  Linux VPN Masquerade HOWTO
  John D. Hardin <jhardin@wolfenet.com>
  v2.2, 9 September 1999

  This document describes how to configure a Linux firewall to masquer­
  ade IPsec- and PPTP-based Virtual Private Network traffic, allowing
  you to establish a VPN connection without losing the security and
  flexibility of your Linux firewall's internet connection and allowing
  you to make available a VPN server that does not have a registered
  internet IP address.  Information on configuring the VPN client and
  server is also given.
  ______________________________________________________________________

  Table of Contents

  1. Introduction

     1.1 Introduction
     1.2 Feedback, Credits & Resources
     1.3 Copyright & Disclaimer

  2. Background Knowledge

     2.1 What is a VPN?
     2.2 What is IPsec?
     2.3 What is PPTP?
     2.4 Why masquerade a VPN client?
     2.5 Can several clients on my local network use IPsec simultaneously?
     2.6 Can several clients on my local network use PPTP simultaneously?
     2.7 Can I access the remote network from my entire local network?
     2.8 Why masquerade the VPN server?
     2.9 Why patch the Linux kernel?
     2.10 Current Status

  3. Configuring the Linux firewall

     3.1 Example network
     3.2 Determining what needs to be done on the firewall
     3.3 Patching and configuring the 2.0.x kernel for VPN Masquerade support
     3.4 Patching and configuring the 2.2.x kernel for VPN Masquerade support
     3.5 ipfwadm setup for a Private-IP VPN Client or Server
     3.6 ipchains setup for a Private-IP VPN Client or Server
     3.7 A note about dynamic IP addressing
     3.8 Additional setup for a Private-IP VPN Server
     3.9 ipfwadm setup for a Registered-IP VPN Server
     3.10 ipfwadm setup for a Registered-IP VPN Client

  4. Configuring the VPN client

     4.1 Configuring a MS W'95 or W'98 client
     4.2 Configuring a MS NT client

  5. Troubleshooting

     5.1 Testing
     5.2 Possible problems
     5.3 Troubleshooting
     5.4 MS PPTP Clients and domain-name issues
     5.5 MS PPTP Clients and Novell IPX
     5.6 MS network password issues
     5.7 If your IPsec session always dies after a certain amount of time

  6. IPsec masquerade technical notes and special security considerations

     6.1 Limitations and weaknesses of IPsec masquerade
     6.2 Proper routing of inbound encrypted traffic

  ______________________________________________________________________

  1.  Introduction

  1.1.  Introduction

  This document describes how to configure masquerading of IPsec and
  PPTP VPN traffic. SSH-based VPNs (such as that sold by F-Secure and
  outlined in the VPN mini-HOWTO) are based on standard TCP traffic and
  do not need any special kernel modifications.

  VPN Masquerade allows you to establish one or more IPsec and/or PPTP
  sessions to internet-accessible VPN servers via your Linux internet
  firewall without forcing you to connect to your ISP directly from the
  VPN client system - thus retaining all of the benefits of your Linux
  internet firewall. It also allows you to set up a VPN server with a
  Private Network IP address (as described in RFC1918) behind a
  masquerading Linux firewall, permitting you to provide relatively
  secure access to a private network via only one registered IP address
  - even if that IP address represents a dynamic dial-up link.

  It is strongly recommended that you understand, configure and test
  regular IP Masquerading before you attempt to set up VPN masquerading.
  Please see the IP Masquerade HOWTO and the IP Masquerade Resource page
  at <http://ipmasq.cjb.net/> before proceeding.

  The patch for the 2.0.x-series kernels works well on Linux kernel
  version 2.0.36, has been incorporated into the 2.0.37 release, may
  work on versions earlier than 2.0.36, and should work on Linux kernels
  up to about version 2.1.102. The IP masquerade code in the kernel was
  restructured at about version 2.1.103, requiring a different patch for
  the 2.1.105+ and 2.2.x series of kernels. A patch is available for
  kernels from 2.2.5 to 2.2.12, and it may work on earlier kernels.

  1.2.  Feedback, Credits & Resources

  Please feel free to send any feedback or comments regarding this
  document to me at <jhardin@wolfenet.com>.

  I personally have experience with masquerading a MS NT-Server-based
  PPTP client, configuring a registered-IP PPTP server, and using PPTP
  for network-to-network routing.  The information on masquerading a
  Private-IP PPTP server is from discussions with Len Bayles
  <len@isdi.com>, Simon Cocking <simon@ibs.com.au> and C. Scott Ananian
  <cananian@lcs.mit.edu>.

  The current version of this document can be found at
  <ftp://ftp.rubyriver.com/pub/jhardin/masquerade/VPN-Masquerade.sgml> -
  it and other formats (such as HTML) can be found via the Linux
  Documentation Project's HOWTO repository and in the /usr/doc/HOWTO/
  directory on your nearest Linux system.

  The home page for the Linux VPN Masquerade kernel patch is
  <http://www.wolfenet.com/~jhardin/ip_masq_vpn.html>

  The home page for the PPTP-only Masquerade kernel patch for the
  2.1.105+ and 2.2.x kernel series is
  <http://bmrc.berkeley.edu/people/chaffee/linux_pptp.html>.

  The home page for the ipportfw port-forwarding kernel patch and
  configuration tool is
  <http://www.ox.compsoc.org.uk/~steve/portforwarding.html>.

  The home page for the ipfwd generic IP redirector is
  <http://www.pdos.lcs.mit.edu/~cananian/Projects/IPfwd/>.

  Profuse thanks to Gordon Chaffee <chaffee@cs.berkeley.edu> for coding
  and sharing a patch to traceroute that allows tracing GRE traffic. It
  should prove invaluable in troubleshooting if your GRE traffic is
  being blocked somewhere. The patch is available at
  <http://www.wolfenet.com/~jhardin/pptp-traceroute.patch.gz>

  More thanks to Steve Chinatti <chinatti@alumni.Princeton.EDU> for
  contributing his original IPsec masquerade hack, from which I
  shamelessly stole some very important ideas...
  More information on setting up firewall rules to run automatically -
  including how to automatically use the correct IP address in a
  dynamic-IP environment - can be found at
  <http://www.wolfenet.com/~jhardin/ipfwadm/invocation.html>

  The home page for Linux FreeS/WAN (IPsec for Linux) is
  <http://www.xs4all.nl/~freeswan>

  The home page for the Linux PPTP project is
  <http://www.pdos.lcs.mit.edu/~cananian/Projects/PPTP> and a patch to
  add PPTP server capability is available at
  <http://debs.fuller.edu/cgi-bin/display?list=pptp&msg=222>

  A second Linux PPTP server called PoPToP is available at
  <http://www.moretonbay.com/vpn/pptp.html>

  Paul Cadach <paul@odt.east.telecom.kz> has made patches that add MS-
  CHAP-v2, MPPE and Multilink support to Linux pppd. See
  <ftp://ftp.east.telecom.kz/pub/src/networking/ppp/ppp-2.3.5-my.tgz>
  for MS-CHAP and MPPE, and
  <ftp://ftp.east.telecom.kz/pub/src/networking/ppp/multilink/ppp-2.3.5-mp.tgz>
  for Multilink.

  1.3.  Copyright & Disclaimer

  This document is copyright © 1999 by John D. Hardin.  Permission is
  granted to redistribute it under the terms of the GNU General Public
  License.

  The information presented in this document is correct to the best of
  my knowledge. IP Masquerading is experimental, and it is possible that
  I have made a mistake in writing or testing the kernel patch or
  composing the instructions in this document; you should determine for
  yourself if you want to make the changes outlined in this document.

       THE AUTHOR IS NOT RESPONSIBLE FOR ANY DAMAGES INCURRED DUE
       TO ACTIONS TAKEN BASED ON THE INFORMATION IN THIS DOCUMENT.
       BACK UP ANY AND ALL CRITICAL INFORMATION BEFORE IMPLEMENTING
       THE CHANGES OUTLINED IN THIS DOCUMENT. MAKE SURE YOU HAVE A
       WORKING, BOOTABLE KERNEL AVAILABLE BEFORE PATCHING AND
       RECOMPILING YOUR KERNEL AS OUTLINED IN THIS DOCUMENT.

  In other words, take sensible precautions.

  2.  Background Knowledge

  2.1.  What is a VPN?

  A Virtual Private Network, or "VPN", is a tunnel that carries private
  network traffic from one endpoint system to another over a public
  network (such as the Internet) without the traffic being aware that
  there are intermediate hops between the endpoints, or the intermediate
  hops being aware they are carrying the network packets that are
  traversing the tunnel.  The tunnel may optionally compress and/or
  encrypt the data, providing enhanced performance and some measure of
  security.

  The "Virtual" part stems from the fact that you are constructing a
  private link over a public network, rather than actually buying a
  direct hardwired link over leased lines. The VPN allows you to pretend
  you are using a leased line to communicate between the endpoints.

  You may find the VPN FAQ at
  <http://kubarb.phsx.ukans.edu/~tbird/FAQ.html> informative.

  2.2.  What is IPsec?

  IPsec is a set of standard protocols for implementing secure
  communications and encryption key exchange between computers. It can
  be used to implement a VPN.

  An IPsec VPN generally consists of two communications channels between
  the the endpoints: a key-exchange channel over which authentication
  and encryption key information is passed, and one or more data
  channels over which private network traffic is carried.

  The key-exchange channel is a standard UDP connection to and from port
  500. The data channels carrying the traffic between the client and
  server use IP protocol number 50 (ESP).

  More information is available in F-Secure's IPsec FAQ at
  <http://www.data-fellows.com/f-secure/support/vpn-
  plus/faq/techfaq.htm>, and in RFC2402 (the AH protocol), RFC2406 (the
  ESP protocol), and RFC2408 (the ISAKMP key-exchange protocol).

