Linux 2.2.0

[davej-history.git] / net / ipv4 / ip_output.c

/*
 * 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 Internet Protocol (IP) output module.
 *
 * Version:     $Id: ip_output.c,v 1.64 1999/01/04 20:05:33 davem Exp $
 *
 * Authors:     Ross Biro, <bir7@leland.Stanford.Edu>
 *              Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
 *              Donald Becker, <becker@super.org>
 *              Alan Cox, <Alan.Cox@linux.org>
 *              Richard Underwood
 *              Stefan Becker, <stefanb@yello.ping.de>
 *              Jorge Cwik, <jorge@laser.satlink.net>
 *              Arnt Gulbrandsen, <agulbra@nvg.unit.no>
 *
 *      See ip_input.c for original log
 *
 *      Fixes:
 *              Alan Cox        :       Missing nonblock feature in ip_build_xmit.
 *              Mike Kilburn    :       htons() missing in ip_build_xmit.
 *              Bradford Johnson:       Fix faulty handling of some frames when 
 *                                      no route is found.
 *              Alexander Demenshin:    Missing sk/skb free in ip_queue_xmit
 *                                      (in case if packet not accepted by
 *                                      output firewall rules)
 *              Mike McLagan    :       Routing by source
 *              Alexey Kuznetsov:       use new route cache
 *              Andi Kleen:             Fix broken PMTU recovery and remove
 *                                      some redundant tests.
 *      Vitaly E. Lavrov        :       Transparent proxy revived after year coma.
 *              Andi Kleen      :       Replace ip_reply with ip_send_reply.
 *              Andi Kleen      :       Split fast and slow ip_build_xmit path 
 *                                      for decreased register pressure on x86 
 *                                      and more readibility. 
 *              Marc Boucher    :       When call_out_firewall returns FW_QUEUE,
 *                                      silently abort send instead of failing
 *                                      with -EPERM.
 */

#include <asm/uaccess.h>
#include <asm/system.h>
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/string.h>
#include <linux/errno.h>
#include <linux/config.h>

#include <linux/socket.h>
#include <linux/sockios.h>
#include <linux/in.h>
#include <linux/inet.h>
#include <linux/netdevice.h>
#include <linux/etherdevice.h>
#include <linux/proc_fs.h>
#include <linux/stat.h>
#include <linux/init.h>

#include <net/snmp.h>
#include <net/ip.h>
#include <net/protocol.h>
#include <net/route.h>
#include <net/tcp.h>
#include <net/udp.h>
#include <linux/skbuff.h>
#include <net/sock.h>
#include <net/arp.h>
#include <net/icmp.h>
#include <net/raw.h>
#include <net/checksum.h>
#include <linux/igmp.h>
#include <linux/ip_fw.h>
#include <linux/firewall.h>
#include <linux/mroute.h>
#include <linux/netlink.h>

/*
 *      Shall we try to damage output packets if routing dev changes?
 */

int sysctl_ip_dynaddr = 0;


int ip_id_count = 0;

/* Generate a checksum for an outgoing IP datagram. */
__inline__ void ip_send_check(struct iphdr *iph)
{
        iph->check = 0;
        iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
}

/* 
 *              Add an ip header to a skbuff and send it out.
 */
void ip_build_and_send_pkt(struct sk_buff *skb, struct sock *sk,
                           u32 saddr, u32 daddr, struct ip_options *opt)
{
        struct rtable *rt = (struct rtable *)skb->dst;
        struct iphdr *iph;
        struct device *dev;
        
        /* Build the IP header. */
        if (opt)
                iph=(struct iphdr *)skb_push(skb,sizeof(struct iphdr) + opt->optlen);
        else
                iph=(struct iphdr *)skb_push(skb,sizeof(struct iphdr));

        iph->version  = 4;
        iph->ihl      = 5;
        iph->tos      = sk->ip_tos;
        iph->frag_off = 0;
        if (ip_dont_fragment(sk, &rt->u.dst))
                iph->frag_off |= htons(IP_DF);
        iph->ttl      = sk->ip_ttl;
        iph->daddr    = rt->rt_dst;
        iph->saddr    = rt->rt_src;
        iph->protocol = sk->protocol;
        iph->tot_len  = htons(skb->len);
        iph->id       = htons(ip_id_count++);
        skb->nh.iph   = iph;

        if (opt && opt->optlen) {
                iph->ihl += opt->optlen>>2;
                ip_options_build(skb, opt, daddr, rt, 0);
        }

        dev = rt->u.dst.dev;

