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/*

 * This is a module which is used for setting the MSS option in TCP packets.

 *

 * Copyright (c) 2000 Marc Boucher

 */

#include <linux/module.h>

#include <linux/skbuff.h>

#include <linux/ip.h>

#include <net/tcp.h>

#include <linux/netfilter_ipv4/ip_tables.h>

#include <linux/netfilter_ipv4/ipt_TCPMSS.h>

#if 0

#define DEBUGP printk

#else

#define DEBUGP(format, args...)

#endif

static u_int16_t

cheat_check(u_int32_t oldvalinv, u_int32_t newval, u_int16_t oldcheck)

{

        u_int32_t diffs[] = { oldvalinv, newval };

        return csum_fold(csum_partial((char *)diffs, sizeof(diffs),

                                      oldcheck^0xFFFF));

}

static inline unsigned int

optlen(const u_int8_t *opt, unsigned int offset)

{

        /* Beware zero-length options: make finite progress */

        if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) return 1;

        else return opt[offset+1];

}

static unsigned int

ipt_tcpmss_target(struct sk_buff **pskb,

                  unsigned int hooknum,

                  const struct net_device *in,

                  const struct net_device *out,

                  const void *targinfo,

                  void *userinfo)

{

        const struct ipt_tcpmss_info *tcpmssinfo = targinfo;

        struct tcphdr *tcph;

        struct iphdr *iph;

        u_int16_t tcplen, newtotlen, oldval, newmss;

        unsigned int i;

        u_int8_t *opt;

        /* raw socket (tcpdump) may have clone of incoming skb: don't

           disturb it --RR */

        if (skb_cloned(*pskb) && !(*pskb)->sk) {

                struct sk_buff *nskb = skb_copy(*pskb, GFP_ATOMIC);

                if (!nskb)

                        return NF_DROP;

                kfree_skb(*pskb);

                *pskb = nskb;

        }

        iph = (*pskb)->nh.iph;

        tcplen = (*pskb)->len - iph->ihl*4;

        tcph = (void *)iph + iph->ihl*4;

        /* Since it passed flags test in tcp match, we know it is is

           not a fragment, and has data >= tcp header length.  SYN

           packets should not contain data: if they did, then we risk

           running over MTU, sending Frag Needed and breaking things

           badly. --RR */

        if (tcplen != tcph->doff*4) {

                if (net_ratelimit())

                        printk(KERN_ERR

                               "ipt_tcpmss_target: bad length (%d bytes)\n",

                               (*pskb)->len);

                return NF_DROP;

        }

        if(tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {

                if(!(*pskb)->dst) {

                        if (net_ratelimit())

                                printk(KERN_ERR

                                        "ipt_tcpmss_target: no dst?! can't determine path-MTU\n");

                        return NF_DROP; /* or IPT_CONTINUE ?? */

                }

                if((*pskb)->dst->pmtu <= (sizeof(struct iphdr) + sizeof(struct tcphdr))) {

                        if (net_ratelimit())

                                printk(KERN_ERR

                                        "ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", (*pskb)->dst->pmtu);

                        return NF_DROP; /* or IPT_CONTINUE ?? */

                }

                newmss = (*pskb)->dst->pmtu - sizeof(struct iphdr) - sizeof(struct tcphdr);

        } else

                newmss = tcpmssinfo->mss;

        opt = (u_int8_t *)tcph;

        for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)){

                if ((opt[i] == TCPOPT_MSS) &&

                    ((tcph->doff*4 - i) >= TCPOLEN_MSS) &&

                    (opt[i+1] == TCPOLEN_MSS)) {

                        u_int16_t oldmss;

                        oldmss = (opt[i+2] << 8) | opt[i+3];

                        if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&

                                (oldmss <= newmss))

                                        return IPT_CONTINUE;

                        opt[i+2] = (newmss & 0xff00) >> 8;

                        opt[i+3] = (newmss & 0x00ff);

                        tcph->check = cheat_check(htons(oldmss)^0xFFFF,

                                                  htons(newmss),

                                                  tcph->check);

                        DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"

                               "->%u.%u.%u.%u:%hu changed TCP MSS option"

                               " (from %u to %u)\n",

                               NIPQUAD((*pskb)->nh.iph->saddr),

                               ntohs(tcph->source),

                               NIPQUAD((*pskb)->nh.iph->daddr),

                               ntohs(tcph->dest),

                               oldmss, newmss);

                        goto retmodified;

                }

        }

        /*

         * MSS Option not found ?! add it..

