2005-04-17 06:20:36 +08:00
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/*
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* This is a module which is used for setting the MSS option in TCP packets.
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*
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* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*/
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#include <linux/module.h>
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#include <linux/skbuff.h>
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#include <linux/ip.h>
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#include <net/tcp.h>
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#include <linux/netfilter_ipv4/ip_tables.h>
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#include <linux/netfilter_ipv4/ipt_TCPMSS.h>
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MODULE_LICENSE("GPL");
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MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
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MODULE_DESCRIPTION("iptables TCP MSS modification module");
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#if 0
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#define DEBUGP printk
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#else
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#define DEBUGP(format, args...)
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#endif
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static u_int16_t
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cheat_check(u_int32_t oldvalinv, u_int32_t newval, u_int16_t oldcheck)
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{
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u_int32_t diffs[] = { oldvalinv, newval };
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return csum_fold(csum_partial((char *)diffs, sizeof(diffs),
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oldcheck^0xFFFF));
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}
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static inline unsigned int
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optlen(const u_int8_t *opt, unsigned int offset)
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{
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/* Beware zero-length options: make finite progress */
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if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) return 1;
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else return opt[offset+1];
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}
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static unsigned int
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ipt_tcpmss_target(struct sk_buff **pskb,
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const struct net_device *in,
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const struct net_device *out,
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unsigned int hooknum,
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const void *targinfo,
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void *userinfo)
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{
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const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
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struct tcphdr *tcph;
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struct iphdr *iph;
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u_int16_t tcplen, newtotlen, oldval, newmss;
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unsigned int i;
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u_int8_t *opt;
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2005-08-10 10:37:23 +08:00
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if (!skb_make_writable(pskb, (*pskb)->len))
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2005-04-17 06:20:36 +08:00
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return NF_DROP;
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2005-08-21 08:40:41 +08:00
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if ((*pskb)->ip_summed == CHECKSUM_HW &&
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skb_checksum_help(*pskb, out == NULL))
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return NF_DROP;
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2005-04-17 06:20:36 +08:00
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iph = (*pskb)->nh.iph;
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tcplen = (*pskb)->len - iph->ihl*4;
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tcph = (void *)iph + iph->ihl*4;
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/* Since it passed flags test in tcp match, we know it is is
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not a fragment, and has data >= tcp header length. SYN
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packets should not contain data: if they did, then we risk
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running over MTU, sending Frag Needed and breaking things
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badly. --RR */
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if (tcplen != tcph->doff*4) {
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if (net_ratelimit())
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printk(KERN_ERR
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"ipt_tcpmss_target: bad length (%d bytes)\n",
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(*pskb)->len);
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return NF_DROP;
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}
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if(tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) {
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if(!(*pskb)->dst) {
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if (net_ratelimit())
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printk(KERN_ERR
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"ipt_tcpmss_target: no dst?! can't determine path-MTU\n");
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return NF_DROP; /* or IPT_CONTINUE ?? */
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}
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if(dst_mtu((*pskb)->dst) <= (sizeof(struct iphdr) + sizeof(struct tcphdr))) {
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if (net_ratelimit())
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printk(KERN_ERR
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"ipt_tcpmss_target: unknown or invalid path-MTU (%d)\n", dst_mtu((*pskb)->dst));
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return NF_DROP; /* or IPT_CONTINUE ?? */
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}
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newmss = dst_mtu((*pskb)->dst) - sizeof(struct iphdr) - sizeof(struct tcphdr);
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} else
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newmss = tcpmssinfo->mss;
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opt = (u_int8_t *)tcph;
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for (i = sizeof(struct tcphdr); i < tcph->doff*4; i += optlen(opt, i)){
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if ((opt[i] == TCPOPT_MSS) &&
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((tcph->doff*4 - i) >= TCPOLEN_MSS) &&
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(opt[i+1] == TCPOLEN_MSS)) {
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u_int16_t oldmss;
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oldmss = (opt[i+2] << 8) | opt[i+3];
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if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&
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(oldmss <= newmss))
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return IPT_CONTINUE;
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opt[i+2] = (newmss & 0xff00) >> 8;
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opt[i+3] = (newmss & 0x00ff);
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tcph->check = cheat_check(htons(oldmss)^0xFFFF,
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htons(newmss),
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tcph->check);
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DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
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"->%u.%u.%u.%u:%hu changed TCP MSS option"
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" (from %u to %u)\n",
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NIPQUAD((*pskb)->nh.iph->saddr),
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ntohs(tcph->source),
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NIPQUAD((*pskb)->nh.iph->daddr),
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ntohs(tcph->dest),
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oldmss, newmss);
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goto retmodified;
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}
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}
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/*
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* MSS Option not found ?! add it..
