l2tpns/docs/html/practical-vpns.html

<h1 id="overview">Overview of VPNs and IPsec</h1>
<h2 id="vpns">Virtual Private Networks</h2>
<p>The purpose of a VPN is to create a secure channel ontop of an
un-secure medium, where a computer or a device are put in each end-point
in order to establish communication, each of these end-points are often
reffered to as Point of Presense, or POP. This kind of a communication
allows the capability of creating a Virtual Private Network, which is
accesable over a medium such as the Internet and thus, extend the
physical boundaries of an existing local network.</p>
<p>VPNs have three forms:</p>
<dl>
<dt>Site-To-Site VPNs</dt>
<dd>
<p>these setups exist in order to extend the local network to create a
much bigger LAN over the Internet.</p>
</dd>
<dt>Network-To-Host or Remote access VPNs</dt>
<dd>
<p>where a central VPN server is able to achieve multiple connections,
often reffered to as RoadWarrior VPNs. (This setup is very common among
ISPs)</p>
</dd>
<dt>Network-To-Network</dt>
<dd>
<p>extranet VPNs allow secure connections within branches and business
partners, they are an extension of a Site-To-Site VPNs.</p>
</dd>
</dl>
<p><img src="./images/site-to-site-vpn.png" alt="site to site" /><br />
shows a Site-To-Site VPN diagram.</p>
<p>IP/VPNs are connections which are based upon IP tunnels. A tunnel is
a way to encapsulate an IP packet inside another IP packet or some other
type of packet. Why do we need tunneling? A Virtual Private Network is
identified by IANA's private IP assignments and so such packet can not
go beyond the uplink Internet interface.</p>
<p><img src="./images/tunneling-process.png"
alt="tunneling process" /><br />
shows the tunneling process.</p>
<p>Several tunneling protocols are available for manifesting VPNs.</p>
<dl>
<dt>L2F</dt>
<dd>
<p>Layer 2 Forwarding, an older implementation which assume position at
the link layer of the OSI. It has no encryption capabilities and hence,
deprecated.</p>
</dd>
<dt>L2TP</dt>
<dd>
<p>Layer 2 Tunneling Protocol, still no encryption capabilities.</p>
</dd>
<dt>PPTP</dt>
<dd>
<p>Point-to-Point Tunneling Protocol, and yet again, no encryption.</p>
</dd>
</dl>
<p>As seen, the requirement of encryption enhancement is urgent in order
to assure authentication, data integrity and privacy. IPsec solves this
by providing a suite of security measures implemented at layer 3.</p>
<h2 id="ipsec">IP Security Suite (IPsec)</h2>
<p>VPN Security is now appearing, this complex things. How so? VPN
tunnels by themselves are easily maintained by single-standalone tools
like pppd, l2tpns, stunnel and others. Involving security with VPNs
though requires more:</p>
<ul>
<li><p>authentication, data integrity and privacy</p></li>
<li><p>keying management</p></li>
</ul>
<div class="note">
<p>Keys are secrets being shared by two end-points to provide a secure
mean of communication against a third-party connection from sniffing the
actual data.</p>
</div>
<p>Different ways to handle key management include RADIUS (Remote
Authentication Dial In User Service) systems which provide AAA
(Authentication, Authorization and Accounting). Another solution is
ISAKMP/Oackly - Internet Security Association and Key Management
Protocol. This solution requires you to posess one of the following:</p>
<ul>
<li><p>something you have</p></li>
<li><p>something you know</p></li>
<li><p>something you are</p></li>
</ul>
<p>The more requirements you meet the more secure is the medium, once
established. Let's review, something we have is like a certificate, it
proves who we are. Something we know, is a key, a secret password which
we were told in a whisper, and something we are is our-fingerprint which
identifies ourselves from other individuals.</p>
<h3 id="ipsec-in-depth">IPsec in Depth</h3>
<p>IPsec consists of two main protocols, an Authentication Header and
Encapsulation Security Payload, also known as AH and ESP. Although it is
not bound to these and can be extended (and often is) to other standarts
such as</p>
<ul>
<li><p>Data Encryption Standart (DES and 3DES)</p></li>
<li><p>Diffie-Hellman (DH)</p></li>
<li><p>Secure Hash Algorithm-1 (SHA1)</p></li>
<li><p>Message Digest 5 (MD5)</p></li>
<li><p>Internet Key Exchange (IKE)</p></li>
<li><p>Certification Authorities (CA)</p></li>
</ul>
<p>We'll be deploying an IKE daemon to handle the key management, which
uses the Diffie- Hellman cryptography protocol in order to allow two
parties to establish a connection based upon a shared secret key that
both parties posess. (Authentication within IKE is handled by MD5
hashing)</p>
<p>IKE is responsible for authentication of two IPsec parties,
negotiation of keys for encryption algorithms and security associations.
