Setting up the SSTP listener and verifying it

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 We have seen the steps to configure a SSTP server in one of the previous posts. However, we will concentrate on on aspect of the configuration in this post in detail and the most important one too, because without this your server is not yet ready to accept SSTP connections - Setting up the SSTP listener and verifying if it is set up correctly.

    As all of you know, SSTP works over HTTPS and so the SSTP listener that Routing and Remote Access Server sets up is very similar to a HTTPS site that you create using IIS. When you create a HTTPS site in IIS, you specify the IP address to listen on (default is INADDR_ANY), port to listen on and also the web server certificate that should be bound to the HTTPS site. Once you do this, a HTTPS listener is setup for the IP:port pair you specified and the certificate you specified to that IP:port pair.

  Now, a similar thing happens when you configure Routing and Remote Access server using the steps given in the previous post. The HTTPS listener is setup. The IP:port pair on which it is setup and the certificate it binds to the listener are as follows:

  • The IP address for the listener is INADDR_ANY i.e. 0.0.0.0 for IPv4 and [::] for IPv6
  • The default port used is 443. However you can change this value to a different port using the registry value 'ListenerPort' at HKEYLocal  MachineSystemCurrentControlSetServicesSstpSvcParameters to the desired value if needed.
  • For the certificate to bind to this listener, it looks in the Local Computer --> Personal store and picks up the first valid certificate that is returned while querying the certificates in the store.

           A valid certificate should satisfy the following:

                        -   Enhanced key usage(EKU) should be either 'Server Authentication' or 'All purpose'

                        -   The certificate should have a private key

          Also, a certificate with EKU 'Server authentication' is preferred over a certificate with EKU 'All purpose'

  As the certificate is mandatory to setup a HTTPS listener, if there is no valid certificate in the Local Computer -->Personal store when Routing and Remote Access starts, the listener will not be setup. And hence SSTP connections cannot be established to the server. This will be informed to the user through an event log.

 Also, it is very important to see that the correct certificate is bound to the listener if there are more than one valid certificates in the Local Computer --> Personal store. This is because, the server sends this certificate bound to the listener to the client when it connects, just as it happens when we access HTTPS sites. When we access HTTPS sites, if the name of the website on the certificate i.e. its subject name is not the same as what we typed in the address bar, we get a warning as below:

"There is a problem with this website's security certificate.

The security certificate presented by this website was issued for a different website's address.

Security certificate problems may indicate an attempt to fool you or intercept any data you send to the server.  

We recommend that you close this webpage and do not continue to this website. "

     The same can occur in the case of SSTP also. If we have a certificate whose subject name is say 'ServerName1' bound to the SSTP listener and we use the name 'ServerName2' for hostname in the client's VPN connection, then the certificate returned to the client will not have the subject name that it expects.

In the case of HTTPS sites, Internet explorer gives us the choice of continuing to the site inspite of knowing the security issue. However, in the case of SSTP connections, this might pose a greater risk as you are exposed to the full network access through the tunnel. If the subject name of the certificate does not match the hostname specified, the SSTP VPN connection cannot be established.

Troubleshooting the listener:

Keeping all the above points in mind, these are the issues that can occur

  • Default port - Is TCP port listening?
  • No valid certificate to bind to the listener
  • More than one valid certificate. Should check if the right one was picked up
  • The listener port specified is not available

Lets take up each one of these separately.

Default port - Is TCP port listening?

On a command prompt, type the command 'netstat -aon |findstr 443'. If you see the below line displayed, then the TCP port is listening for HTTPS requests. You can go to the next step now.

TCP    [::]:443                [::]:0                 LISTENING       4

No valid certificate to bind to the listener

On a command prompt, type the command, 'netsh http show sslcert'. If you see the message that there are no SSL certificate bindings, then it means that there was no valid certificate for SSTP to bind to the listener.

Look at the event viewer (Start --> Run --> eventvwr) under Windows Logs --> System for any log from RasSSTP. You will see an event if this was the case.

Install a valid certificate in the Local Computer --> Personal store and then restart the Routing and Remote Access server configuration.

More than one valid certificate. Should check if the right one was picked up

 On a command prompt, type the command, 'netsh http show sslcert'. If a certificate is bound to the listener, you will see a message as below.

