/*
   * Connection tracking support for PPTP (Point to Point Tunneling Protocol).
   * PPTP is a 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/)
   *
   * Limitations:
   * 	 - We blindly assume that control connections are always
   * 	   established in PNS->PAC direction.  This is a violation
   * 	   of RFFC2673
   * 	 - We can only support one single call within each session
   * TODO:
   *	 - testing of incoming PPTP calls
   */
  
  #include <linux/module.h>
  #include <linux/skbuff.h>
  #include <linux/in.h>
  #include <linux/tcp.h>
  
  #include <net/netfilter/nf_conntrack.h>
  #include <net/netfilter/nf_conntrack_core.h>
  #include <net/netfilter/nf_conntrack_helper.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_CT_PPTP_VERSION "3.1"
  
  MODULE_LICENSE("GPL");
  MODULE_AUTHOR("Harald Welte <laforge@gnumonks.org>");
  MODULE_DESCRIPTION("Netfilter connection tracking helper module for PPTP");
  MODULE_ALIAS("ip_conntrack_pptp");

  MODULE_ALIAS_NFCT_HELPER("pptp");

  
  static DEFINE_SPINLOCK(nf_pptp_lock);
  
  int

  (*nf_nat_pptp_hook_outbound)(struct sk_buff *skb,

  			     struct nf_conn *ct, enum ip_conntrack_info ctinfo,
  			     struct PptpControlHeader *ctlh,
  			     union pptp_ctrl_union *pptpReq) __read_mostly;
  EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_outbound);
  
  int

  (*nf_nat_pptp_hook_inbound)(struct sk_buff *skb,

  			    struct nf_conn *ct, enum ip_conntrack_info ctinfo,
  			    struct PptpControlHeader *ctlh,
  			    union pptp_ctrl_union *pptpReq) __read_mostly;
  EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_inbound);
  
  void
  (*nf_nat_pptp_hook_exp_gre)(struct nf_conntrack_expect *expect_orig,
  			    struct nf_conntrack_expect *expect_reply)
  			    __read_mostly;
  EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_exp_gre);
  
  void
  (*nf_nat_pptp_hook_expectfn)(struct nf_conn *ct,
  			     struct nf_conntrack_expect *exp) __read_mostly;
  EXPORT_SYMBOL_GPL(nf_nat_pptp_hook_expectfn);

  #if defined(DEBUG) || defined(CONFIG_DYNAMIC_DEBUG)

  /* PptpControlMessageType names */

  const char *const pptp_msg_name[] = {

  	"UNKNOWN_MESSAGE",
  	"START_SESSION_REQUEST",
  	"START_SESSION_REPLY",
  	"STOP_SESSION_REQUEST",
  	"STOP_SESSION_REPLY",
  	"ECHO_REQUEST",
  	"ECHO_REPLY",
  	"OUT_CALL_REQUEST",
  	"OUT_CALL_REPLY",
  	"IN_CALL_REQUEST",
  	"IN_CALL_REPLY",
  	"IN_CALL_CONNECT",
  	"CALL_CLEAR_REQUEST",
  	"CALL_DISCONNECT_NOTIFY",
  	"WAN_ERROR_NOTIFY",
  	"SET_LINK_INFO"
  };
  EXPORT_SYMBOL(pptp_msg_name);

  #endif
  
  #define SECS *HZ
  #define MINS * 60 SECS
  #define HOURS * 60 MINS
  
  #define PPTP_GRE_TIMEOUT 		(10 MINS)
  #define PPTP_GRE_STREAM_TIMEOUT 	(5 HOURS)
  
  static void pptp_expectfn(struct nf_conn *ct,
  			 struct nf_conntrack_expect *exp)
  {

  	struct net *net = nf_ct_net(ct);

  	typeof(nf_nat_pptp_hook_expectfn) nf_nat_pptp_expectfn;

  	pr_debug("increasing timeouts
  ");