  Important note: If your VPN is based on the AH protocol, it cannot be
  masqueraded. The AH protocol specifies a cryptographic checksum across
  portions of the IP header, including the IP addresses. IP Masquerade
  is implemented by modifying the source IP address for outbound packets
  and the destination IP address for inbound packets. Since the
  masquerading gateway cannot participate in the encryption key
  exchange, it cannot generate the correct cryptographic checksums for
  the modified IP headers. Thus the modified IP packets will be
  discarded by the recipient as invalid, because they fail the
  cryptographic checksum test.

  2.3.  What is PPTP?

  PPTP stands for Point-to-Point Tunnelling Protocol. It is a Microsoft-
  proposed protocol for implementing a VPN.

  The PPTP VPN protocol consists of two communications channels between
  the client and server: a control channel over which link-management
  information is passed, and a data channel over which (possibly
  encrypted) private network traffic is carried.

  The control channel is a standard TCP connection to port 1723 on the
  server. The data channel carrying the private network traffic uses IP
  protocol number 47 (GRE), a generic encapsulation protocol described
  in RFC1701. The transparent transmission of data over the data channel
  is achieved by negotiating a standard PPP connection over it, just as
  if it were a dialup connection directly from the client to the server.
  The options negotiated over the tunnel by PPP control whether the data
  is compressed and/or encrypted, thus PPTP itself has nothing to do
  with encryption.

  The details of the PPTP protocol are documented at
  <http://www.ietf.org/internet-drafts/draft-ietf-pppext-pptp-08.txt>.

  Microsoft's implementation of the PPTP protocol is not considered very
  secure. If you're interested in the details, here are three seperate
  analyses:

  <http://www.counterpane.com/pptp.html>

  <http://www.geek-girl.com/bugtraq/1999_1/0664.html>

  <http://oliver.efri.hr/~crv/security/bugs/NT/pptp2.html>

  2.4.  Why masquerade a VPN client?

  Most current VPN clients assume you will be connecting the client
  computer directly to the internet. Doing this when you have only a
  single connection for internet access bypasses your Linux firewall and
  the security and access-sharing capabilities that it provides.
  Extending the Linux firewall to also masquerade VPN traffic allows you
  to retain the firewalling security provided by the Linux firewall as
  well as permitting the other systems on your local network to access
  the internet regardless of whether or not the VPN network connection
  is active.

  If your firewall is being used in a corporate setting you may also
  wish to require your VPN client users to go through that firewall for
  security reasons, rather than providing them with modems so they can
  dial out on their own when they need to use VPN. VPN Masquerade allows
  you to do so even if the desktops do not have registered IP addresses.

  2.5.  Can several clients on my local network use IPsec simultane­
  ously?

  Yes, though there may occasionally be minor problems.

  The IPsec protocols define a method for identifying the traffic
  streams called the Security Parameters Index ("SPI"). Unfortunately
  the SPI used by outbound traffic is different from the SPI used by
  inbound traffic, and there is no other identifying information
  available that is not encrypted, so association of the inbound and
  outbound data streams is difficult and not perfectly reliable.

  IPsec Masquerade attempts to associate inbound and outbound ESP
  traffic by serializing new connections. While this has worked well in
  testing, it cannot be guaranteed to be perfectly reliable, and the
  serialization of new traffic may result in some timeouts if the link
  is saturated or if many local IPsec hosts attempt to initiate
  communications or rekey with the same remote IPsec host
  simultaneously.

  It is also assumed that should this association scheme fail to
  associate the traffic streams correctly, the IPsec hosts themselves
  will discard the incorrectly routed traffic because it will have the
  wrong SPI values. This is required by the IPsec RFC.

  These problems could be eliminated if there was some way to sniff the
  new SPI values from the ISAKMP key exchange before any ESP traffic
  appears, but unfortunately that portion of the key exchange is
  encrypted.

  To minimize the problems associated with this, it is recommended that
  you open a command window on your masqueraded IPsec host and run the
  "ping" program pinging a host on the remote network for as long as you
  have the tunnel up.
  See the IPsec technical notes at the end of the document for more
  details.

  2.6.  Can several clients on my local network use PPTP simultaneously?

  Yes.

  You must enable PPTP Call ID masquerade when configuring your kernel
  in order to distinguish between multiple data streams from the same
  server.  PPTP masq with Call ID masq enabled will support many
  concurrent masqueraded sessions with no restrictions on which server a
  client can call.

  For another alternative, see the next question...

  2.7.  Can I access the remote network from my entire local network?

  Yes. However, your VPN client must be able to forward IP traffic.

  This means that you'll either have to use a Linux VPN client or a MS
  NT VPN client. The IP stack in W'95 and W'98 does not support IP
  forwarding. NT Workstation will work for this, and is less expensive
  than NT Server if you're only using it to route encrypted traffic.

  If you cannot install a Linux or NT-based VPN client, then you'll have
  to enable PPTP Call-ID masquerade if you are using PPTP, and install
  VPN client software on every system you want to provide access for.
  This is inefficient, aesthetically revolting and a security weakness,
  but it's cheaper than licensing NT.

  Network-to-network routing this way works very well. This is how I
  have my home network set up for telecommuting. It does require a
  little more networking knowhow than simply giving everybody their own
  VPN client.

  In my experience, network-to-network routing in a pure-MS environment
  requires RRAS be installed at both ends of the tunnel.

  2.8.  Why masquerade the VPN server?

  If your VPN server has a registered IP address you do not need to
  masquerade it, simply configure your firewall to route the VPN traffic
  properly as described below.

  If your VPN server has a Private-Network IP address you will need to
  redirect the inbound traffic to it and masquerade the outbound traffic
  from it. Masquerading allows you to make a VPN server available to the
  internet even if you only have one assigned IP address. This should
  work even if your IP address is dynamically assigned: you would
  publicize the IP address for clients through the use of a third-party
  dynamic DNS service such as that provided by DDNS.ORG and configure
  the clients to connect to a system named our-company.ddns.org or
  something similar. Note that this is a security risk, because it is
  possible for an incorrect IP address to be retrieved from the dynamic
  DNS server through timing problems, a failure to properly register the
  current dynamic IP address, or a third party registering a different
  IP address under the system name.

  2.9.  Why patch the Linux kernel?

  The largest problem in masquerading VPN traffic is that the stock
  2.0.x Linux kernel has no special awareness of IP protocols other than
  TCP, UDP and ICMP.

  All IP traffic may be forwarded and filtered by IP address, but
  filtering on or masquerading IP protocols other than TCP, UDP and ICMP
  requires modifying the kernel.

  The PPTP control channel is plain TCP and requires no special setup
  beyond letting it through the firewall and masquerading it.

  Masquerading the IPsec and PPTP data channels requires a modification
  that adds support for the ESP and GRE protocols to the masquerading
  code, and masquerading the ISAKMP key exchange protocol requires a
  modification that prevents masquerade from altering the UDP source
  port number and adds tracking of the ISAKMP cookie values.

  2.10.  Current Status

  The 2.0.x kernel patch works on kernel 2.0.36 and is incorporated into
  the standard 2.0.37 kernel release. It may work on earlier kernels but
  I have not tested it, and I recommend you upgrade to the latest kernel
  anyway for security reasons if you are running a kernel that old.

  The 2.2.x kernel patch works on kernels from 2.2.5 to 2.2.12 and may
  work on earlier kernels, but that has not been tested. It will be
  submitted for inclusion in the standard 2.2.13 release if all goes
  well.

  The 2.0.x kernel patch has been tested and works on x86 and Sparc
  systems, and the 2.2.x kernel patch has been tested and works on x86
  systems, but there should be no major problems in porting to other
  architectures. I believe the architecture dependencies are only in
  endian-ness within the bitmaps in the GRE header definition used to
  format debugging log messages.  If anyone ports this to a non-Intel
  architecture I'd appreciate hearing about it so I can merge any
  changes into the master copy.

  A PPTP-only kernel patch for the 2.1.105+ and 2.2.x kernel series is
  available at
  <http://bmrc.berkeley.edu/people/chaffee/linux_pptp.html>.

  See the VPN Masquerade home page at
  <http://www.wolfenet.com/~jhardin/ip_masq_vpn.html> for the status of
  the VPN Masq patches, and
  <http://bmrc.berkeley.edu/people/chaffee/linux_pptp.html> for the
  status of the 2.1.105+/2.2.x PPTP-only Masq patch.

  3.  Configuring the Linux firewall

  3.1.  Example network

  For the Private-IP configuration examples in this document we will use
  this sample network:

  Internet-------- 200.200.200.*   ppp0 or  200.200.200.200 eth1
                                   Dual-Homed Linux Firewall
              .--- 10.0.0.1        eth0
              |
              |--- 10.0.0.2        VPN client or server
              |

  For the registered-IP configuration examples in this document we will
  use this sample network:

       Internet-------- 200.200.200.200 eth1
                                        Dual-Homed Linux Firewall
                   .--- 222.0.0.1       eth0
                   |
                   |--- 222.0.0.2       VPN client or server
                   |

  The VPN server that the example clients connect to will be 199.0.0.1

  The VPN clients that the connect to the example server will be
  199.0.0.2 and 199.0.0.3

  3.2.  Determining what needs to be done on the firewall

  If your VPN client or server has a registered internet IP address you
  do not need to masquerade or modify your kernel - the stock kernel
  will successfully route all VPN traffic. You can skip directly to the
  registered-IP setup sections below.

  If your VPN client or server has a Private-Network IP address as
  described in RFC1918 you will need to patch your kernel (unless your
  kernel is 2.0.37 or higher in the 2.0.x series).

  If you are setting up a masqueraded VPN server, you will also have to
  obtain and install the following two packages:

  ·  To redirect the inbound TCP/UDP traffic (the 1723/tcp PPTP control
     channel or the 500/udp ISAKMP channel), you need the appropriate
     ipportfw port-forwarding kernel patch and configuration tool from
     <http://www.ox.compsoc.org.uk/~steve/portforwarding.html>.  Port
     forwarding has been incorporated into the 2.2.x kernel. See man
     ipmasqadm for configuration details.

