linux/net/ipv4/ip_forward.c
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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/sched.h>
  27#include <linux/skbuff.h>
  28#include <linux/ip.h>
  29#include <linux/icmp.h>
  30#include <linux/netdevice.h>
  31#include <net/sock.h>
  32#include <net/ip.h>
  33#include <net/tcp.h>
  34#include <net/udp.h>
  35#include <net/icmp.h>
  36#include <linux/tcp.h>
  37#include <linux/udp.h>
  38#include <linux/netfilter_ipv4.h>
  39#include <net/checksum.h>
  40#include <linux/route.h>
  41#include <net/route.h>
  42#include <net/xfrm.h>
  43
  44static inline int ip_forward_finish(struct sk_buff *skb)
  45{
  46        struct ip_options * opt = &(IPCB(skb)->opt);
  47
  48        IP_INC_STATS_BH(IPSTATS_MIB_OUTFORWDATAGRAMS);
  49
  50        if (unlikely(opt->optlen))
  51                ip_forward_options(skb);
  52
  53        return dst_output(skb);
  54}
  55
  56int ip_forward(struct sk_buff *skb)
  57{
  58        struct iphdr *iph;      /* Our header */
  59        struct rtable *rt;      /* Route we use */
  60        struct ip_options * opt = &(IPCB(skb)->opt);
  61
  62        if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
  63                goto drop;
  64
  65        if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  66                return NET_RX_SUCCESS;
  67
  68        if (skb->pkt_type != PACKET_HOST)
  69                goto drop;
  70
  71        skb->ip_summed = CHECKSUM_NONE;
  72        
  73        /*
  74         *      According to the RFC, we must first decrease the TTL field. If
  75         *      that reaches zero, we must reply an ICMP control message telling
  76         *      that the packet's lifetime expired.
  77         */
  78        if (skb->nh.iph->ttl <= 1)
  79                goto too_many_hops;
  80
  81        if (!xfrm4_route_forward(skb))
  82                goto drop;
  83
  84        rt = (struct rtable*)skb->dst;
  85
  86        if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
  87                goto sr_failed;
  88
  89        /* We are about to mangle packet. Copy it! */
  90        if (skb_cow(skb, LL_RESERVED_SPACE(rt->u.dst.dev)+rt->u.dst.header_len))
  91                goto drop;
  92        iph = skb->nh.iph;
  93
  94        /* Decrease ttl after skb cow done */
  95        ip_decrease_ttl(iph);
  96
  97        /*
  98         *      We now generate an ICMP HOST REDIRECT giving the route
  99         *      we calculated.
 100         */
 101        if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr)
 102                ip_rt_send_redirect(skb);
 103
 104        skb->priority = rt_tos2priority(iph->tos);
 105
 106        return NF_HOOK(PF_INET, NF_IP_FORWARD, skb, skb->dev, rt->u.dst.dev,
 107                       ip_forward_finish);
 108
 109sr_failed:
 110        /*
 111         *      Strict routing permits no gatewaying
 112         */
 113         icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 114         goto drop;
 115
 116too_many_hops:
 117        /* Tell the sender its packet died... */
 118        IP_INC_STATS_BH(IPSTATS_MIB_INHDRERRORS);
 119        icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 120drop:
 121        kfree_skb(skb);
 122        return NET_RX_DROP;
 123}
 124
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