#ifdef CONFIG_FIREWALL
        if (call_out_firewall(PF_INET, dev, iph, NULL, &skb) < FW_ACCEPT)
                goto drop;
#endif

        ip_send_check(iph);

        /* Send it out. */
        skb->dst->output(skb);
        return;

#ifdef CONFIG_FIREWALL
drop:
        kfree_skb(skb);
#endif
}

int __ip_finish_output(struct sk_buff *skb)
{
        return ip_finish_output(skb);
}

int ip_mc_output(struct sk_buff *skb)
{
        struct sock *sk = skb->sk;
        struct rtable *rt = (struct rtable*)skb->dst;
        struct device *dev = rt->u.dst.dev;

        /*
         *      If the indicated interface is up and running, send the packet.
         */
         
        ip_statistics.IpOutRequests++;
#ifdef CONFIG_IP_ROUTE_NAT
        if (rt->rt_flags & RTCF_NAT)
                ip_do_nat(skb);
#endif

        skb->dev = dev;
        skb->protocol = __constant_htons(ETH_P_IP);

        /*
         *      Multicasts are looped back for other local users
         */

        if (rt->rt_flags&RTCF_MULTICAST && (!sk || sk->ip_mc_loop)) {
#ifdef CONFIG_IP_MROUTE
                /* Small optimization: do not loopback not local frames,
                   which returned after forwarding; they will be  dropped
                   by ip_mr_input in any case.
                   Note, that local frames are looped back to be delivered
                   to local recipients.

                   This check is duplicated in ip_mr_input at the moment.
                 */
                if ((rt->rt_flags&RTCF_LOCAL) || !(IPCB(skb)->flags&IPSKB_FORWARDED))
#endif
                dev_loopback_xmit(skb);

                /* Multicasts with ttl 0 must not go beyond the host */

                if (skb->nh.iph->ttl == 0) {
                        kfree_skb(skb);
                        return 0;
                }
        }

        if (rt->rt_flags&RTCF_BROADCAST)
                dev_loopback_xmit(skb);

        return ip_finish_output(skb);
}

int ip_output(struct sk_buff *skb)
{
#ifdef CONFIG_IP_ROUTE_NAT
        struct rtable *rt = (struct rtable*)skb->dst;
#endif

        ip_statistics.IpOutRequests++;

#ifdef CONFIG_IP_ROUTE_NAT
        if (rt->rt_flags&RTCF_NAT)
                ip_do_nat(skb);
#endif

        return ip_finish_output(skb);
}

/* Queues a packet to be sent, and starts the transmitter if necessary.  
 * This routine also needs to put in the total length and compute the 
 * checksum.  We use to do this in two stages, ip_build_header() then
 * this, but that scheme created a mess when routes disappeared etc.
 * So we do it all here, and the TCP send engine has been changed to
 * match. (No more unroutable FIN disasters, etc. wheee...)  This will
 * most likely make other reliable transport layers above IP easier
 * to implement under Linux.
 */
void ip_queue_xmit(struct sk_buff *skb)
{
        struct sock *sk = skb->sk;
        struct ip_options *opt = sk->opt;
        struct rtable *rt;
        struct device *dev;
        struct iphdr *iph;
        unsigned int tot_len;

        /* Make sure we can route this packet. */
        rt = (struct rtable *) sk->dst_cache;
        if(rt == NULL || rt->u.dst.obsolete) {
                u32 daddr;

                sk->dst_cache = NULL;
                ip_rt_put(rt);

                /* Use correct destination address if we have options. */
                daddr = sk->daddr;
                if(opt && opt->srr)
                        daddr = opt->faddr;

                /* If this fails, retransmit mechanism of transport layer will
                 * keep trying until route appears or the connection times itself
                 * out.
                 */
                if(ip_route_output(&rt, daddr, sk->saddr,
                                   RT_TOS(sk->ip_tos) | RTO_CONN | sk->localroute,
                                   sk->bound_dev_if))
                        goto drop;
                sk->dst_cache = &rt->u.dst;
        }
        if(opt && opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
                goto no_route;

