LXR linux/net/ipv4/ip

   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * INET         An implementation of the TCP/IP protocol suite for the LINUX
   4 *              operating system.  INET is implemented using the  BSD Socket
   5 *              interface as the means of communication with the user level.
   6 *
   7 *              The IP forwarding functionality.
   8 *
   9 * Authors:     see ip.c
  10 *
  11 * Fixes:
  12 *              Many            :       Split from ip.c , see ip_input.c for
  13 *                                      history.
  14 *              Dave Gregorich  :       NULL ip_rt_put fix for multicast
  15 *                                      routing.
  16 *              Jos Vos         :       Add call_out_firewall before sending,
  17 *                                      use output device for accounting.
  18 *              Jos Vos         :       Call forward firewall after routing
  19 *                                      (always use output device).
  20 *              Mike McLagan    :       Routing by source
  21 */
  22
  23#include <linux/types.h>
  24#include <linux/mm.h>
  25#include <linux/skbuff.h>
  26#include <linux/ip.h>
  27#include <linux/icmp.h>
  28#include <linux/netdevice.h>
  29#include <linux/slab.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
  43static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
  44{
  45        if (skb->len <= mtu)
  46                return false;
  47
  48        if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0))
  49                return false;
  50
  51        /* original fragment exceeds mtu and DF is set */
  52        if (unlikely(IPCB(skb)->frag_max_size > mtu))
  53                return true;
  54
  55        if (skb->ignore_df)
  56                return false;
  57
  58        if (skb_is_gso(skb) && skb_gso_validate_network_len(skb, mtu))
  59                return false;
  60
  61        return true;
  62}
  63
  64
  65static int ip_forward_finish(struct net *net, struct sock *sk, struct sk_buff *skb)
  66{
  67        struct ip_options *opt  = &(IPCB(skb)->opt);
  68
  69        __IP_INC_STATS(net, IPSTATS_MIB_OUTFORWDATAGRAMS);
  70        __IP_ADD_STATS(net, IPSTATS_MIB_OUTOCTETS, skb->len);
  71
  72#ifdef CONFIG_NET_SWITCHDEV
  73        if (skb->offload_l3_fwd_mark) {
  74                consume_skb(skb);
  75                return 0;
  76        }
  77#endif
  78
  79        if (unlikely(opt->optlen))
  80                ip_forward_options(skb);
  81
  82        skb->tstamp = 0;
  83        return dst_output(net, sk, skb);
  84}
  85
  86int ip_forward(struct sk_buff *skb)
  87{
  88        u32 mtu;
  89        struct iphdr *iph;      /* Our header */
  90        struct rtable *rt;      /* Route we use */
  91        struct ip_options *opt  = &(IPCB(skb)->opt);
  92        struct net *net;
  93
  94        /* that should never happen */
  95        if (skb->pkt_type != PACKET_HOST)
  96                goto drop;
  97
  98        if (unlikely(skb->sk))
  99                goto drop;
 100
 101        if (skb_warn_if_lro(skb))
 102                goto drop;
 103
 104        if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
 105                goto drop;
 106
 107        if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
 108                return NET_RX_SUCCESS;
 109
 110        skb_forward_csum(skb);
 111        net = dev_net(skb->dev);
 112
 113        /*
 114         *      According to the RFC, we must first decrease the TTL field. If
 115         *      that reaches zero, we must reply an ICMP control message telling
 116         *      that the packet's lifetime expired.
 117         */
 118        if (ip_hdr(skb)->ttl <= 1)
 119                goto too_many_hops;
 120
 121        if (!xfrm4_route_forward(skb))
 122                goto drop;
 123
 124        rt = skb_rtable(skb);
 125
 126        if (opt->is_strictroute && rt->rt_uses_gateway)
 127                goto sr_failed;
 128
 129        IPCB(skb)->flags |= IPSKB_FORWARDED;
 130        mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
 131        if (ip_exceeds_mtu(skb, mtu)) {
 132                IP_INC_STATS(net, IPSTATS_MIB_FRAGFAILS);
 133                icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
 134                          htonl(mtu));
 135                goto drop;
 136        }
 137
 138        /* We are about to mangle packet. Copy it! */
 139        if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
 140                goto drop;
 141        iph = ip_hdr(skb);
 142
 143        /* Decrease ttl after skb cow done */
 144        ip_decrease_ttl(iph);
 145
 146        /*
 147         *      We now generate an ICMP HOST REDIRECT giving the route
 148         *      we calculated.
 149         */
 150        if (IPCB(skb)->flags & IPSKB_DOREDIRECT && !opt->srr &&
 151            !skb_sec_path(skb))
 152                ip_rt_send_redirect(skb);
 153
 154        if (net->ipv4.sysctl_ip_fwd_update_priority)
 155                skb->priority = rt_tos2priority(iph->tos);
 156
 157        return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD,
 158                       net, NULL, skb, skb->dev, rt->dst.dev,
 159                       ip_forward_finish);
 160
 161sr_failed:
 162        /*
 163         *      Strict routing permits no gatewaying
 164         */
 165         icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
 166         goto drop;
 167
 168too_many_hops:
 169        /* Tell the sender its packet died... */
 170        __IP_INC_STATS(net, IPSTATS_MIB_INHDRERRORS);
 171        icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
 172drop:
 173        kfree_skb(skb);
 174        return NET_RX_DROP;
 175}
 176