net/ipv4/netfilter/nf_nat_pptp.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0-only
 /*
  * nf_nat_pptp.c
  *
  * NAT support for PPTP (Point to Point Tunneling Protocol).
  * PPTP is a protocol for creating virtual private networks.
  * It is a specification defined by Microsoft and some vendors
  * working with Microsoft.  PPTP is built on top of a modified
  * version of the Internet Generic Routing Encapsulation Protocol.
  * GRE is defined in RFC 1701 and RFC 1702.  Documentation of
  * PPTP can be found in RFC 2637
  *
  * (C) 2000-2005 by Harald Welte <laforge@gnumonks.org>
  *
  * Development of this code funded by Astaro AG (http://www.astaro.com/)
  *
  * (C) 2006-2012 Patrick McHardy <kaber@trash.net>
  *
  * TODO: - NAT to a unique tuple, not to TCP source port
  * 	   (needs netfilter tuple reservation)
  */

 #include <linux/module.h>
 #include <linux/tcp.h>

 #include <net/netfilter/nf_nat.h>
 #include <net/netfilter/nf_nat_helper.h>
 #include <net/netfilter/nf_conntrack_helper.h>
 #include <net/netfilter/nf_conntrack_expect.h>
 #include <net/netfilter/nf_conntrack_zones.h>
 #include <linux/netfilter/nf_conntrack_proto_gre.h>
 #include <linux/netfilter/nf_conntrack_pptp.h>

 #define NF_NAT_PPTP_VERSION "3.0"

 #define REQ_CID(req, off)		(*(__be16 *)((char *)(req) + (off)))

 MODULE_LICENSE("GPL");
 MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
 MODULE_DESCRIPTION("Netfilter NAT helper module for PPTP");
 MODULE_ALIAS_NF_NAT_HELPER("pptp");

 static void pptp_nat_expected(struct nf_conn *ct,
 			      struct nf_conntrack_expect *exp)
 {
 	struct net *net = nf_ct_net(ct);
 	const struct nf_conn *master = ct->master;
 	struct nf_conntrack_expect *other_exp;
 	struct nf_conntrack_tuple t = {};
 	const struct nf_ct_pptp_master *ct_pptp_info;
 	const struct nf_nat_pptp *nat_pptp_info;
 	struct nf_nat_range2 range;
 	struct nf_conn_nat *nat;

 	nat = nf_ct_nat_ext_add(ct);
 	if (WARN_ON_ONCE(!nat))
 		return;

 	nat_pptp_info = &nat->help.nat_pptp_info;
 	ct_pptp_info = nfct_help_data(master);

 	/* And here goes the grand finale of corrosion... */
 	if (exp->dir == IP_CT_DIR_ORIGINAL) {
 		pr_debug("we are PNS->PAC\n");
 		/* therefore, build tuple for PAC->PNS */
 		t.src.l3num = AF_INET;
 		t.src.u3.ip = master->tuplehash[!exp->dir].tuple.src.u3.ip;
 		t.src.u.gre.key = ct_pptp_info->pac_call_id;
 		t.dst.u3.ip = master->tuplehash[!exp->dir].tuple.dst.u3.ip;
 		t.dst.u.gre.key = ct_pptp_info->pns_call_id;
 		t.dst.protonum = IPPROTO_GRE;
 	} else {
 		pr_debug("we are PAC->PNS\n");
 		/* build tuple for PNS->PAC */
 		t.src.l3num = AF_INET;
 		t.src.u3.ip = master->tuplehash[!exp->dir].tuple.src.u3.ip;
 		t.src.u.gre.key = nat_pptp_info->pns_call_id;
 		t.dst.u3.ip = master->tuplehash[!exp->dir].tuple.dst.u3.ip;
 		t.dst.u.gre.key = nat_pptp_info->pac_call_id;
 		t.dst.protonum = IPPROTO_GRE;
 	}

 	pr_debug("trying to unexpect other dir: ");
 	nf_ct_dump_tuple_ip(&t);
 	other_exp = nf_ct_expect_find_get(net, nf_ct_zone(ct), &t);
 	if (other_exp) {
 		nf_ct_unexpect_related(other_exp);
 		nf_ct_expect_put(other_exp);
 		pr_debug("success\n");
 	} else {
 		pr_debug("not found!\n");
 	}

