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
18 MODULE_LICENSE("GPL");
19 MODULE_AUTHOR("Marc Boucher <marc@mbsi.ca>");
20 MODULE_DESCRIPTION("Xtables: TCP MSS match");
21 MODULE_ALIAS("ipt_tcpmss");
22 MODULE_ALIAS("ip6t_tcpmss");
23
24 static bool
25 tcpmss_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 }
69 out:
70 return info->invert;
71
72 dropit:
73 par->hotdrop = true;
74 return false;
75 }
76
77 static 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
96 static int __init tcpmss_mt_init(void)
97 {
98 return xt_register_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
99 }
100
101 static void __exit tcpmss_mt_exit(void)
102 {
103 xt_unregister_matches(tcpmss_mt_reg, ARRAY_SIZE(tcpmss_mt_reg));
104 }
105
106 module_init(tcpmss_mt_init);
107 module_exit(tcpmss_mt_exit);
108
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