Configuring Windows Server 2008 as a Remote Access SSL

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					Configuring Windows Server 2008 as a Remote Access SSL VPN Server

A high level overview of VPN networking technologies and a description of Microsoft VPN
protocols, highlighting the advantages of the new SSTP VPN protocol.

Remote Access is one of today’s “big things”. As an increasing number of people need access to
information stored on work and home computers, the ability to access that information from
anywhere is critical. Gone are the days when you could say “I’ll get that information to you when I
get to my computer”. You need that information now if you want to be competitive in today’s
business environment.

In the stone age of computing, the way to remotely access information on your computer was to use
a dial-up connection. RAS dial-up connections worked over regular POTS (Plain Old Telephone
Service) lines and had speeds that ranged up to around 56kbps. Speed was a major problem with
dial-up RAS connections, but an even bigger problem was the cost of the connections when a long
distance number was required for access.

With the introduction and growth of the Internet, dial-up RAS connections became less relevant.
The reason for this was the introduction of virtual private network (VPN) connections. VPN
connections provided the same point to point connectivity that the dial-up RAS connections
provided, but did so faster and cheaper, as the speed of the VPN connection could be as fast as the
Internet link and the cost of the connection is independent of the destination. The only cost is that of
the Internet link.

Virtual Private Networking

A VPN connection allows a computer to establish a virtual and private connection to a network
over the Internet. The connection is virtual because when the computer establishes a VPN
connection over the Internet, the computer making the VPN connection acts like a node that’s
directly connected to the network, as if it had an Ethernet cable connected to that network. The user
can access all the same resources he could connect to as if he were directly connected to the
network. However, in the case of the VPN client connection to a VPN server, the connection is a
virtual one because there is no actual Ethernet connection to the destination network. The
connection is private because the contents of the datastream moving inside the VPN connection are
encrypted so that no one over the Internet is able to intercept and read the contents of the
communications moving over the VPN link.

Windows Servers and clients have supported VPN connections since the days of Windows NT and
Windows 95. While Windows clients and servers have supported VPN connections for over a
decade, the type of VPN support has evolved over time. Windows Vista Service Pack 1 and
Windows Server 2008 now support three types of VPN connections. These are:

      PPTP
      L2TP/IPSec
      SSTP

PPTP is the Point to Point tunneling protocol. PPTP is the simplest method you can use to establish
a VPN connection, but unfortunately it is also the least secure. The reason why PPTP is the least
secure option is that user credentials are not exchanged over a secure link. That is to say, encryption
of the VPN connection takes place after credentials are exchanged. Even though actual credential
information is not transmitted between VPN client and server, the hash values exchanged can be
leveraged by sophisticated hackers to gain access to VPN servers and connect to corporate
networks.

A more secure VPN protocol is L2TP/IPSec. L2TP/IPSec was a joint development between
Microsoft and Cisco. L2TP/IPSec is more secure than PPTP because a secure IPSec session is
established before credentials are sent over the wire. Hackers are not able to access the user
credentials and thus cannot steal them to use them later. More importantly, IPSec provides for
mutual machine authentication, so that untrusted machines are not able to connect to the
L2TP/IPSec VPN gateway. IPSec provides for mutual machine authentication, data integrity,
confidentiality, and non-repudiation. L2TP supports PPP and EAP user authentication mechanisms,
which allows for a high level of log on security because both user and machine authentication is
required.

Windows Vista SP1 and Windows Server 2008 now support a new VPN protocol – Secure Socket
Tunneling Protocol or SSTP. SSTP uses SSL encrypted HTTP connections to establish a VPN
connection to the VPN gateway. SSTP is secure because user credentials are not sent until after a
secure SSL tunnel is established with the VPN gateway. SSTP is also known as PPP over SSL, so
this means that you can use PPP and EAP authentication mechanisms to make your SSTP
connection more secure.

