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338e8a79 SS |
1 | /* |
2 | * A module for stripping a specific TCP option from TCP packets. | |
3 | * | |
4 | * Copyright (C) 2007 Sven Schnelle <svens@bitebene.org> | |
5 | * Copyright © CC Computer Consultants GmbH, 2007 | |
6 | * Contact: Jan Engelhardt <jengelh@computergmbh.de> | |
7 | * | |
8 | * This program is free software; you can redistribute it and/or modify | |
9 | * it under the terms of the GNU General Public License version 2 as | |
10 | * published by the Free Software Foundation. | |
11 | */ | |
12 | ||
13 | #include <linux/module.h> | |
14 | #include <linux/skbuff.h> | |
15 | #include <linux/ip.h> | |
16 | #include <linux/ipv6.h> | |
17 | #include <linux/tcp.h> | |
18 | #include <net/ipv6.h> | |
19 | #include <net/tcp.h> | |
20 | #include <linux/netfilter/x_tables.h> | |
21 | #include <linux/netfilter/xt_TCPOPTSTRIP.h> | |
22 | ||
23 | static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset) | |
24 | { | |
25 | /* Beware zero-length options: make finite progress */ | |
26 | if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0) | |
27 | return 1; | |
28 | else | |
29 | return opt[offset+1]; | |
30 | } | |
31 | ||
32 | static unsigned int | |
33 | tcpoptstrip_mangle_packet(struct sk_buff *skb, | |
34 | const struct xt_tcpoptstrip_target_info *info, | |
35 | unsigned int tcphoff, unsigned int minlen) | |
36 | { | |
37 | unsigned int optl, i, j; | |
38 | struct tcphdr *tcph; | |
39 | u_int16_t n, o; | |
40 | u_int8_t *opt; | |
41 | ||
42 | if (!skb_make_writable(skb, skb->len)) | |
43 | return NF_DROP; | |
44 | ||
45 | tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff); | |
46 | opt = (u_int8_t *)tcph; | |
47 | ||
48 | /* | |
49 | * Walk through all TCP options - if we find some option to remove, | |
50 | * set all octets to %TCPOPT_NOP and adjust checksum. | |
51 | */ | |
52 | for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) { | |
53 | optl = optlen(opt, i); | |
54 | ||
55 | if (i + optl > tcp_hdrlen(skb)) | |
56 | break; | |
57 | ||
58 | if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i])) | |
59 | continue; | |
60 | ||
61 | for (j = 0; j < optl; ++j) { | |
62 | o = opt[i+j]; | |
63 | n = TCPOPT_NOP; | |
64 | if ((i + j) % 2 == 0) { | |
65 | o <<= 8; | |
66 | n <<= 8; | |
67 | } | |
68 | inet_proto_csum_replace2(&tcph->check, skb, htons(o), | |
69 | htons(n), 0); | |
70 | } | |
71 | memset(opt + i, TCPOPT_NOP, optl); | |
72 | } | |
73 | ||
74 | return XT_CONTINUE; | |
75 | } | |
76 | ||
77 | static unsigned int | |
78 | tcpoptstrip_tg4(struct sk_buff *skb, const struct net_device *in, | |
79 | const struct net_device *out, unsigned int hooknum, | |
80 | const struct xt_target *target, const void *targinfo) | |
81 | { | |
82 | return tcpoptstrip_mangle_packet(skb, targinfo, ip_hdrlen(skb), | |
83 | sizeof(struct iphdr) + sizeof(struct tcphdr)); | |
84 | } | |
85 | ||
86 | #if defined(CONFIG_IP6_NF_MANGLE) || defined(CONFIG_IP6_NF_MANGLE_MODULE) | |
87 | static unsigned int | |
88 | tcpoptstrip_tg6(struct sk_buff *skb, const struct net_device *in, | |
89 | const struct net_device *out, unsigned int hooknum, | |
90 | const struct xt_target *target, const void *targinfo) | |
91 | { | |
92 | struct ipv6hdr *ipv6h = ipv6_hdr(skb); | |
be8d0d79 | 93 | int tcphoff; |
338e8a79 SS |
94 | u_int8_t nexthdr; |
95 | ||
96 | nexthdr = ipv6h->nexthdr; | |
97 | tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr); | |
98 | if (tcphoff < 0) | |
99 | return NF_DROP; | |
100 | ||
101 | return tcpoptstrip_mangle_packet(skb, targinfo, tcphoff, | |
102 | sizeof(*ipv6h) + sizeof(struct tcphdr)); | |
103 | } | |
104 | #endif | |
105 | ||
106 | static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = { | |
107 | { | |
108 | .name = "TCPOPTSTRIP", | |
ee999d8b | 109 | .family = NFPROTO_IPV4, |
338e8a79 SS |
110 | .table = "mangle", |
111 | .proto = IPPROTO_TCP, | |
112 | .target = tcpoptstrip_tg4, | |
113 | .targetsize = sizeof(struct xt_tcpoptstrip_target_info), | |
114 | .me = THIS_MODULE, | |
115 | }, | |
116 | #if defined(CONFIG_IP6_NF_MANGLE) || defined(CONFIG_IP6_NF_MANGLE_MODULE) | |
117 | { | |
118 | .name = "TCPOPTSTRIP", | |
ee999d8b | 119 | .family = NFPROTO_IPV6, |
338e8a79 SS |
120 | .table = "mangle", |
121 | .proto = IPPROTO_TCP, | |
122 | .target = tcpoptstrip_tg6, | |
123 | .targetsize = sizeof(struct xt_tcpoptstrip_target_info), | |
124 | .me = THIS_MODULE, | |
125 | }, | |
126 | #endif | |
127 | }; | |
128 | ||
129 | static int __init tcpoptstrip_tg_init(void) | |
130 | { | |
131 | return xt_register_targets(tcpoptstrip_tg_reg, | |
132 | ARRAY_SIZE(tcpoptstrip_tg_reg)); | |
133 | } | |
134 | ||
135 | static void __exit tcpoptstrip_tg_exit(void) | |
136 | { | |
137 | xt_unregister_targets(tcpoptstrip_tg_reg, | |
138 | ARRAY_SIZE(tcpoptstrip_tg_reg)); | |
139 | } | |
140 | ||
141 | module_init(tcpoptstrip_tg_init); | |
142 | module_exit(tcpoptstrip_tg_exit); | |
143 | MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@computergmbh.de>"); | |
2ae15b64 | 144 | MODULE_DESCRIPTION("Xtables: TCP option stripping"); |
338e8a79 SS |
145 | MODULE_LICENSE("GPL"); |
146 | MODULE_ALIAS("ipt_TCPOPTSTRIP"); | |
147 | MODULE_ALIAS("ip6t_TCPOPTSTRIP"); |