        /* We have a route, so grab a reference. */
        skb->dst = dst_clone(sk->dst_cache);

        /* OK, we know where to send it, allocate and build IP header. */
        iph = (struct iphdr *) skb_push(skb, sizeof(struct iphdr) + (opt ? opt->optlen : 0));
        iph->version  = 4;
        iph->ihl      = 5;
        iph->tos      = sk->ip_tos;
        iph->frag_off = 0;
        iph->ttl      = sk->ip_ttl;
        iph->daddr    = rt->rt_dst;
        iph->saddr    = rt->rt_src;
        iph->protocol = sk->protocol;
        skb->nh.iph   = iph;
        /* Transport layer set skb->h.foo itself. */

        if(opt && opt->optlen) {
                iph->ihl += opt->optlen >> 2;
                ip_options_build(skb, opt, sk->daddr, rt, 0);
        }

        tot_len = skb->len;
        iph->tot_len = htons(tot_len);
        iph->id = htons(ip_id_count++);

        dev = rt->u.dst.dev;

#ifdef CONFIG_FIREWALL
        if (call_out_firewall(PF_INET, dev, iph, NULL, &skb) < FW_ACCEPT) 
                goto drop;
#endif

        /* This can happen when the transport layer has segments queued
         * with a cached route, and by the time we get here things are
         * re-routed to a device with a different MTU than the original
         * device.  Sick, but we must cover it.
         */
        if (skb_headroom(skb) < dev->hard_header_len && dev->hard_header) {
                struct sk_buff *skb2;

                skb2 = skb_realloc_headroom(skb, (dev->hard_header_len + 15) & ~15);
                kfree_skb(skb);
                if (skb2 == NULL)
                        return;
                if (sk)
                        skb_set_owner_w(skb, sk);
                skb = skb2;
                iph = skb->nh.iph;
        }

        /* Do we need to fragment.  Again this is inefficient.  We
         * need to somehow lock the original buffer and use bits of it.
         */
        if (tot_len > rt->u.dst.pmtu)
                goto fragment;

        if (ip_dont_fragment(sk, &rt->u.dst))
                iph->frag_off |= __constant_htons(IP_DF);

        /* Add an IP checksum. */
        ip_send_check(iph);

        skb->priority = sk->priority;
        skb->dst->output(skb);
        return;

fragment:
        if (ip_dont_fragment(sk, &rt->u.dst) &&
            tot_len > (iph->ihl<<2) + sizeof(struct tcphdr)+16) {
                /* Reject packet ONLY if TCP might fragment
                   it itself, if were careful enough.
                   Test is not precise (f.e. it does not take sacks
                   into account). Actually, tcp should make it. --ANK (980801)
                 */
                iph->frag_off |= __constant_htons(IP_DF);
                printk(KERN_DEBUG "sending pkt_too_big to self\n");
                icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
                          htonl(rt->u.dst.pmtu));
                goto drop;
        }
        ip_fragment(skb, skb->dst->output);
        return;

no_route:
        sk->dst_cache = NULL;
        ip_rt_put(rt);
        ip_statistics.IpOutNoRoutes++;
        /* Fall through... */
drop:
        kfree_skb(skb);
}

/*
 *      Build and send a packet, with as little as one copy
 *
 *      Doesn't care much about ip options... option length can be
 *      different for fragment at 0 and other fragments.
 *
 *      Note that the fragment at the highest offset is sent first,
 *      so the getfrag routine can fill in the TCP/UDP checksum header
 *      field in the last fragment it sends... actually it also helps
 *      the reassemblers, they can put most packets in at the head of
 *      the fragment queue, and they know the total size in advance. This
 *      last feature will measurably improve the Linux fragment handler one
 *      day.
 *
 *      The callback has five args, an arbitrary pointer (copy of frag),
 *      the source IP address (may depend on the routing table), the 
 *      destination address (char *), the offset to copy from, and the
 *      length to be copied.
 */