  
  	/* increase timeout of GRE data channel conntrack entry */
  	ct->proto.gre.timeout	     = PPTP_GRE_TIMEOUT;
  	ct->proto.gre.stream_timeout = PPTP_GRE_STREAM_TIMEOUT;
  
  	/* Can you see how rusty this code is, compared with the pre-2.6.11
  	 * one? That's what happened to my shiny newnat of 2002 ;( -HW */
  
  	rcu_read_lock();
  	nf_nat_pptp_expectfn = rcu_dereference(nf_nat_pptp_hook_expectfn);

  	if (nf_nat_pptp_expectfn && ct->master->status & IPS_NAT_MASK)

  		nf_nat_pptp_expectfn(ct, exp);
  	else {
  		struct nf_conntrack_tuple inv_t;
  		struct nf_conntrack_expect *exp_other;
  
  		/* obviously this tuple inversion only works until you do NAT */
  		nf_ct_invert_tuplepr(&inv_t, &exp->tuple);

  		pr_debug("trying to unexpect other dir: ");

  		nf_ct_dump_tuple(&inv_t);

  		exp_other = nf_ct_expect_find_get(net, nf_ct_zone(ct), &inv_t);

  		if (exp_other) {
  			/* delete other expectation.  */

  			pr_debug("found
  ");

  			nf_ct_unexpect_related(exp_other);
  			nf_ct_expect_put(exp_other);

  		} else {

  			pr_debug("not found
  ");

  		}
  	}
  	rcu_read_unlock();
  }

  static int destroy_sibling_or_exp(struct net *net, struct nf_conn *ct,

  				  const struct nf_conntrack_tuple *t)

  {

  	const struct nf_conntrack_tuple_hash *h;

  	struct nf_conntrack_expect *exp;
  	struct nf_conn *sibling;

  	u16 zone = nf_ct_zone(ct);

  	pr_debug("trying to timeout ct or exp for tuple ");

  	nf_ct_dump_tuple(t);

  	h = nf_conntrack_find_get(net, zone, t);

  	if (h)  {
  		sibling = nf_ct_tuplehash_to_ctrack(h);

  		pr_debug("setting timeout of conntrack %p to 0
  ", sibling);

  		sibling->proto.gre.timeout	  = 0;
  		sibling->proto.gre.stream_timeout = 0;
  		if (del_timer(&sibling->timeout))
  			sibling->timeout.function((unsigned long)sibling);
  		nf_ct_put(sibling);
  		return 1;
  	} else {

  		exp = nf_ct_expect_find_get(net, zone, t);

  		if (exp) {

  			pr_debug("unexpect_related of expect %p
  ", exp);

  			nf_ct_unexpect_related(exp);
  			nf_ct_expect_put(exp);

  			return 1;
  		}
  	}
  	return 0;
  }
  
  /* timeout GRE data connections */
  static void pptp_destroy_siblings(struct nf_conn *ct)
  {

  	struct net *net = nf_ct_net(ct);

  	const struct nf_conn_help *help = nfct_help(ct);

  	struct nf_conntrack_tuple t;
  
  	nf_ct_gre_keymap_destroy(ct);
  
  	/* try original (pns->pac) tuple */
  	memcpy(&t, &ct->tuplehash[IP_CT_DIR_ORIGINAL].tuple, sizeof(t));
  	t.dst.protonum = IPPROTO_GRE;
  	t.src.u.gre.key = help->help.ct_pptp_info.pns_call_id;
  	t.dst.u.gre.key = help->help.ct_pptp_info.pac_call_id;

  	if (!destroy_sibling_or_exp(net, ct, &t))

  		pr_debug("failed to timeout original pns->pac ct/exp
  ");

  
  	/* try reply (pac->pns) tuple */
  	memcpy(&t, &ct->tuplehash[IP_CT_DIR_REPLY].tuple, sizeof(t));
  	t.dst.protonum = IPPROTO_GRE;
  	t.src.u.gre.key = help->help.ct_pptp_info.pac_call_id;
  	t.dst.u.gre.key = help->help.ct_pptp_info.pns_call_id;

  	if (!destroy_sibling_or_exp(net, ct, &t))

  		pr_debug("failed to timeout reply pac->pns ct/exp
  ");