Privacy is Not Security

I should note here that VPN connections are more about privacy than security. While I do recognize
that privacy is a major component of secure communications, privacy in and of itself does not
provide security. VPN technologies provide for privacy of communications over the Internet, which
prevents intruders from reading the contents of your communications. VPN technologies also allow
you to make sure that only authorized users can connect to the network through the VPN gateway.
However, privacy, authentication and authorization do not provide a comprehensive security
solution.

For example, suppose you have an employee who you have granted VPN access. Since your
Windows Server 2008 VPN protocols support EAP user authentication, you decided to deploy
smart cards for your users and use the L2TP/IPSec VPN protocol. The combination of smart cards
and L2TP/IPSec help insure that strong machine and user authentication is required. Your smart
card and L2TP/IPSec solution works well and everyone is happy.

Everyone is happy until one day one of your users connects to your SQL server to access payroll
information and starts to share that information with other employees. What happened? Wasn’t the
VPN connection secure? Yes, the VPN connection was secure to the extent that it provided privacy,
authentication and authorization – but one thing it did not provide was access control, and access
control is the most pivotal aspects of computer security. In fact, it can be argued that without access
control, all other security measure are of relatively little value.

For a VPN solution to be truly secure, you need to make sure your VPN gateway is able to perform
user/group based access controls so that you can implement least privilege access to VPN users.
Advanced VPN gateways and firewalls like the ISA Firewall can perform this type of strong
user/group based access control on VPN connections. In addition, advanced firewalls like the ISA
Firewall can perform stateful packet and application layer inspection on VPN client connections.

Even though the Windows Server 2008 VPN server does not provide for user/group access controls,
there are other ways you can implement strong access controls on the data servers themselves if you
do not want to pay for an advanced firewall and VPN gateway. In this article we are focusing only
the VPN server component. If you would like to learn more about the ISA firewall and its advanced
VPN server capabilities, check out www.isaserver.org

Why Introduce a New VPN Protocol?

Microsoft already had two viable VPN protocols that allowed users to connect to the corporate
network, so why introduce a third one? SSTP is a great advance for Windows VPN users because
SSTP does not have the problems with firewalls and NAT devices that PPTP and L2TP/IPSec have.
In order for PPTP to work through a NAT device, the NAT device needs to support PPTP through a
PPTP “NAT editor”. If there is no NAT editor for PPTP on the NAT device, the PPTP connections
will fail.

L2TP/IPSec has problems with NAT devices and firewalls because the firewall needs to have the
L2TP port UDP 1701 open outbound, the IPSec IKE port, UDP 500 open outbound, and the IPSec
NAT traversal port, UDP 4500 open outbound (the L2TP port is not required when using NAT-T).
Most firewalls in public places, such as hotels, conference centers, restaurants, and other locations
only allow a small number of ports open outbound, such as HTTP, TCP port 80 and HTTPS (SSL),
TCP port 443. If you need support for protocols other than HTTP and SSL when you leave the
office, you are playing a game of dice. You may or may not get the required ports needed for PPTP
or L2TP/IPSec.

In contrast, SSTP VPN connections are tunneled over SSL using TCP port 443. Since all firewalls
and NAT devices have TCP port 443 open, you will be able to use SSTP from anywhere. This
greatly simplifies the life of the road warrior who needs to use VPN connections to connect to the
office, and also makes life a lot easier on the lives of the corporate admin who needs to support the
road warrior, as well as the help desk people at the service providers who provide Internet access
for hotels, conference centers, and other public locations.