     If ipmasqadm is not included with your distribution it can be
     obtained at  <http://juanjox.linuxhq.com/>

  ·  To redirect the initial inbound tunnel traffic (GRE for PPTP and
     ESP for IPsec), you need the ipfwd generic-IP proxy from
     <http://www.pdos.lcs.mit.edu/~cananian/Projects/IPfwd/>.

  You do not need port forwarding or ipfwd if you are only masquerading
  clients.

  3.3.  Patching and configuring the 2.0.x kernel for VPN Masquerade
  support

  1. Install the kernel source (preferably version 2.0.37), which you
     can obtain from  <http://www.kernel.org/> or a mirror. The sources
     should be automatically extracted into a directory named
     /usr/src/linux.

  2. Configure and test standard IP Masquerading (see the IP Masquerade
     HOWTO). Doing this will familiarize you with recompiling your
     kernel and introduce you to IP Masquerading in general.

  3. Back up your kernel sources.

  4. Obtain the kernel patch if necessary.

     If your kernel version is 2.0.36 or lower, obtain the 2.0.x VPN
     Masquerade kernel patch from the VPN Masquerade home page in the
     "Resources" section above.

     If your kernel version is 2.0.37 or higher in the 2.0.x series, you
     do not need to apply any patches. The VPN Masquerade code is
     included in the kernel. Skip the discussion of patching the kernel.

     For the purposes of this document we'll assume you've saved the
     appropriate patch in /usr/src/ip_masq_vpn.patch.gz.

  5. Apply the VPN Masquerade patch to your kernel if necessary:

  ·  Change to the kernel source directory:

       cd /usr/src/linux

  ·  Apply the patch:

       gzcat ../ip_masq_vpn.patch.gz | patch -l -p0 > vpn-patch.log

       Note that the options are "dash lowercase L, dash lowercase
       P zero".

       Also, you may get odd results if you change the order of the
       arguments. Patch seems to be sensitive to the order they
       appear on the command line.

  ·  Check the vpn-patch.log file to see if any hunks failed.  If you
     get failed hunks, then you probably either omitted the options or
     ran the patch program from the wrong directory. Restore your kernel
     from the backup and try again.

  6. If you are masquerading a VPN server, obtain and install the
     ipportfw patch from the site given above.
     There is a known conflict between the VPN Masquerade patch and two
     other networking patches: the IP Firewall Chains patch and the
     ipportfw patch.  They are all trying to add options at the same
     location in net/ipv4/Config.in, and the changes made by one patch
     alter the context that the other patches are looking for.

     If you're applying the VPN Masquerade patch and the IP Firewall
     Chains or ipportfw patches to your 2.0.x kernel, you will have to
     manually edit net/ipv4/Config.in and add the block of configuration
     options from the patch file that fails to work. Looking at the
     patch file should show you where in net/ipv4/Config.in the new
     options should be added.

     The syntax of patch files is simple. For each block of changes to
     make, there are two sections: the first shows the "before" state,
     with an indication of lines to be changed or deleted; the second
     shows the "after" state, with an indication of the lines that have
     been changed or added. Use the first section to find where to add
     the lines, and add the lines that are indicated in the second
     section.

     This should not be a problem once those patches are updated for
     2.0.37+

  7. Configure your kernel and select the following options - say YES to
     the following:

    * Prompt for development and/or incomplete code/drivers
      CONFIG_EXPERIMENTAL
      - You must enable this to see the VPN Masq options.

    * Networking support
      CONFIG_NET

    * Network firewalls
      CONFIG_FIREWALL

    * TCP/IP networking
      CONFIG_INET

    * IP: forwarding/gatewaying
      CONFIG_IP_FORWARD

    * IP: firewalling
      CONFIG_IP_FIREWALL

    * IP: masquerading (EXPERIMENTAL)
      CONFIG_IP_MASQUERADE
      - This is required.

    * IP: PPTP masq support (EXPERIMENTAL)
      CONFIG_IP_MASQUERADE_PPTP
      - Enables PPTP data channel masquerading, if you are
        masquerading a PPTP client or server.

    * IP: PPTP Call ID masq support (EXPERIMENTAL)
      CONFIG_IP_MASQUERADE_PPTP_MULTICLIENT
      - Enables PPTP Call ID masquerading; only necessary if
        you will be masquerading more than one client trying
        to connect to the same remote server. DO NOT enable
        this option if you will be masquerading a PPTP server.

    * IP: IPsec ESP & ISAKMP masq support (EXPERIMENTAL)
      CONFIG_IP_MASQUERADE_IPSEC
      - Enables IPsec masquerade, if you are masquerading an
        IPsec host.

    * IP: IPSEC masq table lifetime (minutes)
      - See your network administrator to determine what the
        "rekey interval" or "key lifetime" is set to. The
        default lifetime of masq table entries is thirty minutes.
        If your rekey interval is greater than thirty minutes,
        then you should increase the lifetime to a value slightly
        greater than the rekey interval.

    * IP: always defragment
      CONFIG_IP_ALWAYS_DEFRAG
      - Highly recommended for a firewall.

  NOTE: These are just the settings you need for masquerading.  Select
  whatever other options you need for your specific setup.

  8. Recompile the kernel and install it for testing. Don't replace a
     known working kernel with your new kernel until you have proven it
     works.

  To determine whether the running kernel includes VPN Masquerade
  support, run the following command:

       grep masq /proc/ksyms

  Look for the following entries:

  ·  IPsec masquerade: ip_masq_out_get_isakmp, ip_masq_in_get_isakmp,
     ip_fw_masq_esp and ip_fw_demasq_esp

  ·  PPTP masquerade: ip_fw_masq_gre and ip_fw_demasq_gre

  ·  PPTP Call-ID masquerade: ip_masq_pptp

  If you don't see these entries, VPN Masquerade support is probably not
  available. If you get complaints about /proc/ksyms not being available
  or /proc not being available, make sure that you have enabled the
  /proc filesystem in your kernel configuration.

  See the Kernel HOWTO for more details on configuring and recompiling
  your kernel.

  3.4.  Patching and configuring the 2.2.x kernel for VPN Masquerade
  support

  1. Install the kernel source (preferably version 2.2.10 or later),
     which you can obtain from  <http://www.kernel.org/> or a mirror.
     The sources should be automatically extracted into a directory
     named /usr/src/linux.

  2. Configure and test standard IP Masquerading (see the IP Masquerade
     HOWTO). Doing this will familiarize you with recompiling your
     kernel and introduce you to IP Masquerading in general.

  3. Back up your kernel sources.

  4. Obtain the kernel patch from the VPN Masquerade home page in the
     "Resources" section above.

     For the purposes of this document we'll assume you've saved the
     appropriate patch in /usr/src/ip_masq_vpn.patch.gz.

  5. Apply the VPN Masquerade patch to your kernel if necessary:

  ·  Change to the source directory:

       cd /usr/src

  ·  Apply the patch:

  gzcat ip_masq_vpn.patch.gz | patch -l -p0 > vpn-patch.log

       Note that the options are "dash lowercase L, dash lowercase
       P zero".

       Also, you may get odd results if you change the order of the
       arguments. Patch seems to be sensitive to the order they
       appear on the command line.

       Also note that the directory you run the patch command in is
       different for the 2.2.x kernel patch

  ·  Check the vpn-patch.log file to see if any hunks failed.  If you
     get failed hunks, then you probably either omitted the options or
     ran the patch program from the wrong directory. Restore your kernel
     from the backup and try again.

  6. If you are masquerading a VPN server you do not need the ipportfw
     patch as port forwarding is now built-in. See the ipmasqadm man
     page for more details.

  7. Configure your kernel and select the following options - say YES to
     the following:

    * Prompt for development and/or incomplete code/drivers
      CONFIG_EXPERIMENTAL
      - You must enable this to see the VPN Masq options.

    * Networking support
      CONFIG_NET

    * Network firewalls
      CONFIG_FIREWALL

    * TCP/IP networking
      CONFIG_INET

    * IP: firewalling
      CONFIG_IP_FIREWALL

    * IP: always defragment
      CONFIG_IP_ALWAYS_DEFRAG
      - Required for masquerading

    * IP: masquerading (EXPERIMENTAL)
      CONFIG_IP_MASQUERADE
      - This is required.

    * IP: masquerading special modules support
      CONFIG_IP_MASQUERADE_MOD
      - This is required.

    * IP: ipportfw masq support (EXPERIMENTAL)
      CONFIG_IP_MASQUERADE_IPPORTFW
      - Enable this if you will be masquerading a VPN server.

    * IP: PPTP masq support
      CONFIG_IP_MASQUERADE_PPTP
      - Enables PPTP data channel masquerading, if you are
        masquerading a PPTP client or server. This is now
        available as a module.
        Note that you no longer need to specify Call-ID masquerade.

    * IP: IPsec ESP & ISAKMP masq support (EXPERIMENTAL)
      CONFIG_IP_MASQUERADE_IPSEC
      - Enables IPsec masquerade, if you are masquerading an
        IPsec host. This is now available as a module.

    * IP: IPsec masq table lifetime (minutes)
      - See your network administrator to determine what the
        "rekey interval" or "key lifetime" is set to. The default
        lifetime of masq table entries is thirty minutes.
        If your rekey interval is greater than thirty minutes,
        then you should increase the lifetime to a value slightly
        greater than the rekey interval.

    * IP: Enable parallel sessions (possible security risk - see help)
      CONFIG_IP_MASQUERADE_IPSEC_PAROK
      - See the IPsec masquerade technical notes and
        special security considerations section of the HOWTO for
        security considerations to be aware of when
        masquerading IPsec traffic. If you are only masquerading
        one IPsec client this setting has no effect.

