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   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  * Version:     $Id: ip_forward.c,v 1.48 2000/12/13 18:31:48 davem Exp $
   9  *
  10  * Authors:     see ip.c
  11  *
  12  * Fixes:
  13  *              Many            :       Split from ip.c , see ip_input.c for
  14  *                                      history.
  15  *              Dave Gregorich  :       NULL ip_rt_put fix for multicast
  16  *                                      routing.
  17  *              Jos Vos         :       Add call_out_firewall before sending,
  18  *                                      use output device for accounting.
  19  *              Jos Vos         :       Call forward firewall after routing
  20  *                                      (always use output device).
  21  *              Mike McLagan    :       Routing by source
  22  */
  23
  24 #include <linux/types.h>
  25 #include <linux/mm.h>
  26 #include <linux/skbuff.h>
  27 #include <linux/ip.h>
  28 #include <linux/icmp.h>
  29 #include <linux/netdevice.h>
  30 #include <net/sock.h>
  31 #include <net/ip.h>
  32 #include <net/tcp.h>
  33 #include <net/udp.h>
  34 #include <net/icmp.h>
  35 #include <linux/tcp.h>
  36 #include <linux/udp.h>
  37 #include <linux/netfilter_ipv4.h>
  38 #include <net/checksum.h>
  39 #include <linux/route.h>
  40 #include <net/route.h>
  41 #include <net/xfrm.h>
  42
  43 static inline int ip_forward_finish(struct sk_buff *skb)
  44 {
  45         struct ip_options * opt = &(IPCB(skb)->opt);
  46
  47         IP_INC_STATS_BH(IPSTATS_MIB_OUTFORWDATAGRAMS);
  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         struct iphdr *iph;      /* Our header */
  58         struct rtable *rt;      /* Route we use */
  59         struct ip_options * opt = &(IPCB(skb)->opt);
  60
  61         if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
  62                 goto drop;
  63
  64         if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  65                 return NET_RX_SUCCESS;
  66
  67         if (skb->pkt_type != PACKET_HOST)
  68                 goto drop;
  69
  70         skb->ip_summed = CHECKSUM_NONE;
  71
  72         /*
  73          *      According to the RFC, we must first decrease the TTL field. If
  74          *      that reaches zero, we must reply an ICMP control message telling
  75          *      that the packet's lifetime expired.
  76          */
  77         if (ip_hdr(skb)->ttl <= 1)
  78                 goto too_many_hops;
  79
  80         if (!xfrm4_route_forward(skb))
  81                 goto drop;
  82
  83         rt = (struct rtable*)skb->dst;
  84
  85         if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
  86                 goto sr_failed;
  87
  88         /* We are about to mangle packet. Copy it! */
  89         if (skb_cow(skb, LL_RESERVED_SPACE(rt->u.dst.dev)+rt->u.dst.header_len))
  90                 goto drop;
  91         iph = ip_hdr(skb);
  92
  93         /* Decrease ttl after skb cow done */
  94         ip_decrease_ttl(iph);
  95
  96         /*
  97          *      We now generate an ICMP HOST REDIRECT giving the route
  98          *      we calculated.
  99          */
 100         if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr)
 101                 ip_rt_send_redirect(skb);
 102
 103         skb->priority = rt_tos2priority(iph->tos);
 104
 105         return NF_HOOK(PF_INET, NF_IP_FORWARD, skb, skb->dev, rt->u.dst.dev,
 106                        ip_forward_finish);
 107
 108 sr_failed:
 109         /*
 110          *      Strict routing permits no gatewaying
 111          */
 112          icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 113          goto drop;
 114
 115 too_many_hops:
 116         /* Tell the sender its packet died... */
 117         IP_INC_STATS_BH(IPSTATS_MIB_INHDRERRORS);
 118         icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 119 drop:
 120         kfree_skb(skb);
 121         return NET_RX_DROP;
 122 }