int ip_build_xmit_slow(struct sock *sk,
                  int getfrag (const void *,
                               char *,
                               unsigned int,    
                               unsigned int),
                  const void *frag,
                  unsigned length,
                  struct ipcm_cookie *ipc,
                  struct rtable *rt,
                  int flags)
{
        unsigned int fraglen, maxfraglen, fragheaderlen;
        int err;
        int offset, mf;
        int mtu;
        unsigned short id;

        int hh_len = (rt->u.dst.dev->hard_header_len + 15)&~15;
        int nfrags=0;
        struct ip_options *opt = ipc->opt;
        int df = 0;

        mtu = rt->u.dst.pmtu;
        if (ip_dont_fragment(sk, &rt->u.dst))
                df = htons(IP_DF);
  
        if (!sk->ip_hdrincl)
                length -= sizeof(struct iphdr);

        if (opt) {
                fragheaderlen = sizeof(struct iphdr) + opt->optlen;
                maxfraglen = ((mtu-sizeof(struct iphdr)-opt->optlen) & ~7) + fragheaderlen;
        } else {
                fragheaderlen = sk->ip_hdrincl ? 0 : sizeof(struct iphdr);
                
                /*
                 *      Fragheaderlen is the size of 'overhead' on each buffer. Now work
                 *      out the size of the frames to send.
                 */
         
                maxfraglen = ((mtu-sizeof(struct iphdr)) & ~7) + fragheaderlen;
        }

        if (length + fragheaderlen > 0xFFFF) {
                ip_local_error(sk, EMSGSIZE, rt->rt_dst, sk->dport, mtu);
                return -EMSGSIZE;
        }

        /*
         *      Start at the end of the frame by handling the remainder.
         */
         
        offset = length - (length % (maxfraglen - fragheaderlen));
        
        /*
         *      Amount of memory to allocate for final fragment.
         */
         
        fraglen = length - offset + fragheaderlen;
        
        if (length-offset==0) {
                fraglen = maxfraglen;
                offset -= maxfraglen-fragheaderlen;
        }
        
        
        /*
         *      The last fragment will not have MF (more fragments) set.
         */
         
        mf = 0;

        /*
         *      Don't fragment packets for path mtu discovery.
         */
         
        if (offset > 0 && df) { 
                ip_local_error(sk, EMSGSIZE, rt->rt_dst, sk->dport, mtu);
                return(-EMSGSIZE);
        }

        /*
         *      Lock the device lists.
         */

        dev_lock_list();
        
        /*
         *      Get an identifier
         */
         
        id = htons(ip_id_count++);

        /*
         *      Begin outputting the bytes.
         */
         
        do {
                int error;
                char *data;
                struct sk_buff * skb;

                /*
                 *      Get the memory we require with some space left for alignment.
                 */

                skb = sock_alloc_send_skb(sk, fraglen+hh_len+15, 0, flags&MSG_DONTWAIT, &error);
                if (skb == NULL) {
                        ip_statistics.IpOutDiscards++;
                        if(nfrags>1)
                                ip_statistics.IpFragCreates++;                  
                        dev_unlock_list();
                        return(error);
                }
                
                /*
                 *      Fill in the control structures
                 */
                 
                skb->priority = sk->priority;
                skb->dst = dst_clone(&rt->u.dst);
                skb_reserve(skb, hh_len);

                /*
                 *      Find where to start putting bytes.
                 */
                 
                data = skb_put(skb, fraglen);
                skb->nh.iph = (struct iphdr *)data;

                /*
                 *      Only write IP header onto non-raw packets 
                 */
                 
                if(!sk->ip_hdrincl) {
                        struct iphdr *iph = (struct iphdr *)data;

                        iph->version = 4;
                        iph->ihl = 5;
                        if (opt) {
                                iph->ihl += opt->optlen>>2;
                                ip_options_build(skb, opt,
                                                 ipc->addr, rt, offset);
                        }
                        iph->tos = sk->ip_tos;
                        iph->tot_len = htons(fraglen - fragheaderlen + iph->ihl*4);
                        iph->id = id;
                        iph->frag_off = htons(offset>>3);
                        iph->frag_off |= mf|df;
                        if (rt->rt_type == RTN_MULTICAST)
                                iph->ttl = sk->ip_mc_ttl;
                        else
                                iph->ttl = sk->ip_ttl;
                        iph->protocol = sk->protocol;
                        iph->check = 0;
                        iph->saddr = rt->rt_src;
                        iph->daddr = rt->rt_dst;
                        iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
                        data += iph->ihl*4;
                        