  NOTE: These are just the settings you need for masquerading.  Select
  whatever other options you need for your specific setup.
  8. Recompile the kernel and install it for testing. Don't replace a
     known working kernel with your new kernel until you have proven it
     works.

  To determine whether the running kernel includes VPN Masquerade
  support, run the following command:

       grep masq /proc/ksyms

  Look for the following entries:

  ·  IPsec masquerade: ip_fw_masq_esp and ip_fw_demasq_esp

  ·  PPTP masquerade: ip_fw_masq_gre and ip_fw_demasq_gre

  If you don't see these entries, VPN Masquerade support is probably not
  available (did you remember to modprobe ip_masq_pptp or modprobe
  ip_masq_ipsec if you compiled them as modules?). If you get complaints
  about /proc/ksyms not being available or /proc not being available,
  make sure that you have enabled the /proc filesystem in your kernel
  configuration.

  See the Kernel HOWTO for more details on configuring and recompiling
  your kernel.

  3.5.  ipfwadm setup for a Private-IP VPN Client or Server

  The firewall must now be configured to masquerade the outbound VPN
  traffic.  You may wish to visit
  <http://www.wolfenet.com/~jhardin/ipfwadm.html> to take a look at a
  GUI wrapper around the ipfwadm command that automates a lot of
  security-related packet filtering setup.

  The minimum firewall rules are:

       # Set the default forwarding policy to DENY:
       ipfwadm -F -p deny
       # Allow local-network traffic
       ipfwadm -I -a accept    -S 10.0.0.0/8 -D 0.0.0.0/0  -W eth0
       ipfwadm -O -a accept    -S 0.0.0.0/0  -D 10.0.0.0/8 -W eth0
       # Masquerade traffic for internet addresses and allow internet traffic
       ipfwadm -F -a accept -m -S 10.0.0.0/8 -D 0.0.0.0/0  -W ppp0
       ipfwadm -O -a accept    -S 0.0.0.0/0  -D 0.0.0.0/0  -W ppp0
       ipfwadm -I -a accept    -S 0.0.0.0/0  -D 0.0.0.0/0  -W ppp0

       or, if you have a permanent connection,

       ipfwadm -F -a accept -m -S 10.0.0.0/8 -D 0.0.0.0/0  -W eth1
       ipfwadm -O -a accept    -S 0.0.0.0/0  -D 0.0.0.0/0  -W eth1
       ipfwadm -I -a accept    -S 0.0.0.0/0  -D 0.0.0.0/0  -W eth1

  This is a completely open setup, though. It will masquerade any
  traffic from any host on the local network destined for any host on
  the internet, and provides no security at all.

  A tight firewall setup would only allow traffic between the client and
  the server, and would block everything else:

       # Set the default policy to DENY:
       ipfwadm -I -p deny
       ipfwadm -O -p deny
       ipfwadm -F -p deny
       # Allow local-network traffic
       ipfwadm -I -a accept -S 10.0.0.0/8 -D 0.0.0.0/0  -W eth0
       ipfwadm -O -a accept -S 0.0.0.0/0  -D 10.0.0.0/8 -W eth0
       # Masquerade only VPN traffic between the VPN client and the VPN server
       ipfwadm -F -a accept -m -P udp -S 10.0.0.2/32 500 -D 199.0.0.1/32 500  -W ppp0
       ipfwadm -F -a accept -m -P tcp -S 10.0.0.2/32     -D 199.0.0.1/32 1723 -W ppp0
       ipfwadm -F -a deny      -P tcp -S 10.0.0.2/32     -D 199.0.0.1/32      -W ppp0
       ipfwadm -F -a deny      -P udp -S 10.0.0.2/32     -D 199.0.0.1/32      -W ppp0
       ipfwadm -F -a accept -m -P all -S 10.0.0.2/32     -D 199.0.0.1/32      -W ppp0
       ipfwadm -O -a accept    -P udp -S 200.200.200.0/24 500 -D 199.0.0.1/32 500  -W ppp0
       ipfwadm -O -a accept    -P tcp -S 200.200.200.0/24     -D 199.0.0.1/32 1723 -W ppp0
       ipfwadm -O -a deny      -P tcp -S 200.200.200.0/24     -D 199.0.0.1/32      -W ppp0
       ipfwadm -O -a deny      -P udp -S 200.200.200.0/24     -D 199.0.0.1/32      -W ppp0
       ipfwadm -O -a accept    -P all -S 200.200.200.0/24     -D 199.0.0.1/32      -W ppp0
       ipfwadm -I -a accept    -P udp -S 199.0.0.1/32 500     -D 200.200.200.0/24 500 -W ppp0
       ipfwadm -I -a accept    -P tcp -S 199.0.0.1/32 1723    -D 200.200.200.0/24     -W ppp0
       ipfwadm -I -a deny      -P tcp -S 199.0.0.1/32         -D 200.200.200.0/24     -W ppp0
       ipfwadm -I -a deny      -P udp -S 199.0.0.1/32         -D 200.200.200.0/24     -W ppp0
       ipfwadm -I -a accept    -P all -S 199.0.0.1/32         -D 200.200.200.0/24     -W ppp0

       or, if you have a permanent connection,

       ipfwadm -F -a accept -m -P udp -S 10.0.0.2/32 500 -D 199.0.0.1/32 500  -W eth1
       ipfwadm -F -a accept -m -P tcp -S 10.0.0.2/32     -D 199.0.0.1/32 1723 -W eth1
       ipfwadm -F -a deny      -P tcp -S 10.0.0.2/32     -D 199.0.0.1/32      -W eth1
       ipfwadm -F -a deny      -P udp -S 10.0.0.2/32     -D 199.0.0.1/32      -W eth1
       ipfwadm -F -a accept -m -P all -S 10.0.0.2/32     -D 199.0.0.1/32      -W eth1
       ipfwadm -O -a accept    -P udp -S 200.200.200.200/32 500 -D 199.0.0.1/32 500  -W eth1
       ipfwadm -O -a accept    -P tcp -S 200.200.200.200/32     -D 199.0.0.1/32 1723 -W eth1
       ipfwadm -O -a deny      -P tcp -S 200.200.200.200/32     -D 199.0.0.1/32      -W eth1
       ipfwadm -O -a deny      -P udp -S 200.200.200.200/32     -D 199.0.0.1/32      -W eth1
       ipfwadm -O -a accept    -P all -S 200.200.200.200/32     -D 199.0.0.1/32      -W eth1
       ipfwadm -I -a accept    -P udp -S 199.0.0.1/32 500  -D 200.200.200.200/32 500 -W eth1
       ipfwadm -I -a accept    -P tcp -S 199.0.0.1/32 1723 -D 200.200.200.200/32     -W eth1
       ipfwadm -I -a deny      -P tcp -S 199.0.0.1/32      -D 200.200.200.200/32     -W eth1
       ipfwadm -I -a deny      -P udp -S 199.0.0.1/32      -D 200.200.200.200/32     -W eth1
       ipfwadm -I -a accept    -P all -S 199.0.0.1/32      -D 200.200.200.200/32     -W eth1

  Note: these rules only allow VPN traffic and block everything else.
  You will have to add rules for any other traffic you wish to permit,
  such as DNS, HTTP, POP, IMAP, etc.

  3.6.  ipchains setup for a Private-IP VPN Client or Server

  The minimum ipchains firewall rules are:

  # Set the default forwarding policy to DENY:
  ipchains -P forward DENY
  # Allow local-network traffic
  ipchains -A input   -j ACCEPT -s 10.0.0.0/8 -d 0.0.0.0/0  -i eth0
  ipchains -A output  -j ACCEPT -s 0.0.0.0/0  -d 10.0.0.0/8 -i eth0
  # Masquerade traffic for internet addresses and allow internet traffic
  ipchains -A forward -j MASQ   -s 10.0.0.0/8 -d 0.0.0.0/0  -i ppp0
  ipchains -A output  -j ACCEPT -s 0.0.0.0/0  -d 0.0.0.0/0  -i ppp0
  ipchains -A input   -j ACCEPT -s 0.0.0.0/0  -d 0.0.0.0/0  -i ppp0

  or, if you have a permanent connection,

  ipchains -A forward -j MASQ   -s 10.0.0.0/8 -d 0.0.0.0/0  -i eth1
  ipchains -A output  -j ACCEPT -s 0.0.0.0/0  -d 0.0.0.0/0  -i eth1
  ipchains -A input   -j ACCEPT -s 0.0.0.0/0  -d 0.0.0.0/0  -i eth1

  This is a completely open setup, though. It will masquerade any traf­
  fic from any host on the local network destined for any host on the
  internet, and provides no security at all.

