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/* Kernel module to match TCP MSS values. */
#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
/* 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 tcphdr *tcp,
u_int16_t datalen,
int invert,
int *hotdrop)
{
unsigned int i;
const u_int8_t *opt = (u_int8_t *)tcp;
/* If we don't have the whole header, drop packet. */
if (tcp->doff * 4 > datalen) {
*hotdrop = 1;
return 0;
}
for (i = sizeof(struct tcphdr); i < tcp->doff * 4; ) {
if ((opt[i] == TCPOPT_MSS)
&& ((tcp->doff * 4 - 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;
}
static int
match(const struct sk_buff *skb,
const struct net_device *in,
const struct net_device *out,
const void *matchinfo,
int offset,
const void *hdr,
u_int16_t datalen,
int *hotdrop)
{
const struct ipt_tcpmss_match_info *info = matchinfo;
const struct tcphdr *tcph = (void *)skb->nh.iph + skb->nh.iph->ihl*4;
return mssoption_match(info->mss_min, info->mss_max, tcph,
skb->len - skb->nh.iph->ihl*4,
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 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
= { { NULL, NULL }, "tcpmss", &match, &checkentry, NULL, 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);
MODULE_LICENSE("GPL");
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