For several years now, users at the Institution where I currently work part-time have expressed a need for a robust and secure connection to the internal network. The internal network actually has some clients that are on private networks and some clients that are Internet-routeable.
Being mostly a Microsoft shop, the options that have been available were not deemed appropriate for various reasons, usually due to security concerns. After being introduced to the problem a few months ago and listening to the concerns surrounding it, I suggested using a boot CD, which would alleviate all of their security concerns. They gave me the go ahead to give it a try. I now have a working prototype and this document describes the process used to create it.
Copyright © 2006-11-13 by Jeffery Douglas Waddell. You are free:
Use the information in this document at your own risk. I disavow any potential liability for the contents of this document. Use of the concepts, examples, and/or other content of this document is entirely at your own risk.
All copyrights are owned by their owners, unless specifically noted otherwise. Use of a term in this document should not be regarded as affecting the validity of any trademark or service mark.
Naming of particular products or brands should not be seen as endorsements.
You are strongly recommended to take a backup of your system before major installation and backups at regular intervals.
I used many resources to do what I did. A big thank you to all those that have contributed to these projects. They include but are not limited to:
Currently there are no translations. If you would you like to translate this, Please contact me at jefferydouglaswaddell (at) gmail (dot) com .
To begin, I will attempt to lay out the problem a little more thoroughly and give the theory behind the prototype.
In the case of the Institution, there is a set of machines that are behind a very robust firewall. This firewall allows VERY little to go through. As far as connecting out to the Internet, many things, including access to HTTP, FTP, secure shell, etc., are allowed. As far as connecting in, none of those are allowed.
You can imagine the firewall as a set of machines on the edge of our internal network. There is a lot of unsecured traffic within our internal network, which has both private networks and internet addressed clients within it. To be allowed to do any of the things that require a connection INTO the network from the Internet, a server allowing the connection must be on the periphery of the network (i.e. part of the firewall).
The desire has been to be able to access the internal network sufficiently to do work from home (on par with doing work in the office), but without opening up the firewall such that our unsecured traffic would be exposed to the Internet. There is also sensitive, internal data within the internal network that can be accessed, that should neither be copied to the home machine (laptop, etc.) nor printed at the home machine.
So the question becomes: how do you allow enough access to give certain people the ability to do their jobs from home and still protect the sensitive information and the network as a whole? Other security considerations that came up were:
The answer that came to my mind was to give the users a boot CD that they could place in a machine (whether at Uncle Joe's, their own house, or the internet cafe at the airport) and use to boot into their internal work desktop. The theory is that connecting to a server on the periphery of the network, which then gives them access to their desktop, is at least as secure as if they had accessed the network from their internal desktop, and that a great deal of control can be exercised over when and how that access is granted. Being a boot CD, no on-site OS configuration will be required, nor will extra precautions concerning malware, spy-ware, or viruses need to be taken. In the next section, Technologies, I will examine some of the ways this can be implemented, and in the Implementation section I will explain how we actually did it.
Since many people are already familiar with openVPN, this seemed like a good idea. However, in and of itself openVPN is not sufficient. The most convenient way for people to be able to work is for them to be able to directly connect to their already existing desktop. All of the users here run either Windows XP (tm) or Windows 2000 (tm), which suggests rdesktop as a solution. But rdesktop can't get through the firewall and we won't open our firewall for that traffic as it would be too hard to secure. Adding openVPN allows for more security, but it runs into the following problems within the scope of our implementation:
In order to eliminate the security issues above and to make it less of a difficult system to maintain into the future, I suggested creating a Linux live CD that boots, logs into an openVPN server that connects the external and internal networks, and then automatically opens the individuals internal desktop using rdesktop.
Many decisions had to be made as to which direction to go. The following subsections detail some of the paths I took to get to a working prototype. Please modify to suit your environment. Where appropriate I will make clarifying comments.