SSL Certificate bindings:
-------------------------

    IP:port                 : 0.0.0.0:443
    Certificate Hash        : c14e9c7ffe2f292ef4367eed10317f4c1ba20df0
    Application ID          : {ba195980-cd49-458b-9e23-c84ee0adcd75}
    Certificate Store Name  : MY
    Verify Client Certificate Revocation    : Enabled
    Verify Revocation Using Cached Client Certificate Only    : Disabled
    Usage Check    : Enabled
    Revocation Freshness Time : 0
    URL Retrieval Timeout   : 0
    Ctl Identifier          :
    Ctl Store Name          :
    DS Mapper Usage    : Disabled
    Negotiate Client Certificate    : Disabled

    IP:port                 : [::]:443
    Certificate Hash        : c14e9c7ffe2f292ef4367eed10317f4c1ba20df0
    Application ID          : {ba195980-cd49-458b-9e23-c84ee0adcd75}
    Certificate Store Name  : MY
    Verify Client Certificate Revocation    : Enabled
    Verify Revocation Using Cached Client Certificate Only    : Disabled
    Usage Check    : Enabled
    Revocation Freshness Time : 0
    URL Retrieval Timeout   : 0
    Ctl Identifier          :
    Ctl Store Name          :
    DS Mapper Usage    : Disabled
    Negotiate Client Certificate    : Disabled

If the Application ID is {ba195980-cd49-458b-9e23-c84ee0adcd75} , then it means that this is a binding added by SSTP. So this command shows that there is a certificate which is bound to 0.0.0.0:443 IP:port listener and also a certificate which is bound to [::]::443 IP:port listener. The certificate hash value specifies which certificate is actually bound. This is the SHA1 certificate hash of the certificate. Here, we see that the SHA1 certificate hash of the certificate is c14e9c7ffe2f292ef4367eed10317f4c1ba20df0

We will use this hash to verify if the correct certificate has been bound to the listener.

  • Open the Microsoft Management Console (Start --> Run --> mmc).
  • Add the Local Computer certificates snap-in (Click on File -->Add/Remove snap-in -->Select 'Certificates' from the list of Available snap-ins --> Click on Add --> Select 'Computer account' --> Click on Next --> Ensure 'Local computer' is selected' --> Click on Finish --> OK
  • Expand the 'Certificates (Local Computer)' node (Doubleclick on the node)
  • Expand the 'Personal' node ( Doubleclick on the node). Click on 'Certificates' subnode under this.
  • On the certificates pane, you will see a list of certificates in the store. Doubleclick on the certificate which you want to be bound to the SSTP listener i.e. the certificate with the subject name matching the hostname used in the client VPN connection.
  • Click on 'Details' tab. Make sure '<All>' is selected in the drop down for 'Show:'
  • Ensure that the value for the field 'Thumbprint Algorithm' is sha1
  • Note the value of the field 'Thumbprint'.
  • Compare to see if this value is the same as the certificate hash we saw in the netsh message.
  • If it is, then it means that the right certificate has been bound to the listener.
  • If it is not the same, then some other certificate has been bound to the listener. We can bind the required certificate to the listener using the following commands. These commands will delete the currently cound certificate and bind the certificate the we want to the listeners.

              Say, the value of the 'Thumbprint' field for the required certificate is 'xxx', type the following command on an elevated command prompt:

                 netsh http delete sslcert ipport=0.0.0.0:443

                 netsh http delete sslcert ipport=[::]:443

                 netsh http add sslcert ipport=0.0.0.0:443 certhash=xxx appid={ba195980-cd49-458b-9e23-c84ee0adcd75} certstorename=MY

                 netsh http add sslcert ipport=[::]:443 certhash=xxx appid={ba195980-cd49-458b-9e23-c84ee0adcd75} certstorename=MY

The listener port specified is not available

  If the listener port that you hav e configured in the registry is not available, SSTP will not be able to set up a listener on that port. There will be an event logged in the event viewer in this case. Open event viewer (Start --> Run --> eventvwr). Navigate to Windows Logs --> System and look for logs from RasSstp.

Janani Vasudevan
Software Design Engineer/Test
RRAS, Windows Enterprise Networking

[This posting is provided "AS IS" with no warranties, and confers no rights.]