  A tight firewall setup would only allow traffic between the client and
  the server, and would block everything else:

       # Set the default policy to DENY:
       ipchains -P input   DENY
       ipchains -P output  DENY
       ipchains -P forward DENY
       # Allow local-network traffic
       ipchains -A input  -j ACCEPT -s 10.0.0.0/8 -d 0.0.0.0/0  -i eth0
       ipchains -A output -j ACCEPT -s 0.0.0.0/0  -d 10.0.0.0/8 -i eth0
       # Masquerade only VPN traffic between the VPN client and the VPN server
       # IPsec
       ipchains -A forward -j MASQ   -p udp -s 10.0.0.2/32 500      -d 199.0.0.1/32 500     -i ppp0
       ipchains -A output  -j ACCEPT -p udp -s 200.200.200.0/24 500 -d 199.0.0.1/32 500     -i ppp0
       ipchains -A input   -j ACCEPT -p udp -s 199.0.0.1/32 500     -d 200.200.200.0/24 500 -i ppp0
       ipchains -A forward -j MASQ   -p 50  -s 10.0.0.2/32          -d 199.0.0.1/32         -i ppp0
       ipchains -A output  -j ACCEPT -p 50  -s 200.200.200.0/24     -d 199.0.0.1/32         -i ppp0
       ipchains -A input   -j ACCEPT -p 50  -s 199.0.0.1/32         -d 200.200.200.0/24     -i ppp0
       # PPTP
       ipchains -A forward -j MASQ   -p tcp -s 10.0.0.2/32       -d 199.0.0.1/32 1723 -i ppp0
       ipchains -A output  -j ACCEPT -p tcp -s 200.200.200.0/24  -d 199.0.0.1/32 1723 -i ppp0
       ipchains -A input   -j ACCEPT -p tcp -s 199.0.0.1/32 1723 -d 200.200.200.0/24  -i ppp0
       ipchains -A forward -j MASQ   -p 47  -s 10.0.0.2/32       -d 199.0.0.1/32      -i ppp0
       ipchains -A output  -j ACCEPT -p 47  -s 200.200.200.0/24  -d 199.0.0.1/32      -i ppp0
       ipchains -A input   -j ACCEPT -p 47  -s 199.0.0.1/32      -d 200.200.200.0/24  -i ppp0

       or, if you have a permanent connection,

  # IPsec
  ipchains -A forward -j MASQ   -p udp -s 10.0.0.2/32 500        -d 199.0.0.1/32 500       -i eth1
  ipchains -A output  -j ACCEPT -p udp -s 200.200.200.200/32 500 -d 199.0.0.1/32 500       -i eth1
  ipchains -A input   -j ACCEPT -p udp -s 199.0.0.1/32 500       -d 200.200.200.200/32 500 -i eth1
  ipchains -A forward -j MASQ   -p 50  -s 10.0.0.2/32            -d 199.0.0.1/32           -i eth1
  ipchains -A output  -j ACCEPT -p 50  -s 200.200.200.200/32     -d 199.0.0.1/32           -i eth1
  ipchains -A input   -j ACCEPT -p 50  -s 199.0.0.1/32           -d 200.200.200.200/32     -i eth1
  # PPTP
  ipchains -A forward -j MASQ   -p tcp -s 10.0.0.2/32        -d 199.0.0.1/32 1723  -i eth1
  ipchains -A output  -j ACCEPT -p tcp -s 200.200.200.200/32 -d 199.0.0.1/32 1723  -i eth1
  ipchains -A input   -j ACCEPT -p tcp -s 199.0.0.1/32 1723  -d 200.200.200.200/32 -i eth1
  ipchains -A forward -j MASQ   -p 47  -s 10.0.0.2/32        -d 199.0.0.1/32       -i eth1
  ipchains -A output  -j ACCEPT -p 47  -s 200.200.200.200/32 -d 199.0.0.1/32       -i eth1
  ipchains -A input   -j ACCEPT -p 47  -s 199.0.0.1/32       -d 200.200.200.200/32 -i eth1

  Note: these rules only allow VPN traffic. You will have to add rules
  for any other traffic you wish to permit, such as DNS, HTTP, POP,
  IMAP, etc.

  Also note how there rules are much neater and easier to make sense of
  than the equivalent ipfwadm rules. This is because ipchains allows
  specification of all IP protocols, not just TCP, UDP, ICMP or ALL.

  3.7.  A note about dynamic IP addressing

  If your firewall is assigned a dynamic IP address by your ISP (dialup
  accounts are this way, as are some cable internet services), then you
  should add the following to the startup script /etc/rc.d/rc.local:

       echo 7 > /proc/sys/net/ipv4/ip_dynaddr

  This enables dynamic IP address following, which means that should
  your connection drop and be reestablished, any active sessions will be
  updated to the new IP address rather than using the old IP address.
  This does not mean that the session will continue across the interrup­
  tion, rather that it will be closed down quickly.

  If you do not do this, then there may be a "dead period" after you
  redial and before old masq table entries expire where you're being
  masqueraded with the wrong IP address, which will prevent your
  establishing a connection.

  This is particularly helpful if you are using a demand-dial daemon
  such as diald to manage your dialup connection.

  See /usr/src/linux/Documentation/networking/ip_dynaddr.txt for more
  details.

  3.8.  Additional setup for a Private-IP VPN Server

  If you are setting up VPN masquerade for a Private-IP VPN server (that
  is, you wish to provide for inbound connections as well as outbound
  connections), you also need to install two packet-forwarding
  utilities. One (ipportfw) forwards inbound TCP or UDP traffic
  addressed to a specific port on the firewall system to a system on the
  local network behind the firewall. This is used to redirect the
  initial inbound 1723/tcp PPTP control channel or 500/udp ISAKMP
  traffic to the VPN server. The other (ipfwd) is a more generic
  forwarding utility that allows you to do this for any IP protocol. It
  is used to forward the initial inbound 47/ip (GRE) or 50/ip (ESP) data
  channel traffic to the VPN server.

  Outbound responses to the inbound 1723/tcp or 500/udp traffic are
  masqueraded using the normal IP-Masquerade facilities in the Linux
  kernel.  The outbound 47/ip or 50/ip traffic is masqueraded using the
  VPN-Masquerade kernel patch you installed earlier.

  Once these utilities are installed, you must configure them to forward
  the traffic to the VPN server.

  ·  Configuring ipportfw

     The following commands will set up ipportfw to forward the initial
     inbound 500/udp traffic to the IPsec server:

       # Static-IP ipportfw setup for IPsec
       # Clear the ipportfw forwarding table
       /sbin/ipportfw -C
       # Forward traffic addressed to the firewall's 500/udp port
       # to the IPsec server's 500/udp port
       /sbin/ipportfw -A -u 200.200.200.200/500 -R 10.0.0.2/500

  The following commands will set up ipportfw to forward the initial
  inbound 1723/tcp traffic to the PPTP server:

       # Static-IP ipportfw setup for PPTP
       # Clear the ipportfw forwarding table
       /sbin/ipportfw -C
       # Forward traffic addressed to the firewall's 1723/tcp port
       # to the PPTP server's 1723/tcp port
       /sbin/ipportfw -A -t 200.200.200.200/1723 -R 10.0.0.2/1723

  Note that the ipportfw command line requires the internet IP address
  of the firewall, and you cannot specify the interface (e.g. ppp0) as
  you can with ipfwadm. This means that for a dynamic-IP connection
  (such as a typical dialup PPP connection) you have to run these com­
  mands every time you connect to the internet and are assigned a new IP
  address. You can do this quite easily - simply add the following to
  your /etc/ppp/ip-up or /etc/ppp/ip-up.local script:

       # Dynamic-IP ipportfw setup for IPsec
       # Clear the ipportfw forwarding table
       /sbin/ipportfw -C
       # Forward traffic addressed to the firewall's 500/udp port
       # to the IPsec server's 500/udp port
       /sbin/ipportfw -A -u ${4}/500 -R 10.0.0.2/500

  or:

       # Dynamic-IP ipportfw setup for PPTP
       # Clear the ipportfw forwarding table
       /sbin/ipportfw -C
       # Forward traffic addressed to the firewall's 1723/tcp port
       # to the PPTP server's 1723/tcp port
       /sbin/ipportfw -A -t ${4}/1723 -R 10.0.0.2/1723

  See  <http://www.wolfenet.com/~jhardin/ipfwadm/invocation.html> for
  more information on firewalling with a dynamic IP.

  ·  Configuring ipfwd

     The following command will set up ipfwd to forward the initial
     inbound 50/ip traffic to the IPsec server:

       /sbin/ipfwd --masq 10.0.0.2 50 &

  The following command will set up ipfwd to forward the initial inbound
  47/ip traffic to the PPTP server:

       /sbin/ipfwd --masq 10.0.0.2 47 &

  It should only be run once, from your /etc/rc.d/rc.local script.

  The techniques described here can be generalized to allow masquerading
  of most any type of server - HTTP, FTP, SMTP, and so forth. Servers
  that are purely TCP- or UDP-based will not require ipfwd.

  If you are masquerading a PPTP server you also need to make sure that
  you have not enabled PPTP Call ID masquerade in the kernel. Enabling
  PPTP Call ID masquerade builds in some assumptions that you're
  masquerading only PPTP clients, so enabling it will prevent proper
  masquerade of the PPTP server traffic. This also means that with this
  version of the patch you cannot simultaneously masquerade a PPTP
  server and PPTP clients. Contact me if this limitation causes you
  problems.

  3.9.  ipfwadm setup for a Registered-IP VPN Server

  Setting up a registered-IP VPN server behind a Linux firewall is a
  simple matter of making sure the appropriate routing and packet-filter
  commands are in place. Masquerading is not required.

  Unfortunately the 2.0.x-series kernels will not let us specify IP
  protocol 47 or 50 directly, so this firewall is less secure than it
  could be. If this is a problem for you, then install the IP Firewall
  Chains kernel patch or move to the 2.1.x or 2.2.x series kernel, where
  you can filter by IP protocol.

