Added true support for PPTP aliasing. Some nice features include:

- Multiple PPTP clients behind NAT to the same or different servers.

- Single PPTP server behind NAT -- you just need to redirect TCP
  port 1723 to a local machine.  Multiple servers behind NAT is
  possible but would require a simple API change.

- No API changes!

For more information on how this works see comments at the start of
the alias_pptp.c.

PacketAliasPptp() is no longer necessary and will be removed soon.

Submitted by:	Erik Salander <erik@whistle.com>
Reviewed by:	ru
Rewritten by:	ru
Reviewed by:	Erik Salander <erik@whistle.com>

7 files changed, 422 insertions, 11 deletions

diff --git a/sys/netinet/libalias/HISTORY b/sys/netinet/libalias/HISTORY
index f26299c..8760d92 100644
--- a/sys/netinet/libalias/HISTORY
+++ b/sys/netinet/libalias/HISTORY
@@ -134,3 +134,8 @@ Version 3.0: January 1, 1999
     - Transparent proxying support added.
     - PPTP redirecting support added based on patches
       contributed by Dru Nelson <dnelson@redwoodsoft.com>.
+
+Version 3.1: May, 2000 (Erik Salander, erik@whistle.com)
+    - Added support to alias 227 replies, allows aliasing for
+      FTP servers in passive mode.
+    - Added support for PPTP aliasing.
diff --git a/sys/netinet/libalias/Makefile b/sys/netinet/libalias/Makefile
index 05411f5..3208014 100644
--- a/sys/netinet/libalias/Makefile
+++ b/sys/netinet/libalias/Makefile
@@ -5,7 +5,7 @@ SHLIB_MAJOR=	3
 SHLIB_MINOR=	0
 CFLAGS+=	-Wall -I${.CURDIR}
 SRCS=		alias.c alias_cuseeme.c alias_db.c alias_ftp.c alias_irc.c \
-		alias_nbt.c alias_proxy.c alias_util.c
+		alias_nbt.c alias_pptp.c alias_proxy.c alias_util.c
 INCS=		alias.h
 MAN3=		libalias.3 
 
diff --git a/sys/netinet/libalias/alias.c b/sys/netinet/libalias/alias.c
index eaaedac..e4770fe 100644
--- a/sys/netinet/libalias/alias.c
+++ b/sys/netinet/libalias/alias.c
@@ -76,6 +76,9 @@
     Version 2.3 Dec 1998 (dillon)
 	- Major bounds checking additions, see FreeBSD/CVS
 
+    Version 3.1 May, 2000 (eds)
+	- Added hooks to handle PPTP.
+
     See HISTORY file for additional revisions.
 
     $FreeBSD$
@@ -90,12 +93,6 @@
 #include <netinet/tcp.h>
 #include <netinet/udp.h>
 
-#ifndef IPPROTO_GRE
-#define IPPROTO_GRE 47
-#define IPPROTO_ESP 50
-#define IPPROTO_AH  51
-#endif
-
 #include "alias_local.h"
 #include "alias.h"
 
@@ -105,6 +102,7 @@
 #define IRC_CONTROL_PORT_NUMBER_1 6667
 #define IRC_CONTROL_PORT_NUMBER_2 6668
 #define CUSEEME_PORT_NUMBER 7648
+#define PPTP_CONTROL_PORT_NUMBER 1723
 
 
 
@@ -181,6 +179,7 @@ TcpMonitorOut(struct ip *pip, struct alias_link *link)
     ProtoAliasIn(), ProtoAliasOut()
     UdpAliasIn(), UdpAliasOut()
     TcpAliasIn(), TcpAliasOut()
+    GreAliasIn()
 
 These routines handle protocol specific details of packet aliasing.
 One may observe a certain amount of repetitive arithmetic in these
@@ -234,6 +233,8 @@ static int UdpAliasIn (struct ip *);
 static int TcpAliasOut(struct ip *, int);
 static int TcpAliasIn (struct ip *);
 
+static int GreAliasIn(struct ip *);
+
 
 static int
 IcmpAliasIn1(struct ip *pip)
@@ -725,6 +726,39 @@ ProtoAliasOut(struct ip *pip)
 }
 