I looked at several live CD distributions and concluded that DSL would work best for the purposes at hand. I considered the following:
The choice of DSL means that we are relying on DSL's built in ability to automatically find, configure and attach to a network via DHCP. DSL's wireless support is very minimal and thus we do not support wireless at this time. The end user will need a machine that normally attachs to the Internet through DHCP from their provider and uses a normal wired network card to do so.
mkdir /tmp/workingiso
mount -t iso9660 -o loop dsl-3.0.iso /tmp/working.iso
extract_compressed_fs /tmp/workingiso/KNOPPIX/KNOPPIX > /var/tmp/KNOPPIX-cloop
mkdir /tmp/workingiso.cloop
mount -o loop /var/tmp/KNOPPIX-cloop /tmp/workingiso.cloop
tar -C /tmp/workingiso.cloop -cf - . | tar -C /home/jeff/Desktop/vpn-tree -xvpf -
tar -C /tmp/workingiso -cf - . | tar -C /home/jeff/Desktop/vpn-cd-tree -xvpf -
mkisofs -pad -l -r -J -V "YOURVPN v0.1" -no-emul-boot -boot-load-size 4 -boot-info-table -b boot/isolinux/isolinux.bin -c boot/isolinux/boot.cat -hide-rr-moved -o yourvpn.iso /home/jeff/Desktop/vpn-cd-tree/
qemu -boot d -cdrom yourvpn.iso
mount -t proc none /home/jeff/Desktop/vpn-tree/proc
chroot /home/jeff/Desktop/vpn-tree
mkisofs -L -R -l -V "YOURVPN ISO9660" -v -allow-multidot /home/jeff/Desktop/vpn-tree/ | create_compressed_fs - 65536 > /home/jeff/Desktop/vpn-cd-tree/KNOPPIX/KNOPPIX
mkisofs -pad -l -r -J -V "YOURVPN v0.2" -no-emul-boot -boot-load-size 4 -boot-info-table -b boot/isolinux/isolinux.bin -c boot/isolinux/boot.cat -hide-rr-moved -o yourvpn.iso /home/jeff/Desktop/vpn-cd-tree/
qemu -boot d -cdrom yourvpn.iso
Once built there is NO maintenance to the CDs. If you need to change the private key password for the individual user, burn them a new CD. If they lose a CD, give them a fresh burn. If the CD gets destroyed, give them a new copy.
The openVPN server requires little maintenance. It is recommended that you periodically check the openVPN logs on the server to determine the likelihood of nefarious activity and act accordingly. Usage-tracking is beyond the scope of this document.
I believe that the majority of security and ease of use issues are dealt with by the technologies I have described here. Certainly there are many issues that this technology could expose (mostly internal political or policy issues within your own institution).
If you discover any flaws in the technology. Please contact me about it.
Also, if you can explain the proper pf rules to get *bsd to properly forward (masquerade the 10. network) packets to Internet-routed network segments on the internal side of the VPN, I would definitely like to hear from you. I could not get it to work, whereas the Linux masquerading rule I found just works.
There are several troubleshooting techniques that I used in the course of this project. I would expect people attempting this to be familiar with most or all of them. Here are a few:
As mentioned elsewhere, please see http://openvpn.net for more information on openVPN...the HOWTO is especially good.
Please see http://damnsmalllinux.org for more info on DSL.
For live CD information, Google may be your best bet, although there is now a live CD book that seems fairly good.
Obviously the above mentioned communities can help you with varying aspects of the respective pieces of this puzzle.
If you need help understanding how to put it all together, feel free to contact me after you've thoroughly read (and maybe tried) this HOWTO.
There you have it. May it be of use.
When I receive questions generated from this document I will compile them and insert them here.
Due to a policy decision, we will not be deploying this, although it does work. The security concerns over this method include the following:
This is what you do to create one. This method is likely useful for other projects.
mkdir win-qemu-yourvpn-cd
[autorun]
icon=youricon.ico
open=yourvpn.bat
Here is the config file (with IP address and key names removed for the CD). Text in upper case is used to indicate that you need to change whatever is there to your setup.