 	/* This must be a fresh one. */
 	BUG_ON(ct->status & IPS_NAT_DONE_MASK);

 	/* Change src to where master sends to */
 	range.flags = NF_NAT_RANGE_MAP_IPS;
 	range.min_addr = range.max_addr
 		= ct->master->tuplehash[!exp->dir].tuple.dst.u3;
 	if (exp->dir == IP_CT_DIR_ORIGINAL) {
 		range.flags |= NF_NAT_RANGE_PROTO_SPECIFIED;
 		range.min_proto = range.max_proto = exp->saved_proto;
 	}
 	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);

 	/* For DST manip, map port here to where it's expected. */
 	range.flags = NF_NAT_RANGE_MAP_IPS;
 	range.min_addr = range.max_addr
 		= ct->master->tuplehash[!exp->dir].tuple.src.u3;
 	if (exp->dir == IP_CT_DIR_REPLY) {
 		range.flags |= NF_NAT_RANGE_PROTO_SPECIFIED;
 		range.min_proto = range.max_proto = exp->saved_proto;
 	}
 	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
 }

 /* outbound packets == from PNS to PAC */
 static int
 pptp_outbound_pkt(struct sk_buff *skb,
 		  struct nf_conn *ct,
 		  enum ip_conntrack_info ctinfo,
 		  unsigned int protoff,
 		  struct PptpControlHeader *ctlh,
 		  union pptp_ctrl_union *pptpReq)

 {
 	struct nf_ct_pptp_master *ct_pptp_info;
 	struct nf_conn_nat *nat = nfct_nat(ct);
 	struct nf_nat_pptp *nat_pptp_info;
 	u_int16_t msg;
 	__be16 new_callid;
 	unsigned int cid_off;

 	if (WARN_ON_ONCE(!nat))
 		return NF_DROP;

 	nat_pptp_info = &nat->help.nat_pptp_info;
 	ct_pptp_info = nfct_help_data(ct);

 	new_callid = ct_pptp_info->pns_call_id;

 	switch (msg = ntohs(ctlh->messageType)) {
 	case PPTP_OUT_CALL_REQUEST:
 		cid_off = offsetof(union pptp_ctrl_union, ocreq.callID);
 		/* FIXME: ideally we would want to reserve a call ID
 		 * here.  current netfilter NAT core is not able to do
 		 * this :( For now we use TCP source port. This breaks
 		 * multiple calls within one control session */

 		/* save original call ID in nat_info */
 		nat_pptp_info->pns_call_id = ct_pptp_info->pns_call_id;

 		/* don't use tcph->source since we are at a DSTmanip
 		 * hook (e.g. PREROUTING) and pkt is not mangled yet */
 		new_callid = ct->tuplehash[IP_CT_DIR_REPLY].tuple.dst.u.tcp.port;

 		/* save new call ID in ct info */
 		ct_pptp_info->pns_call_id = new_callid;
 		break;
 	case PPTP_IN_CALL_REPLY:
 		cid_off = offsetof(union pptp_ctrl_union, icack.callID);
 		break;
 	case PPTP_CALL_CLEAR_REQUEST:
 		cid_off = offsetof(union pptp_ctrl_union, clrreq.callID);
 		break;
 	default:
 		pr_debug("unknown outbound packet 0x%04x:%s\n", msg,
 			 pptp_msg_name(msg));
 		fallthrough;
 	case PPTP_SET_LINK_INFO:
 		/* only need to NAT in case PAC is behind NAT box */
 	case PPTP_START_SESSION_REQUEST:
 	case PPTP_START_SESSION_REPLY:
 	case PPTP_STOP_SESSION_REQUEST:
 	case PPTP_STOP_SESSION_REPLY:
 	case PPTP_ECHO_REQUEST:
 	case PPTP_ECHO_REPLY:
 		/* no need to alter packet */
 		return NF_ACCEPT;
 	}

 	/* only OUT_CALL_REQUEST, IN_CALL_REPLY, CALL_CLEAR_REQUEST pass
 	 * down to here */
 	pr_debug("altering call id from 0x%04x to 0x%04x\n",
 		 ntohs(REQ_CID(pptpReq, cid_off)), ntohs(new_callid));

