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>

1 files changed, 279 insertions, 0 deletions

diff --git a/lib/libalias/alias_pptp.c b/lib/libalias/alias_pptp.c
new file mode 100644
index 0000000..822cfa6
--- /dev/null
+++ b/lib/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)));
+}