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, 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
91 static unsigned int
tcpoptstrip_tg4(struct sk_buff * skb,const struct xt_action_param * par)92 tcpoptstrip_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)
99 static unsigned int
tcpoptstrip_tg6(struct sk_buff * skb,const struct xt_action_param * par)100 tcpoptstrip_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
117 static 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
tcpoptstrip_tg_init(void)140 static int __init tcpoptstrip_tg_init(void)
141 {
142 return xt_register_targets(tcpoptstrip_tg_reg,
143 ARRAY_SIZE(tcpoptstrip_tg_reg));
144 }
145
tcpoptstrip_tg_exit(void)146 static void __exit tcpoptstrip_tg_exit(void)
147 {
148 xt_unregister_targets(tcpoptstrip_tg_reg,
149 ARRAY_SIZE(tcpoptstrip_tg_reg));
150 }
151
152 module_init(tcpoptstrip_tg_init);
153 module_exit(tcpoptstrip_tg_exit);
154 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
155 MODULE_DESCRIPTION("Xtables: TCP option stripping");
156 MODULE_LICENSE("GPL");
157 MODULE_ALIAS("ipt_TCPOPTSTRIP");
158 MODULE_ALIAS("ip6t_TCPOPTSTRIP");
159