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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 *
7 * This program is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License version 2 as
9 * published by the Free Software Foundation.
10 */
11
12#include <linux/module.h>
13#include <linux/skbuff.h>
14#include <linux/ip.h>
15#include <linux/ipv6.h>
16#include <linux/tcp.h>
17#include <net/ipv6.h>
18#include <net/tcp.h>
19#include <linux/netfilter/x_tables.h>
20#include <linux/netfilter/xt_TCPOPTSTRIP.h>
21
22static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
23{
24 /* Beware zero-length options: make finite progress */
25 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
26 return 1;
27 else
28 return opt[offset+1];
29}
30
31static unsigned int
32tcpoptstrip_mangle_packet(struct sk_buff *skb,
33 const struct xt_action_param *par,
34 unsigned int tcphoff, unsigned int minlen)
35{
36 const struct xt_tcpoptstrip_target_info *info = par->targinfo;
37 unsigned int optl, i, j;
38 struct tcphdr *tcph;
39 u_int16_t n, o;
40 u_int8_t *opt;
41 int len, tcp_hdrlen;
42
43 /* This is a fragment, no TCP header is available */
44 if (par->fragoff != 0)
45 return XT_CONTINUE;
46
47 if (!skb_make_writable(skb, skb->len))
48 return NF_DROP;
49
50 len = skb->len - tcphoff;
51 if (len < (int)sizeof(struct tcphdr))
52 return NF_DROP;
53
54 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
55 tcp_hdrlen = tcph->doff * 4;
56
57 if (len < tcp_hdrlen)
58 return NF_DROP;
59
60 opt = (u_int8_t *)tcph;
61
62 /*
63 * Walk through all TCP options - if we find some option to remove,
64 * set all octets to %TCPOPT_NOP and adjust checksum.
65 */
66 for (i = sizeof(struct tcphdr); i < tcp_hdrlen - 1; i += optl) {
67 optl = optlen(opt, i);
68
69 if (i + optl > tcp_hdrlen)
70 break;
71
72 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
73 continue;
74
75 for (j = 0; j < optl; ++j) {
76 o = opt[i+j];
77 n = TCPOPT_NOP;
78 if ((i + j) % 2 == 0) {
79 o <<= 8;
80 n <<= 8;
81 }
82 inet_proto_csum_replace2(&tcph->check, skb, htons(o),
83 htons(n), false);
84 }
85 memset(opt + i, TCPOPT_NOP, optl);
86 }
87
88 return XT_CONTINUE;
89}
90
91static unsigned int
92tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
93{
94 return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
95 sizeof(struct iphdr) + sizeof(struct tcphdr));
96}
97
98#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
99static unsigned int
100tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
101{
102 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
103 int tcphoff;
104 u_int8_t nexthdr;
105 __be16 frag_off;
106
107 nexthdr = ipv6h->nexthdr;
108 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
109 if (tcphoff < 0)
110 return NF_DROP;
111
112 return tcpoptstrip_mangle_packet(skb, par, tcphoff,
113 sizeof(*ipv6h) + sizeof(struct tcphdr));
114}
115#endif
116
117static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
118 {
119 .name = "TCPOPTSTRIP",
120 .family = NFPROTO_IPV4,
121 .table = "mangle",
122 .proto = IPPROTO_TCP,
123 .target = tcpoptstrip_tg4,
124 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
125 .me = THIS_MODULE,
126 },
127#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
128 {
129 .name = "TCPOPTSTRIP",
130 .family = NFPROTO_IPV6,
131 .table = "mangle",
132 .proto = IPPROTO_TCP,
133 .target = tcpoptstrip_tg6,
134 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
135 .me = THIS_MODULE,
136 },
137#endif
138};
139
140static int __init tcpoptstrip_tg_init(void)
141{
142 return xt_register_targets(tcpoptstrip_tg_reg,
143 ARRAY_SIZE(tcpoptstrip_tg_reg));
144}
145
146static void __exit tcpoptstrip_tg_exit(void)
147{
148 xt_unregister_targets(tcpoptstrip_tg_reg,
149 ARRAY_SIZE(tcpoptstrip_tg_reg));
150}
151
152module_init(tcpoptstrip_tg_init);
153module_exit(tcpoptstrip_tg_exit);
154MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
155MODULE_DESCRIPTION("Xtables: TCP option stripping");
156MODULE_LICENSE("GPL");
157MODULE_ALIAS("ipt_TCPOPTSTRIP");
158MODULE_ALIAS("ip6t_TCPOPTSTRIP");
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 *
7 * This program is free software; you can redistribute it and/or modify
8 * it under the terms of the GNU General Public License version 2 as
9 * published by the Free Software Foundation.