  }
  
  /* expect GRE connections (PNS->PAC and PAC->PNS direction) */
  static int exp_gre(struct nf_conn *ct, __be16 callid, __be16 peer_callid)
  {
  	struct nf_conntrack_expect *exp_orig, *exp_reply;
  	enum ip_conntrack_dir dir;
  	int ret = 1;
  	typeof(nf_nat_pptp_hook_exp_gre) nf_nat_pptp_exp_gre;

  	exp_orig = nf_ct_expect_alloc(ct);

  	if (exp_orig == NULL)
  		goto out;

  	exp_reply = nf_ct_expect_alloc(ct);

  	if (exp_reply == NULL)
  		goto out_put_orig;
  
  	/* original direction, PNS->PAC */
  	dir = IP_CT_DIR_ORIGINAL;

  	nf_ct_expect_init(exp_orig, NF_CT_EXPECT_CLASS_DEFAULT,

  			  nf_ct_l3num(ct),

  			  &ct->tuplehash[dir].tuple.src.u3,
  			  &ct->tuplehash[dir].tuple.dst.u3,
  			  IPPROTO_GRE, &peer_callid, &callid);

  	exp_orig->expectfn = pptp_expectfn;
  
  	/* reply direction, PAC->PNS */
  	dir = IP_CT_DIR_REPLY;

  	nf_ct_expect_init(exp_reply, NF_CT_EXPECT_CLASS_DEFAULT,

  			  nf_ct_l3num(ct),

  			  &ct->tuplehash[dir].tuple.src.u3,
  			  &ct->tuplehash[dir].tuple.dst.u3,
  			  IPPROTO_GRE, &callid, &peer_callid);

  	exp_reply->expectfn = pptp_expectfn;
  
  	nf_nat_pptp_exp_gre = rcu_dereference(nf_nat_pptp_hook_exp_gre);
  	if (nf_nat_pptp_exp_gre && ct->status & IPS_NAT_MASK)
  		nf_nat_pptp_exp_gre(exp_orig, exp_reply);

  	if (nf_ct_expect_related(exp_orig) != 0)

  		goto out_put_both;

  	if (nf_ct_expect_related(exp_reply) != 0)

  		goto out_unexpect_orig;
  
  	/* Add GRE keymap entries */
  	if (nf_ct_gre_keymap_add(ct, IP_CT_DIR_ORIGINAL, &exp_orig->tuple) != 0)
  		goto out_unexpect_both;
  	if (nf_ct_gre_keymap_add(ct, IP_CT_DIR_REPLY, &exp_reply->tuple) != 0) {
  		nf_ct_gre_keymap_destroy(ct);
  		goto out_unexpect_both;
  	}
  	ret = 0;
  
  out_put_both:

  	nf_ct_expect_put(exp_reply);

  out_put_orig:

  	nf_ct_expect_put(exp_orig);

  out:
  	return ret;
  
  out_unexpect_both:

  	nf_ct_unexpect_related(exp_reply);

  out_unexpect_orig:

  	nf_ct_unexpect_related(exp_orig);

  	goto out_put_both;
  }
  
  static inline int

  pptp_inbound_pkt(struct sk_buff *skb,

  		 struct PptpControlHeader *ctlh,
  		 union pptp_ctrl_union *pptpReq,
  		 unsigned int reqlen,
  		 struct nf_conn *ct,
  		 enum ip_conntrack_info ctinfo)
  {
  	struct nf_ct_pptp_master *info = &nfct_help(ct)->help.ct_pptp_info;
  	u_int16_t msg;
  	__be16 cid = 0, pcid = 0;
  	typeof(nf_nat_pptp_hook_inbound) nf_nat_pptp_inbound;
  
  	msg = ntohs(ctlh->messageType);

  	pr_debug("inbound control message %s
  ", pptp_msg_name[msg]);