This process is commonly regarded as two phases:</p>
<dl>
<dt>Phase 1: IKE Security Association</dt>
<dd>
<p>The IKE daemon authenticates against the peers in order to achieve a
secure channel, according to the Diffie-Hellman key agreement.</p>
</dd>
<dt>Phase 2: IKE IPsec Negotiation</dt>
<dd>
<p>After achieving an authenticated channel, the parties now negotiate a
secure transform (the way to encrypt and secure the medium) where the
sender is offering his/hers transform set after which the receiver
decides upon one. An IPsec session can now safely begin.</p>
</dd>
</dl>
<p>Just to be clear, a Security Association is an agreed relation
between two parties which describes how they will use security services
(from IPsec) to communicate.</p>
<h3 id="ipsec-modes">IPsec Modes</h3>
<p>IPsec can operate in two different modes:</p>
<dl>
<dt>Transport mode</dt>
<dd>
<p>takes place when two devices (like a station and a gateway (now
considered a host)) are establishing a connection which upon they both
support IPsec.</p>
</dd>
<dt>Tunnel mode</dt>
<dd>
<p>we require tunnel mode when we proxy IPsec connetions between two
stations behind the IPsec gateway. For example, in a Site-to-Site VPN a
tunnel mode lives, since it exists in order to provide the stations
behind these gateways runing the VPN/IPsec to communicate securely. In
this situation, both end-points are runing an IPsec software.</p>
</dd>
</dl>
<p>In definition, a tunnel mode IPsec is better secured than transport.
Without going too deep into the ins-and-outs of the technical side,
transport mode doesn't encapsulate the actual IP layer but only the
tcp/udp (Transport layer of the OSI) where-as a tunnel mode encapsulate
both the Transport layer and the IP layer into a new IP packet.</p>
<p>To summarize, we need VPNs for data-exchange methods and a set of
IPsec tools for security reasons.</p>
<h1 id="deployment">VPN Deployment</h1>
<p>I've assembled another diagram to view the actual VPN setup.<br />
<img src="./images/vpn-deployment.png" alt="vpn deployment" /><br />
gives a general description of how the network will be layed out in
real-world scenario.</p>
<p>We notice that a single Linux box is acting as a Gateway and has all
the services included with it. This is a bad idea from a security
prespective but it's easy to just deploy the FreeRADIUS and MySQL
servers on another machine. Of course the L2TPns and the rest of the
IPsec tools suite would have to remain on the Gateway box (not
necessarily the Firewall).</p>
<p><img src="./images/vpn-process.png" alt="vpn process" /><br />
attempts to explain the actual process that the VPN takes and to detail
the place that each of that application-in-charge takes place.</p>
<h2 id="deployment-requirements">Requirements</h2>
<h3 id="deployment-requirements-toolbox">The Toolbox</h3>
<p>Following is a description of the requirements you will have to
meet:</p>
<dl>
<dt>A Linux box</dt>
<dd>
<p>preferably a 2.4.27 kernel or higher.</p>
<p>Debian is the chosen distribution which means we'll be using apt-get
for installation, but I'll also focus on basic source tarballs
installation.</p>
<p>Dependencies:</p>
<ul>
<li>ipsec configuration in the kernel</li>
</ul>
</dd>
<dt>L2TPns</dt>
<dd>
<p>an L2TP PPP Termination tool.</p>
<p>Dependencies:</p>
<ul>
<li><p>libcli 1.8.0 or greater</p></li>
<li><p>tun/tap interface compiled in the kernel or as a module</p></li>
</ul>
</dd>
<dt>FreeRADIUS</dt>
<dd>
<p>For authentication, and accounting.</p>
</dd>
<dt>MySQL</dt>
<dd>
<p>To act as a back-end database for the RADIUS.</p>
</dd>
<dt>OpenSwan</dt>
<dd>
<p>Provides the ipsec suite package.</p>
</dd>
</dl>
<h3 id="deployment-requirements-kernel">Kernel Support</h3>
<p>Debian stock kernel 2.4.27 and up are ipsec compatible although if
you think otherwise check for the kernel-patch-openswan package.</p>