 	/* mangle packet */
 	if (!nf_nat_mangle_tcp_packet(skb, ct, ctinfo, protoff,
 				      cid_off + sizeof(struct pptp_pkt_hdr) +
 				      sizeof(struct PptpControlHeader),
 				      sizeof(new_callid), (char *)&new_callid,
 				      sizeof(new_callid)))
 		return NF_DROP;
 	return NF_ACCEPT;
 }

 static void
 pptp_exp_gre(struct nf_conntrack_expect *expect_orig,
 	     struct nf_conntrack_expect *expect_reply)
 {
 	const struct nf_conn *ct = expect_orig->master;
 	struct nf_conn_nat *nat = nfct_nat(ct);
 	struct nf_ct_pptp_master *ct_pptp_info;
 	struct nf_nat_pptp *nat_pptp_info;

 	if (WARN_ON_ONCE(!nat))
 		return;

 	nat_pptp_info = &nat->help.nat_pptp_info;
 	ct_pptp_info = nfct_help_data(ct);

 	/* save original PAC call ID in nat_info */
 	nat_pptp_info->pac_call_id = ct_pptp_info->pac_call_id;

 	/* alter expectation for PNS->PAC direction */
 	expect_orig->saved_proto.gre.key = ct_pptp_info->pns_call_id;
 	expect_orig->tuple.src.u.gre.key = nat_pptp_info->pns_call_id;
 	expect_orig->tuple.dst.u.gre.key = ct_pptp_info->pac_call_id;
 	expect_orig->dir = IP_CT_DIR_ORIGINAL;

 	/* alter expectation for PAC->PNS direction */
 	expect_reply->saved_proto.gre.key = nat_pptp_info->pns_call_id;
 	expect_reply->tuple.src.u.gre.key = nat_pptp_info->pac_call_id;
 	expect_reply->tuple.dst.u.gre.key = ct_pptp_info->pns_call_id;
 	expect_reply->dir = IP_CT_DIR_REPLY;
 }

 /* inbound packets == from PAC to PNS */
 static int
 pptp_inbound_pkt(struct sk_buff *skb,
 		 struct nf_conn *ct,
 		 enum ip_conntrack_info ctinfo,
 		 unsigned int protoff,
 		 struct PptpControlHeader *ctlh,
 		 union pptp_ctrl_union *pptpReq)
 {
 	const struct nf_nat_pptp *nat_pptp_info;
 	struct nf_conn_nat *nat = nfct_nat(ct);
 	u_int16_t msg;
 	__be16 new_pcid;
 	unsigned int pcid_off;

 	if (WARN_ON_ONCE(!nat))
 		return NF_DROP;

 	nat_pptp_info = &nat->help.nat_pptp_info;
 	new_pcid = nat_pptp_info->pns_call_id;

 	switch (msg = ntohs(ctlh->messageType)) {
 	case PPTP_OUT_CALL_REPLY:
 		pcid_off = offsetof(union pptp_ctrl_union, ocack.peersCallID);
 		break;
 	case PPTP_IN_CALL_CONNECT:
 		pcid_off = offsetof(union pptp_ctrl_union, iccon.peersCallID);
 		break;
 	case PPTP_IN_CALL_REQUEST:
 		/* only need to nat in case PAC is behind NAT box */
 		return NF_ACCEPT;
 	case PPTP_WAN_ERROR_NOTIFY:
 		pcid_off = offsetof(union pptp_ctrl_union, wanerr.peersCallID);
 		break;
 	case PPTP_CALL_DISCONNECT_NOTIFY:
 		pcid_off = offsetof(union pptp_ctrl_union, disc.callID);
 		break;
 	case PPTP_SET_LINK_INFO:
 		pcid_off = offsetof(union pptp_ctrl_union, setlink.peersCallID);
 		break;
 	default:
 		pr_debug("unknown inbound packet %s\n", pptp_msg_name(msg));
 		fallthrough;
 	case PPTP_START_SESSION_REQUEST:
 	case PPTP_START_SESSION_REPLY:
 	case PPTP_STOP_SESSION_REQUEST:
 	case PPTP_STOP_SESSION_REPLY:
 	case PPTP_ECHO_REQUEST:
 	case PPTP_ECHO_REPLY:
 		/* no need to alter packet */
 		return NF_ACCEPT;
 	}