The SSTP Connection Process

The following shows how the SSTP connection process works:

   1. The SSTP VPN client establishes a TCP connection with the SSTP VPN gateway between a
      random TCP source port on the SSTP VPN client and TCP port 443 on the SSTP VPN
      gateway.
   2. The SSTP VPN client sends an SSL Client-Hello message, indicating that the SSTP VPN
      client wants to establish an SSL session with the SSTP VPN gateway.
   3. The SSTP VPN gateway sends its computer certificate to the SSTP VPN client.
   4. The SSTP VPN client validates the computer certificate by checking its Trusted Root
      Certification Authorities certificates store to see if the CA certificate that signed the server
      certificate is located in that store. The SSTP VPN client then determines the encryption
      method for the SSL session, generates an SSL session key and encrypts it with the SSTP
      VPN gateway’s public key, and then sends the encrypted form of the SSL session key to the
      SSTP VPN gateway.
   5. The SSTP VPN gateway decrypts the encrypted SSL session key with the private key of its
      computer certificate’s private key. All future communication between the SSTP VPN client
      and the SSTP VPN gateway is encrypted with the negotiated encryption method and SSL
      session key.
   6. The SSTP VPN client sends an HTTP over SSL (HTTPS) request message to the SSTP
      VPN gateway.
   7. The SSTP VPN client negotiates an SSTP tunnel with the SSTP VPN gateway.
   8. The SSTP VPN client negotiates a PPP connection with the SSTP server. This negotiation
      includes authenticating the user’s credentials using standard PPP authentication methods (or
      even EAP authentication) and configuring settings for Internet Protocol version 4 (IPv4) or
      Internet Protocol version 6 (IPv6) traffic.
   9. The SSTP client begins sending IPv4 or IPv6 traffic over the PPP link.

For those of you who are interested in the characteristics of the VPN protocol architecture, you can
see that in the figure below. Notice that SSTP has an additional header compared to the other two
VPN protocols. That because there is HTTPS encapsulation in addition to the SSTP header. L2TP
and PPTP don’t have application layer headers encapsulating the communication.




Figure 1

We will use a simple three machine example network to show how SSTP works. The names and
characteristics of the three machines are:

Vista:
Vista Business Edition
Vista Service Pack 1
Non-domain member

W2008RC0-VPNGW:
Windows Server 2008 Enterprise Edition
Two NICs – Internal and External
Domain member

WIN2008RC-DC:
Windows Server 2008 Enterprise Edition
Domain Controller of MSFIREWALL.ORG domain
DHCP Server
DNS Server
Certificate Server (Enterprise CA)

Notice that you must use Vista Service Pack 1 as the VPN client. While there have been discussions
in the past about Windows XP Service Pack 3 supporting SSTP, this may not end up being the case.
I recently installed the release candidate for Windows XP Service Pack 3 on a test machine and
found no evidence of SSTP support. This is a real shame, as there is a large installed based of
Windows XP on laptop computers, and the common consensus at this time is that Vista is too slow
for laptop use at this time. Perhaps the Vista performance problems will be rectified with Vista
Service Pack 1.

The high level configuration of the example network is seen in the figure below.




Figure 2



The steps required to get a simple three machine SSTP VPN client/server solution to work.


In the first part of this article series on how to configure Windows Server 2008 as a SSL VPN
server, I went over some of the history of Microsoft VPN servers and VPN protocols. We finished
that article up with a description of the example network that we’ll use in this and subsequent
articles on configuring the VPN gateway to support SSTP connections from Vista SP1 clients.

Before we begin, I need to say that I know that there is a step by step guide on how to configure
SSTP connections to Windows Server 2008 on the www.microsoft.com Web site. The problem
with that article is that I felt it did not reflect a real world environment that uses an enterprise CA
for certificate assignment. Because of that, and some of the issues that were left out of the Microsoft
step by step guide, I decided to do this article. I think you will learn a few new things along the way
as you follow along with me.
I’m not going to go through all the steps from the ground up. I will assume that you have installed a
DC and enabled the DHCP, DNS and Certificate Services roles on that server. The certificate server
type should be Enterprise, so that you are hosting an enterprise CA on your network. The VPN
server should be joined to the domain before you begin the following steps. The Vista client needs
to have SP1 installed before you get started.

