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1// SPDX-License-Identifier: GPL-2.0-only
2/* Kernel module to match TCP MSS values. */
3
4/* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
5 * Portions (C) 2005 by Harald Welte <laforge@netfilter.org>
6 */
7
8#include <linux/module.h>
9#include <linux/skbuff.h>
10#include <net/tcp.h>
11
12#include <linux/netfilter/xt_tcpmss.h>
13#include <linux/netfilter/x_tables.h>
14
15#include <linux/netfilter_ipv4/ip_tables.h>
16#include <linux/netfilter_ipv6/ip6_tables.h>
17
18MODULE_LICENSE("GPL");
19MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
20MODULE_DESCRIPTION("Xtables: TCP MSS match");
21MODULE_ALIAS("ipt_tcpmss");
22MODULE_ALIAS("ip6t_tcpmss");
23
24static bool
25tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)
26{
27 const struct xt_tcpmss_match_info *info = par->matchinfo;
28 const struct tcphdr *th;
29 struct tcphdr _tcph;
30 /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
31 const u_int8_t *op;
32 u8 _opt[15 * 4 - sizeof(_tcph)];
33 unsigned int i, optlen;
34
35 /* If we don't have the whole header, drop packet. */
36 th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
37 if (th == NULL)
38 goto dropit;
39
40 /* Malformed. */
41 if (th->doff*4 < sizeof(*th))
42 goto dropit;
43
44 optlen = th->doff*4 - sizeof(*th);
45 if (!optlen)
46 goto out;
47
48 /* Truncated options. */
49 op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);
50 if (op == NULL)
51 goto dropit;
52
53 for (i = 0; i < optlen; ) {
54 if (op[i] == TCPOPT_MSS
55 && (optlen - i) >= TCPOLEN_MSS
56 && op[i+1] == TCPOLEN_MSS) {
57 u_int16_t mssval;
58
59 mssval = (op[i+2] << 8) | op[i+3];
60
61 return (mssval >= info->mss_min &&
62 mssval <= info->mss_max) ^ info->invert;
63 }
64 if (op[i] < 2)
65 i++;
66 else
67 i += op[i+1] ? : 1;
68 }
69out:
70 return info->invert;
71
72dropit:
73 par->hotdrop = true;
74 return false;
75}
76
77static struct xt_match tcpmss_mt_reg[] __read_mostly = {
78 {
79 .name = "tcpmss",
80 .family = NFPROTO_IPV4,
81 .match = tcpmss_mt,
82 .matchsize = sizeof(struct xt_tcpmss_match_info),
83 .proto = IPPROTO_TCP,
84 .me = THIS_MODULE,
85 },
86 {
87 .name = "tcpmss",
88 .family = NFPROTO_IPV6,
89 .match = tcpmss_mt,
90 .matchsize = sizeof(struct xt_tcpmss_match_info),
91 .proto = IPPROTO_TCP,
92 .me = THIS_MODULE,
93 },
94};
95
96static int __init tcpmss_mt_init(void)
97{
98 return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
99}
100
101static void __exit tcpmss_mt_exit(void)
102{
103 xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
104}
105
106module_init(tcpmss_mt_init);
107module_exit(tcpmss_mt_exit);
1/* Kernel module to match TCP MSS values. */
2
3/* Copyright (C) 2000 Marc Boucher <marc@mbsi.ca>
4 * Portions (C) 2005 by Harald Welte <laforge@netfilter.org>
5 *
6 * This program is free software; you can redistribute it and/or modify
7 * it under the terms of the GNU General Public License version 2 as
8 * published by the Free Software Foundation.
9 */
10
11#include <linux/module.h>
12#include <linux/skbuff.h>
13#include <net/tcp.h>
14
15#include <linux/netfilter/xt_tcpmss.h>
16#include <linux/netfilter/x_tables.h>
17
18#include <linux/netfilter_ipv4/ip_tables.h>
19#include <linux/netfilter_ipv6/ip6_tables.h>
20
21MODULE_LICENSE("GPL");
22MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
23MODULE_DESCRIPTION("Xtables: TCP MSS match");
24MODULE_ALIAS("ipt_tcpmss");
25MODULE_ALIAS("ip6t_tcpmss");
26
27static bool
28tcpmss_mt(const struct sk_buff *skb, struct xt_action_param *par)
29{
30 const struct xt_tcpmss_match_info *info = par->matchinfo;
31 const struct tcphdr *th;
32 struct tcphdr _tcph;
33 /* tcp.doff is only 4 bits, ie. max 15 * 4 bytes */
34 const u_int8_t *op;
35 u8 _opt[15 * 4 - sizeof(_tcph)];
36 unsigned int i, optlen;
37
38 /* If we don't have the whole header, drop packet. */
39 th = skb_header_pointer(skb, par->thoff, sizeof(_tcph), &_tcph);
40 if (th == NULL)
41 goto dropit;
42
43 /* Malformed. */
44 if (th->doff*4 < sizeof(*th))
45 goto dropit;
46
47 optlen = th->doff*4 - sizeof(*th);
48 if (!optlen)
49 goto out;
50
51 /* Truncated options. */
52 op = skb_header_pointer(skb, par->thoff + sizeof(*th), optlen, _opt);
53 if (op == NULL)
54 goto dropit;
55
56 for (i = 0; i < optlen; ) {
57 if (op[i] == TCPOPT_MSS
58 && (optlen - i) >= TCPOLEN_MSS
59 && op[i+1] == TCPOLEN_MSS) {
60 u_int16_t mssval;
61
62 mssval = (op[i+2] << 8) | op[i+3];
63
64 return (mssval >= info->mss_min &&
65 mssval <= info->mss_max) ^ info->invert;
66 }
67 if (op[i] < 2)
68 i++;
69 else
70 i += op[i+1] ? : 1;
71 }
72out:
73 return info->invert;
74
75dropit:
76 par->hotdrop = true;
77 return false;
78}
79
80static struct xt_match tcpmss_mt_reg[] __read_mostly = {
81 {
82 .name = "tcpmss",
83 .family = NFPROTO_IPV4,
84 .match = tcpmss_mt,
85 .matchsize = sizeof(struct xt_tcpmss_match_info),
86 .proto = IPPROTO_TCP,
87 .me = THIS_MODULE,
88 },
89 {
90 .name = "tcpmss",
91 .family = NFPROTO_IPV6,
92 .match = tcpmss_mt,
93 .matchsize = sizeof(struct xt_tcpmss_match_info),
94 .proto = IPPROTO_TCP,
95 .me = THIS_MODULE,
96 },
97};
98
99static int __init tcpmss_mt_init(void)
100{
101 return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
102}
103
104static void __exit tcpmss_mt_exit(void)
105{
106 xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
107}
108
109module_init(tcpmss_mt_init);
110module_exit(tcpmss_mt_exit);