2 * A module for stripping a specific TCP option from TCP packets.
4 * Copyright (C) 2007 Sven Schnelle <svens@bitebene.org>
5 * Copyright © CC Computer Consultants GmbH, 2007
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.
12 #include <linux/module.h>
13 #include <linux/skbuff.h>
15 #include <linux/ipv6.h>
16 #include <linux/tcp.h>
19 #include <linux/netfilter/x_tables.h>
20 #include <linux/netfilter/xt_TCPOPTSTRIP.h>
22 static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
24 /* Beware zero-length options: make finite progress */
25 if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
32 tcpoptstrip_mangle_packet(struct sk_buff *skb,
33 const struct xt_action_param *par,
34 unsigned int tcphoff, unsigned int minlen)
36 const struct xt_tcpoptstrip_target_info *info = par->targinfo;
37 unsigned int optl, i, j;
43 /* This is a fragment, no TCP header is available */
44 if (par->fragoff != 0)
47 if (!skb_make_writable(skb, skb->len))
50 len = skb->len - tcphoff;
51 if (len < (int)sizeof(struct tcphdr) ||
52 tcp_hdr(skb)->doff * 4 > len)
55 tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
56 opt = (u_int8_t *)tcph;
59 * Walk through all TCP options - if we find some option to remove,
60 * set all octets to %TCPOPT_NOP and adjust checksum.
62 for (i = sizeof(struct tcphdr); i < tcp_hdrlen(skb); i += optl) {
63 optl = optlen(opt, i);
65 if (i + optl > tcp_hdrlen(skb))
68 if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
71 for (j = 0; j < optl; ++j) {
74 if ((i + j) % 2 == 0) {
78 inet_proto_csum_replace2(&tcph->check, skb, htons(o),
81 memset(opt + i, TCPOPT_NOP, optl);
88 tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
90 return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb),
91 sizeof(struct iphdr) + sizeof(struct tcphdr));
94 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
96 tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
98 struct ipv6hdr *ipv6h = ipv6_hdr(skb);
103 nexthdr = ipv6h->nexthdr;
104 tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
108 return tcpoptstrip_mangle_packet(skb, par, tcphoff,
109 sizeof(*ipv6h) + sizeof(struct tcphdr));
113 static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
115 .name = "TCPOPTSTRIP",
116 .family = NFPROTO_IPV4,
118 .proto = IPPROTO_TCP,
119 .target = tcpoptstrip_tg4,
120 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
123 #if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
125 .name = "TCPOPTSTRIP",
126 .family = NFPROTO_IPV6,
128 .proto = IPPROTO_TCP,
129 .target = tcpoptstrip_tg6,
130 .targetsize = sizeof(struct xt_tcpoptstrip_target_info),
136 static int __init tcpoptstrip_tg_init(void)
138 return xt_register_targets(tcpoptstrip_tg_reg,
139 ARRAY_SIZE(tcpoptstrip_tg_reg));
142 static void __exit tcpoptstrip_tg_exit(void)
144 xt_unregister_targets(tcpoptstrip_tg_reg,
145 ARRAY_SIZE(tcpoptstrip_tg_reg));
148 module_init(tcpoptstrip_tg_init);
149 module_exit(tcpoptstrip_tg_exit);
150 MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
151 MODULE_DESCRIPTION("Xtables: TCP option stripping");
152 MODULE_LICENSE("GPL");
153 MODULE_ALIAS("ipt_TCPOPTSTRIP");
154 MODULE_ALIAS("ip6t_TCPOPTSTRIP");