ip_forward.c

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/*
 * INET     An implementation of the TCP/IP protocol suite for the LINUX
 *      operating system.  INET is implemented using the  BSD Socket
 *      interface as the means of communication with the user level.
 *
 *      The IP forwarding functionality.
 *
 * Authors: see ip.c
 *
 * Fixes:
 *      Many        :   Split from ip.c , see ip_input.c for
 *                  history.
 *      Dave Gregorich  :   NULL ip_rt_put fix for multicast
 *                  routing.
 *      Jos Vos     :   Add call_out_firewall before sending,
 *                  use output device for accounting.
 *      Jos Vos     :   Call forward firewall after routing
 *                  (always use output device).
 *      Mike McLagan    :   Routing by source
 */

#include <linux/types.h>
#include <linux/mm.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/icmp.h>
#include <linux/netdevice.h>
#include <linux/slab.h>
#include <net/sock.h>
#include <net/ip.h>
#include <net/tcp.h>
#include <net/udp.h>
#include <net/icmp.h>
#include <linux/tcp.h>
#include <linux/udp.h>
#include <linux/netfilter_ipv4.h>
#include <net/checksum.h>
#include <linux/route.h>
#include <net/route.h>
#include <net/xfrm.h>

static int ip_forward_finish(struct sk_buff *skb)
{
    struct ip_options *opt  = &(IPCB(skb)->opt);

    IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTFORWDATAGRAMS);
    IP_ADD_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_OUTOCTETS, skb->len);

    if (unlikely(opt->optlen))
        ip_forward_options(skb);

    return dst_output(skb);
}

int ip_forward(struct sk_buff *skb)
{
    struct iphdr *iph;  /* Our header */
    struct rtable *rt;  /* Route we use */
    struct ip_options *opt  = &(IPCB(skb)->opt);

    if (skb_warn_if_lro(skb))
        goto drop;

    if (!xfrm4_policy_check(NULL, XFRM_POLICY_FWD, skb))
        goto drop;

    if (IPCB(skb)->opt.router_alert && ip_call_ra_chain(skb))
        return NET_RX_SUCCESS;

    if (skb->pkt_type != PACKET_HOST)
        goto drop;

    skb_forward_csum(skb);

    /*
     *  According to the RFC, we must first decrease the TTL field. If
     *  that reaches zero, we must reply an ICMP control message telling
     *  that the packet's lifetime expired.
     */
    if (ip_hdr(skb)->ttl <= 1)
        goto too_many_hops;

    if (!xfrm4_route_forward(skb))
        goto drop;

    rt = skb_rtable(skb);

    if (opt->is_strictroute && rt->rt_uses_gateway)
        goto sr_failed;

    if (unlikely(skb->len > dst_mtu(&rt->dst) && !skb_is_gso(skb) &&
             (ip_hdr(skb)->frag_off & htons(IP_DF))) && !skb->local_df) {
        IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
        icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
              htonl(dst_mtu(&rt->dst)));
        goto drop;
    }

    /* We are about to mangle packet. Copy it! */
    if (skb_cow(skb, LL_RESERVED_SPACE(rt->dst.dev)+rt->dst.header_len))
        goto drop;
    iph = ip_hdr(skb);

    /* Decrease ttl after skb cow done */
    ip_decrease_ttl(iph);

    /*
     *  We now generate an ICMP HOST REDIRECT giving the route
     *  we calculated.
     */
    if (rt->rt_flags&RTCF_DOREDIRECT && !opt->srr && !skb_sec_path(skb))
        ip_rt_send_redirect(skb);

    skb->priority = rt_tos2priority(iph->tos);

    return NF_HOOK(NFPROTO_IPV4, NF_INET_FORWARD, skb, skb->dev,
               rt->dst.dev, ip_forward_finish);

sr_failed:
    /*
     *  Strict routing permits no gatewaying
     */
     icmp_send(skb, ICMP_DEST_UNREACH, ICMP_SR_FAILED, 0);
     goto drop;

too_many_hops:
    /* Tell the sender its packet died... */
    IP_INC_STATS_BH(dev_net(skb_dst(skb)->dev), IPSTATS_MIB_INHDRERRORS);
    icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
drop:
    kfree_skb(skb);
    return NET_RX_DROP;
}