 	/* only OUT_CALL_REPLY, IN_CALL_CONNECT, IN_CALL_REQUEST,
 	 * WAN_ERROR_NOTIFY, CALL_DISCONNECT_NOTIFY pass down here */

 	/* mangle packet */
 	pr_debug("altering peer call id from 0x%04x to 0x%04x\n",
 		 ntohs(REQ_CID(pptpReq, pcid_off)), ntohs(new_pcid));

 	if (!nf_nat_mangle_tcp_packet(skb, ct, ctinfo, protoff,
 				      pcid_off + sizeof(struct pptp_pkt_hdr) +
 				      sizeof(struct PptpControlHeader),
 				      sizeof(new_pcid), (char *)&new_pcid,
 				      sizeof(new_pcid)))
 		return NF_DROP;
 	return NF_ACCEPT;
 }

 static int __init nf_nat_helper_pptp_init(void)
 {
 	BUG_ON(nf_nat_pptp_hook_outbound != NULL);
 	RCU_INIT_POINTER(nf_nat_pptp_hook_outbound, pptp_outbound_pkt);

 	BUG_ON(nf_nat_pptp_hook_inbound != NULL);
 	RCU_INIT_POINTER(nf_nat_pptp_hook_inbound, pptp_inbound_pkt);

 	BUG_ON(nf_nat_pptp_hook_exp_gre != NULL);
 	RCU_INIT_POINTER(nf_nat_pptp_hook_exp_gre, pptp_exp_gre);

 	BUG_ON(nf_nat_pptp_hook_expectfn != NULL);
 	RCU_INIT_POINTER(nf_nat_pptp_hook_expectfn, pptp_nat_expected);
 	return 0;
 }

 static void __exit nf_nat_helper_pptp_fini(void)
 {
 	RCU_INIT_POINTER(nf_nat_pptp_hook_expectfn, NULL);
 	RCU_INIT_POINTER(nf_nat_pptp_hook_exp_gre, NULL);
 	RCU_INIT_POINTER(nf_nat_pptp_hook_inbound, NULL);
 	RCU_INIT_POINTER(nf_nat_pptp_hook_outbound, NULL);
 	synchronize_rcu();
 }

 module_init(nf_nat_helper_pptp_init);
 module_exit(nf_nat_helper_pptp_fini);
	// SPDX-License-Identifier: GPL-2.0-only
	/*
	* nf_nat_pptp.c
	*
	* NAT support for PPTP (Point to Point Tunneling Protocol).
	* PPTP is a protocol for creating virtual private networks.
	* It is a specification defined by Microsoft and some vendors
	* working with Microsoft. PPTP is built on top of a modified
	* version of the Internet Generic Routing Encapsulation Protocol.
	* GRE is defined in RFC 1701 and RFC 1702. Documentation of
	* PPTP can be found in RFC 2637
	*
	* (C) 2000-2005 by Harald Welte <laforge@gnumonks.org>
	*
	* Development of this code funded by Astaro AG (http://www.astaro.com/)
	*
	* (C) 2006-2012 Patrick McHardy <kaber@trash.net>
	*
	* TODO: - NAT to a unique tuple, not to TCP source port
	* (needs netfilter tuple reservation)
	*/

	#include <linux/module.h>
	#include <linux/tcp.h>

	#include <net/netfilter/nf_nat.h>
	#include <net/netfilter/nf_nat_helper.h>
	#include <net/netfilter/nf_conntrack_helper.h>
	#include <net/netfilter/nf_conntrack_expect.h>
	#include <net/netfilter/nf_conntrack_zones.h>
	#include <linux/netfilter/nf_conntrack_proto_gre.h>
	#include <linux/netfilter/nf_conntrack_pptp.h>

	#define NF_NAT_PPTP_VERSION "3.0"

	#define REQ_CID(req, off) ((__be16 )((char *)(req) + (off)))