----------------
##############################################
# Sample client-side OpenVPN 2.0 config file #
# for connecting to multi-client server. #
# #
# This configuration can be used by multiple #
# clients, however each client should have #
# its own cert and key files. #
# #
# On Windows, you might want to rename this #
# file so it has a .ovpn extension #
##############################################
# Specify that we are a client and that we
# will be pulling certain config file directives
# from the server.
client
# Use the same setting as you are using on
# the server.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun
# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel
# if you have more than one. On XP SP2,
# you may need to disable the firewall
# for the TAP adapter.
;dev-node MyTap
# Are we connecting to a TCP or
# UDP server? Use the same setting as
# on the server.
;proto tcp
proto udp
# The hostname/IP and port of the server.
# You can have multiple remote entries
# to load balance between the servers.
remote PUT_YOUR_OPENVPN_SERVER_IP_HERE 1194
;remote my-server-2 1194
# Choose a random host from the remote
# list for load-balancing. Otherwise
# try hosts in the order specified.
;remote-random
# Keep trying indefinitely to resolve the
# host name of the OpenVPN server. Very useful
# on machines which are not permanently connected
# to the internet such as laptops.
resolv-retry infinite
# Most clients don't need to bind to
# a specific local port number.
nobind
# Downgrade privileges after initialization (non-Windows only)
user nobody
group nogroup
# Try to preserve some state across restarts.
persist-key
persist-tun
# If you are connecting through an
# HTTP proxy to reach the actual OpenVPN
# server, put the proxy server/IP and
# port number here. See the man page
# if your proxy server requires
# authentication.
;http-proxy-retry # retry on connection failures
;http-proxy [proxy server] [proxy port #]
# Wireless networks often produce a lot
# of duplicate packets. Set this flag
# to silence duplicate packet warnings.
;mute-replay-warnings
# SSL/TLS parms.
# See the server config file for more
# description. It's best to use
# a separate .crt/.key file pair
# for each client. A single ca
# file can be used for all clients.
ca /etc/openvpn/keys/ca.crt
cert /etc/openvpn/keys/THECDUSERSCRTFILE.crt
key /etc/openvpn/keys/THECDUSERSPASSWORDPROTECTEDPUBLICKEY.key
# Verify server certificate by checking
# that the certicate has the nsCertType
# field set to "server". This is an
# important precaution to protect against
# a potential attack discussed here:
# http://openvpn.net/howto.html#mitm
#
# To use this feature, you will need to generate
# your server certificates with the nsCertType
# field set to "server". The build-key-server
# script in the easy-rsa folder will do this.
ns-cert-type server
# If a tls-auth key is used on the server
# then every client must also have the key.
;tls-auth ta.key 1
# Select a cryptographic cipher.
# If the cipher option is used on the server
# then you must also specify it here.
;cipher x
# Enable compression on the VPN link.
# Don't enable this unless it is also
# enabled in the server config file.
comp-lzo
# Set log file verbosity.
verb 3
# Silence repeating messages
mute 20
#added in hopes of removing a command line option
auth-retry interact
Here is the openVPN configuration file for the server.
