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 * 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
  42static 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
  55int 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 (skb_warn_if_lro(skb))
  62                goto drop;
  63
  64        if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
  65                goto drop;
  66
  67        if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  68                return NET_RX_SUCCESS;
  69
  70        if (skb->pkt_type != PACKET_HOST)
  71                goto drop;
  72
  73        skb_forward_csum(skb);
  74
  75        /*
  76         *      According to the RFC, we must first decrease the TTL field. If
  77         *      that reaches zero, we must reply an ICMP control message telling
  78         *      that the packet's lifetime expired.
  79         */
  80        if (ip_hdr(skb)->ttl <= 1)
  81                goto too_many_hops;
  82
  83        if (!xfrm4_route_forward(skb))
  84                goto drop;
  85
  86        rt = skb_rtable(skb);
  87
  88        if (opt->is_strictroute && rt->rt_uses_gateway)
  89                goto sr_failed;
  90
  91        if (unlikely(skb->len > dst_mtu(&rt->dst) && !skb_is_gso(skb) &&
  92                     (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) {
  93                IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
  94                icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
  95                          htonl(dst_mtu(&rt->dst)));
  96                goto drop;
  97        }
  98
  99        /* We are about to mangle packet. Copy it! */
 100        if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
 101                goto drop;
 102        iph = ip_hdr(skb);
 103
 104        /* Decrease ttl after skb cow done */
 105        ip_decrease_ttl(iph);
 106
 107        /*
 108         *      We now generate an ICMP HOST REDIRECT giving the route
 109         *      we calculated.
 110         */
 111        if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))
 112                ip_rt_send_redirect(skb);
 113
 114        skb->priority = rt_tos2priority(iph->tos);
 115
 116        return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, skb, skb->dev,
 117                       rt->dst.dev, ip_forward_finish);
 118
 119sr_failed:
 120        /*
 121         *      Strict routing permits no gatewaying
 122         */
 123         icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 124         goto drop;
 125
 126too_many_hops:
 127        /* Tell the sender its packet died... */
 128        IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_INHDRERRORS);
 129        icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 130drop:
 131        kfree_skb(skb);
 132        return NET_RX_DROP;
 133}
 134
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