We will need to perform the following procedures to get the solution working:

      Install IIS on the VPN server
      Request a machine certificate for the VPN server using the IIS Certificate Request Wizard
      Install the RRAS server role on the VPN server
      Enable the RRAS Server and configure it to be a VPN and NAT server
      Configure the NAT server to publish the CRL
      Configure the User Account to allow dial-up connections
      Configure IIS on the Certificate Server to allow HTTP connections for the CRL directory
      Configure the HOSTS file on the VPN client
      Use PPTP to connect to the VPN server
      Obtain a CA Certificate from the Enterprise CA
      Configure the Client to use SSTP and Connect to the VPN Server using SSTP

Install IIS on the VPN Server

This might sound like a strange way to get things started, as I normally suggest that you never put a
Web server on a network security device. The good news is that we do not need to keep the Web
server on the VPN server, we just need to use it for a little while. The reason for this is that the Web
enrollment site included with the Windows Server 2008 Certificate Server is no longer very useful
for requesting computer certificates. In fact, it is no use at all. What is interesting about this is that
you can still try to get a computer certificate using the Web enrollment site, and it will look like it
was installed, but in fact, the certificate is not installed.

To solve this problem, we will take advantage of the fact that we are using an enterprise CA. When
using an Enterprise CA, you can make a request to an online certificate server. The online request
for a computer certificate is allowed when you use the IIS Certificate Request Wizard and request
what they now call a “Domain Certificate”. This only works when the machine requesting the
certificate belongs to the same domain as the Enterprise CA.

Perform the following steps on the VPN server to install the IIS Web server role:

   1. Open the Windows 2008 Server Manager.
   2. In the left pane of the console, click the Roles node.
Figure 1

   3. Click the Add Roles link on the right side of the right pane.
   4. Click Next on the Before You Begin page.
   5. Put a checkmark in the Web Server (IIS) checkbox on the Select Server Roles page. Click
      Next.
Figure 2

   6. Read the information on the Web Server (IIS) page if you like. This is good general
      information about using IIS 7 as a Web server, but since we are not going to use the IIS Web
      server on the VPN server, this information does not really apply to our scenario. Click Next.
   7. On the Select Role Services page, a number of options are already selected. However, if
      you use the default options, it does not seem that you will get the option of using the
      Certificate Request Wizard. This was the case when I tested it. There is no Role Service for
      the Certificate Request Wizard, so I tried putting a checkmark in each of the Security
      options and that seemed to work. Do the same on yours and click Next.
Figure 3

   8. Review the information on the Confirm Installation Selections page and click Install.
   9. Click Close on the Installation Results page.
Figure 4

Request a Machine Certificate for the VPN Server using the IIS Certificate Request Wizard

The next step is to request a machine certificate for the VPN server. The VPN server needs a
machine certificate to create the SSL VPN connection with the SSL VPN client computer. The
common name on the certificate must match the name that the VPN client will use to connect to the
SSL VPN gateway computer. This means that you will need to create a public DNS entry for the
name on the certificate so that resolves to the external IP address on the VPN server, or the IP
address of a NAT device in front of the VPN server that will forward the connection to the SSL
VPN server.

Perform the following steps to request and install the computer certificate on the SSL VPN server:

   1. In the Server Manager, expand the Roles node in the left pane and then expand the Web
      Server (IIS) node. Click on Internet Information Services (IIS) Manager.
Figure 5

   2. In the Internet Information Services (IIS) Manager console that appears in the panes to
      the right of the left pane, click on the name of the server. In this example, the name of the
      server is W2008RC0-VPNGW. Click on the Server Certificates icon in the right pane of
      the IIS console.




Figure 6
   3. In the right pane of the console, click the Create Domain Certificate link.