 
+static int
+GreAliasIn(struct ip *pip)
+{
+    u_short call_id;
+    struct alias_link *link;
+
+/* Return if proxy-only mode is enabled. */
+    if (packetAliasMode & PKT_ALIAS_PROXY_ONLY)
+        return (PKT_ALIAS_OK);
+
+    if (PptpGetCallID(pip, &call_id)) {
+	if ((link = FindPptpIn(pip->ip_src, pip->ip_dst, call_id)) != NULL) {
+	    struct in_addr alias_address;
+	    struct in_addr original_address;
+
+	    alias_address = GetAliasAddress(link);
+	    original_address = GetOriginalAddress(link);
+	    PptpSetCallID(pip, GetOriginalPort(link));
+
+	    /* Restore original IP address. */
+	    DifferentialChecksum(&pip->ip_sum,
+				 (u_short *)&original_address,
+				 (u_short *)&pip->ip_dst,
+				 2);
+	    pip->ip_dst = original_address;
+
+	    return (PKT_ALIAS_OK);
+	} else
+	    return (PKT_ALIAS_IGNORED);
+    } else
+	return ProtoAliasIn(pip);
+}
+
 
 static int
 UdpAliasIn(struct ip *pip)
@@ -903,6 +937,11 @@ TcpAliasIn(struct ip *pip)
         int accumulate;
         u_short *sptr;
 
+/* Special processing for IP encoding protocols */
+        if (ntohs(tc->th_dport) == PPTP_CONTROL_PORT_NUMBER
+         || ntohs(tc->th_sport) == PPTP_CONTROL_PORT_NUMBER)
+            AliasHandlePptpIn(pip, link);
+
         alias_address = GetAliasAddress(link);
         original_address = GetOriginalAddress(link);
         proxy_address = GetProxyAddress(link);
@@ -1070,9 +1109,12 @@ TcpAliasOut(struct ip *pip, int maxpacketsize)
         if (ntohs(tc->th_dport) == FTP_CONTROL_PORT_NUMBER
          || ntohs(tc->th_sport) == FTP_CONTROL_PORT_NUMBER)
             AliasHandleFtpOut(pip, link, maxpacketsize);
-        if (ntohs(tc->th_dport) == IRC_CONTROL_PORT_NUMBER_1
+        else if (ntohs(tc->th_dport) == IRC_CONTROL_PORT_NUMBER_1
          || ntohs(tc->th_dport) == IRC_CONTROL_PORT_NUMBER_2)
             AliasHandleIrcOut(pip, link, maxpacketsize);
+        else if (ntohs(tc->th_dport) == PPTP_CONTROL_PORT_NUMBER
+         || ntohs(tc->th_sport) == PPTP_CONTROL_PORT_NUMBER)
+            AliasHandlePptpOut(pip, link);
 
 /* Adjust TCP checksum since source port is being aliased */
 /* and source address is being altered                    */
@@ -1301,6 +1343,9 @@ PacketAliasIn(char *ptr, int maxpacketsize)
             case IPPROTO_TCP:
                 iresult = TcpAliasIn(pip);
                 break;
+            case IPPROTO_GRE:
+                iresult = GreAliasIn(pip);
+                break;
 	    default:
 		iresult = ProtoAliasIn(pip);
                 break;
diff --git a/sys/netinet/libalias/alias_db.c b/sys/netinet/libalias/alias_db.c
index 211a7c2..408fcdd 100644
--- a/sys/netinet/libalias/alias_db.c
+++ b/sys/netinet/libalias/alias_db.c
@@ -147,6 +147,7 @@
 /* Timeouts (in seconds) for different link types */
 #define ICMP_EXPIRE_TIME             60
 #define UDP_EXPIRE_TIME              60
+#define PPTP_EXPIRE_TIME             60
 #define PROTO_EXPIRE_TIME            60
 #define FRAGMENT_ID_EXPIRE_TIME      10
 #define FRAGMENT_PTR_EXPIRE_TIME     30
@@ -265,6 +266,7 @@ struct alias_link                /* Main data structure */
 #define LINK_FRAGMENT_ID              (IPPROTO_MAX + 1)
 #define LINK_FRAGMENT_PTR             (IPPROTO_MAX + 2)
 #define LINK_ADDR                     (IPPROTO_MAX + 3)
+#define LINK_PPTP                     (IPPROTO_MAX + 4)
 