  
  	switch (msg) {
  	case PPTP_START_SESSION_REPLY:
  		/* server confirms new control session */
  		if (info->sstate < PPTP_SESSION_REQUESTED)
  			goto invalid;
  		if (pptpReq->srep.resultCode == PPTP_START_OK)
  			info->sstate = PPTP_SESSION_CONFIRMED;
  		else
  			info->sstate = PPTP_SESSION_ERROR;
  		break;
  
  	case PPTP_STOP_SESSION_REPLY:
  		/* server confirms end of control session */
  		if (info->sstate > PPTP_SESSION_STOPREQ)
  			goto invalid;
  		if (pptpReq->strep.resultCode == PPTP_STOP_OK)
  			info->sstate = PPTP_SESSION_NONE;
  		else
  			info->sstate = PPTP_SESSION_ERROR;
  		break;
  
  	case PPTP_OUT_CALL_REPLY:
  		/* server accepted call, we now expect GRE frames */
  		if (info->sstate != PPTP_SESSION_CONFIRMED)
  			goto invalid;
  		if (info->cstate != PPTP_CALL_OUT_REQ &&
  		    info->cstate != PPTP_CALL_OUT_CONF)
  			goto invalid;
  
  		cid = pptpReq->ocack.callID;
  		pcid = pptpReq->ocack.peersCallID;
  		if (info->pns_call_id != pcid)
  			goto invalid;

  		pr_debug("%s, CID=%X, PCID=%X
  ", pptp_msg_name[msg],
  			 ntohs(cid), ntohs(pcid));

  
  		if (pptpReq->ocack.resultCode == PPTP_OUTCALL_CONNECT) {
  			info->cstate = PPTP_CALL_OUT_CONF;
  			info->pac_call_id = cid;
  			exp_gre(ct, cid, pcid);
  		} else
  			info->cstate = PPTP_CALL_NONE;
  		break;
  
  	case PPTP_IN_CALL_REQUEST:
  		/* server tells us about incoming call request */
  		if (info->sstate != PPTP_SESSION_CONFIRMED)
  			goto invalid;
  
  		cid = pptpReq->icreq.callID;

  		pr_debug("%s, CID=%X
  ", pptp_msg_name[msg], ntohs(cid));

  		info->cstate = PPTP_CALL_IN_REQ;
  		info->pac_call_id = cid;
  		break;
  
  	case PPTP_IN_CALL_CONNECT:
  		/* server tells us about incoming call established */
  		if (info->sstate != PPTP_SESSION_CONFIRMED)
  			goto invalid;
  		if (info->cstate != PPTP_CALL_IN_REP &&
  		    info->cstate != PPTP_CALL_IN_CONF)
  			goto invalid;
  
  		pcid = pptpReq->iccon.peersCallID;
  		cid = info->pac_call_id;
  
  		if (info->pns_call_id != pcid)
  			goto invalid;

  		pr_debug("%s, PCID=%X
  ", pptp_msg_name[msg], ntohs(pcid));

  		info->cstate = PPTP_CALL_IN_CONF;
  
  		/* we expect a GRE connection from PAC to PNS */
  		exp_gre(ct, cid, pcid);
  		break;
  
  	case PPTP_CALL_DISCONNECT_NOTIFY:
  		/* server confirms disconnect */
  		cid = pptpReq->disc.callID;

  		pr_debug("%s, CID=%X
  ", pptp_msg_name[msg], ntohs(cid));

  		info->cstate = PPTP_CALL_NONE;
  
