LXR linux/net/ipv4/ip

   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 bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
  43{
  44        if (skb->len <= mtu)
  45                return false;
  46
  47        if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0))
  48                return false;
  49
  50        /* original fragment exceeds mtu and DF is set */
  51        if (unlikely(IPCB(skb)->frag_max_size > mtu))
  52                return true;
  53
  54        if (skb->ignore_df)
  55                return false;
  56
  57        if (skb_is_gso(skb) && skb_gso_validate_mtu(skb, mtu))
  58                return false;
  59
  60        return true;
  61}
  62
  63
  64static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
  65{
  66        struct ip_options *opt  = &(IPCB(skb)->opt);
  67
  68        __IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
  69        __IP_ADD_STATS(net, IPSTATS_MIB_OUTOCTETS, skb->len);
  70
  71        if (unlikely(opt->optlen))
  72                ip_forward_options(skb);
  73
  74        return dst_output(net, sk, skb);
  75}
  76
  77int ip_forward(struct sk_buff *skb)
  78{
  79        u32 mtu;
  80        struct iphdr *iph;      /* Our header */
  81        struct rtable *rt;      /* Route we use */
  82        struct ip_options *opt  = &(IPCB(skb)->opt);
  83        struct net *net;
  84
  85        /* that should never happen */
  86        if (skb->pkt_type != PACKET_HOST)
  87                goto drop;
  88
  89        if (unlikely(skb->sk))
  90                goto drop;
  91
  92        if (skb_warn_if_lro(skb))
  93                goto drop;
  94
  95        if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
  96                goto drop;
  97
  98        if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
  99                return NET_RX_SUCCESS;
 100
 101        skb_forward_csum(skb);
 102        net = dev_net(skb->dev);
 103
 104        /*
 105         *      According to the RFC, we must first decrease the TTL field. If
 106         *      that reaches zero, we must reply an ICMP control message telling
 107         *      that the packet's lifetime expired.
 108         */
 109        if (ip_hdr(skb)->ttl <= 1)
 110                goto too_many_hops;
 111
 112        if (!xfrm4_route_forward(skb))
 113                goto drop;
 114
 115        rt = skb_rtable(skb);
 116
 117        if (opt->is_strictroute && rt->rt_uses_gateway)
 118                goto sr_failed;
 119
 120        IPCB(skb)->flags |= IPSKB_FORWARDED;
 121        mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
 122        if (ip_exceeds_mtu(skb, mtu)) {
 123                IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
 124                icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
 125                          htonl(mtu));
 126                goto drop;
 127        }
 128
 129        /* We are about to mangle packet. Copy it! */
 130        if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
 131                goto drop;
 132        iph = ip_hdr(skb);
 133
 134        /* Decrease ttl after skb cow done */
 135        ip_decrease_ttl(iph);
 136
 137        /*
 138         *      We now generate an ICMP HOST REDIRECT giving the route
 139         *      we calculated.
 140         */
 141        if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
 142            !skb_sec_path(skb))
 143                ip_rt_send_redirect(skb);
 144
 145        skb->priority = rt_tos2priority(iph->tos);
 146
 147        return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
 148                       net, NULL, skb, skb->dev, rt->dst.dev,
 149                       ip_forward_finish);
 150
 151sr_failed:
 152        /*
 153         *      Strict routing permits no gatewaying
 154         */
 155         icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 156         goto drop;
 157
 158too_many_hops:
 159        /* Tell the sender its packet died... */
 160        __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
 161        icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 162drop:
 163        kfree_skb(skb);
 164        return NET_RX_DROP;
 165}
 166