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*/
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if (skb_tailroom((*pskb)) < TCPOLEN_MSS) {
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struct sk_buff *newskb;
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newskb = skb_copy_expand(*pskb, skb_headroom(*pskb),
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TCPOLEN_MSS, GFP_ATOMIC);
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if (!newskb) {
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if (net_ratelimit())
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printk(KERN_ERR "ipt_tcpmss_target:"
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" unable to allocate larger skb\n");
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return NF_DROP;
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}
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kfree_skb(*pskb);
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*pskb = newskb;
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iph = (*pskb)->nh.iph;
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tcph = (void *)iph + iph->ihl*4;
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}
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skb_put((*pskb), TCPOLEN_MSS);
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opt = (u_int8_t *)tcph + sizeof(struct tcphdr);
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memmove(opt + TCPOLEN_MSS, opt, tcplen - sizeof(struct tcphdr));
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tcph->check = cheat_check(htons(tcplen) ^ 0xFFFF,
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htons(tcplen + TCPOLEN_MSS), tcph->check);
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tcplen += TCPOLEN_MSS;
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opt[0] = TCPOPT_MSS;
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opt[1] = TCPOLEN_MSS;
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opt[2] = (newmss & 0xff00) >> 8;
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opt[3] = (newmss & 0x00ff);
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tcph->check = cheat_check(~0, *((u_int32_t *)opt), tcph->check);
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oldval = ((u_int16_t *)tcph)[6];
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tcph->doff += TCPOLEN_MSS/4;
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tcph->check = cheat_check(oldval ^ 0xFFFF,
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((u_int16_t *)tcph)[6], tcph->check);
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newtotlen = htons(ntohs(iph->tot_len) + TCPOLEN_MSS);
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iph->check = cheat_check(iph->tot_len ^ 0xFFFF,
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newtotlen, iph->check);
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iph->tot_len = newtotlen;
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DEBUGP(KERN_INFO "ipt_tcpmss_target: %u.%u.%u.%u:%hu"
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"->%u.%u.%u.%u:%hu added TCP MSS option (%u)\n",
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NIPQUAD((*pskb)->nh.iph->saddr),
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ntohs(tcph->source),
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NIPQUAD((*pskb)->nh.iph->daddr),
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ntohs(tcph->dest),
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newmss);
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retmodified:
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return IPT_CONTINUE;
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}
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#define TH_SYN 0x02
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static inline int find_syn_match(const struct ipt_entry_match *m)
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{
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const struct ipt_tcp *tcpinfo = (const struct ipt_tcp *)m->data;
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if (strcmp(m->u.kernel.match->name, "tcp") == 0
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&& (tcpinfo->flg_cmp & TH_SYN)
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&& !(tcpinfo->invflags & IPT_TCP_INV_FLAGS))
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return 1;
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return 0;
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}
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/* Must specify -p tcp --syn/--tcp-flags SYN */
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static int
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ipt_tcpmss_checkentry(const char *tablename,
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const struct ipt_entry *e,
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void *targinfo,
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unsigned int targinfosize,
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unsigned int hook_mask)
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{
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const struct ipt_tcpmss_info *tcpmssinfo = targinfo;
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if (targinfosize != IPT_ALIGN(sizeof(struct ipt_tcpmss_info))) {
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DEBUGP("ipt_tcpmss_checkentry: targinfosize %u != %u\n",
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targinfosize, IPT_ALIGN(sizeof(struct ipt_tcpmss_info)));
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return 0;
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}
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if((tcpmssinfo->mss == IPT_TCPMSS_CLAMP_PMTU) &&
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((hook_mask & ~((1 << NF_IP_FORWARD)
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| (1 << NF_IP_LOCAL_OUT)
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| (1 << NF_IP_POST_ROUTING))) != 0)) {
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printk("TCPMSS: path-MTU clamping only supported in FORWARD, OUTPUT and POSTROUTING hooks\n");
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return 0;
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}
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if (e->ip.proto == IPPROTO_TCP
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&& !(e->ip.invflags & IPT_INV_PROTO)
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&& IPT_MATCH_ITERATE(e, find_syn_match))
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return 1;
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printk("TCPMSS: Only works on TCP SYN packets\n");
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return 0;
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}
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static struct ipt_target ipt_tcpmss_reg = {
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.name = "TCPMSS",
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.target = ipt_tcpmss_target,
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.checkentry = ipt_tcpmss_checkentry,
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.me = THIS_MODULE,
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};
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static int __init init(void)
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{
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return ipt_register_target(&ipt_tcpmss_reg);
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}
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static void __exit fini(void)
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{
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ipt_unregister_target(&ipt_tcpmss_reg);
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}
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module_init(init);
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module_exit(fini);
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