  The firewall rules will look something like this:

       # This section should follow your other firewall rules.

       # Specify the acceptable clients explicitly for tighter security.
       # Allow the IPsec ISAKMP traffic in and out.
       ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P udp -S 199.0.0.2/32 500 -D 222.0.0.2/32 500
       ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P udp -D 199.0.0.2/32 500 -S 222.0.0.2/32 500
       ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P udp -S 199.0.0.3/32 500 -D 222.0.0.2/32 500
       ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P udp -D 199.0.0.3/32 500 -S 222.0.0.2/32 500
       # Allow the PPTP control channel in and out.
       ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P tcp -S 199.0.0.2/32 -D 222.0.0.2/32 1723
       ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P tcp -D 199.0.0.2/32 -S 222.0.0.2/32 1723
       ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P tcp -S 199.0.0.3/32 -D 222.0.0.2/32 1723
       ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P tcp -D 199.0.0.3/32 -S 222.0.0.2/32 1723

       # Block all other TCP and UDP traffic from the internet.
       # This is essentially a "default deny TCP/UDP" that
       # only applies to the internet interface.
       ipfwadm -I -a deny -W eth1 -V 200.200.200.200 -P tcp
       ipfwadm -I -a deny -W eth1 -V 200.200.200.200 -P udp

       # Specify the acceptable clients explicitly for tighter security.
       # Note that this is too open since we're forced to
       # specify "-P all" rather than "-P 47" or "-P 50"...
       # Allow the PPTP data channel and IPsec ESP traffic in and out.
       ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P all -S 199.0.0.2/32 -D 222.0.0.2/32
       ipfwadm -0 -a accept -W eth1 -V 200.200.200.200 -P all -D 199.0.0.2/32 -S 222.0.0.2/32
       ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P all -S 199.0.0.3/32 -D 222.0.0.2/32
       ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P all -D 199.0.0.3/32 -S 222.0.0.2/32

       # Block all other traffic from the internet.
       # This is essentially a "default deny" that
       # only applies to the internet interface.
       ipfwadm -I -a deny -W eth1 -V 200.200.200.200

  If you are installing firewall rules on forwarding and/or output, you
  will have do do something similar. The above example only covers VPN
  traffic; you will have to merge it into your existing firewall setup
  to allow any other traffic you need.

  3.10.  ipfwadm setup for a Registered-IP VPN Client

  Setting up a registered-IP VPN client behind a Linux firewall is
  similar to setting up a registered-IP VPN server.

  The firewall rules will look something like this:

  # Allow the IPsec ISAKMP traffic out and in.
  ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P udp -S 222.0.0.2/32 500 -D 199.0.0.1/32 500
  ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P udp -D 222.0.0.2/32 500 -S 199.0.0.1/32 500
  # Allow the PPTP control channel out and in.
  ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P tcp -S 222.0.0.2/32 -D 199.0.0.1/32 1723
  ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P tcp -D 222.0.0.2/32 -S 199.0.0.1/32 1723

  # Block all other TCP and UDP traffic from the internet.
  # This is essentially a "default deny TCP/UDP" that
  # only applies to the internet interface.
  ipfwadm -I -a deny -W eth1 -V 200.200.200.200 -P tcp
  ipfwadm -I -a deny -W eth1 -V 200.200.200.200 -P udp

  # Note that this is too open since we're forced to
  # specify "-P all" rather than "-P 47" or "-P 50"...
  # Allow the PPTP data channel and IPsec ESP traffic out and in
  ipfwadm -O -a accept -W eth1 -V 200.200.200.200 -P all -S 222.0.0.2/32 -D 199.0.0.1/32
  ipfwadm -I -a accept -W eth1 -V 200.200.200.200 -P all -D 222.0.0.2/32 -S 199.0.0.1/32

  # Block all other traffic from the internet.
  # This is essentially a "default deny" that
  # only applies to the internet interface.
  ipfwadm -I -a deny -W eth1 -V 200.200.200.200

  4.  Configuring the VPN client

  4.1.  Configuring a MS W'95 or W'98 client

  1. Set up your routing so that the Linux firewall is your default
     gateway:

     a. Open Control Panel/Network or right-click "Network Neighborhood"
        and click on Properties.

     b. Click on the Configuration tab.

     c. In the list of installed network components, double-click on the
        "TCP/IP -> whatever-NIC-you-have" line.

     d. Click on the Gateway tab.

     e. Enter the local-network IP address of your Linux firewall.
        Delete any other gateways.

     f. Click on the "OK" button.

  2. Test masquerading. For example, run "telnet my.isp.mail.server
     smtp" and you should see the mail server's welcome banner.

  3. Install and configure the VPN software. For IPsec software follow
     the manufacturer's instructions. For MS PPTP:

     a. Open Control Panel/Network or right-click "Network Neighborhood"
        and click on Properties.

     b. Click on the Configuration tab.

     c. Click on the "Add" button, then double-click on the "Adapter"
        line.

     d. Select "Microsoft" as the manufacturer and add the "Virtual
        Private Networking Adapter" adapter.

     e. Reboot when prompted to.

     f. If you need to use strong (128-bit) encryption, download the
        strong encryption DUN update from the MS secure site at
        <http://mssecure.www.conxion.com/cgi-bin/ntitar.pl> and install
        it, then reboot again when prompted to.

     g. Create a new dial-up phonebook entry for your PPTP server.

     h. Select the VPN adapter as the device to use, and enter the PPTP
        server's internet IP address as the telephone number.

     i. Select the Server Types tab, and check the compression and
        encryption checkboxes.

     j. Click on the "TCP/IP Settings" button.

     k. Set the dynamic/static IP address information for your client as
        instructed to by your PPTP server's administrator.

     l. If you wish to have access to your local network while the PPTP
        connection is up, uncheck the "Use default gateway on remote
        network" checkbox.

     m. Reboot a few more times, just from habit... :)

  4.2.  Configuring a MS NT client

       Note: this section may be incomplete as it's been a while
       since I've installed PPTP on an NT system.

  1. Set up your routing so that the Linux firewall is your default
     gateway:

     a. Open Control Panel/Network or right-click "Network Neighborhood"
        and click on Properties.

     b. Click on the Protocols tab and double-click on the "TCP/IP"
        line.

     c. Enter the local-network IP address of your Linux firewall in the
        "Default Gateway" box.

     d. Click on the "OK" button.

  2. Test masquerading. For example, run "telnet my.isp.mail.server
     smtp" and you should see the mail server's welcome banner.

  3. Install and configure the VPN software. For IPsec software follow
     the manufacturer's instructions. For MS PPTP:

     a. Open Control Panel/Network or right-click "Network Neighborhood"
        and click on Properties.

     b. Click on the Protocols tab.

     c. Click on the "Add" button, then double-click on the "Point-to-
        Point Tunneling Protocol" line.

     d. When it asks for the number of Virtual Private Networks, enter
        the number of PPTP servers you could possibly be communicating
        with.

     e. Reboot when prompted to.

     f. If you need to use strong (128-bit) encryption, download the
        strong encryption PPTP update from the MS secure site at
        <http://mssecure.www.conxion.com/cgi-bin/ntitar.pl> and install
        it, then reboot again when prompted to.

     g. Create a new dial-up phonebook entry for your PPTP server.

     h. Select the VPN adapter as the device to use, and enter the PPTP
        server's internet IP address as the telephone number.

     i. Select the Server Types tab, and check the compression and
        encryption checkboxes.

     j. Click on the "TCP/IP Settings" button.

     k. Set the dynamic/static IP address information for your client as
        instructed to by your PPTP server's administrator.

     l. If you wish to have access to your local network while the PPTP
        connection is up, see MS Knowledge Base article Q143168 for a
        registry fix.  (Sigh.)

     m. Make sure you reapply the most recent Service Pack, to ensure
        that your RAS and PPTP libraries are up-to-date for security and
        performance enhancements.

  5.  Troubleshooting

  5.1.  Testing

  To test VPN Masquerade:

  1. Bring up your ISP connection from your Linux box and verify that it
     still works properly.

  2. Verify that regular masquerading still works properly by, for
     example, trying to browse a Web site or access an FTP server from a
     masqueraded box on your local network.

  3. PPTP: Verify that you have masquerading of the PPTP control channel
     properly configured: try to telnet from the PPTP client system to
     port 1723 on your PPTP server. Don't expect to see anything, but if
     you get a timeout or an error saying the connection failed, take a
     look at the masquerade rules on your Linux box to ensure that you
     are indeed masquerading traffic from your PPTP client to TCP port
     1723 on your PPTP server.
  4. PPTP: Attempt to establish a PPTP connection. I recommend you also
     run RASMON if it is available, as this will give you a minimal
     amount of information about the status of the connection. If you
     establish a PPTP connection on the first try, congratulations!
     You're done!

  5. IPsec: Attempt to establish an IPsec connection.

  5.2.  Possible problems

  There are several things that may prevent a VPN session from being
  established.  We'll work through them going from the client to the
  server and back again.  We will assume you're using a Windows-based
  client for the examples, as that's the most common case.

  1. Connect information: the "telephone number" in the VPN dialup
     configuration must be the Internet IP address of the VPN server, or
     the IP address of the firewall if the server is being masqueraded.

  2. PPTP and strong encryption: unless both client and server have the
     128-bit NDISWAN.SYS or W'95/'98 PPTP software, you will not be able
     to establish a strongly-encrypted session. Unfortunately in my
     experience this problem does not generate any obvious error
     messages, it just keeps trying and trying and trying... The strong
     encryption update can be obtained from the Microsoft secure site
     URL given int he "Configuring a MS Client" section.

     This may also affect IPsec clients, if they use the MS-supplied
     encryption libraries rather than using their own libraries.

  3. Routing: verify that the default route on your VPN client is
     pointing at the Linux masquerade box. Run the route print command
     and look for an 0.0.0.0 entry.

     If other masqueraded services (such as HTTP, FTP, IRC, etc.) work
     from your VPN client system then this probably is not the problem.

  4. Masquerading: there are two parts to the VPN session.

     For IPsec, the authentication and key exchange service (ISAKMP),
     which is a normal UDP session to port 500 on the remote IPsec host,
     must be configured for masquerading as you would any other UDP
     service (such as DNS).

     For PPTP, the control channel, which is a normal TCP session to
     port 1723 on the PPTP server, must be configured for masquerading
     as you would any other TCP service (such as HTTP).

     The encrypted data channel in IPsec is carried over ESP, IP
     protocol 50.  The encrypted data channel in PPTP is carried over
     GRE, IP protocol 47.  (Note that these are not TCP or UDP port
     numbers!)  Since the 2.0 Linux kernel only lets you specify TCP,
     UDP, ICMP and ALL IP protocols when creating masquerade rules, you
     must also masquerade ALL protocol traffic if you are masquerading
     only specific services. If you are masquerading everything, you
     don't need to worry about this.

