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.
 *              
 * Version:     $Id: ip_forward.c,v 1.48 2000/12/13 18:31:48 davem Exp $
 *
 * 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/config.h>
#include <linux/types.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/skbuff.h>
#include <linux/ip.h>
#include <linux/icmp.h>
#include <linux/netdevice.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>

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

        IP_INC_STATS_BH(IpForwDatagrams);

        if (opt->optlen == 0) {
#ifdef CONFIG_NET_FASTROUTE
                struct rtable *rt = (struct rtable*)skb->dst;

                if (rt->rt_flags&RTCF_FAST && !netdev_fastroute_obstacles) {
                        struct dst_entry *old_dst;
                        unsigned h = ((*(u8*)&rt->key.dst)^(*(u8*)&rt->key.src))&NETDEV_FASTROUTE_HMASK;

                        write_lock_irq(&skb->dev->fastpath_lock);
                        old_dst = skb->dev->fastpath[h];
                        skb->dev->fastpath[h] = dst_clone(&rt->u.dst);
                        write_unlock_irq(&skb->dev->fastpath_lock);

                        dst_release(old_dst);
                }
#endif
                return (ip_send(skb));
        }

        ip_forward_options(skb);
        return (ip_send(skb));
}

int ip_forward(struct sk_buff *skb)
{
        struct net_device *dev2;        /* Output device */
        struct iphdr *iph;      /* Our header */
        struct rtable *rt;      /* Route we use */
        struct ip_options * opt = &(IPCB(skb)->opt);
        unsigned short mtu;

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

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

        skb->ip_summed = CHECKSUM_NONE;
        
        /*
         *      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.
         */

        iph = skb->nh.iph;
        rt = (struct rtable*)skb->dst;

        if (iph->ttl <= 1)
                goto too_many_hops;

        if (opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
                goto sr_failed;

        /*
         *      Having picked a route we can now send the frame out
         *      after asking the firewall permission to do so.
         */

        skb->priority = rt_tos2priority(iph->tos);
        dev2 = rt->u.dst.dev;
        mtu = rt->u.dst.pmtu;

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

        /* We are about to mangle packet. Copy it! */
        if (skb_cow(skb, dev2->hard_header_len))
                goto drop;
        iph = skb->nh.iph;

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

        /*
         * We now may allocate a new buffer, and copy the datagram into it.
         * If the indicated interface is up and running, kick it.
         */

        if (skb->len > mtu && (ntohs(iph->frag_off) & IP_DF))
                goto frag_needed;

#ifdef CONFIG_IP_ROUTE_NAT
        if (rt->rt_flags & RTCF_NAT) {
                if (ip_do_nat(skb)) {
                        kfree_skb(skb);
                        return NET_RX_BAD;
                }
        }
#endif

        return NF_HOOK(PF_INET, NF_IP_FORWARD, skb, skb->dev, dev2,
                       ip_forward_finish);

frag_needed:
        IP_INC_STATS_BH(IpFragFails);
        icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(mtu));
        goto drop;

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... */
        icmp_send(skb, ICMP_TIME_EXCEEDED, ICMP_EXC_TTL, 0);
drop:
        kfree_skb(skb);
        return NET_RX_DROP;
}

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