     int flags;                   /* indicates special characteristics   */
 
@@ -329,6 +331,7 @@ linkTableIn[LINK_TABLE_IN_SIZE];     /*   into input and output lookup  */
 static int icmpLinkCount;            /* Link statistics                 */
 static int udpLinkCount;
 static int tcpLinkCount;
+static int pptpLinkCount;
 static int protoLinkCount;
 static int fragmentIdLinkCount;
 static int fragmentPtrLinkCount;
@@ -455,10 +458,11 @@ ShowAliasStats(void)
 
    if (monitorFile)
    {
-      fprintf(monitorFile, "icmp=%d, udp=%d, tcp=%d, proto=%d, frag_id=%d frag_ptr=%d",
+      fprintf(monitorFile, "icmp=%d, udp=%d, tcp=%d, pptp=%d, proto=%d, frag_id=%d frag_ptr=%d",
               icmpLinkCount,
               udpLinkCount,
               tcpLinkCount,
+              pptpLinkCount,
               protoLinkCount,
               fragmentIdLinkCount,
               fragmentPtrLinkCount);
@@ -466,6 +470,7 @@ ShowAliasStats(void)
       fprintf(monitorFile, " / tot=%d  (sock=%d)\n",
               icmpLinkCount + udpLinkCount
                             + tcpLinkCount
+                            + pptpLinkCount
                             + protoLinkCount
                             + fragmentIdLinkCount
                             + fragmentPtrLinkCount,
@@ -550,7 +555,7 @@ GetNewPort(struct alias_link *link, int alias_port_param)
    the port number.  GetNewPort() will return this number
    without check that it is in use.
 
-   When this parameter is -1, it indicates to get a randomly
+   When this parameter is GET_ALIAS_PORT, it indicates to get a randomly
    selected port number.
 */
  
@@ -618,7 +623,9 @@ GetNewPort(struct alias_link *link, int alias_port_param)
         if (go_ahead)
         {
             if ((packetAliasMode & PKT_ALIAS_USE_SOCKETS)
-             && (link->flags & LINK_PARTIALLY_SPECIFIED))
+             && (link->flags & LINK_PARTIALLY_SPECIFIED)
+	     && ((link->link_type == LINK_TCP) || 
+		 (link->link_type == LINK_UDP)))
             {
                 if (GetSocket(port_net, &link->sockfd, link->link_type))
                 {
@@ -838,6 +845,9 @@ DeleteLink(struct alias_link *link)
             if (link->data.tcp != NULL)
                 free(link->data.tcp);
             break;
+        case LINK_PPTP:
+            pptpLinkCount--;
+            break;
         case LINK_FRAGMENT_ID:
             fragmentIdLinkCount--;
             break;
@@ -906,6 +916,9 @@ AddLink(struct in_addr  src_addr,
         case LINK_TCP:
             link->expire_time = TCP_EXPIRE_INITIAL;
             break;
+        case LINK_PPTP:
+            link->expire_time = PPTP_EXPIRE_TIME;
+            break;
         case LINK_FRAGMENT_ID:
             link->expire_time = FRAGMENT_ID_EXPIRE_TIME;
             break;
@@ -994,6 +1007,9 @@ AddLink(struct in_addr  src_addr,
 #endif
                 }
                 break;
+            case LINK_PPTP:
+                pptpLinkCount++;
+                break;
             case LINK_FRAGMENT_ID:
                 fragmentIdLinkCount++;
                 break;
@@ -1307,6 +1323,7 @@ FindLinkIn(struct in_addr dst_addr,
     AddFragmentPtrLink(), FindFragmentPtr()
     FindProtoIn(), FindProtoOut()
     FindUdpTcpIn(), FindUdpTcpOut()
+    FindPptpIn(), FindPptpOut()
     FindOriginalAddress(), FindAliasAddress()
 