  		/* untrack this call id, unexpect GRE packets */
  		pptp_destroy_siblings(ct);
  		break;
  
  	case PPTP_WAN_ERROR_NOTIFY:

  	case PPTP_SET_LINK_INFO:

  	case PPTP_ECHO_REQUEST:
  	case PPTP_ECHO_REPLY:
  		/* I don't have to explain these ;) */
  		break;
  
  	default:
  		goto invalid;
  	}
  
  	nf_nat_pptp_inbound = rcu_dereference(nf_nat_pptp_hook_inbound);
  	if (nf_nat_pptp_inbound && ct->status & IPS_NAT_MASK)

  		return nf_nat_pptp_inbound(skb, ct, ctinfo, ctlh, pptpReq);

  	return NF_ACCEPT;
  
  invalid:

  	pr_debug("invalid %s: type=%d cid=%u pcid=%u "
  		 "cstate=%d sstate=%d pns_cid=%u pac_cid=%u
  ",
  		 msg <= PPTP_MSG_MAX ? pptp_msg_name[msg] : pptp_msg_name[0],
  		 msg, ntohs(cid), ntohs(pcid),  info->cstate, info->sstate,
  		 ntohs(info->pns_call_id), ntohs(info->pac_call_id));

  	return NF_ACCEPT;
  }
  
  static inline int

  pptp_outbound_pkt(struct sk_buff *skb,

  		  struct PptpControlHeader *ctlh,
  		  union pptp_ctrl_union *pptpReq,
  		  unsigned int reqlen,
  		  struct nf_conn *ct,
  		  enum ip_conntrack_info ctinfo)
  {
  	struct nf_ct_pptp_master *info = &nfct_help(ct)->help.ct_pptp_info;
  	u_int16_t msg;
  	__be16 cid = 0, pcid = 0;
  	typeof(nf_nat_pptp_hook_outbound) nf_nat_pptp_outbound;
  
  	msg = ntohs(ctlh->messageType);

  	pr_debug("outbound control message %s
  ", pptp_msg_name[msg]);

  
  	switch (msg) {
  	case PPTP_START_SESSION_REQUEST:
  		/* client requests for new control session */
  		if (info->sstate != PPTP_SESSION_NONE)
  			goto invalid;
  		info->sstate = PPTP_SESSION_REQUESTED;
  		break;
  
  	case PPTP_STOP_SESSION_REQUEST:
  		/* client requests end of control session */
  		info->sstate = PPTP_SESSION_STOPREQ;
  		break;
  
  	case PPTP_OUT_CALL_REQUEST:
  		/* client initiating connection to server */
  		if (info->sstate != PPTP_SESSION_CONFIRMED)
  			goto invalid;
  		info->cstate = PPTP_CALL_OUT_REQ;
  		/* track PNS call id */
  		cid = pptpReq->ocreq.callID;

  		pr_debug("%s, CID=%X
  ", pptp_msg_name[msg], ntohs(cid));

  		info->pns_call_id = cid;
  		break;
  
  	case PPTP_IN_CALL_REPLY:
  		/* client answers incoming call */
  		if (info->cstate != PPTP_CALL_IN_REQ &&
  		    info->cstate != PPTP_CALL_IN_REP)
  			goto invalid;
  
  		cid = pptpReq->icack.callID;
  		pcid = pptpReq->icack.peersCallID;
  		if (info->pac_call_id != pcid)
  			goto invalid;

  		pr_debug("%s, CID=%X PCID=%X
  ", pptp_msg_name[msg],
  			 ntohs(cid), ntohs(pcid));

  
  		if (pptpReq->icack.resultCode == PPTP_INCALL_ACCEPT) {
  			/* part two of the three-way handshake */
  			info->cstate = PPTP_CALL_IN_REP;
  			info->pns_call_id = cid;
  		} else
  			info->cstate = PPTP_CALL_NONE;
  		break;
  
  	case PPTP_CALL_CLEAR_REQUEST:
  		/* client requests hangup of call */
  		if (info->sstate != PPTP_SESSION_CONFIRMED)
  			goto invalid;
  		/* FUTURE: iterate over all calls and check if
  		 * call ID is valid.  We don't do this without newnat,
  		 * because we only know about last call */
  		info->cstate = PPTP_CALL_CLEAR_REQ;
  		break;
  
  	case PPTP_SET_LINK_INFO:
  	case PPTP_ECHO_REQUEST:
  	case PPTP_ECHO_REPLY:
  		/* I don't have to explain these ;) */
  		break;
  