	MODULE_LICENSE("GPL");
	MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
	MODULE_DESCRIPTION("Netfilter NAT helper module for PPTP");
	MODULE_ALIAS_NF_NAT_HELPER("pptp");

	static void pptp_nat_expected(struct nf_conn *ct,
	struct nf_conntrack_expect *exp)
	{
	struct net *net = nf_ct_net(ct);
	const struct nf_conn *master = ct->master;
	struct nf_conntrack_expect *other_exp;
	struct nf_conntrack_tuple t = {};
	const struct nf_ct_pptp_master *ct_pptp_info;
	const struct nf_nat_pptp *nat_pptp_info;
	struct nf_nat_range2 range;
	struct nf_conn_nat *nat;

	nat = nf_ct_nat_ext_add(ct);
	if (WARN_ON_ONCE(!nat))
	return;

	nat_pptp_info = &nat->help.nat_pptp_info;
	ct_pptp_info = nfct_help_data(master);

	/* And here goes the grand finale of corrosion... */
	if (exp->dir == IP_CT_DIR_ORIGINAL) {
	pr_debug("we are PNS->PAC\n");
	/* therefore, build tuple for PAC->PNS */
	t.src.l3num = AF_INET;
	t.src.u3.ip = master->tuplehash[!exp->dir].tuple.src.u3.ip;
	t.src.u.gre.key = ct_pptp_info->pac_call_id;
	t.dst.u3.ip = master->tuplehash[!exp->dir].tuple.dst.u3.ip;
	t.dst.u.gre.key = ct_pptp_info->pns_call_id;
	t.dst.protonum = IPPROTO_GRE;
	} else {
	pr_debug("we are PAC->PNS\n");
	/* build tuple for PNS->PAC */
	t.src.l3num = AF_INET;
	t.src.u3.ip = master->tuplehash[!exp->dir].tuple.src.u3.ip;
	t.src.u.gre.key = nat_pptp_info->pns_call_id;
	t.dst.u3.ip = master->tuplehash[!exp->dir].tuple.dst.u3.ip;
	t.dst.u.gre.key = nat_pptp_info->pac_call_id;
	t.dst.protonum = IPPROTO_GRE;
	}

	pr_debug("trying to unexpect other dir: ");
	nf_ct_dump_tuple_ip(&t);
	other_exp = nf_ct_expect_find_get(net, nf_ct_zone(ct), &t);
	if (other_exp) {
	nf_ct_unexpect_related(other_exp);
	nf_ct_expect_put(other_exp);
	pr_debug("success\n");
	} else {
	pr_debug("not found!\n");
	}

	/* This must be a fresh one. */
	BUG_ON(ct->status & IPS_NAT_DONE_MASK);

	/* Change src to where master sends to */
	range.flags = NF_NAT_RANGE_MAP_IPS;
	range.min_addr = range.max_addr
	= ct->master->tuplehash[!exp->dir].tuple.dst.u3;
	if (exp->dir == IP_CT_DIR_ORIGINAL) {
	range.flags \|= NF_NAT_RANGE_PROTO_SPECIFIED;
	range.min_proto = range.max_proto = exp->saved_proto;
	}
	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_SRC);

	/* For DST manip, map port here to where it's expected. */
	range.flags = NF_NAT_RANGE_MAP_IPS;
	range.min_addr = range.max_addr
	= ct->master->tuplehash[!exp->dir].tuple.src.u3;
	if (exp->dir == IP_CT_DIR_REPLY) {
	range.flags \|= NF_NAT_RANGE_PROTO_SPECIFIED;
	range.min_proto = range.max_proto = exp->saved_proto;
	}
	nf_nat_setup_info(ct, &range, NF_NAT_MANIP_DST);
	}

	/* outbound packets == from PNS to PAC */
	static int
	pptp_outbound_pkt(struct sk_buff *skb,
	struct nf_conn *ct,
	enum ip_conntrack_info ctinfo,
	unsigned int protoff,
	struct PptpControlHeader *ctlh,
	union pptp_ctrl_union *pptpReq)