----------------
#################################################
# Sample OpenVPN 2.0 config file for #
# multi-client server. #
# #
# Edited on 9/12/2006 by Jeff Waddell #
# #
# This file is for the server side #
# of a many-clients <-> one-server #
# OpenVPN configuration. #
# #
# OpenVPN also supports #
# single-machine <-> single-machine #
# configurations (See the Examples page #
# on the web site for more info). #
# #
# This config should work on Windows #
# or Linux/BSD systems. Remember on #
# Windows to quote pathnames and use #
# double backslashes, e.g.: #
# "C:\\Program Files\\OpenVPN\\config\\foo.key" #
# #
# Comments are preceded with '#' or ';' #
#################################################
# Which local IP address should OpenVPN
# listen on? (optional)
;local a.b.c.d
local YOUR_OPENVPN_SERVER_IP_GOES_HERE_NEEDS_TO_BE_AN_IP_THAT_THIS_BOX_IS
# Which TCP/UDP port should OpenVPN listen on?
# If you want to run multiple OpenVPN instances
# on the same machine, use a different port
# number for each one. You will need to
# open up this port on your firewall.
port 1194
# TCP or UDP server?
;proto tcp
proto udp
# "dev tun" will create a routed IP tunnel,
# "dev tap" will create an ethernet tunnel.
# Use "dev tap0" if you are ethernet bridging
# and have precreated a tap0 virtual interface
# and bridged it with your ethernet interface.
# If you want to control access policies
# over the VPN, you must create firewall
# rules for the TUN/TAP interface.
# On non-Windows systems, you can give
# an explicit unit number, such as tun0.
# On Windows, use "dev-node" for this.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun
# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel if you
# have more than one. On XP SP2 or higher,
# you may need to selectively disable the
# Windows firewall for the TAP adapter.
# Non-Windows systems usually don't need this.
;dev-node MyTap
# SSL/TLS root certificate (ca), certificate
# (cert), and private key (key). Each client
# and the server must have their own cert and
# key file. The server and all clients will
# use the same ca file.
#
# See the "easy-rsa" directory for a series
# of scripts for generating RSA certificates
# and private keys. Remember to use
# a unique Common Name for the server
# and each of the client certificates.
#
# Any X509 key management system can be used.
# OpenVPN can also use a PKCS #12 formatted key file
# (see "pkcs12" directive in man page).
ca /etc/openvpn/keys/ca.crt
cert /etc/openvpn/keys/server.crt
key /etc/openvpn/keys/server.key # This file should be kept secret
# Diffie hellman parameters.
# Generate your own with:
# openssl dhparam -out dh1024.pem 1024
# Substitute 2048 for 1024 if you are using
# 2048 bit keys.
dh /etc/openvpn/keys/dh1024.pem
# Configure server mode and supply a VPN subnet
# for OpenVPN to draw client addresses from.
# The server will take 10.8.0.1 for itself,
# the rest will be made available to clients.
# Each client will be able to reach the server
# on 10.8.0.1. Comment this line out if you are
# ethernet bridging. See the man page for more info.
server 10.8.0.0 255.255.255.0
# Maintain a record of client <-> virtual IP address
# associations in this file. If OpenVPN goes down or
# is restarted, reconnecting clients can be assigned
# the same virtual IP address from the pool that was
# previously assigned.
ifconfig-pool-persist ipp.txt
# Configure server mode for ethernet bridging.
# You must first use your OS's bridging capability
# to bridge the TAP interface with the ethernet
# NIC interface. Then you must manually set the
# IP/netmask on the bridge interface, here we
# assume 10.8.0.4/255.255.255.0. Finally we
# must set aside an IP range in this subnet
# (start=10.8.0.50 end=10.8.0.100) to allocate
# to connecting clients. Leave this line commented
# out unless you are ethernet bridging.
;server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100
# Push routes to the client to allow it
# to reach other private subnets behind
# the server. Remember that these
# private subnets will also need
# to know to route the OpenVPN client
# address pool (10.8.0.0/255.255.255.0)
# back to the OpenVPN server.
;push "route 192.168.10.0 255.255.255.0"
;push "route 192.168.20.0 255.255.255.0"