 (prototypes in alias_local.h)
@@ -1533,6 +1550,56 @@ FindUdpTcpOut(struct in_addr  src_addr,
 }
 
 
+struct alias_link *
+FindPptpIn(struct in_addr dst_addr,
+          struct in_addr alias_addr,
+          u_short call_id)
+{
+    struct alias_link *link;
+
+    link = FindLinkIn(dst_addr, alias_addr,
+                      NO_DEST_PORT, call_id,
+                      LINK_PPTP, 1);
+
+    if (link == NULL && !(packetAliasMode & PKT_ALIAS_DENY_INCOMING))
+    {
+        struct in_addr target_addr;
+
+        target_addr = FindOriginalAddress(alias_addr);
+        link = AddLink(target_addr, dst_addr, alias_addr,
+                       call_id, NO_DEST_PORT, call_id,
+                       LINK_PPTP);
+    }
+
+    return(link);
+}
+
+
+struct alias_link * 
+FindPptpOut(struct in_addr  src_addr,
+            struct in_addr  dst_addr,
+            u_short         call_id)
+{
+    struct alias_link *link;
+
+    link = FindLinkOut(src_addr, dst_addr,
+                       call_id, NO_DEST_PORT,
+                       LINK_PPTP, 1);
+
+    if (link == NULL)
+    {
+        struct in_addr alias_addr;
+
+        alias_addr = FindAliasAddress(src_addr);
+        link = AddLink(src_addr, dst_addr, alias_addr,
+                       call_id, NO_DEST_PORT, GET_ALIAS_PORT,
+                       LINK_PPTP);
+    }
+
+    return(link);
+}
+
+
 struct in_addr
 FindOriginalAddress(struct in_addr alias_addr)
 {
@@ -2308,6 +2375,7 @@ PacketAliasInit(void)
     icmpLinkCount = 0;
     udpLinkCount = 0;
     tcpLinkCount = 0;
+    pptpLinkCount = 0;
     protoLinkCount = 0;
     fragmentIdLinkCount = 0;
     fragmentPtrLinkCount = 0;
diff --git a/sys/netinet/libalias/alias_local.h b/sys/netinet/libalias/alias_local.h
index 783f6b2..439c9ea 100644
--- a/sys/netinet/libalias/alias_local.h
+++ b/sys/netinet/libalias/alias_local.h
@@ -107,6 +107,12 @@ FindUdpTcpIn (struct in_addr, struct in_addr, u_short, u_short, u_char);
 struct alias_link *
 FindUdpTcpOut(struct in_addr, struct in_addr, u_short, u_short, u_char);
 
+struct alias_link *
+FindPptpIn(struct in_addr, struct in_addr, u_short);
+
+struct alias_link *
+FindPptpOut(struct in_addr, struct in_addr, u_short);
+
 struct in_addr
 FindOriginalAddress(struct in_addr);
 
@@ -159,6 +165,12 @@ void AliasHandleFtpOut(struct ip *, struct alias_link *, int);
 /* IRC routines */
 void AliasHandleIrcOut(struct ip *, struct alias_link *, int);
 