<h2 id="deployment-installation">Installation</h2>
<h3 id="deployment-installation-l2tpns">L2TPns</h3>
<h4 id="deployment-installation-l2tpns-install">Installation</h4>
<blockquote>
<p>L2TPns is a layer 2 tunneling protocol network server (LNS). It
supports up to 65535 concurrent sessions per server/cluster plus ISP
features such as rate limiting, walled garden, usage accounting, and
more.</p>
</blockquote>
<p>In a personal note - L2TPns is highly configurable for many cases,
and extremely reliable for production/commerical use.</p>
<dl>
<dt>Step 1:</dt>
<dd>
<p>Make sure you have libcli-1.8 development package installed:</p>
<pre><code># apt-cache search libcli
libcli-dev - emulates a cisco style telnet command-line interface (dev files)
libcli1 - emulates a cisco style telnet command-line interface
# apt-get install libcli-dev</code></pre>
</dd>
<dt>Step 2:</dt>
<dd>
<p>Download the source from <a
href="http://sourceforge.net/projects/l2tpns/">SourceForge</a>.</p>
</dd>
<dt>Step 3:</dt>
<dd>
<p>Build and install: <code>make &amp;&amp; make install</code></p>
</dd>
</dl>
<div class="note">
<p>Alternately, you can skip these steps and simply
<code>apt-get install l2tpns</code>.</p>
</div>
<div class="note">
<p>On RPM-based distributions, you should be able to make packages from
the libcli and l2tpns source tarballs with
<code>rpmbuild -ta</code>.</p>
</div>
<p>Once compiliation is done you will have l2tpns in
<code>/usr/sbin/l2tpns</code>, and all configuration files can be found
in <code>/etc/l2tpns/</code>.</p>
<h4 id="deployment-installation-l2tpns-config">Configuration</h4>
<p>The only configuration that L2TPns takes is centralized in the
configuration file <code>/etc/l2tpns/startup-config</code>.</p>
<pre><code>set debug 2                               # Debugging level
set log_file &quot;/var/log/l2tpns&quot;            # Log file: comment out to use stderr, use
                                          # &quot;syslog:facility&quot; for syslog
set pid_file &quot;/var/run/l2tpns.pid&quot;        # Write pid to this file
set l2tp_secret &quot;secret&quot;                  # shared secret
set primary_dns 212.117.128.6             # Only 2 DNS server entries are allowed
set secondary_dns 212.117.129.3
set primary_radius 192.168.0.1            # Can have multiple radius server entries,
                                          # but ony one radius secret
set primary_radius_port 1812
set radius_secret &quot;radius_secret&quot;
set radius_accounting yes
set radius_dae_port 3799
set accounting_dir &quot;/var/run/l2tpns/acct&quot; # Write usage accounting files into specified
                      # directory
set peer_address 192.168.0.1              # Gateway address given to clients
load plugin &quot;sessionctl&quot;                  # Drop/kill sessions
load plugin &quot;autothrottle&quot;                # Throttle/snoop based on RADIUS
load plugin &quot;throttlectl&quot;                 # Control throttle/snoop with nsctl
load plugin &quot;snoopctl&quot;</code></pre>
<p>This is the trimmed down version of probably most of the common
configuration and even some extra options.</p>
<p>Important configuration options are highlited and you should adjust
these to meet your network needs. We can deploy all of the environment
into one box which is of course not a very good idea from a security
point of view, but will function just fine. Moreover, we will be using
aliased IP addresses so once you've decided to move the FreeRADIUS
daemon to another computer on the LAN it will be fairly easy and won't
take too much configuration into it.</p>
<p>Next, we need to setup the IP pool that L2TPns will provide to each
VPN client. The configuration file is located at
<code>/etc/l2tpns/ip_pool</code> and should look like the following:</p>
<pre><code>172.16.21.0/24</code></pre>
<div class="important">