	{
	struct nf_ct_pptp_master *ct_pptp_info;
	struct nf_conn_nat *nat = nfct_nat(ct);
	struct nf_nat_pptp *nat_pptp_info;
	u_int16_t msg;
	__be16 new_callid;
	unsigned int cid_off;

	if (WARN_ON_ONCE(!nat))
	return NF_DROP;

	nat_pptp_info = &nat->help.nat_pptp_info;
	ct_pptp_info = nfct_help_data(ct);

	new_callid = ct_pptp_info->pns_call_id;

	switch (msg = ntohs(ctlh->messageType)) {
	case PPTP_OUT_CALL_REQUEST:
	cid_off = offsetof(union pptp_ctrl_union, ocreq.callID);
	/* FIXME: ideally we would want to reserve a call ID
	* here. current netfilter NAT core is not able to do
	* this :( For now we use TCP source port. This breaks
	* multiple calls within one control session */

	/* save original call ID in nat_info */
	nat_pptp_info->pns_call_id = ct_pptp_info->pns_call_id;

	/* don't use tcph->source since we are at a DSTmanip
	* hook (e.g. PREROUTING) and pkt is not mangled yet */
	new_callid = ct->tuplehash[IP_CT_DIR_REPLY].tuple.dst.u.tcp.port;

	/* save new call ID in ct info */
	ct_pptp_info->pns_call_id = new_callid;
	break;
	case PPTP_IN_CALL_REPLY:
	cid_off = offsetof(union pptp_ctrl_union, icack.callID);
	break;
	case PPTP_CALL_CLEAR_REQUEST:
	cid_off = offsetof(union pptp_ctrl_union, clrreq.callID);
	break;
	default:
	pr_debug("unknown outbound packet 0x%04x:%s\n", msg,
	pptp_msg_name(msg));
	fallthrough;
	case PPTP_SET_LINK_INFO:
	/* only need to NAT in case PAC is behind NAT box */
	case PPTP_START_SESSION_REQUEST:
	case PPTP_START_SESSION_REPLY:
	case PPTP_STOP_SESSION_REQUEST:
	case PPTP_STOP_SESSION_REPLY:
	case PPTP_ECHO_REQUEST:
	case PPTP_ECHO_REPLY:
	/* no need to alter packet */
	return NF_ACCEPT;
	}

	/* only OUT_CALL_REQUEST, IN_CALL_REPLY, CALL_CLEAR_REQUEST pass
	* down to here */
	pr_debug("altering call id from 0x%04x to 0x%04x\n",
	ntohs(REQ_CID(pptpReq, cid_off)), ntohs(new_callid));

	/* mangle packet */
	if (!nf_nat_mangle_tcp_packet(skb, ct, ctinfo, protoff,
	cid_off + sizeof(struct pptp_pkt_hdr) +
	sizeof(struct PptpControlHeader),
	sizeof(new_callid), (char *)&new_callid,
	sizeof(new_callid)))
	return NF_DROP;
	return NF_ACCEPT;
	}

	static void
	pptp_exp_gre(struct nf_conntrack_expect *expect_orig,
	struct nf_conntrack_expect *expect_reply)
	{
	const struct nf_conn *ct = expect_orig->master;
	struct nf_conn_nat *nat = nfct_nat(ct);
	struct nf_ct_pptp_master *ct_pptp_info;
	struct nf_nat_pptp *nat_pptp_info;

	if (WARN_ON_ONCE(!nat))
	return;

	nat_pptp_info = &nat->help.nat_pptp_info;
	ct_pptp_info = nfct_help_data(ct);

	/* save original PAC call ID in nat_info */
	nat_pptp_info->pac_call_id = ct_pptp_info->pac_call_id;

	/* alter expectation for PNS->PAC direction */
	expect_orig->saved_proto.gre.key = ct_pptp_info->pns_call_id;
	expect_orig->tuple.src.u.gre.key = nat_pptp_info->pns_call_id;
	expect_orig->tuple.dst.u.gre.key = ct_pptp_info->pac_call_id;
	expect_orig->dir = IP_CT_DIR_ORIGINAL;

	/* alter expectation for PAC->PNS direction */
	expect_reply->saved_proto.gre.key = nat_pptp_info->pns_call_id;
	expect_reply->tuple.src.u.gre.key = nat_pptp_info->pac_call_id;
	expect_reply->tuple.dst.u.gre.key = ct_pptp_info->pns_call_id;
	expect_reply->dir = IP_CT_DIR_REPLY;
	}