+/* PPTP routines */
+int  PptpGetCallID(struct ip *, u_short *);
+void PptpSetCallID(struct ip *, u_short);
+void AliasHandlePptpOut(struct ip *, struct alias_link *);
+void AliasHandlePptpIn(struct ip *, struct alias_link *);
+
 /* NetBIOS routines */
 int AliasHandleUdpNbt(struct ip *, struct alias_link *, struct in_addr *, u_short);
 int AliasHandleUdpNbtNS(struct ip *, struct alias_link *, struct in_addr *, u_short *, struct in_addr *, u_short *);
diff --git a/sys/netinet/libalias/alias_pptp.c b/sys/netinet/libalias/alias_pptp.c
new file mode 100644
index 0000000..822cfa6
--- /dev/null
+++ b/sys/netinet/libalias/alias_pptp.c
@@ -0,0 +1,279 @@
+/*
+ * alias_pptp.c
+ *
+ * Copyright (c) 2000 Whistle Communications, Inc.
+ * All rights reserved.
+ *
+ * Subject to the following obligations and disclaimer of warranty, use and
+ * redistribution of this software, in source or object code forms, with or
+ * without modifications are expressly permitted by Whistle Communications;
+ * provided, however, that:
+ * 1. Any and all reproductions of the source or object code must include the
+ *    copyright notice above and the following disclaimer of warranties; and
+ * 2. No rights are granted, in any manner or form, to use Whistle
+ *    Communications, Inc. trademarks, including the mark "WHISTLE
+ *    COMMUNICATIONS" on advertising, endorsements, or otherwise except as
+ *    such appears in the above copyright notice or in the software.
+ *
+ * THIS SOFTWARE IS BEING PROVIDED BY WHISTLE COMMUNICATIONS "AS IS", AND
+ * TO THE MAXIMUM EXTENT PERMITTED BY LAW, WHISTLE COMMUNICATIONS MAKES NO
+ * REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED, REGARDING THIS SOFTWARE,
+ * INCLUDING WITHOUT LIMITATION, ANY AND ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, OR NON-INFRINGEMENT.
+ * WHISTLE COMMUNICATIONS DOES NOT WARRANT, GUARANTEE, OR MAKE ANY
+ * REPRESENTATIONS REGARDING THE USE OF, OR THE RESULTS OF THE USE OF THIS
+ * SOFTWARE IN TERMS OF ITS CORRECTNESS, ACCURACY, RELIABILITY OR OTHERWISE.
+ * IN NO EVENT SHALL WHISTLE COMMUNICATIONS BE LIABLE FOR ANY DAMAGES
+ * RESULTING FROM OR ARISING OUT OF ANY USE OF THIS SOFTWARE, INCLUDING
+ * WITHOUT LIMITATION, ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
+ * PUNITIVE, OR CONSEQUENTIAL DAMAGES, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES, LOSS OF USE, DATA OR PROFITS, HOWEVER CAUSED AND UNDER ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF WHISTLE COMMUNICATIONS IS ADVISED OF THE POSSIBILITY
+ * OF SUCH DAMAGE.
+ *
+ * Author: Erik Salander <erik@whistle.com>
+ *
+ * $FreeBSD$
+ */
+
+/*
+   Alias_pptp.c performs special processing for PPTP sessions under TCP.
+   Specifically, watch PPTP control messages and alias the Call ID or the
+   Peer's Call ID in the appropriate messages.  Note, PPTP requires
+   "de-aliasing" of incoming packets, this is different than any other
+   TCP applications that are currently (ie. FTP, IRC and RTSP) aliased.
+
+   For Call IDs encountered for the first time, a GRE alias link is created.
+   The GRE alias link uses the Call ID in place of the original port number.
+   An alias Call ID is created.
+
+   For this routine to work, the PPTP control messages must fit entirely
+   into a single TCP packet.  This is typically the case, but is not
+   required by the spec.
+
+   Unlike some of the other TCP applications that are aliased (ie. FTP,
+   IRC and RTSP), the PPTP control messages that need to be aliased are