                        /*
                         *      Any further fragments will have MF set.
                         */
                         
                        mf = htons(IP_MF);
                }
                
                /*
                 *      User data callback
                 */

                err = 0;
                if (getfrag(frag, data, offset, fraglen-fragheaderlen))
                        err = -EFAULT;
                
                /*
                 *      Account for the fragment.
                 */

#ifdef CONFIG_FIREWALL
                if(!err) {
                        int fw_res;

                        fw_res = call_out_firewall(PF_INET, rt->u.dst.dev, skb->nh.iph, NULL, &skb);
                        if(fw_res == FW_QUEUE) {
                                kfree_skb(skb);
                                skb = NULL;
                        } else if(fw_res < FW_ACCEPT) {
                                err = -EPERM;
                        }
                }
#endif

                if (err) { 
                        ip_statistics.IpOutDiscards++;
                        kfree_skb(skb);
                        dev_unlock_list();
                        return err; 
                }
                        

                offset -= (maxfraglen-fragheaderlen);
                fraglen = maxfraglen;

                nfrags++;

                err = 0; 
                if (skb && rt->u.dst.output(skb)) {
                        err = -ENETDOWN;
                        ip_statistics.IpOutDiscards++;  
                        break;
                }
        } while (offset >= 0);

        if (nfrags>1)
                ip_statistics.IpFragCreates += nfrags;
        dev_unlock_list();
        return err;
}


/*
 *      Fast path for unfragmented packets.
 */
int ip_build_xmit(struct sock *sk, 
                  int getfrag (const void *,
                               char *,
                               unsigned int,    
                               unsigned int),
                  const void *frag,
                  unsigned length,
                  struct ipcm_cookie *ipc,
                  struct rtable *rt,
                  int flags)
{
        int err;
        struct sk_buff *skb;
        int df;
        struct iphdr *iph;

        /*
         *      Try the simple case first. This leaves fragmented frames, and by
         *      choice RAW frames within 20 bytes of maximum size(rare) to the long path
         */

        if (!sk->ip_hdrincl)
                length += sizeof(struct iphdr);

        /*
         *      Check for slow path.
         */
        if (length > rt->u.dst.pmtu || ipc->opt != NULL)  
                return ip_build_xmit_slow(sk,getfrag,frag,length,ipc,rt,flags); 

        /*
         *      Do path mtu discovery if needed.
         */
        df = 0;
        if (ip_dont_fragment(sk, &rt->u.dst))
                df = htons(IP_DF);
                
        /* 
         *      Fast path for unfragmented frames without options. 
         */ 
        {
        int hh_len = (rt->u.dst.dev->hard_header_len + 15)&~15;

        skb = sock_alloc_send_skb(sk, length+hh_len+15,
                                  0, flags&MSG_DONTWAIT, &err);
        if(skb==NULL)
                goto error; 
        skb_reserve(skb, hh_len);
        }
        
        skb->priority = sk->priority;
        skb->dst = dst_clone(&rt->u.dst);

        skb->nh.iph = iph = (struct iphdr *)skb_put(skb, length);
        
        dev_lock_list();
        
        if(!sk->ip_hdrincl) {
                iph->version=4;
                iph->ihl=5;
                iph->tos=sk->ip_tos;
                iph->tot_len = htons(length);
                iph->id=htons(ip_id_count++);
                iph->frag_off = df;
                iph->ttl=sk->ip_mc_ttl;
                if (rt->rt_type != RTN_MULTICAST)
                        iph->ttl=sk->ip_ttl;
                iph->protocol=sk->protocol;
                iph->saddr=rt->rt_src;
                iph->daddr=rt->rt_dst;
                iph->check=0;
                iph->check = ip_fast_csum((unsigned char *)iph, iph->ihl);
                err = getfrag(frag, ((char *)iph)+iph->ihl*4,0, length-iph->ihl*4);
        }
        else
                err = getfrag(frag, (void *)iph, 0, length);

        dev_unlock_list();

        if (err) 
                err = -EFAULT;