10 */
11
12#include <linux/module.h>
13#include <linux/skbuff.h>
14#include <linux/ip.h>
15#include <linux/ipv6.h>
16#include <linux/tcp.h>
17#include <net/ipv6.h>
18#include <net/tcp.h>
19#include <linux/netfilter/x_tables.h>
20#include <linux/netfilter/xt_TCPOPTSTRIP.h>
21
22static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
23{
24 /* Beware zero-length options: make finite progress */
25 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
26 return 1;
27 else
28 return opt[offset+1];
29}
30
31static unsigned int
32tcpoptstrip_mangle_packet(struct sk_buff *skb,
33 const struct xt_tcpoptstrip_target_info *info,
34 unsigned int tcphoff, unsigned int minlen)
35{
36 unsigned int optl, i, j;
37 struct tcphdr *tcph;
38 u_int16_t n, o;
39 u_int8_t *opt;
40
41 if (!skb_make_writable(skb, skb->len))
42 return NF_DROP;
43
44 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
45 opt = (u_int8_t *)tcph;
46
47 /*
48 * Walk through all TCP options - if we find some option to remove,
49 * set all octets to %TCPOPT_NOP and adjust checksum.
50 */
51 for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
52 optl = optlen(opt, i);
53
54 if (i + optl > tcp_hdrlen(skb))
55 break;
56
57 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
58 continue;
59
60 for (j = 0; j < optl; ++j) {
61 o = opt[i+j];
62 n = TCPOPT_NOP;
63 if ((i + j) % 2 == 0) {
64 o <<= 8;
65 n <<= 8;
66 }
67 inet_proto_csum_replace2(&tcph->check, skb, htons(o),
68 htons(n), 0);
69 }
70 memset(opt + i, TCPOPT_NOP, optl);
71 }
72
73 return XT_CONTINUE;
74}
75
76static unsigned int
77tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
78{
79 return tcpoptstrip_mangle_packet(skb, par->targinfo, ip_hdrlen(skb),
80 sizeof(struct iphdr) + sizeof(struct tcphdr));
81}
82
83#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
84static unsigned int
85tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
86{
87 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
88 int tcphoff;
89 u_int8_t nexthdr;
90 __be16 frag_off;
91
92 nexthdr = ipv6h->nexthdr;
93 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
94 if (tcphoff < 0)
95 return NF_DROP;
96
97 return tcpoptstrip_mangle_packet(skb, par->targinfo, tcphoff,
98 sizeof(*ipv6h) + sizeof(struct tcphdr));
99}
100#endif
101
102static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
103 {
104 .name = "TCPOPTSTRIP",
105 .family = NFPROTO_IPV4,
106 .table = "mangle",
107 .proto = IPPROTO_TCP,
108 .target = tcpoptstrip_tg4,
109 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
110 .me = THIS_MODULE,
111 },
112#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
113 {
114 .name = "TCPOPTSTRIP",
115 .family = NFPROTO_IPV6,
116 .table = "mangle",
117 .proto = IPPROTO_TCP,
118 .target = tcpoptstrip_tg6,
119 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
120 .me = THIS_MODULE,
121 },
122#endif
123};
124
125static int __init tcpoptstrip_tg_init(void)
126{
127 return xt_register_targets(tcpoptstrip_tg_reg,
128 ARRAY_SIZE(tcpoptstrip_tg_reg));
129}
130
131static void __exit tcpoptstrip_tg_exit(void)
132{
133 xt_unregister_targets(tcpoptstrip_tg_reg,
134 ARRAY_SIZE(tcpoptstrip_tg_reg));
135}
136
137module_init(tcpoptstrip_tg_init);
138module_exit(tcpoptstrip_tg_exit);
139MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
140MODULE_DESCRIPTION("Xtables: TCP option stripping");
141MODULE_LICENSE("GPL");
142MODULE_ALIAS("ipt_TCPOPTSTRIP");
143MODULE_ALIAS("ip6t_TCPOPTSTRIP");