+   guaranteed to remain the same length.  The aliased Call ID is a fixed
+   length field.
+
+   Reference: RFC 2637
+
+   Initial version:  May, 2000 (eds)
+
+*/
+
+/* Includes */
+#include <ctype.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/types.h>
+#include <netinet/in_systm.h>
+#include <netinet/in.h>
+#include <netinet/ip.h>
+#include <netinet/tcp.h>
+
+#include "alias_local.h"
+
+/*
+ * PPTP definitions
+ */
+
+struct grehdr			/* Enhanced GRE header. */
+{
+    u_char    gh_recursion:3,	/* Recursion control. */
+              gh_ssr_flag:1,	/* Strict source route present. */
+              gh_seq_no_flag:1,	/* Sequence number present. */
+              gh_key_flag:1,	/* Key present. */
+              gh_rt_flag:1,	/* Routing present. */
+              gh_cksum_flag:1;	/* Checksum present. */
+    u_char    gh_version:3,	/* GRE version. */
+              gh_flags:4,	/* Flags. */
+              gh_ack_no_flag:1;	/* Acknowledgment sequence number present. */
+    u_short   gh_protocol;	/* Protocol type. */
+    u_short   gh_length;	/* Payload length. */
+    u_short   gh_call_id;	/* Call ID. */
+    u_int32_t gh_seq_no;	/* Sequence number (optional). */
+    u_int32_t gh_ack_no;	/* Acknowledgment number (optional). */
+};
+
+/* The PPTP protocol ID used in the GRE 'proto' field. */
+#define PPTP_GRE_PROTO          0x880b
+
+/* Bits that must be set a certain way in all PPTP/GRE packets. */
+#define PPTP_INIT_VALUE		((0x2001 << 16) | PPTP_GRE_PROTO)
+#define PPTP_INIT_MASK		0xef7fffff
+
+#define PPTP_MAGIC		0x1a2b3c4d
+#define PPTP_CTRL_MSG_TYPE	1
+
+enum {
+  PPTP_StartCtrlConnRequest = 1,
+  PPTP_StartCtrlConnReply = 2,
+  PPTP_StopCtrlConnRequest = 3,
+  PPTP_StopCtrlConnReply = 4,
+  PPTP_EchoRequest = 5,
+  PPTP_statoReply = 6,
+  PPTP_OutCallRequest = 7,
+  PPTP_OutCallReply = 8,
+  PPTP_InCallRequest = 9,
+  PPTP_InCallReply = 10,
+  PPTP_InCallConn = 11,
+  PPTP_CallClearRequest = 12,
+  PPTP_CallDiscNotify = 13,
+  PPTP_WanErrorNotify = 14,
+  PPTP_SetLinkInfo = 15,
+};
+
+  /* Message structures */
+  struct pptpMsgHead {
+    u_int16_t   length;         /* total length */
+    u_int16_t   msgType;        /* PPTP message type */
+    u_int32_t   magic;          /* magic cookie */
+    u_int16_t   type;           /* control message type */
+    u_int16_t   resv0;          /* reserved */
+  };
+  typedef struct pptpMsgHead    *PptpMsgHead;
+
+  struct pptpCallIds {
+    u_int16_t   cid1;           /* Call ID field #1 */
+    u_int16_t   cid2;           /* Call ID field #2 */
+  };
+  typedef struct pptpCallIds    *PptpCallId;
+
+static PptpCallId AliasVerifyPptp(struct ip *, u_int16_t *);
+
+int
+PptpGetCallID(struct ip *pip,
+              u_short *call_id)
+{
+    struct grehdr *gr;
+
+    gr = (struct grehdr *)((char *)pip + (pip->ip_hl << 2));
+
+    /* Check GRE header bits. */
+    if ((ntohl(*((u_int32_t *)gr)) & PPTP_INIT_MASK) == PPTP_INIT_VALUE) {
+	*call_id = gr->gh_call_id;
+	return 1;
+    } else
+	return 0;
+};
+
+void PptpSetCallID(struct ip *pip, u_short call_id)
+{
+    struct grehdr *gr;
+
+    gr = (struct grehdr *)((char *)pip + (pip->ip_hl << 2));
+    gr->gh_call_id = call_id;
+};
+
+void
+AliasHandlePptpOut(struct ip *pip,	    /* IP packet to examine/patch */
+                   struct alias_link *link) /* The PPTP control link */
+{
+    struct alias_link   *gre_link;
+    PptpCallId    	cptr;
+    u_int16_t           ctl_type;           /* control message type */
+    struct tcphdr 	*tc;
+
+    /* Verify valid PPTP control message */
