// SPDX-License-Identifier: GPL-2.0-only
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/*
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* A module for stripping a specific TCP option from TCP packets.
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*
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* Copyright (C) 2007 Sven Schnelle <svens@bitebene.org>
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* Copyright © CC Computer Consultants GmbH, 2007
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*/
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#include <linux/module.h>
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#include <linux/skbuff.h>
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#include <linux/ip.h>
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#include <linux/ipv6.h>
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#include <linux/tcp.h>
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#include <net/ipv6.h>
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#include <net/tcp.h>
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#include <linux/netfilter/x_tables.h>
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#include <linux/netfilter/xt_TCPOPTSTRIP.h>
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static inline unsigned int optlen(const u_int8_t *opt, unsigned int offset)
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{
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/* Beware zero-length options: make finite progress */
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if (opt[offset] <= TCPOPT_NOP || opt[offset+1] == 0)
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return 1;
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else
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return opt[offset+1];
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}
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static unsigned int
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tcpoptstrip_mangle_packet(struct sk_buff *skb,
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const struct xt_action_param *par,
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unsigned int tcphoff)
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{
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const struct xt_tcpoptstrip_target_info *info = par->targinfo;
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struct tcphdr *tcph, _th;
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unsigned int optl, i, j;
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u_int16_t n, o;
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u_int8_t *opt;
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int tcp_hdrlen;
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/* This is a fragment, no TCP header is available */
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if (par->fragoff != 0)
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return XT_CONTINUE;
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tcph = skb_header_pointer(skb, tcphoff, sizeof(_th), &_th);
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if (!tcph)
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return NF_DROP;
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tcp_hdrlen = tcph->doff * 4;
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if (tcp_hdrlen < sizeof(struct tcphdr))
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return NF_DROP;
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if (skb_ensure_writable(skb, tcphoff + tcp_hdrlen))
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return NF_DROP;
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/* must reload tcph, might have been moved */
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tcph = (struct tcphdr *)(skb_network_header(skb) + tcphoff);
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opt = (u8 *)tcph;
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/*
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* Walk through all TCP options - if we find some option to remove,
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* set all octets to %TCPOPT_NOP and adjust checksum.
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*/
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for (i = sizeof(struct tcphdr); i < tcp_hdrlen - 1; i += optl) {
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optl = optlen(opt, i);
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if (i + optl > tcp_hdrlen)
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break;
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if (!tcpoptstrip_test_bit(info->strip_bmap, opt[i]))
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continue;
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for (j = 0; j < optl; ++j) {
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o = opt[i+j];
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n = TCPOPT_NOP;
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if ((i + j) % 2 == 0) {
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o <<= 8;
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n <<= 8;
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}
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inet_proto_csum_replace2(&tcph->check, skb, htons(o),
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htons(n), false);
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}
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memset(opt + i, TCPOPT_NOP, optl);
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}
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return XT_CONTINUE;
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}
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static unsigned int
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tcpoptstrip_tg4(struct sk_buff *skb, const struct xt_action_param *par)
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{
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return tcpoptstrip_mangle_packet(skb, par, ip_hdrlen(skb));
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}
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#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
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static unsigned int
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tcpoptstrip_tg6(struct sk_buff *skb, const struct xt_action_param *par)
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{
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struct ipv6hdr *ipv6h = ipv6_hdr(skb);
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int tcphoff;
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u_int8_t nexthdr;
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__be16 frag_off;
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nexthdr = ipv6h->nexthdr;
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tcphoff = ipv6_skip_exthdr(skb, sizeof(*ipv6h), &nexthdr, &frag_off);
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if (tcphoff < 0)
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return NF_DROP;
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return tcpoptstrip_mangle_packet(skb, par, tcphoff);
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}
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#endif
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static struct xt_target tcpoptstrip_tg_reg[] __read_mostly = {
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{
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.name = "TCPOPTSTRIP",
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.family = NFPROTO_IPV4,
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.table = "mangle",
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.proto = IPPROTO_TCP,
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.target = tcpoptstrip_tg4,
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.targetsize = sizeof(struct xt_tcpoptstrip_target_info),
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.me = THIS_MODULE,
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},
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#if IS_ENABLED(CONFIG_IP6_NF_MANGLE)
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{
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.name = "TCPOPTSTRIP",
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.family = NFPROTO_IPV6,
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.table = "mangle",
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.proto = IPPROTO_TCP,
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.target = tcpoptstrip_tg6,
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.targetsize = sizeof(struct xt_tcpoptstrip_target_info),
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.me = THIS_MODULE,
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},
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#endif
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};
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static int __init tcpoptstrip_tg_init(void)
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{
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return xt_register_targets(tcpoptstrip_tg_reg,
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ARRAY_SIZE(tcpoptstrip_tg_reg));
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}
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static void __exit tcpoptstrip_tg_exit(void)
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{
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xt_unregister_targets(tcpoptstrip_tg_reg,
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ARRAY_SIZE(tcpoptstrip_tg_reg));
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}
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module_init(tcpoptstrip_tg_init);
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module_exit(tcpoptstrip_tg_exit);
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MODULE_AUTHOR("Sven Schnelle <svens@bitebene.org>, Jan Engelhardt <jengelh@medozas.de>");
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MODULE_DESCRIPTION("Xtables: TCP option stripping");
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MODULE_LICENSE("GPL");
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MODULE_ALIAS("ipt_TCPOPTSTRIP");
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MODULE_ALIAS("ip6t_TCPOPTSTRIP");
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