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Securing Web Applications through a Secure Reverse Proxy

This article describes recommended practices for setting up the Sun ONE Proxy Server software to represent a secure content server to outside clients, preventing direct, unmonitored access to your server's data from outside your company. This article uses recommended practices to secure your web applications behind a firewall and leverage access and authentication using the Sun ONE platform products.

This article assumes an intermediate reader who is familiar with installing and configuring the Sun ONE Proxy Server. It also assumes that the reader can configure the firewall router to allow a specific server on a specific port access through the firewall without allowing any other machines in or out.

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In today's complicated, heterogeneous back-end environments, organizations search for ways to manage their diverse corporate infrastructure. One way to accomplish this task is to centralize all Web resource requests to multiple back-end systems. With the large volume of different web servers available, all with multiple versions, updates, and platforms, it is very easy to become vunerable to attacks. For example, different versions of web servers used by companies today have security alerts issue to the public that describe these problems and how to exploit them

Placing the web servers behind the firewall and denying access to them greatly reduces their vulnerability to attacks. The actual web server is accessed using reverse proxy servers. The reverse proxy server retrieves the Web resources through the firewall from the actual web server. In addition, the data returned from the actual web server can be cached locally at the reverse proxy level to improve the access time of the web site. Having a firewall working in tandem with a reverse proxy server greatly reduces the vulnerabilities.

This article covers the following topics:

  • "Understanding the Reverse Proxy Server"

  • "Setting Up the Proxy Server"

  • "Understanding the Secure Reverse Proxy"

  • "Configuring the Proxy Server to Handle Client Authentication with Digital Certificates"

This article is intended for use by intermediate-level system administrators. It assumes that you are familiar with installing and configuring the Sun™ ONE Proxy Server software. It also assumes that you understand network filtering and can properly configure your firewall. This article describes basic capabilities that are found in many common proxy implementations. As such, this approach is not limited to the Sun™ ONE implementation although the examples in this article are.

This article describes recommended practices for installing and configuring the Sun ONE Proxy Server software for the purpose of constructing a secure-content gateway that prevents direct, unrestricted, and unmonitored access to your company's internal systems and data from outside clients. The article also provides recommended practices to secure your web applications behind a firewall and leverage access and authentication with the Sun ONE platform products. In this article, security is accomplished two ways:

  • Secure reverse proxy, which acts as a stand-in for your content server to provide an additional barrier between back-end environments and the possibility of malicious attack.

  • Authentication based on Secure Socket Layers (SSL) certificates is used to guarantee the identity of the clients requesting data from the back-end systems.

This article assumes that the reader has taken the necessary measures to ensure that the suggested recommended practices are built on a secure platform.

Understanding the Reverse Proxy Server

A reverse proxy server is a server that acts as a broker between two entities, validating and processing a transaction in such a way that the actual parties to the transaction do not directly communicate with one another. This means that the proxy acts on behalf of the content web server. A reverse proxy server can represent one or several content servers. Random servers cannot be accessed through a reverse proxy server. Only the predetermined set of files that are available from the content server can be accessed. A reverse proxy server is a designated proxy server for those servers, and it is used by all clients for access to the specific site that it is servicing.

Having a firewall working in tandem with a reverse proxy server can greatly reduce the possibility of exposing your back-end data resources. The firewall must be configured to only allow specific types of access (HTTP/HTTPS) from the reverse proxy server to the back-end web servers. This configuration ensures that requests coming from the proxy are valid requests and all other requests are seen as potential hackers. A properly configured firewall will accept requests from the proxy server and route it appropriately to your back-end resources.

One of the biggest benefits of having a reverse proxy configuration is that your clients have a single point of access to your content web servers. This obviously adds a second layer of security that allows you to track and contain an attack against your content servers. The second benefit is that you, as a system administrator, have a single point of control over who can access the servers and what content you allow the users to access. Another great benefit is that outsiders are not aware of the names of the content servers you are proxying. This allows you to easily replace content servers or make host name changes since the rules or "mappings" are handled by the reverse proxy. This does not affect outside clients. The idea of setting up an architecture with a single point of access helps in the load balancing and failover. For companies concerned with hardware costs, leveraging a reverse proxy can significantly lower hardware cost because it eliminates the need to have separate hardware and software for internal and external users. Internal and external users can access the same servers using the same HTTP requests. This method also eliminates the need to have different hardware to store data for internal and external users. The reverse proxy is capable of securing the back-end data that is required to service an HTTP application without exposing any information to outside world.

So, what exactly does a reverse proxy do? When a client makes a request to your web site, the request goes to the proxy server. The proxy server then sends the client's request through a specific path in the firewall back to the content web server. The content web server passes the result through the path back to the proxy. The proxy sends the retrieved information to the client, as if the proxy were the actual content server (see Figure 1).

Figure 1FIGURE 1 Reverse Proxy Deployed at Firewall

If the content web server returns an error message, the proxy server can intercept the message and change any URLs listed in the headers before sending the message to the client. This prevents external clients from getting redirection URLs to the internal content server.

Since a reverse proxy server potentially allows access to internal hosts, disabling generic (forward) proxying on the proxy server, or applying appropriate access controls if they are enabled, is important. The firewall should be configured so that it allows connections from the reverse proxy to the content web servers exclusively, and not to any other internal resources. The proxy server's configuration should not allow generic proxy requests. It should only allow reverse proxy requests and remap them appropriately to the content web servers.

In addition, the reverse proxy can be configured to secure data utilizing Secure Socket Layers (SSL). This type of configuration is known as a secure reverse proxy. A secure reverse proxy can provide an encrypted connection from a proxy server outside a firewall to a secure content server inside the firewall. It also allows clients to connect securely to the proxy server, facilitating the secure transmission of information (such as credit card numbers).

A reverse proxy configuration has specific components. These components and their functions are:

  • Request URL remapping. The reverse proxy server must map the request URLs to URLs that point to the back-end web server. The reverse proxy server only sends the path portion of the URL to the back-end web server. If the URL being accessed is:

  • http://myproxy.sun.com/dir/file.html

    The back-end web server only receives the portion:

  • Request header remapping. Certain request headers may contain information that was constructed with the assumption that the reverse proxy server is the content server. One such header is the ''Host:" header which carries the hostname that was in the URL that is being requested. Obviously, these headers need to be remapped correctly so that the true content server will receive the proper "Host:" value.

  • Response header remapping. The response headers may contain information that explicitly points to the content web server. One example is the "Location:" header that is used with redirections. The reverse proxy servers remap the "Location:" field, replacing the reference to the content web server with its own address.

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