  	default:
  		goto invalid;
  	}
  
  	nf_nat_pptp_outbound = rcu_dereference(nf_nat_pptp_hook_outbound);
  	if (nf_nat_pptp_outbound && ct->status & IPS_NAT_MASK)

  		return nf_nat_pptp_outbound(skb, ct, ctinfo, ctlh, pptpReq);

  	return NF_ACCEPT;
  
  invalid:

  	pr_debug("invalid %s: type=%d cid=%u pcid=%u "
  		 "cstate=%d sstate=%d pns_cid=%u pac_cid=%u
  ",
  		 msg <= PPTP_MSG_MAX ? pptp_msg_name[msg] : pptp_msg_name[0],
  		 msg, ntohs(cid), ntohs(pcid),  info->cstate, info->sstate,
  		 ntohs(info->pns_call_id), ntohs(info->pac_call_id));

  	return NF_ACCEPT;
  }
  
  static const unsigned int pptp_msg_size[] = {
  	[PPTP_START_SESSION_REQUEST]  = sizeof(struct PptpStartSessionRequest),
  	[PPTP_START_SESSION_REPLY]    = sizeof(struct PptpStartSessionReply),
  	[PPTP_STOP_SESSION_REQUEST]   = sizeof(struct PptpStopSessionRequest),
  	[PPTP_STOP_SESSION_REPLY]     = sizeof(struct PptpStopSessionReply),
  	[PPTP_OUT_CALL_REQUEST]       = sizeof(struct PptpOutCallRequest),
  	[PPTP_OUT_CALL_REPLY]	      = sizeof(struct PptpOutCallReply),
  	[PPTP_IN_CALL_REQUEST]	      = sizeof(struct PptpInCallRequest),
  	[PPTP_IN_CALL_REPLY]	      = sizeof(struct PptpInCallReply),
  	[PPTP_IN_CALL_CONNECT]	      = sizeof(struct PptpInCallConnected),
  	[PPTP_CALL_CLEAR_REQUEST]     = sizeof(struct PptpClearCallRequest),
  	[PPTP_CALL_DISCONNECT_NOTIFY] = sizeof(struct PptpCallDisconnectNotify),
  	[PPTP_WAN_ERROR_NOTIFY]	      = sizeof(struct PptpWanErrorNotify),
  	[PPTP_SET_LINK_INFO]	      = sizeof(struct PptpSetLinkInfo),
  };
  
  /* track caller id inside control connection, call expect_related */
  static int

  conntrack_pptp_help(struct sk_buff *skb, unsigned int protoff,

  		    struct nf_conn *ct, enum ip_conntrack_info ctinfo)
  
  {
  	int dir = CTINFO2DIR(ctinfo);

  	const struct nf_ct_pptp_master *info = &nfct_help(ct)->help.ct_pptp_info;
  	const struct tcphdr *tcph;
  	struct tcphdr _tcph;
  	const struct pptp_pkt_hdr *pptph;
  	struct pptp_pkt_hdr _pptph;

  	struct PptpControlHeader _ctlh, *ctlh;
  	union pptp_ctrl_union _pptpReq, *pptpReq;

  	unsigned int tcplen = skb->len - protoff;

  	unsigned int datalen, reqlen, nexthdr_off;
  	int oldsstate, oldcstate;
  	int ret;
  	u_int16_t msg;
  
  	/* don't do any tracking before tcp handshake complete */

  	if (ctinfo != IP_CT_ESTABLISHED && ctinfo != IP_CT_ESTABLISHED_REPLY)

  		return NF_ACCEPT;
  
  	nexthdr_off = protoff;

  	tcph = skb_header_pointer(skb, nexthdr_off, sizeof(_tcph), &_tcph);

  	BUG_ON(!tcph);
  	nexthdr_off += tcph->doff * 4;

  	datalen = tcplen - tcph->doff * 4;

  	pptph = skb_header_pointer(skb, nexthdr_off, sizeof(_pptph), &_pptph);

  	if (!pptph) {

  		pr_debug("no full PPTP header, can't track
  ");

  		return NF_ACCEPT;
  	}
  	nexthdr_off += sizeof(_pptph);
  	datalen -= sizeof(_pptph);
  