+    if ((cptr = AliasVerifyPptp(pip, &ctl_type)) == NULL)
+      return;
+
+    /* Modify certain PPTP messages */
+    if ((ctl_type >= PPTP_OutCallRequest) &&
+       (ctl_type <= PPTP_CallDiscNotify)) {
+
+      /* Establish GRE link for address and Call ID found in PPTP Control Msg */
+      gre_link = FindPptpOut(GetOriginalAddress(link), GetDestAddress(link),
+                             cptr->cid1);
+
+      if (gre_link != NULL) {
+	/* alias the Call Id */
+	cptr->cid1 = GetAliasPort(gre_link);
+
+	/* Compute TCP checksum for revised packet */
+	tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
+	tc->th_sum = 0;
+	tc->th_sum = TcpChecksum(pip);
+      }
+    }
+}
+
+void
+AliasHandlePptpIn(struct ip *pip,	   /* IP packet to examine/patch */
+                  struct alias_link *link) /* The PPTP control link */
+{
+    struct alias_link   *gre_link;
+    PptpCallId    	cptr;
+    u_int16_t     	*pcall_id;
+    u_int16_t           ctl_type;           /* control message type */
+    struct tcphdr 	*tc;
+
+    /* Verify valid PPTP control message */
+    if ((cptr = AliasVerifyPptp(pip, &ctl_type)) == NULL)
+      return;
+
+    /* Modify certain PPTP messages */
+    switch (ctl_type)
+    {
+    case PPTP_InCallConn:
+    case PPTP_WanErrorNotify:
+    case PPTP_SetLinkInfo:
+      pcall_id = &cptr->cid1;
+      break;
+    case PPTP_OutCallReply:
+    case PPTP_InCallReply:
+      pcall_id = &cptr->cid2;
+      break;
+    default:
+      return;
+      break;
+    }
+
+    /* Find GRE link for address and Call ID found in PPTP Control Msg */
+    gre_link = FindPptpIn(GetDestAddress(link), GetAliasAddress(link),
+	                  *pcall_id);
+
+    if (gre_link != NULL) {
+      /* alias the Call Id */
+      *pcall_id = GetOriginalPort(gre_link);
+
+      /* Compute TCP checksum for modified packet */
+      tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
+      tc->th_sum = 0;
+      tc->th_sum = TcpChecksum(pip);
+    }
+}
+
+PptpCallId
+AliasVerifyPptp(struct ip *pip, u_int16_t *ptype) /* IP packet to examine/patch */
+{
+    int           	hlen, tlen, dlen;
+    PptpMsgHead   	hptr;
+    struct tcphdr 	*tc;
+
+    /* Calculate some lengths */
+    tc = (struct tcphdr *) ((char *) pip + (pip->ip_hl << 2));
+    hlen = (pip->ip_hl + tc->th_off) << 2;
+    tlen = ntohs(pip->ip_len);
+    dlen = tlen - hlen;
+
+    /* Verify data length */
+    if (dlen < (sizeof(struct pptpMsgHead) + sizeof(struct pptpCallIds)))
+      return(NULL);
+
+    /* Move up to PPTP message header */
+    hptr = (PptpMsgHead)(((char *) pip) + hlen);
+
+    /* Return the control message type */
+    *ptype = ntohs(hptr->type);
+
+    /* Verify PPTP Control Message */
+    if ((ntohs(hptr->msgType) != PPTP_CTRL_MSG_TYPE) ||
+        (ntohl(hptr->magic) != PPTP_MAGIC))
+      return(NULL);
+    else
+      return((PptpCallId)(((char *)hptr) + sizeof(struct pptpMsgHead)));
+}
diff --git a/sys/netinet/libalias/libalias.3 b/sys/netinet/libalias/libalias.3
index 3b69983..53c0d6b 100644
--- a/sys/netinet/libalias/libalias.3
+++ b/sys/netinet/libalias/libalias.3
@@ -835,6 +835,8 @@ versions 1.0 - 1.8, 2.0 - 2.4.
 versions 1.8b, 1.9 and 2.5.
 Added IRC DCC support as well as contributing a number of architectural
 improvements; added the firewall bypass for FTP/IRC DCC.
+.An Erik Salander Aq erik@whistle.com
+added support for PPTP.
 .Sh ACKNOWLEDGMENTS
 Listed below, in approximate chronological order, are individuals who
 have provided valuable comments and/or debugging assistance.