Tricks to do client certificate authentications behind a reverse proxy

The most classical reverse proxies utilizations are:

• The reverse proxy is located on a DMZ (public Internet exposed area)
• Web applications are located on a VLAN (private network)
• The reverse proxy reads the initial request, then it initiates a similar (but new) request to the internal Web applications.

The problem we are tackling in this article is about X509 client certificate authentications. By definition and for security, a HTTPS request clear content cannot be spied. This is why when putting a reverse proxy behind the client and the internal web application, the HTTPS stream will be broken and we will loose all the client certificate data.

Here is some tips to forward without many efforts the client certificate data to the web application:

In this situation, the reverse proxy is an apache and the internal web application is also an apache. The tip is to use the headers modules to manually forward the wanted client cert data. Of course for security reasons, you have to configure your reverse proxy to only allow wanted client certificate (based on the AC for example).

On debian, to activate the headers module, just type this command:

sudo a2enmod headers

Then you have to edit the appropriate reverse proxy virtual host directive this way:

Listen 1981
NameVirtualHost *:1981
<VirtualHost *:1981>
   ServerName localhost
 
   ErrorLog /var/log/apache2/1981.error.log
   CustomLog /var/log/apache2/1981.access.log combined
 
   # activate HTTPS on the reverse proxy
   SSLEngine On
   SSLCertificateFile    /etc/apache2/ssl/mycert.crt
   SSLCertificateKeyFile /etc/apache2/ssl/mycert.key
 
   # activate the client certificate authentication 
   SSLCACertificateFile /etc/apache2/ssl/client-accepted-ca-chain.crt
   SSLVerifyClient require
   SSLVerifyDepth 2
 
   <Proxy *>
     AddDefaultCharset Off
     Order deny,allow
     Allow from all
   </Proxy>
 
   # initialize the special headers to a blank value to avoid http header forgeries
   RequestHeader set SSL_CLIENT_S_DN    ""
   RequestHeader set SSL_CLIENT_I_DN    ""
   RequestHeader set SSL_SERVER_S_DN_OU ""
   RequestHeader set SSL_CLIENT_VERIFY  ""
 
   <Location />
     # add all the SSL_* you need in the internal web application
     RequestHeader set SSL_CLIENT_S_DN "%{SSL_CLIENT_S_DN}s"
     RequestHeader set SSL_CLIENT_I_DN "%{SSL_CLIENT_I_DN}s"
     RequestHeader set SSL_SERVER_S_DN_OU "%{SSL_SERVER_S_DN_OU}s"
     RequestHeader set SSL_CLIENT_VERIFY "%{SSL_CLIENT_VERIFY}s"
 
     ProxyPass          http://localhost:50161/
     ProxyPassReverse   http://localhost:50161/
   </Location>
</VirtualHost>

The important directives are the RequestHeader lines. You can found a complete list of the SSL environement variables at the online apache documentation.

In this situation, the reverse proxy is an apache again and the internal web application is a tomcat server. The tip is to use the AJP protocol. Once your tomcat is configured with an AJP connector, you just have to configure HTTPS with a special option (+ExportCertData) on your apache reverse proxy.

Listen 1979
NameVirtualHost *:1979
<VirtualHost *:1979>
   ServerName localhost
 
   ErrorLog /var/log/apache2/1979.error.log
   CustomLog /var/log/apache2/1979.access.log combined
 
   SSLEngine On
   SSLCertificateFile    /etc/apache2/ssl/mycert.crt
   SSLCertificateKeyFile /etc/apache2/ssl/mycert.key
 
   SSLCACertificateFile /etc/apache2/ssl/client-accepted-ca-chain.crt
   SSLVerifyClient optional
   SSLVerifyDepth 2
 
   # this option is mandatory to force apache to forward the client cert data to tomcat
   SSLOptions +ExportCertData
 
   <Proxy *>
     AddDefaultCharset Off
     Order deny,allow
     Allow from all
   </Proxy>
 
   ProxyPass / ajp://localhost:8009/
   ProxyPassReverse / ajp://localhost:8009/
 
</VirtualHost>