xt_tcpmss.c

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/* Kernel module to match TCP MSS values. */

/* Copyright (C) 2000 Marc Boucher <[email protected]>
 * Portions (C) 2005 by Harald Welte <[email protected]>
 *
 * 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/xt_tcpmss.h>
#include <linux/netfilter/x_tables.h>

#include <linux/netfilter_ipv4/ip_tables.h>
#include <linux/netfilter_ipv6/ip6_tables.h>

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Marc Boucher <[email protected]>");
MODULE_DESCRIPTION("Xtables: TCP MSS match");
MODULE_ALIAS("ipt_tcpmss");
MODULE_ALIAS("ip6t_tcpmss");

static bool
tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)
{
    const struct xt_tcpmss_match_info *info = par->matchinfo;
    const struct tcphdr *th;
    struct tcphdr _tcph;
    /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
    const u_int8_t *op;
    u8 _opt[15 * 4 - sizeof(_tcph)];
    unsigned int i, optlen;

    /* If we don't have the whole header, drop packet. */
    th = skb_header_pointer(skb, par->thoff, 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, par->thoff + 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 >= info->mss_min &&
                mssval <= info->mss_max) ^ info->invert;
        }
        if (op[i] < 2)
            i++;
        else
            i += op[i+1] ? : 1;
    }
out:
    return info->invert;

dropit:
    par->hotdrop = true;
    return false;
}

static struct xt_match tcpmss_mt_reg[] __read_mostly = {
    {
        .name       = "tcpmss",
        .family     = NFPROTO_IPV4,
        .match      = tcpmss_mt,
        .matchsize  = sizeof(struct xt_tcpmss_match_info),
        .proto      = IPPROTO_TCP,
        .me     = THIS_MODULE,
    },
    {
        .name       = "tcpmss",
        .family     = NFPROTO_IPV6,
        .match      = tcpmss_mt,
        .matchsize  = sizeof(struct xt_tcpmss_match_info),
        .proto      = IPPROTO_TCP,
        .me     = THIS_MODULE,
    },
};

static int __init tcpmss_mt_init(void)
{
    return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
}

static void __exit tcpmss_mt_exit(void)
{
    xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
}

module_init(tcpmss_mt_init);
module_exit(tcpmss_mt_exit);