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Proxy Firewalls

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Tools for Proxying

Many available applications provide proxy capabilities. We've already mentioned some of them while describing proxy capabilities. However, it's becoming harder to find pure proxy products on the market. The major commercial vendors have embraced hybrid technologies that combine proxy and stateful technologies, making it harder to identify when proxy techniques are used in their products. Mergers and acquisitions in the field have also added confusion to this subject. A classic example of this is the Gauntlet firewall. Gauntlet was one of the first, and most popular, proxy firewalls. Originally created by TIS, its technology was first acquired in 1998 by Network Associates, Inc (NAI). NAI continued to sell the products under the Gauntlet name. In 2002 though, NAI sold Gauntlet to Secure Computing, which already owned a competing firewall called Sidewinder. Secure Computing then integrated both products together to create a hybrid product. If you were to read the marketing literature for this product (which retains the Sidewinder name), you would not see any reference to proxies. How do we know there is any proxy technology left in it? Well, certain keywords keep popping up that can clue you in. These include terms such as secure gateway services and application layer protocol analysis. The bottom-line impact of all this market activity is you are going to need to ask and to experiment to determine how commercial products protect your network.

In the rest of this section, we'll talk about some important proxy technologies. We'll start with one of the proxies that started it all: the Firewall Toolkit. In addition, we'll cover an important proxy-enabling technology called SOCKS. Finally, we'll cover Squid, the most popular open source web proxy.

Firewall Toolkit (FWTK)

Firewall Toolkit was one of the first proxy firewalls. It was developed by Trusted Information Systems (TIS) under an Advanced Research Projects Agency (ARPA) grant, and it was first released in October of 1993. The key technology of FWTK was used to create the first version of the Gauntlet firewall. FWTK is still available at http://www.fwtk.org, but has not been updated for many years. In fact, version 2.1, the last update, was released in early 1998, and there are no current plans to extend it further. Still, it can be used to implement a useful proxy firewall, especially if you do not need to support many protocols.

FWTK is available in source code, which is an important part of its appeal. Anyone from a security analyst to a potential attacker can review its design to look for defects. TIS referred to this as a crystal box design, a term coined by one of its first customers. In a crystal box design, nothing about the design is hidden. Therefore, the security of the system is totally dependent on the quality of the design, not any secrets buried inside the design. Put another way, FWTK does not depend on security through obscurity.

This same approach has been followed in the cryptographic community for decades. The belief is that if a design has not been peer-reviewed by the cryptographic community, no one should have any confidence in its security. This might seem an arrogant point of view, until you look at the history of proprietary cryptographic systems. To save you some research, they have not faired very well. This is something to consider when you select the products you will use to secure your network. It is important to remember that FWTK is currently unsupported. This, and the fact that it does not support many modern protocols (such as H.323), would make it a poor choice for an enterprise firewall. However, if your network does not require protocols unsupported by FWTK, and you are interested in learning the nuts and bolts of proxy implementation, FWTK can be an effective product.


As we discussed at the beginning of this chapter, SOCKS is a proxy toolkit that enables applications to be proxied without requiring specific proxy code to be re-created for each client application. Many proxy products support the SOCKS protocol, allowing any SOCKS-enabled client to make use of the proxy's services. This includes providing access to hosts on the other side of the SOCKS server without requiring direct IP connectivity. The SOCKS server performs authentication and authorization functions on requests, establishes proxy connections, and relays data between hosts. A SOCKS proxy server licensed for noncommercial use is available for free from http://www.socks.permeo.com/.

For applications to work with the SOCKS proxy server, they must be "SOCKS-ified." Most of the work involved in doing this has been packaged into the SOCKS software development kit (SDK). A reasonably skilled network application developer would have little difficulty adding SOCKS functionality to an application using the SDK.

SOCKS has evolved over time and gone through several revisions. SOCKS version 4 was the first popular version of SOCKS and is still in use. However version 5 adds important features, including the support of UDP proxying as well as a variety of authentication methods. The Internet Engineering Task Force (IETF) approved SOCKSv5 as the standard (RFC 1928) generic, proxy protocol for TCP/IP-based networking applications.

SOCKS is more of an enabling technology than a product in its own right. Many client software packages already support SOCKS. If they do, you can securely manage connectivity, authentication, and access control to them using any SOCKS-compliant proxy. Examples of common proxy servers that support SOCKS are Squid (described in the following section, "Squid"), Apache's mod_proxy module, and Permeo's proxy products. If you have an application that does not support SOCKS that you would like to add proxy support to, using the SOCKS API is a relatively quick and effective way of adding a robust proxy implementation to your product.

SOCKS Version 4

The SOCKSv4 protocol defines the message format and conventions to allow TCP-based application users transparent access across a firewall. During proxy connection setup, the SOCKS server grants access based on TCP header information, including IP addresses and source and destination host port numbers. The SOCKS server also authorizes users using Ident (RFC 1413) information.

SOCKS Version 5

The SOCKS version 5 protocol, also known as authenticated firewall traversal (AFT), is an open Internet standard (RFC 1928) for performing network proxies at the transport layer. It resolves a few issues that SOCKS version 4 protocol did not fully address or omitted:

  • Strong authentication—The SOCKSv5 authentication method negotiation is handled by the SOCKSv5 client/server communication. The application client identifies the authentication methods it can support to the SOCKSv5 server. The SOCKSv5 server, in turn, sends a message to the client identifying the authentication method the client should use. The authentication method is also determined based on the security policy defined in the SOCKSv5 server configuration. If the client's supported authentication methods fail to meet the security requirements of the proxy's policy, the SOCKSv5 server denies communication.

  • Address resolution proxy—SOCKSv5's built-in address resolution proxy simplifies DNS administration and facilitates IP address hiding and translation. SOCKSv5 clients can pass the name, instead of the resolved address, to the SOCKSv5 server, and the server resolves the address for the client.

  • Proxy for UDP-based applications—SOCKSv5 supports UDP association by creating a virtual proxy circuit for UDP-based application data.

There are two additional SOCKSv5-related standards to support authentication methods:

  • Username/password authentication for SOCKSv5 (RFC 1929)

  • GSS-API (Generic Security Service Application Programming Interface) authentication for SOCKSv5 (RFC 1961)


Squid is a highly regarded open source web proxy project. It provides high-performance proxy caching for HTTP, HTTPS, and FTP. Squid can be used in several web proxy scenarios. Its most frequent use is to cache browser requests for a site to accelerate and control web conversations. However, it is equally useful as a web server accelerator and as a reverse proxy server.

Squid was designed to run under UNIX and has been successfully compiled on a broad set, including Linux, FreeBSD, OpenBSD, Mac OS/X, Sun Solaris, IBM AIX, and HP-UX. (Note that this is only a partial list.) It can also be compiled to run under Windows if used in conjunction with the Cygwin (http://www.cygwin.com) and Mingw (http://www.mingw.org) packages. Squid is available at http://www.squid-cache.org.

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