#ifdef CONFIG_FIREWALL
        if(!err) {
                int fw_res;

                fw_res = call_out_firewall(PF_INET, rt->u.dst.dev, iph, NULL, &skb);
                if(fw_res == FW_QUEUE) {
                        /* re-queued elsewhere; silently abort this send */
                        kfree_skb(skb);
                        return 0;
                }
                if(fw_res < FW_ACCEPT)
                        err = -EPERM;
        }
#endif

        if (err) { 
                kfree_skb(skb);
                goto error;
        }
        
        return rt->u.dst.output(skb);

error:
        ip_statistics.IpOutDiscards++;
        return err; 
}
                       


/*
 *      This IP datagram is too large to be sent in one piece.  Break it up into
 *      smaller pieces (each of size equal to IP header plus
 *      a block of the data of the original IP data part) that will yet fit in a
 *      single device frame, and queue such a frame for sending.
 *
 *      Yes this is inefficient, feel free to submit a quicker one.
 */

void ip_fragment(struct sk_buff *skb, int (*output)(struct sk_buff*))
{
        struct iphdr *iph;
        unsigned char *raw;
        unsigned char *ptr;
        struct device *dev;
        struct sk_buff *skb2;
        unsigned int mtu, hlen, left, len; 
        int offset;
        int not_last_frag;
        struct rtable *rt = (struct rtable*)skb->dst;

        dev = rt->u.dst.dev;

        /*
         *      Point into the IP datagram header.
         */

        raw = skb->nh.raw;
        iph = (struct iphdr*)raw;

        /*
         *      Setup starting values.
         */

        hlen = iph->ihl * 4;
        left = ntohs(iph->tot_len) - hlen;      /* Space per frame */
        mtu = rt->u.dst.pmtu - hlen;    /* Size of data space */
        ptr = raw + hlen;                       /* Where to start from */

        /*
         *      The protocol doesn't seem to say what to do in the case that the
         *      frame + options doesn't fit the mtu. As it used to fall down dead
         *      in this case we were fortunate it didn't happen
         *
         *      It is impossible, because mtu>=68. --ANK (980801)
         */

#ifdef CONFIG_NET_PARANOIA
        if (mtu<8) 
                goto fail;
#endif

        /*
         *      Fragment the datagram.
         */

        offset = (ntohs(iph->frag_off) & IP_OFFSET) << 3;
        not_last_frag = iph->frag_off & htons(IP_MF);

        /*
         *      Keep copying data until we run out.
         */

        while(left > 0) {
                len = left;
                /* IF: it doesn't fit, use 'mtu' - the data space left */
                if (len > mtu)
                        len = mtu;
                /* IF: we are not sending upto and including the packet end
                   then align the next start on an eight byte boundary */
                if (len < left) {
                        len &= ~7;
                }
                /*
                 *      Allocate buffer.
                 */

                if ((skb2 = alloc_skb(len+hlen+dev->hard_header_len+15,GFP_ATOMIC)) == NULL) {
                        NETDEBUG(printk(KERN_INFO "IP: frag: no memory for new fragment!\n"));
                        goto fail;
                }

                /*
                 *      Set up data on packet
                 */

                skb2->pkt_type = skb->pkt_type;
                skb2->priority = skb->priority;
                skb_reserve(skb2, (dev->hard_header_len+15)&~15);
                skb_put(skb2, len + hlen);
                skb2->nh.raw = skb2->data;
                skb2->h.raw = skb2->data + hlen;

                /*
                 *      Charge the memory for the fragment to any owner
                 *      it might possess
                 */

                if (skb->sk)
                        skb_set_owner_w(skb2, skb->sk);
                skb2->dst = dst_clone(skb->dst);

                /*
                 *      Copy the packet header into the new buffer.
                 */

                memcpy(skb2->nh.raw, raw, hlen);

                /*
                 *      Copy a block of the IP datagram.
                 */
                memcpy(skb2->h.raw, ptr, len);
                left -= len;

                /*
                 *      Fill in the new header fields.
                 */
                iph = skb2->nh.iph;
                iph->frag_off = htons((offset >> 3));

                /* ANK: dirty, but effective trick. Upgrade options only if
                 * the segment to be fragmented was THE FIRST (otherwise,
                 * options are already fixed) and make it ONCE
                 * on the initial skb, so that all the following fragments
                 * will inherit fixed options.
                 */
                if (offset == 0)
                        ip_options_fragment(skb);

