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
optlen(const u_int8_t * opt,unsigned int offset)22 static 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
31 static unsigned int
tcpoptstrip_mangle_packet(struct sk_buff * skb,const struct xt_action_param * par,unsigned int tcphoff,unsigned int minlen)32 tcpoptstrip_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;
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 if (tcph->doff * 4 > len)
56 return NF_DROP;
57
58 opt = (u_int8_t *)tcph;
59
60 /*
61 * Walk through all TCP options - if we find some option to remove,
62 * set all octets to %TCPOPT_NOP and adjust checksum.
63 */
64 for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
65 optl = optlen(opt, i);
66
67 if (i + optl > tcp_hdrlen(skb))
68 break;
69
70 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
71 continue;
72
73 for (j = 0; j < optl; ++j) {
74 o = opt[i+j];
75 n = TCPOPT_NOP;
76 if ((i + j) % 2 == 0) {
77 o <<= 8;
78 n <<= 8;
79 }
80 inet_proto_csum_replace2(&tcph->check, skb, htons(o),
81 htons(n), 0);
82 }
83 memset(opt + i, TCPOPT_NOP, optl);
84 }
85
86 return XT_CONTINUE;
87 }
88
89 static unsigned int
tcpoptstrip_tg4(struct sk_buff * skb,const struct xt_action_param * par)90 tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
91 {
92 return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
93 sizeof(struct iphdr) + sizeof(struct tcphdr));
94 }
95
96 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
97 static unsigned int
tcpoptstrip_tg6(struct sk_buff * skb,const struct xt_action_param * par)98 tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
99 {
100 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
101 int tcphoff;
102 u_int8_t nexthdr;
103 __be16 frag_off;
104
105 nexthdr = ipv6h->nexthdr;
106 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
107 if (tcphoff < 0)
108 return NF_DROP;
109
110 return tcpoptstrip_mangle_packet(skb, par, tcphoff,
111 sizeof(*ipv6h) + sizeof(struct tcphdr));
112 }
113 #endif
114
115 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
116 {
117 .name = "TCPOPTSTRIP",
118 .family = NFPROTO_IPV4,
119 .table = "mangle",
120 .proto = IPPROTO_TCP,
121 .target = tcpoptstrip_tg4,
122 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
123 .me = THIS_MODULE,
124 },
125 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
126 {
127 .name = "TCPOPTSTRIP",
128 .family = NFPROTO_IPV6,
129 .table = "mangle",
130 .proto = IPPROTO_TCP,
131 .target = tcpoptstrip_tg6,
132 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
133 .me = THIS_MODULE,
134 },
135 #endif
136 };
137
tcpoptstrip_tg_init(void)138 static int __init tcpoptstrip_tg_init(void)
139 {
140 return xt_register_targets(tcpoptstrip_tg_reg,
141 ARRAY_SIZE(tcpoptstrip_tg_reg));
142 }
143
tcpoptstrip_tg_exit(void)144 static void __exit tcpoptstrip_tg_exit(void)
145 {
146 xt_unregister_targets(tcpoptstrip_tg_reg,
147 ARRAY_SIZE(tcpoptstrip_tg_reg));
148 }
149
150 module_init(tcpoptstrip_tg_init);
151 module_exit(tcpoptstrip_tg_exit);
152 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
153 MODULE_DESCRIPTION("Xtables: TCP option stripping");
154 MODULE_LICENSE("GPL");
155 MODULE_ALIAS("ipt_TCPOPTSTRIP");
156 MODULE_ALIAS("ip6t_TCPOPTSTRIP");
157