     In order to isolate the firewall rules from the kernel masquerade
     code, try establishing a VPN connection with your firewall
     completely open, then if it works, tighten the firewall rules.

     2.0.x ipfwadm completely open firewall:

       ipfwadm -I -p accept
       ipfwadm -O -p accept
       ipfwadm -F -a accept -m

  2.2.x ipchains completely open firewall:

       ipchains -P input   ACCEPT
       ipchains -P output  ACCEPT
       ipchains -P forward MASQ

  Do not leave your firewall completely open for any longer than it
  takes to prove that a masqueraded VPN connection can be established!

  5. Intermediary hops and the Internet: All routers between your Linux
     firewall and the remote IPsec host must forward packets carrying IP
     protocol 50.  All routers between your Linux firewall and the PPTP
     server must forward packets carrying IP protocol 47.  If you had
     IPsec or PPTP working when your VPN client system directly dialled
     your ISP then this probably is not the problem.

     To isolate whether an intermediary hop is blocking GRE traffic, use
     a patched traceroute to trace the progress of GRE packets. See the
     resources section for information on the traceroute patch. A
     similar patch for ESP is in the works.

  6. The remote firewall: the firewall at the server end must allow a
     system with the IP address assigned to your Linux box by your ISP
     to connect to port 500/udp on the IPsec host or port 1723/tcp on
     the PPTP server. If you had the VPN working when your VPN client
     system directly dialled your ISP then this probably is not the
     problem.

  7. The server firewall and ESP: the IPsec encrypted data is carried
     over IP protocol 50. If the firewall the remote IPsec host is
     behind does not forward ESP traffic in both directions, IPsec will
     not work. Again, if you had IPsec working when your IPsec client
     system directly dialled your ISP then this probably is not the
     problem.

  8. The server firewall and GRE: the PPTP data channel is carried as a
     GRE-encapsulated (IP protocol 47) PPP session. If the firewall your
     PPTP server is behind does not forward GRE traffic in both
     directions, PPTP will not work. Again, if you had PPTP working when
     your PPTP client system directly dialed your ISP then this probably
     isn't the problem.

  9. The patch: If your IPsec client successfully authenticates you but
     cannot establish a network connection, the patch may not be
     masquerading ESP traffic properly. If your PPTP client establishes
     the control channel (RASMON beeps and the little telephone lights
     up) and sends GRE traffic (the upper light in RASMON blinks) but
     gets no GRE traffic back (the lower light in RASMON does not blink
     in response) the patch may not be masquerading GRE traffic
     properly.
     Look in /var/log/messages for log entries showing that VPN traffic
     was seen. Turning on VPN debugging may help you to determine
     whether or not the patch is at fault. Also run a sniffer on your
     internet connection and look for outbound VPN traffic (see below).

  10.
     Multiple clients: the older PPTP patch does NOT support
     masquerading of multiple PPTP clients attempting to access the same
     PPTP server. If you're trying to do this, you should take a look at
     your network design and consider whether you should set up a PPTP
     router for your local clients. The 2.0 patch incorporates Call-ID
     masquerading, which allows multiple simultaneous sessions. Note: do
     not enable PPTP Call-ID masquerade if you are masquerading a PPTP
     Server. At the current time this will prevent the server's outbound
     traffic from being masqueraded.

  5.3.  Troubleshooting

  Most problems can be localized by running a packet sniffer (e.g.
  tcpdump with the -v option) on your VPN firewall.  If everything is
  working properly, you'll see the following traffic:

  ·  Client local network:

     IPsec: UDP (destination UDP port 500) and ESP (IP protocol 50)
     traffic from your IPsec client local network IP to the remote IPsec
     host's Internet IP. If you don't see this, your IPsec client is
     misconfigured.

     PPTP: TCP (destination TCP port 1723) and GRE (IP protocol 47)
     traffic from your PPTP client local network IP to the PPTP server's
     Internet IP. If you don't see this, your PPTP client is
     misconfigured.

  ·  ISP side of client firewall: UDP and ESP or TCP and GRE traffic
     from the client firewall Internet IP (remember - we're
     masquerading) to the VPN server's Internet IP. If you don't see
     this, your masquerade is misconfigured or the patch isn't working.

  ·  ISP side of server firewall: UDP and ESP or TCP and GRE traffic
     from the client Internet IP to the VPN server's Internet IP. If you
     don't see this, the Internet is down :) or some intermediary is
     blocking ESP or GRE traffic.

  ·  Boundary network (DMZ) side of server firewall: UDP and ESP or TCP
     and GRE traffic from the client internet IP to the server IP. If
     you don't see this, check your firewall rules for forwarding UDP
     port 500 and IP protocol 50 or TCP port 1723 and IP protocol 47,
     and the configuration of ipportfw and ipfwd if you're masquerading
     the server.

  ·  Boundary network side of server firewall: UDP (source port 500) and
     ESP or TCP (source port 1723) and GRE traffic from the VPN server
     IP to the client internet IP. If you don't see this, check the VPN
     server configuration, including the packet filtering rules on the
     VPN server.

  ·  ISP side of server firewall: UDP and ESP or TCP and GRE traffic
     from the VPN server IP (or firewall IP if the server is
     masqueraded) to the client internet IP. If you don't see this,
     check your firewall rules for forwarding UDP port 500 and IP
     protocol 50 or TCP port 1723 and IP protocol 47.

  ·  ISP side of client firewall: UDP and ESP or TCP and GRE traffic
     from the VPN server IP to the client firewall internet IP. If you
     don't see this, the Internet is acting up again.

  ·  Client local network: UDP and ESP or TCP and GRE traffic from the
     VPN server internet IP to the VPN client local network IP. If you
     see the UDP traffic but not the ESP traffic, or the TCP traffic but
     not the GRE traffic, the patch isn't working or wasn't properly
     installed.

  You may find it helpful to turn on VPN debugging and recompile your
  kernel. Add the following to /etc/syslog.conf

       # debugging
       *.=debug           /var/log/debug

  and watch /var/log/messages and /var/log/debug for log messages about
  the VPN traffic. Note that logging - especially verbose logging - will
  cause a great deal of disk activity and will cause the log files to
  grow very large very quickly. Don't turn on debugging unless you need
  to, and turn it off when you're done.

  5.4.  MS PPTP Clients and domain-name issues

  Thanks to Charles Curley <ccurley@trib.com> for the following:

       If you use PPTP (Point to Point Tunneling Protocol) to
       access a Microsoft Networking (SMB) environment and have
       your own Microsoft Networking environment in your local pri­
       vate network (Samba or Windows), give your local workgroup a
       name that does not show up in the remote environment. The
       reason is that while your PPTP client is logged into the
       remote environment, it will see the remote environment's
       domain name servers, and will only see the remote computers
       in that workgroup.

       You should avoid the lazy option. Microsoft ships Windows
       set up for a default workgroup name of WORKGROUP. Some
       people will be lazy and accept that as their workgroup when
       they set up their computers. So there is a good chance that
       the remote environment will have a workgroup called
       WORKGROUP, administrators willing or not.

  I think that this will apply regardless of the VPN in use, as name
  services aren't dependent on the transport. If your client(s) can see
  the WINS servers on the remote network then you may experience this,
  PPTP or no PPTP.

  5.5.  MS PPTP Clients and Novell IPX

  If you're having trouble with IPX traffic over your PPTP link, please
  see sections 3.5 and 5.2 in this MS Knowledge Base article:
  <http://support.microsoft.com/support/kb/articles/q154/0/91.asp>

  Thanks to David Griswold <dgriswol@ix.netcom.com>

  5.6.  MS network password issues

  When you are using a VPN to access a MS network you should remember
  that you will have to provide two different authentication tokens -
  one to connect to the VPN server (the VPN password) and the other to
  access resources on the remote network once the connection is
  established (the network password).

  The VPN password - the username and password you enter into your VPN
  client when initiating the call to the VPN server - is only used by
  the VPN server to grant you permission to connect to the network via
  the VPN. It isn't used for anything else once you're connected.

  The VPN password is not used to prove your identity to other computers
  on the remote network. You must provide another username/password pair
  - your network password - for that.

  There are two ways to supply a network password. Your network password
  may be the same username/password pair you supplied when logging onto
  the local network when you started your computer up. If it is
  different, you can configure your VPN client to ask you for your
  network password for the remote network once the VPN connection is
  established.

  If you are successfully connecting to the VPN server but you cannot
  access any of the resources provided by the remote network, then you
  aren't providing a valid network username/password pair for the remote
  network.  Verify that the username and password for your local network
  will also work on the remote network, or set your VPN client to prompt
  you for a username and password for use on the remote network and "log
  on" to the remote network once the VPN connection is established.

  5.7.  If your IPsec session always dies after a certain amount of time

  If you're having trouble with your IPsec tunnel regularly dying,
  particularly if checking the system logs on the firewall shows that
  ISAKMP packets with "zero cookie" values are being seen, here's what's
  happening:

  Earlier versions of the IPsec Masq patch did not change the timeout
  for masq table entries for ISAKMP UDP packets. The masq table entries
  for the ISAKMP UDP traffic would time out fairly quickly (relative to
  the data channel) and be removed; if the remote IPsec host then
  decided to initiate rekeying before the local IPsec host did, the
  inbound ISAKMP traffic for the rekey couldn't be routed to the
  masqueraded host. The rekey traffic would be discarded, the remote
  IPsec host would think the link had failed, and the connection would
  eventually be terminated.

  The 2.0.36 patch has been modified to increase the timeout on ISAKMP
  UDP masq table entries. Get the current version of the patch,
  available via the sites given in the Resources section, and repatch
  and recompile your kernel.

  Also verify that your IPsec Masq Table Lifetime parameter is
  configured to be the same as or slightly longer than your rekey
  interval.