# To assign specific IP addresses to specific
# clients or if a connecting client has a private
# subnet behind it that should also have VPN access,
# use the subdirectory "ccd" for client-specific
# configuration files (see man page for more info).
# EXAMPLE: Suppose the client
# having the certificate common name "Thelonious"
# also has a small subnet behind his connecting
# machine, such as 192.168.40.128/255.255.255.248.
# First, uncomment out these lines:
;client-config-dir ccd
;route 192.168.40.128 255.255.255.248
# Then create a file ccd/Thelonious with this line:
# iroute 192.168.40.128 255.255.255.248
# This will allow Thelonious' private subnet to
# access the VPN. This example will only work
# if you are routing, not bridging, i.e. you are
# using "dev tun" and "server" directives.
# EXAMPLE: Suppose you want to give
# Thelonious a fixed VPN IP address of 10.9.0.1.
# First uncomment out these lines:
;client-config-dir ccd
;route 10.9.0.0 255.255.255.252
# Then add this line to ccd/Thelonious:
# ifconfig-push 10.9.0.1 10.9.0.2
# Suppose that you want to enable different
# firewall access policies for different groups
# of clients. There are two methods:
# (1) Run multiple OpenVPN daemons, one for each
# group, and firewall the TUN/TAP interface
# for each group/daemon appropriately.
# (2) (Advanced) Create a script to dynamically
# modify the firewall in response to access
# from different clients. See man
# page for more info on learn-address script.
;learn-address ./script
# If enabled, this directive will configure
# all clients to redirect their default
# network gateway through the VPN, causing
# all IP traffic such as web browsing and
# and DNS lookups to go through the VPN
# (The OpenVPN server machine may need to NAT
# the TUN/TAP interface to the internet in
# order for this to work properly).
# CAVEAT: May break client's network config if
# client's local DHCP server packets get routed
# through the tunnel. Solution: make sure
# client's local DHCP server is reachable via
# a more specific route than the default route
# of 0.0.0.0/0.0.0.0.
;push "redirect-gateway"
# Certain Windows-specific network settings
# can be pushed to clients, such as DNS
# or WINS server addresses. CAVEAT:
# http://openvpn.net/faq.html#dhcpcaveats
;push "dhcp-option DNS 10.8.0.1"
;push "dhcp-option WINS 10.8.0.1"
# Uncomment this directive to allow different
# clients to be able to "see" each other.
# By default, clients will only see the server.
# To force clients to only see the server, you
# will also need to appropriately firewall the
# server's TUN/TAP interface.
;client-to-client
# Uncomment this directive if multiple clients
# might connect with the same certificate/key
# files or common names. This is recommended
# only for testing purposes. For production use,
# each client should have its own certificate/key
# pair.
#
# IF YOU HAVE NOT GENERATED INDIVIDUAL
# CERTIFICATE/KEY PAIRS FOR EACH CLIENT,
# EACH HAVING ITS OWN UNIQUE "COMMON NAME",
# UNCOMMENT THIS LINE OUT.
;duplicate-cn
# The keepalive directive causes ping-like
# messages to be sent back and forth over
# the link so that each side knows when
# the other side has gone down.
# Ping every 10 seconds, assume that remote
# peer is down if no ping received during
# a 120 second time period.
keepalive 10 60
# For extra security beyond that provided
# by SSL/TLS, create an "HMAC firewall"
# to help block DoS attacks and UDP port flooding.
#
# Generate with:
# openvpn --genkey --secret ta.key
#
# The server and each client must have
# a copy of this key.
# The second parameter should be '0'
# on the server and '1' on the clients.
;tls-auth ta.key 0 # This file is secret
# Select a cryptographic cipher.
# This config item must be copied to
# the client config file as well.
;cipher BF-CBC # Blowfish (default)
;cipher AES-128-CBC # AES
;cipher DES-EDE3-CBC # Triple-DES
# Enable compression on the VPN link.
# If you enable it here, you must also
# enable it in the client config file.
comp-lzo
# The maximum number of concurrently connected
# clients we want to allow.
;max-clients 100
# It's a good idea to reduce the OpenVPN
# daemon's privileges after initialization.
#
# You can uncomment this out on
# non-Windows systems.
user nobody
group nogroup
# The persist options will try to avoid
# accessing certain resources on restart
# that may no longer be accessible because
# of the privilege downgrade.
persist-key
persist-tun
# Output a short status file showing
# current connections, truncated
# and rewritten every minute.
status /var/log/openvpn-status.log
# By default, log messages will go to the syslog (or
# on Windows, if running as a service, they will go to
# the "\Program Files\OpenVPN\log" directory).
# Use log or log-append to override this default.
# "log" will truncate the log file on OpenVPN startup,
# while "log-append" will append to it. Use one
# or the other (but not both).
;log openvpn.log
log-append /var/log/openvpn.log
# Set the appropriate level of log
# file verbosity.
#
# 0 is silent, except for fatal errors
# 4 is reasonable for general usage
# 5 and 6 can help to debug connection problems
# 9 is extremely verbose
verb 3
# Silence repeating messages. At most 20
# sequential messages of the same message
# category will be output to the log.
mute 20
----------------
Still looking for Web space to store the .ISO file.