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