<p>Of course you can change this pool to anything else (IANA IPs
assigned for private internets only) just make sure it is not
conflicting with your current LAN network addresses. This means that if
you've assigned addresses of 192.168.0.1 and 192.168.0.2 to your LAN
boxes you can't have a pool of 192.168.0.1/24 defined since L2TPns will
try to route those addresses from the tun device, which is needless to
say a bad idea...</p>
</div>
<p>Next up, creating the access-list for L2TPns.</p>
<p>Add a username and password into <code>/etc/l2tpns/users</code>:</p>
<pre><code>admin:12345</code></pre>
<p>The password may either be plain-text as above, or encrypted with MD5
or DES (to distinguish DES from plain-text passwords, prefix the value
with <code>{crypt}</code>).</p>
<p>L2TPns utilizes a terminal connection on port 23 which you would feel
very comfortable in if you have worked with routers and switches devices
before. The terminal provides control over the ppp termination which is
why we've created an account to log on to.</p>
<h3 id="deployment-installation-ipsec">IPsec</h3>
<h4 id="deployment-installation-ipsec-install">Installation</h4>
<p>User-space IPsec tools for various IPsec implementations exist for
linux, among them is the port of KAME's libipsec, setkey, and racoon.
Others are the OpenSWAN (a successor to the FreeSWAN project).</p>
<p>Getting IPsec installed is fairly easy with Debian:</p>
<pre><code># apt-get install openswan</code></pre>
<p>The OpenSWAN project provides packages for RPM-based
distributions.</p>
<p>Alternately, you may download the <a
href="http://www.openswan.org/code/">source</a> from the OpenSWAN
project:</p>
<pre><code># tar xvzf openswan-2.4.4.tar.gz
# cd openswan-2.4.4
# ./configure &amp;&amp; make &amp;&amp; make install</code></pre>
<h4 id="deployment-installation-ipsec-config">Configuration</h4>
<p>OpenSWAN acts as the IKE daemon (remember IKE? it's job is to
authenticate between the two peers and negotiate a secure medium). We
will be setting up the IKE daemon as a RoadWarrior configuration, a term
for remote access VPNs.</p>
<p>We desire this approach for compatibilty because after our VPN
solution will be complete any user from a Windows machine will be easily
ready to connect without any 3rd party applications, same for Linux.</p>
<p>Configuration files are placed in <code>/etc/ipsec.d/</code>,
<code>/etc/ipsec.conf</code> and <code>/etc/ipsec.secrets</code>.</p>
<p>Let's start by choosing the remote client and it's PSK (Private
Shared Key) <code>/etc/ipsec.secrets</code>:</p>
<pre><code>hostname_or_ipaddress %any : PSK &quot;mysecretkeyisverylong&quot;</code></pre>
<p>This is an IP/key pair. The IP or FQDN defines the local peer (like a
SOHO branch), then the remote host. Here we defined %any for all hosts,
though it's possible to define only a specific IP. At last, we define
the key associated with it.</p>
<p>A better way to create a key is to utilize /dev/random for creating a
unique key.</p>
<pre><code># dd if=/dev/random count=16 bs=1 2&gt;/dev/null | xxd -ps</code></pre>
<p>Next, let's prepare the configuration file
<code>/etc/ipsec.conf</code>:</p>
<pre><code>version 2.0
config setup
     nat_traversal=yes

conn l2tp
     authby=secret
     pfs=no
     keyingtries=3
     left=real_ip_address
     leftnexthop=%defaultroute
     leftprotoport=17/%any
     right=%any
     rightprotoport=17/%any
     auto=add

include /etc/ipsec.d/examples/no_oe.conf</code></pre>
<p>In this file we have first defined version 2 which is a must, then
enabled NAT Traversal. To understand the importance of this feature
think of the following scenario: A remote user attempts to connect while
he's behind a router and therefore NATed. The router has to
de-encapsulate the packet, change things and then build it up again and
send it. IPsec doesn't like other people messing with it's packet.