         */

        if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {

                struct sk_buff *newskb;

                newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),

                                         TCPOLEN_MSS, GFP_ATOMIC);

                if (!newskb) {

                        if (net_ratelimit())

                                printk(KERN_ERR "ipt_tcpmss_target:"

                                       " unable to allocate larger skb\n");

                        return NF_DROP;

                }

                kfree_skb(*pskb);

                *pskb = newskb;

                iph = (*pskb)->nh.iph;

                tcph = (void *)iph + iph->ihl*4;

        }

        skb_put((*pskb), TCPOLEN_MSS);

        opt = (u_int8_t *)tcph + sizeof(struct tcphdr);

        memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));

        tcph->check = cheat_check(htons(tcplen) ^ 0xFFFF,

                                  htons(tcplen + TCPOLEN_MSS), tcph->check);

        tcplen += TCPOLEN_MSS;

        opt[0] = TCPOPT_MSS;

        opt[1] = TCPOLEN_MSS;

        opt[2] = (newmss & 0xff00) >> 8;

        opt[3] = (newmss & 0x00ff);

        tcph->check = cheat_check(~0, *((u_int32_t *)opt), tcph->check);

        oldval = ((u_int16_t *)tcph)[6];

        tcph->doff += TCPOLEN_MSS/4;

        tcph->check = cheat_check(oldval ^ 0xFFFF,

                                  ((u_int16_t *)tcph)[6], tcph->check);

        newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);

        iph->check = cheat_check(iph->tot_len ^ 0xFFFF,

                                 newtotlen, iph->check);

        iph->tot_len = newtotlen;

        DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"

               "->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",

               NIPQUAD((*pskb)->nh.iph->saddr),

               ntohs(tcph->source),

               NIPQUAD((*pskb)->nh.iph->daddr),

               ntohs(tcph->dest),

               newmss);

 retmodified:

        /* If we had a hardware checksum before, it's now invalid */

        (*pskb)->ip_summed = CHECKSUM_NONE;

        (*pskb)->nfcache |= NFC_UNKNOWN | NFC_ALTERED;

        return IPT_CONTINUE;

}

#define TH_SYN 0x02

static inline int find_syn_match(const struct ipt_entry_match *m)

{

        const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;

        if (strcmp(m->u.kernel.match->name, "tcp") == 0

            && (tcpinfo->flg_cmp & TH_SYN)

            && !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))

                return 1;

        return 0;

}

/* Must specify -p tcp --syn/--tcp-flags SYN */

static int

ipt_tcpmss_checkentry(const char *tablename,

                      const struct ipt_entry *e,

                      void *targinfo,

                      unsigned int targinfosize,

                      unsigned int hook_mask)

{

        const struct ipt_tcpmss_info *tcpmssinfo = targinfo;

        if (targinfosize != IPT_ALIGN(sizeof(struct ipt_tcpmss_info))) {

                DEBUGP("ipt_tcpmss_checkentry: targinfosize %u != %u\n",

                       targinfosize, IPT_ALIGN(sizeof(struct ipt_tcpmss_info)));

                return 0;

        }

        if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&

                        ((hook_mask & ~((1 << NF_IP_FORWARD)

                                | (1 << NF_IP_LOCAL_OUT)

                                | (1 << NF_IP_POST_ROUTING))) != 0)) {

                printk("TCPMSS: path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");

                return 0;

        }

        if (e->ip.proto == IPPROTO_TCP

            && !(e->ip.invflags & IPT_INV_PROTO)

            && IPT_MATCH_ITERATE(e, find_syn_match))

                return 1;

        printk("TCPMSS: Only works on TCP SYN packets\n");

        return 0;

}

static struct ipt_target ipt_tcpmss_reg

= { { NULL, NULL }, "TCPMSS",

    ipt_tcpmss_target, ipt_tcpmss_checkentry, NULL, THIS_MODULE };

static int __init init(void)

{

        return ipt_register_target(&ipt_tcpmss_reg);

}

static void __exit fini(void)

{

        ipt_unregister_target(&ipt_tcpmss_reg);

}

module_init(init);

module_exit(fini);

MODULE_LICENSE("GPL");