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/* 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 0x02
MODULE_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 int
mssoption_match(u_int16_t min, u_int16_t max,
const struct sk_buff *skb,
int invert,
int *hotdrop)
{
struct tcphdr _tcph, *th;
/* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
u8 _opt[15 * 4 - sizeof(_tcph)], *op;
unsigned int i, optlen;
/* If we don't have the whole header, drop packet. */
th = skb_header_pointer(skb, skb->nh.iph->ihl * 4,
sizeof(_tcph), &_tcph);
if (th == NULL)
goto dropit;
/* Malformed. */
if (th->doff*4 < sizeof(*th))
goto dropit;
optlen = th->doff*4 - sizeof(*th);
if (!optlen)
goto out;
/* Truncated options. */
op = skb_header_pointer(skb, skb->nh.iph->ihl * 4 + sizeof(*th),
optlen, _opt);
if (op == NULL)
goto dropit;
for (i = 0; i < optlen; ) {
if (op[i] == TCPOPT_MSS
&& (optlen - i) >= TCPOLEN_MSS
&& op[i+1] == TCPOLEN_MSS) {
u_int16_t mssval;
mssval = (op[i+2] << 8) | op[i+3];
return (mssval >= min && mssval <= max) ^ invert;
}
if (op[i] < 2) i++;
else i += op[i+1]?:1;
}
out:
return invert;
dropit:
*hotdrop = 1;
return 0;
}
static int
match(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 int
checkentry(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);
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