// SPDX-License-Identifier: GPL-2.0-only
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/* Kernel module to match TCP MSS values. */
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/* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
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* Portions (C) 2005 by Harald Welte <laforge@netfilter.org>
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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 <net/tcp.h>
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#include <linux/netfilter/xt_tcpmss.h>
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#include <linux/netfilter/x_tables.h>
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#include <linux/netfilter_ipv4/ip_tables.h>
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#include <linux/netfilter_ipv6/ip6_tables.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("Xtables: TCP MSS match");
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MODULE_ALIAS("ipt_tcpmss");
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MODULE_ALIAS("ip6t_tcpmss");
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static bool
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tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)
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{
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const struct xt_tcpmss_match_info *info = par->matchinfo;
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const struct tcphdr *th;
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struct tcphdr _tcph;
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/* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
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const u_int8_t *op;
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u8 _opt[15 * 4 - sizeof(_tcph)];
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unsigned int i, optlen;
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/* If we don't have the whole header, drop packet. */
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th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
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if (th == NULL)
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goto dropit;
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/* Malformed. */
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if (th->doff*4 < sizeof(*th))
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goto dropit;
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optlen = th->doff*4 - sizeof(*th);
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if (!optlen)
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goto out;
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/* Truncated options. */
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op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);
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if (op == NULL)
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goto dropit;
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for (i = 0; i < optlen; ) {
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if (op[i] == TCPOPT_MSS
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&& (optlen - i) >= TCPOLEN_MSS
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&& op[i+1] == TCPOLEN_MSS) {
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u_int16_t mssval;
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mssval = (op[i+2] << 8) | op[i+3];
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return (mssval >= info->mss_min &&
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mssval <= info->mss_max) ^ info->invert;
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}
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if (op[i] < 2)
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i++;
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else
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i += op[i+1] ? : 1;
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}
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out:
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return info->invert;
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dropit:
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par->hotdrop = true;
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return false;
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}
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static struct xt_match tcpmss_mt_reg[] __read_mostly = {
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{
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.name = "tcpmss",
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.family = NFPROTO_IPV4,
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.match = tcpmss_mt,
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.matchsize = sizeof(struct xt_tcpmss_match_info),
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.proto = IPPROTO_TCP,
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.me = THIS_MODULE,
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},
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{
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.name = "tcpmss",
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.family = NFPROTO_IPV6,
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.match = tcpmss_mt,
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.matchsize = sizeof(struct xt_tcpmss_match_info),
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.proto = IPPROTO_TCP,
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.me = THIS_MODULE,
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},
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};
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static int __init tcpmss_mt_init(void)
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{
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return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
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}
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static void __exit tcpmss_mt_exit(void)
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{
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xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
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}
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module_init(tcpmss_mt_init);
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module_exit(tcpmss_mt_exit);
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