That's why we solve this issue with NAT Traversal.</p>
<p>Next up we configure authentication type (certificates, psk, rsa
keys, etc) then the left and right peers. The default mode OpenSWAN
takes is tunnel unless told otherwise. I won't go into in-depth
explanation of every method, you can take a quick look at
<code>/etc/ipsec.d/examples</code> for more explanation and other
variations of working with RSA keys, Certificates, host-to-host, and
more.</p>
<p>In summary:</p>
<ul>
<li><p>We've configured an almost complete IPsec VPN setup.</p></li>
<li><p>We've installed and configured a VPN server (L2TPns) and our
IPsec security suite.</p></li>
<li><p>To control both of them we use: <code>/etc/init.d/l2tpns</code>
and <code>/etc/init.d/racoon</code> (location of start-up scripts may
vary on non-Debian systems, or if you've installed from
source).</p></li>
</ul>
<h3 id="deployment-installation-freeradius">FreeRADIUS</h3>
<p>The VPN setup needs to authenticate against something, that is the
users database which we chose to be a FreeRADIUS server backed with a
MySQL database.</p>
<h4 id="deployment-installation-freeradius-install">Installation</h4>
<blockquote>
<p>FreeRADIUS is the premiere open source RADIUS server. While detailed
statistics are not available, we believe that FreeRADIUS is well within
the top 5 RADIUS servers world-wide, in terms of the number of people
who use it daily for authentication. It scales from embedded systems
with small amounts of memory, to systems with millions of users. It is
fast, flexible, configurable, and supports more authentication protocols
than many commercial servers.</p>
</blockquote>
<p>Installing on Debian:</p>
<pre><code># apt-get install freeradius freeradius-mysql</code></pre>
<p>From source: Download the latest freeradius package from <a
href="http://freeradius.org/getting.html">freeradius.org</a></p>
<pre><code># tar xvzf freeradius.tar.gz
# cd freeradius
# ./configure &amp;&amp; make &amp;&amp; make install</code></pre>
<h4 id="deployment-installation-freeradius-config">Configuration</h4>
<p>This will appear a bit complex but it isn't, it's just a lot of
configuration.</p>
<p>Following are the configurations you need to have in your
<code>/etc/freeradius/</code> files.</p>
<p>In this section I will not give you a dump of the configuration since
they are very long and mostly default. I'll just post which changes to
make.</p>
<p>We haven't yet configured MySQL, but it'll come afterwards, don't
worry.</p>
<p>Make the following changes to the file
<code>/etc/freeradius/sql.conf</code>:</p>
<pre><code>server = &quot;192.168.0.1&quot;
login = &quot;radius&quot;
password = &quot;12345678&quot;</code></pre>
<p>Add the following to the file
<code>/etc/freeradius/clients.conf</code>:</p>
<pre><code>client 192.168.0.1 {
    secret    = my_secret
    shortname = localhost
    nastype   = other
}</code></pre>
<p>Don't confuse the secret directive there with IPsec. RADIUS server
are using secret keys also to identify their allowed NAS (Network Access
Servers), these are the clients that talk to the RADIUS server.</p>
<p>Also, change the <code>client 127.0.0.1 {}</code> directive to hold
the secret "my_secret" like we configured for 192.168.0.1 to avoid
conflicts.</p>
