Paranoid Penguin - Application Proxying with Zorp, Part II

The Zorp proxy server works with the kernel Netfilter to make an application-level proxy that looks transparent to the client.

Otherwise, the definition should consist of one or more Service lines, specifying a service name referenced in one or more zone definitions, and a Zorp proxy module, either a built-in proxy included in the global import statements or defined in a custom class. The last field in a Service line is a router, which specifies where proxied packets should be sent. You can see in Listing 7 that for the red_http service, we've used the forge_addr=TRUE option to pass the source IPs of Web clients intact from the Internet to our Web server. Without this option, all Web traffic hitting the DMZ appears to originate from the firewall itself.

Although in Listing 7 we're using only the HttpProxy and the PlugProxy (a general-service UDP and TCP proxy that copies application data verbatim), Zorp GPL also has proxies for FTP, whois, SSL, telnet and finger. As I mentioned before, you also can create custom classes to alter or augment these proxies. It's easy to create, for example, an HTTP proxy that performs URL filtering or an SSL proxy stacked on an HTTP proxy so HTTPS traffic can be proxied intelligently. Unfortunately, these are advanced topics I can't cover here; fortunately, all of Zorp's Python proxy modules are heavily commented.

The TransparentRouter referenced in Listing 7 simply proxies the packets to the destination IP and port specified by the client. But in the red instance's red_http service, we see that a DirectedRouter, which requires a mandatory destination IP and port, may be specified instead.

Each Service line in a service-instance definition must have a corresponding Listener line. This line tells Zorp to which local (firewall) IP address and port the service should be bound. It may seem counterintuitive that the ports specified in Listing 7's Listener statements are high ports: 50080 instead of 80 and 50022 instead of 22. But remember, each proxy receives its packets from the kernel through Netfilter, not directly from clients. Accordingly, these high ports must correspond to those specified in your tproxy table Netfilter rules (Listing 1).

I mentioned that unlike HttpProxy, which is a fully application-aware proxy that enforces all relevant Internet RFCs for proper HTTP behavior, PlugProxy is a general-service proxy (GSP). Using PlugProxy still gives better protection than does packet filtering on its own, because the very act of proxying, even without application intelligence, insulates your systems from low-level attacks that Netfilter may not catch on its own.

Conclusion

And with that, we've scratched the dense surface of Zorp GPL. This is by far the most complex tool I've covered in these pages, but I think you'll find Zorp to be well worth the time you invest in learning how to use it.

Mick Bauer, CISSP, is Linux Journal's security editor and an IS security consultant in Minneapolis, Minnesota. He's the author of Building Secure Servers With Linux (O'Reilly & Associates, 2002).

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