	/* inbound packets == from PAC to PNS */
	static int
	pptp_inbound_pkt(struct sk_buff *skb,
	struct nf_conn *ct,
	enum ip_conntrack_info ctinfo,
	unsigned int protoff,
	struct PptpControlHeader *ctlh,
	union pptp_ctrl_union *pptpReq)
	{
	const struct nf_nat_pptp *nat_pptp_info;
	struct nf_conn_nat *nat = nfct_nat(ct);
	u_int16_t msg;
	__be16 new_pcid;
	unsigned int pcid_off;

	if (WARN_ON_ONCE(!nat))
	return NF_DROP;

	nat_pptp_info = &nat->help.nat_pptp_info;
	new_pcid = nat_pptp_info->pns_call_id;

	switch (msg = ntohs(ctlh->messageType)) {
	case PPTP_OUT_CALL_REPLY:
	pcid_off = offsetof(union pptp_ctrl_union, ocack.peersCallID);
	break;
	case PPTP_IN_CALL_CONNECT:
	pcid_off = offsetof(union pptp_ctrl_union, iccon.peersCallID);
	break;
	case PPTP_IN_CALL_REQUEST:
	/* only need to nat in case PAC is behind NAT box */
	return NF_ACCEPT;
	case PPTP_WAN_ERROR_NOTIFY:
	pcid_off = offsetof(union pptp_ctrl_union, wanerr.peersCallID);
	break;
	case PPTP_CALL_DISCONNECT_NOTIFY:
	pcid_off = offsetof(union pptp_ctrl_union, disc.callID);
	break;
	case PPTP_SET_LINK_INFO:
	pcid_off = offsetof(union pptp_ctrl_union, setlink.peersCallID);
	break;
	default:
	pr_debug("unknown inbound packet %s\n", pptp_msg_name(msg));
	fallthrough;
	case PPTP_START_SESSION_REQUEST:
	case PPTP_START_SESSION_REPLY:
	case PPTP_STOP_SESSION_REQUEST:
	case PPTP_STOP_SESSION_REPLY:
	case PPTP_ECHO_REQUEST:
	case PPTP_ECHO_REPLY:
	/* no need to alter packet */
	return NF_ACCEPT;
	}

	/* only OUT_CALL_REPLY, IN_CALL_CONNECT, IN_CALL_REQUEST,
	* WAN_ERROR_NOTIFY, CALL_DISCONNECT_NOTIFY pass down here */

	/* mangle packet */
	pr_debug("altering peer call id from 0x%04x to 0x%04x\n",
	ntohs(REQ_CID(pptpReq, pcid_off)), ntohs(new_pcid));

	if (!nf_nat_mangle_tcp_packet(skb, ct, ctinfo, protoff,
	pcid_off + sizeof(struct pptp_pkt_hdr) +
	sizeof(struct PptpControlHeader),
	sizeof(new_pcid), (char *)&new_pcid,
	sizeof(new_pcid)))
	return NF_DROP;
	return NF_ACCEPT;
	}

	static int __init nf_nat_helper_pptp_init(void)
	{
	BUG_ON(nf_nat_pptp_hook_outbound != NULL);
	RCU_INIT_POINTER(nf_nat_pptp_hook_outbound, pptp_outbound_pkt);

	BUG_ON(nf_nat_pptp_hook_inbound != NULL);
	RCU_INIT_POINTER(nf_nat_pptp_hook_inbound, pptp_inbound_pkt);

	BUG_ON(nf_nat_pptp_hook_exp_gre != NULL);
	RCU_INIT_POINTER(nf_nat_pptp_hook_exp_gre, pptp_exp_gre);

	BUG_ON(nf_nat_pptp_hook_expectfn != NULL);
	RCU_INIT_POINTER(nf_nat_pptp_hook_expectfn, pptp_nat_expected);
	return 0;
	}

	static void __exit nf_nat_helper_pptp_fini(void)
	{
	RCU_INIT_POINTER(nf_nat_pptp_hook_expectfn, NULL);
	RCU_INIT_POINTER(nf_nat_pptp_hook_exp_gre, NULL);
	RCU_INIT_POINTER(nf_nat_pptp_hook_inbound, NULL);
	RCU_INIT_POINTER(nf_nat_pptp_hook_outbound, NULL);
	synchronize_rcu();
	}

	module_init(nf_nat_helper_pptp_init);
	module_exit(nf_nat_helper_pptp_fini);