  	/* if it's not a control message we can't do anything with it */
  	if (ntohs(pptph->packetType) != PPTP_PACKET_CONTROL ||
  	    ntohl(pptph->magicCookie) != PPTP_MAGIC_COOKIE) {

  		pr_debug("not a control packet
  ");

  		return NF_ACCEPT;
  	}

  	ctlh = skb_header_pointer(skb, nexthdr_off, sizeof(_ctlh), &_ctlh);

  	if (!ctlh)
  		return NF_ACCEPT;
  	nexthdr_off += sizeof(_ctlh);
  	datalen -= sizeof(_ctlh);
  
  	reqlen = datalen;
  	msg = ntohs(ctlh->messageType);
  	if (msg > 0 && msg <= PPTP_MSG_MAX && reqlen < pptp_msg_size[msg])
  		return NF_ACCEPT;
  	if (reqlen > sizeof(*pptpReq))
  		reqlen = sizeof(*pptpReq);

  	pptpReq = skb_header_pointer(skb, nexthdr_off, reqlen, &_pptpReq);

  	if (!pptpReq)
  		return NF_ACCEPT;
  
  	oldsstate = info->sstate;
  	oldcstate = info->cstate;
  
  	spin_lock_bh(&nf_pptp_lock);
  
  	/* FIXME: We just blindly assume that the control connection is always
  	 * established from PNS->PAC.  However, RFC makes no guarantee */
  	if (dir == IP_CT_DIR_ORIGINAL)
  		/* client -> server (PNS -> PAC) */

  		ret = pptp_outbound_pkt(skb, ctlh, pptpReq, reqlen, ct,

  					ctinfo);
  	else
  		/* server -> client (PAC -> PNS) */

  		ret = pptp_inbound_pkt(skb, ctlh, pptpReq, reqlen, ct,

  				       ctinfo);

  	pr_debug("sstate: %d->%d, cstate: %d->%d
  ",
  		 oldsstate, info->sstate, oldcstate, info->cstate);

  	spin_unlock_bh(&nf_pptp_lock);
  
  	return ret;
  }

  static const struct nf_conntrack_expect_policy pptp_exp_policy = {
  	.max_expected	= 2,
  	.timeout	= 5 * 60,
  };

  /* control protocol helper */
  static struct nf_conntrack_helper pptp __read_mostly = {
  	.name			= "pptp",
  	.me			= THIS_MODULE,

  	.tuple.src.l3num	= AF_INET,

  	.tuple.src.u.tcp.port	= cpu_to_be16(PPTP_CONTROL_PORT),

  	.tuple.dst.protonum	= IPPROTO_TCP,

  	.help			= conntrack_pptp_help,
  	.destroy		= pptp_destroy_siblings,

  	.expect_policy		= &pptp_exp_policy,

  };

  static void nf_conntrack_pptp_net_exit(struct net *net)
  {
  	nf_ct_gre_keymap_flush(net);
  }
  
  static struct pernet_operations nf_conntrack_pptp_net_ops = {
  	.exit = nf_conntrack_pptp_net_exit,
  };

  static int __init nf_conntrack_pptp_init(void)
  {

  	int rv;
  
  	rv = nf_conntrack_helper_register(&pptp);
  	if (rv < 0)
  		return rv;
  	rv = register_pernet_subsys(&nf_conntrack_pptp_net_ops);
  	if (rv < 0)
  		nf_conntrack_helper_unregister(&pptp);
  	return rv;

  }
  
  static void __exit nf_conntrack_pptp_fini(void)
  {
  	nf_conntrack_helper_unregister(&pptp);

  	unregister_pernet_subsys(&nf_conntrack_pptp_net_ops);

  }
  
  module_init(nf_conntrack_pptp_init);
  module_exit(nf_conntrack_pptp_fini);