Figure 7

   4. Fill out the information on the Distinguished Name Properties page. The most important
      entry on this page is the Common Name entry. This name is the name that VPN clients will
      use to connect to the VPN server. You will need a public DNS entry for this name so that it
      resolves either to the external interface of the VPN server, or the public address of a NAT
      device in front of the VPN server. In this example we will use the common name
      sstp.msfirewall.org. Later, we will create HOSTS file entries on the VPN client computer
      so that it can resolve this name. Click Next.
Figure 8

   5. On the Online Certification Authority page, click the Select button. In the Select
      Certification Authority dialog box, click the name of the Enterprise CA and click OK.
      Enter a friendly name for the certificate in the Friendly name text box. In this example
      we’ll use the name SSTP Cert so that we know it is being used for the SSTP VPN gateway.
Figure 9

   6. Click Finish on the Online Certification Authority page.
Figure 10

   7. The wizard will run and then disappear. After this point you will see the certificate appear in
      the IIS console. Double click on the certificate and you can see the common name in the
      Issued to section and that we have a private key that corresponds to the certificate. Click
      OK to close the Certificate dialog box.
Figure 11

Now that we have a certificate, we can install the RRAS Server Role. Note that it is critical that you
install the certificate first, before you install the RRAS Server Role. If you do not, you will end up
being in a world of hurt, because you will have to use a fairly complex command line routine to
bind the certificate to the SSL VPN listener.

Install the RRAS Server Role on the VPN Server

To install the RRAS Server Role, perform the following steps:

   1. In the Server Manager, click the Roles node in the left pane of the console.
   2. In the Roles Summary section, click the Add Roles link.
   3. Click Next on the Before You Begin page.
   4. On the Select Server Roles page, put a checkmark in the Network Policy and Access
      Services checkbox. Click Next.




Figure 12

   5. Read the information on the Network Policy and Access Services page. Most of it is about
      the new Network Policy Server (which used to be called the Internet Authentication Server
      [IAS] which was a RADIUS server) and NAP, neither of which apply to our current
      scenario. Click Next.
   6. On the Select Role Services page, put a checkmark in the Routing and Remote Access
      Services checkbox. This will put checkmarks in the Remote Access Service and Routing
      checkboxes. Click Next.
Figure 13

   7. Click Install on the Confirm Installation Selections page.
   8. Click Close on the Installation Results page.

Enable the RRAS Server and Configure it to be a VPN and NAT Server

Now that the RRAS server role is installed, we need to enable the RRAS service, just like how we
did it in previous versions of Windows. We need to enable the VPN server feature and the NAT
service. While it is clear why we need to enable the VPN server component, you might wonder why
we need to enable the NAT server. The reason for enabling the NAT server is so that external
clients can gain access to the Certificate Server to connect to the CRL. If the SSTP VPN client
cannot download the CRL, the SSTP VPN connection will fail.

In order to allow access to the CRL, we will configure the VPN server as a NAT server and publish
the CRL using reverse NAT. In a production environment you will be more likely to have a
firewall, such as an ISA Firewall, in front of the Certificate Server, so that you would publish the
CRL using the firewall. However, in this example the only firewall we will be using is the Windows
Firewall on the VPN server, so we will need to configure the VPN server as a NAT server in this
example.

Perform the following steps to enable the RRAS service:

   1. In the Server Manager, expand the Roles node in the left pane of the console. Expand the
      Network Policy and Access Services node and click on the Routing and Remote Access
      node. Right click on the Routing and Remote Access node and click Configure and
      Enable Routing and Remote Access.
Figure 14

   2. Click Next on the Welcome to the Routing and Remote Access Server Setup Wizard
      page.
   3. On the Configuration page, select the Virtual private network (VPN) access and NAT
      option and click Next.




Figure 15
   4. On the VPN Connection page, select the NIC in the Network interfaces section that
      represents the external interface of the VPN server. Then click Next.