                /*
                 *      Added AC : If we are fragmenting a fragment that's not the
                 *                 last fragment then keep MF on each bit
                 */
                if (left > 0 || not_last_frag)
                        iph->frag_off |= htons(IP_MF);
                ptr += len;
                offset += len;

                /*
                 *      Put this fragment into the sending queue.
                 */

                ip_statistics.IpFragCreates++;

                iph->tot_len = htons(len + hlen);

                ip_send_check(iph);

                output(skb2);
        }
        kfree_skb(skb);
        ip_statistics.IpFragOKs++;
        return;
        
fail:
        kfree_skb(skb); 
        ip_statistics.IpFragFails++; 
}

/*
 *      Fetch data from kernel space and fill in checksum if needed.
 */
static int ip_reply_glue_bits(const void *dptr, char *to, unsigned int offset, 
                              unsigned int fraglen)
{
        struct ip_reply_arg *dp = (struct ip_reply_arg*)dptr;
        u16 *pktp = (u16 *)to;
        struct iovec *iov; 
        int len; 
        int hdrflag = 1; 

        iov = &dp->iov[0]; 
        if (offset >= iov->iov_len) { 
                offset -= iov->iov_len;
                iov++; 
                hdrflag = 0; 
        }
        len = iov->iov_len - offset;
        if (fraglen > len) { /* overlapping. */ 
                dp->csum = csum_partial_copy_nocheck(iov->iov_base+offset, to, len,
                                             dp->csum);
                offset = 0;
                fraglen -= len; 
                to += len; 
                iov++;
        }

        dp->csum = csum_partial_copy_nocheck(iov->iov_base+offset, to, fraglen, 
                                             dp->csum); 

        if (hdrflag && dp->csumoffset)
                *(pktp + dp->csumoffset) = csum_fold(dp->csum); /* fill in checksum */
        return 0;              
}

/* 
 *      Generic function to send a packet as reply to another packet.
 *      Used to send TCP resets so far. ICMP should use this function too.
 *
 *      Should run single threaded per socket because it uses the sock 
 *      structure to pass arguments.
 */
void ip_send_reply(struct sock *sk, struct sk_buff *skb, struct ip_reply_arg *arg,
                   unsigned int len)
{
        struct {
                struct ip_options       opt;
                char                    data[40];
        } replyopts;
        struct ipcm_cookie ipc;
        u32 daddr;
        struct rtable *rt = (struct rtable*)skb->dst;
        
        if (ip_options_echo(&replyopts.opt, skb))
                return;
        
        sk->ip_tos = skb->nh.iph->tos;
        sk->priority = skb->priority;
        sk->protocol = skb->nh.iph->protocol;

        daddr = ipc.addr = rt->rt_src;
        ipc.opt = &replyopts.opt;
        
        if (ipc.opt->srr)
                daddr = replyopts.opt.faddr;
        if (ip_route_output(&rt, daddr, rt->rt_spec_dst, RT_TOS(skb->nh.iph->tos), 0))
                return;

        /* And let IP do all the hard work. */
        ip_build_xmit(sk, ip_reply_glue_bits, arg, len, &ipc, rt, MSG_DONTWAIT);
        ip_rt_put(rt);
}

/*
 *      IP protocol layer initialiser
 */

static struct packet_type ip_packet_type =
{
        __constant_htons(ETH_P_IP),
        NULL,   /* All devices */
        ip_rcv,
        NULL,
        NULL,
};



#ifdef CONFIG_PROC_FS
#ifdef CONFIG_IP_MULTICAST
static struct proc_dir_entry proc_net_igmp = {
        PROC_NET_IGMP, 4, "igmp",
        S_IFREG | S_IRUGO, 1, 0, 0,
        0, &proc_net_inode_operations,
        ip_mc_procinfo
};
#endif
#endif  

/*
 *      IP registers the packet type and then calls the subprotocol initialisers
 */

__initfunc(void ip_init(void))
{
        dev_add_pack(&ip_packet_type);

        ip_rt_init();

#ifdef CONFIG_PROC_FS
#ifdef CONFIG_IP_MULTICAST
        proc_net_register(&proc_net_igmp);
#endif
#endif  
}