<p>Uncomment the <code>sql</code> directive in the
<code>authorize</code>, <code>accounting</code>, and
<code>session</code> sections of
<code>/etc/freeradius/radiusd.conf</code>.</p>
<p>Now for populating FreeRADIUS with MySQL. If you don't know or
haven't set root password for MySQL you can do it now with:</p>
<pre><code># mysqladmin -u root password password_here</code></pre>
<p>Then add the following to <code>/root/.my.cnf</code>:</p>
<pre><code>[mysqladmin]
user = root
password = password_here</code></pre>
<p>Create the <code>radius</code> database, using the schema given in
<code>/usr/share/doc/freeradius/examples/db_mysql.sql.gz</code>.</p>
<div class="note">
<p>It may be necessary to modify the column definition of
<code>id</code> in the <code>nas</code> table, removing
<code>DEFAULT '0'</code> such that the definition reads:</p>
<pre><code>id int(10) NOT NULL auto_increment,</code></pre>
</div>
<pre><code># mysqladmin create radius
# mysql radius
mysql&gt; source db_mysql.sql
mysql&gt; GRANT ALL ON * TO &#39;radius&#39;@&#39;localhost&#39; IDENTIFIED BY &#39;radius_password&#39;;</code></pre>
<p>All the configuration is now done. Let's add a user to our VPN
database.</p>
<pre><code># mysql radius
mysql&gt; INSERT INTO radcheck values (0, &quot;test&quot;, &quot;User-Password&quot;, &quot;==&quot;, &quot;1234&quot;);</code></pre>
<p>We have now created a user in the database of username
<code>test</code> and password <code>1234</code>.</p>
<p>Testing the RADIUS setup is simple using the radtest utility provided
with it.</p>
<pre><code># radtest
Usage: radtest user passwd radius-server[:port] nas-port-number secret [ppphint] [nasname]
# radtest test 1234 192.168.0.1 1812 my_secret</code></pre>
<p>radtest sends an Access-Request to the RADIUS server and expects an
Access-Accept back from it. If you're not getting an Access-Accept from
the RADIUS you're advised to check the configuration again and see what
you might have missed.</p>
<h3 id="deployment-installation-firewall">Firewall Configuration</h3>
<p>We need to apply a few things to iptables configuration and kernel
networking.</p>
<p>First off, we need to accept VPN-specific packets through the
firewall. Of course you will have to adjust the rules to fits you needs,
in this case, ppp0 is the Internet interface.</p>
<pre><code># iptables --append INPUT --in-interface  ppp0 -p udp --dport 1701 -j ACCEPT
# iptables --append INPUT --in-interface  ppp0 -p udp --dport 500 -j ACCEPT
# iptables --append INPUT --in-interface  ppp0 -p udp --dport 4500 -j ACCEPT
# iptables --append INPUT --in-interface  ppp0 -p 50 -j ACCEPT</code></pre>
<p>If you haven't setup your Linux box as a gateway yet then you have to
allow forwarding/masqing for the boxes on the LAN (and therefore for the
VPN clients):</p>
<pre><code># iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
# iptables --append FORWARD --in-interface eth0 -j ACCEPT
# echo 1 &gt; /proc/sys/net/ipv4/ip_forward</code></pre>
<h1 id="references">References</h1>
<dl>
<dt>VPN Reference</dt>
<dd>
<p><a
href="http://www.jacco2.dds.nl/networking/freeswan-l2tp.html"></a></p>
</dd>
<dt>L2TPns Project</dt>
<dd>
<p><a href="http://l2tpns.sourceforge.net"></a></p>
</dd>
<dt>OpenSWAN Project</dt>
<dd>
<p><a href="http://www.openswan.org"></a></p>
</dd>
</dl>