Figure 16

   5. On the IP Address Assignment page, select the Automatically option. We can select this
      option because we have a DHCP server installed on the domain controller behind the VPN
      server. If you did not have a DHCP server, then you would have to select the From a
      specified range of addresses option and then provide a list of addresses that VPN clients
      could use when connecting to the network through the VPN gateway. Click Next.
Figure 17

   6. On the Managing Multiple Remote Access Servers page, select the No, use Routing and
      Remote Access to authenticate connection requests. This is the option we use when there
      is no NPS or RADIUS server available. Since the VPN server is a member of the domain,
      you can authenticate users using domain accounts. If the VPN server were not a member of
      the domain, then only local accounts on the VPN server could be used, unless you decide to
      use the NPS server. I’ll do an article on how to use an NPS server in the future. Click Next.
Figure 18

   7. Read the summary information on the Completing the Routing and Remote Access
      Server Setup Wizard page and click Finish.
   8. Click OK in the Routing and Remote Access dialog box informing you that relaying of
      DHCP messages requires a DHCP relay agent.
   9. In the left pane of the console, expand the Routing and Remote Access node and then click
      on the Ports node. In the middle pane you will see that WAN Miniport connections for
      SSTP are now available.




Figure 19
Configure the NAT Server to Publish the CRL

As I mentioned earlier, the SSL VPN client needs to be able to download the CRL to confirm that
the server certificate on the VPN server has not been revoked. In order to do this, you need to
configure a device in front of the certificate server to forward HTTP requests for the CRL location
to the Certificate Server.

How do you know what URL the SSL VPN client needs to connect to in order to download the
CRL? That information is contained within certificate itself. If you go to the VPN server again and
double click on the certificate in the IIS console, as you did earlier, you will be able to find this
information.

Click the Details tab of the certificate and scroll down to the CRL Distribution Points entry and
click on that entry. In the lower pane you will see the various distribution points based on the
protocol used to access those points. In the certificate seen in the figure below, you can see that we
need to allow the SSL VPN client access to the CRL via the URL:

http://win2008rc0-dc.msfirewall.org/CertEnroll/WIN2008RC0-DC.msfirewall.org.crl




Figure 20

Because of this, you need to create a public DNS entry for this name so that external VPN clients
can resolve this name to an IP address on a device that will perform reverse NAT or reverse proxy
to allow access to the Certificate Server’s Web site. In this example, we need to have win2008rc0-
dc.msfirewall.org resolve to the IP address on the external interface of the VPN server. When the
connection reaches the external interface of the VPN server, the VPN server will reverse NAT the
connection to the Certificate Server.

If you are using an advanced firewall, such as an ISA Firewall, you could make publishing the CRL
site more secure, by allowing access only to the CRL, and not the entire site. However, in this
article we will limit ourselves to the capabilities of a simple NAT device, such as what the RRAS
NAT provides.

I should note here that using the default CRL site name might not be the more secure option, since it
exposes a private computer name to the Internet. You can create a custom CDP (CRL Distribution
Point) to prevent this if you consider exposing the private name of your CA in your public DNS a
security issue. You can find some information on how to change these values at How to Change the
Policy Settings for a Certification Authority (CA) in Windows 2000.

Perform the following steps to configure RRAS NAT to forward HTTP requests to Certificate
Server:

   1. In the left pane of the Server Manager, expand the Routing and Remote Access node and
      then expand the IPv4 node. Click on the NAT node.
   2. In the NAT node, right click on the external interface in the middle pane of the console. In
      this example, the name of the external interface is Local Area Connection. Click
      Properties.




Figure 21

   3. In the Local Area Connection Properties dialog box, click on the Web Server (HTTP)
      checkbox. That brings up the Edit Service dialog box. In the Private Address text box,
      enter the IP address of the Certificate Server on the internal network. Click OK.
Figure 22

   4. Click OK in the Local Area Connection Properties dialog box.
Figure 23

Now that the NAT server is installed and configured, we can move our attention to configuring the
CA server and the SSTP VPN client.