  6.  IPsec masquerade technical notes and special security considera­
  tions

  6.1.  Limitations and weaknesses of IPsec masquerade

  Traffic that uses the AH protocol cannot be masqueraded. The AH
  protocol incorporates a cryptographic checksum across the IP addresses
  that the masquerade gateway cannot correctly regenerate. Thus, all
  masqueraded AH traffic will be discarded as having invalid checksums.

  IPsec traffic  using transport-mode ESP also cannot be reliably
  masqueraded.  Transport mode ESP essentially encrypts everything after
  the IP header.  Since, for example, the TCP and UDP checksums include
  the IP source and destination addresses, and the TCP/UDP checksum is
  within the encrypted payload and thus cannot be recalculated after the
  masquerade gateway alters the IP addresses, the TCP/UDP header will
  fail the checksum test at the remote gateway and the packet will be
  discarded. Protocols that do not include information about the source
  or destination IP addresses may successfully use masqueraded transport
  mode.

  Apart from these limitations, IPsec masquerade is secure and reliable
  when only one IPsec host is being masqueraded at a time, or when each
  masqueraded host is communicating with a different remote host. When
  more than one masqueraded host is communicating with the same remote
  host, a few weaknesses show up:

  ·  Transport-mode communications are subject to collisions.

     If two or more masqueraded hosts are using transport mode to
     communicate with the same remote host, and the security policy of
     the remote host permits multiple transport-mode sessions with the
     same peer, it is possible for sessions to experience collisions.
     This happens because the IP address of the masquerading gateway
     will be used to identify the sessions, and any other identifying
     information cannot be masqueraded because it is within the
     encrypted portion of the packet.

     If the remote host's security policy does not permit multiple
     transport-mode sessions with the same peer, the situation is even
     worse: the more-recently-negotiated transport mode session will
     likely completely take over all of the traffic from the older
     session, causing the older session to "go dead". While the
     established sessions from the older transport-mode IPsec session
     may be quickly reset if the remote host isn't expecting to receive
     the traffic, at least one packet of information will be sent to the
     wrong host. This information will probably be discarded by the
     recipient, but it will still be sent.

     Thus, a transport-mode collision may result in leaking of
     information between the two sessions or termination of one or both
     sessions. Using IPsec in transport mode via a masquerading gateway
     is not recommended if there is the possibility that other transport
     mode IPsec sessions will be attempted via the same masquerading
     gateway to the same remote IPsec host.

     IPsec using tunnel mode with extruded network addressing (where the
     masqueraded IPsec host is assigned an IP address from the remote
     host's network) is not subject to these problems, as the IP
     addresses assigned from the remote network will be used to identify
     the sessions instead of using the IP address of the masquerading
     host.

  ·  ISAKMP communications are subject to cookie collisions.

     If two or more masqueraded hosts establishing a session to the same
     remote host happen to select the same initiator cookie when
     initiating ISAKMP traffic, the masquerading gateway will route all
     of the ISAKMP traffic to the second host. There is a 1 in 2^64
     (i.e. very small) chance of this collision happening for each host,
     at the time of establishing the initial ISAKMP connection.

     Correcting this requires including the responder cookie in the key
     used to route inbound ISAKMP traffic. This modification will be
     incorporated into IPsec masquerade for the 2.2.x kernel, and the
     short window between the time the masqueraded host initiates the
     ISAKMP exchange and the remote host responds is covered by
     discarding any new ISAKMP traffic that would collide with the
     current outstanding traffic. This modification will be backported
     to the 2.0.x code soon.

  ·  There may be a collision between SPI values on inbound traffic.

     Two or more masqueraded IPsec hosts communicating with the same
     remote IPsec host may negotiate to use the same SPI value for
     inbound traffic. If this happens the masquerading gateway will
     route all of the inbound traffic to the first host to receive any
     inbound traffic using that SPI. The possibility of this happening
     is about 1 in 2^32 for each outstanding ESP session, and may occur
     on any rekey.

     Since the SPI values refer to different SAs having different
     encryption keys the first host will not be able to decrypt the data
     intended for the other hosts, so no data leakage will occur. There
     is no way for the masquerading gateway to detect or prevent this
     collision. The only way to prevent this collision is for the remote
     IPsec host to check the SPI value proposed by the masqueraded host
     to see if that SPI value is already in use by another SA from the
     same IP address. It is not likely that this will be done, since it
     imposes more overhead on an already expensive operation (the rekey)
     to benefit a small percentage of users in case of a relatively rare
     event.

  ·  Inbound and outbound SPI values may be misassociated.

     This is discussed in detail in the next section.

  The 2.2.x code by default prevents the establishment of multiple
  connections to the same remote host. If the weaknesses exposed by
  multiple connections to the same remote host are acceptable, you can
  enable "parallel sessions".

  6.2.  Proper routing of inbound encrypted traffic

  The portion of the ISAKMP key exchange where the ESP SPI values are
  communicated is encrypted, so the ESP SPI values must be determined by
  inspection of the actual ESP traffic. Also, the outbound ESP traffic
  does not contain any indication of what the inbound SPI will be. This
  means there is no perfectly reliable way to associate inbound ESP
  traffic with outbound ESP traffic.

  IPsec masq attempts to associate inbound and outbound ESP traffic by
  serializing initial ESP traffic on a by-remote-host basis. What this
  means is:

  ·  If an outbound ESP packet with an SPI value that has not previously
     been seen (or whose masquerade table entry has expired) is received
     (which shall hereafter be called an "initial packet"), a masquerade
     table entry for that SourceAddr+SPI+DestAddr combination is
     created. It is marked as "outstanding", that is, no inbound ESP
     traffic has been received for it yet. This is done by setting the
     "inbound SPI" value in the masq table entry to zero, which is a
     value reserved for uses such as this. This will happen at the
     initiation of a new ESP connection and at regular intervals when an
     existing ESP connection rekeys.

  ·  As long as the masq table entry is outstanding, no other initial
     ESP packets for the same remote host will be processed. The packets
     are immediately discarded, and a system log entry is made saying
     the traffic is temporarily blocked. This also applies to initial
     traffic from the same masqueraded host going to the same remote
     host if the SPI values differ. Traffic to other remote hosts, and
     traffic where both SPI values are known ("established" traffic) is
     not affected by this.

  ·  This could easily lead to a Denial of Service of the remote host,
     so this outstanding ESP masq table entry is given a short lifetime,
     and only a limited number of retries of the same traffic are
     allowed. This permits round-robin access to the remote host if
     several masqueraded hosts are attempting to initialize
     simultaneously and responses aren't coming back very quickly, for
     example due to network congestion or a slow remote host.  The retry
     limitation begins once there is a collision, so the masqueraded
     IPsec host can wait as long as necessary for a reply until there's
     a need for serialization.

  ·  When an ESP packet from the outstanding remote host is received and
     the SPI value does not appear in any masq table entry, it is
     assumed that the packet is the response to the outstanding initial
     packet. The SPI value is stored in that masq table entry, thus
     associating the SPI values, and the inbound ESP traffic is routed
     to the masqueraded host. At this point another initial packet for
     the remote server may be processed.

  ·  Any ESP traffic with a zero SPI value is discarded as invalid, per
     the RFC requirements.

  There are several ways this can fail to associate traffic properly:

  ·  Network delays or a slow remote host can cause the response to the
     first initial packet to be delayed long enough that the init masq
     table entry expires and a different masqueraded host is given a
     chance to initialize. This could cause the response to be
     associated with the wrong outbound SPI, which would cause inbound
     traffic to be routed to the wrong masqueraded host. If this happens
     the masqueraded host receiving the traffic in error will discard it
     because it has an unexpected SPI value, and everybody will
     eventually time out, rekey and try again. This can be addressed by
     editing /usr/src/linux/net/ipv4/ip_masq.c and increasing the INIT
     lifetime or the number of INIT retries permitted, at the cost of
     increasing the blocking (and DoS) window.

  ·  Sessions idle or semi-idle (with infrequent inbound traffic and no
     outbound traffic) for a long period of time may be idle long enough
     for the masq table entry to expire. If the remote host sends
     traffic to an established yet expired session while an outstanding
     init to the same remote host is underway, the traffic may be
     misrouted for the same reason as described above. This can be
     addressed by making sure the IPsec Masq Table Lifetime kernel
     configuration parameter is slightly longer than the rekey interval,
     which is the longest time any given SPI pair will be used.  The
     problem here is that you may not know all of the rekey intervals if
     you're masquerading for many remote servers, or some may have their
     rekey intervals set to unreasonably high values, such as several
     hours.

  ·  If there is a delay between a rekey and the transmission of
     outbound ESP traffic using the new SPI, and during this delay
     inbound ESP traffic using the new SPI is received, there will be no
     masq table entry describing how to route the inbound traffic. If
     another masqueraded host has a pending init with the same remote
     host, the traffic will be misassociated. Note that serialization of
     ESP initial traffic does not affect ISAKMP rekey traffic.

  The best solution is to have some way to preload the masq table with
  the properly associated out-SPI/in-SPI pair or some other mapping of
  remote_host + inbound_SPI to masqueraded_host. This cannot be done by
  inspecting the ISAKMP key exchange, as it is encrypted. It may be
  possible to use RSIP (a.k.a. Host-NAT) to communicate with the
  masqueraded IPsec host and request notification of SPI information
  once it has been negotiated. This is being investigated. If something
  is done to implement this it will be done no sooner than the 2.3.x
  series, as RSIP is a fairly complex client/server NAT protocol.

  When an inbound ESP packet with a new SPI is received the masquerading
  firewall attempts to guess which masqueraded host(s) the unassociated
  inbound traffic is intended for. If the inbound ESP traffic is not
  matched to an established session or a pending session initialization,
  then the packet is sent to the masqueraded host(s) who most recently
  rekeyed with that remote host. The "incorrect" masqueraded hosts will
  discard the traffic as being improperly encrypted, and the "correct"
  host will get its data. When the "correct" host responds, the normal
  ESP init serialization process occurs.

