = skb->rtable;
  82
  83         if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
  84                 goto sr_failed;
  85
  86         if (unlikely(skb->len > dst_mtu(&rt->u.dst) && !skb_is_gso(skb) &&
  87                      (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) {
  88                 IP_INC_STATS(IPSTATS_MIB_FRAGFAILS);
  89                 icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
  90                           htonl(dst_mtu(&rt->u.dst)));
  91                 goto drop;
  92         }
  93
  94         /* We are about to mangle packet. Copy it! */
  95         if (skb_cow(skb, LL_RESERVED_SPACE(rt->u.dst.dev)+rt->u.dst.header_len))
  96                 goto drop;
  97         iph = ip_hdr(skb);
  98
  99         /* Decrease ttl after skb cow done */
 100         ip_decrease_ttl(iph);
 101
 102         /*
 103          *      We now generate an ICMP HOST REDIRECT giving the route
 104          *      we calculated.
 105          */
 106         if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb->sp)
 107                 ip_rt_send_redirect(skb);
 108
 109         skb->priority = rt_tos2priority(iph->tos);
 110
 111         return NF_HOOK(PF_INET, NF_INET_FORWARD, skb, skb->dev, rt->u.dst.dev,
 112                        ip_forward_finish);
 113
 114 sr_failed:
 115         /*
 116          *      Strict routing permits no gatewaying
 117          */
 118          icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 119          goto drop;
 120
 121 too_many_hops:
 122         /* Tell the sender its packet died... */
 123         IP_INC_STATS_BH(IPSTATS_MIB_INHDRERRORS);
 124         icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 125 drop:
 126         kfree_skb(skb);
 127         return NET_RX_DROP;
 128 }