/* Kernel module to match TCP MSS values. *//* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca> * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License version 2 as * published by the Free Software Foundation. */#include <linux/module.h>#include <linux/skbuff.h>#include <net/tcp.h>#include <linux/netfilter_ipv4/ipt_tcpmss.h>#include <linux/netfilter_ipv4/ip_tables.h>#define TH_SYN 0x02MODULE_LICENSE("GPL");MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");MODULE_DESCRIPTION("iptables TCP MSS match module");/* Returns 1 if the mss option is set and matched by the range, 0 otherwise */static inline intmssoption_match(u_int16_t min, u_int16_t max,		const struct sk_buff *skb,		int invert,		int *hotdrop){	struct tcphdr tcph;	/* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */	u8 opt[15 * 4 - sizeof(tcph)];	unsigned int i, optlen;	/* If we don't have the whole header, drop packet. */	if (skb_copy_bits(skb, skb->nh.iph->ihl*4, &tcph, sizeof(tcph)) < 0)		goto dropit;	/* Malformed. */	if (tcph.doff*4 < sizeof(tcph))		goto dropit;	optlen = tcph.doff*4 - sizeof(tcph);	/* Truncated options. */	if (skb_copy_bits(skb, skb->nh.iph->ihl*4+sizeof(tcph), opt, optlen)<0)		goto dropit;	for (i = 0; i < optlen; ) {		if (opt[i] == TCPOPT_MSS		    && (optlen - i) >= TCPOLEN_MSS		    && opt[i+1] == TCPOLEN_MSS) {			u_int16_t mssval;			mssval = (opt[i+2] << 8) | opt[i+3];						return (mssval >= min && mssval <= max) ^ invert;		}		if (opt[i] < 2) i++;		else i += opt[i+1]?:1;	}	return invert; dropit:	*hotdrop = 1;	return 0;}static intmatch(const struct sk_buff *skb,      const struct net_device *in,      const struct net_device *out,      const void *matchinfo,      int offset,      int *hotdrop){	const struct ipt_tcpmss_match_info *info = matchinfo;	return mssoption_match(info->mss_min, info->mss_max, skb,			       info->invert, hotdrop);}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;}static intcheckentry(const char *tablename,           const struct ipt_ip *ip,           void *matchinfo,           unsigned int matchsize,           unsigned int hook_mask){	if (matchsize != IPT_ALIGN(sizeof(struct ipt_tcpmss_match_info)))		return 0;	/* Must specify -p tcp */	if (ip->proto != IPPROTO_TCP || (ip->invflags & IPT_INV_PROTO)) {		printk("tcpmss: Only works on TCP packets\n");		return 0;	}	return 1;}static struct ipt_match tcpmss_match = {	.name		= "tcpmss",	.match		= &match,	.checkentry	= &checkentry,	.me		= THIS_MODULE,};static int __init init(void){	return ipt_register_match(&tcpmss_match);}static void __exit fini(void){	ipt_unregister_match(&tcpmss_match);}module_init(init);module_exit(fini);