Paranoid Penguin - Application Proxying with Zorp, Part II
Listing 7. policy.py, Part II (Instance Definitions)
def blue(): Service("blue_http", HttpProxy, router=TransparentRouter()) Service("blue_ssh", PlugProxy, router=TransparentRouter()) Listener(SockAddrInet('10.0.1.254', 50080), "blue_http") Listener(SockAddrInet('10.0.1.254', 50022), "blue_ssh") def purple(): pass def red(): Service("red_http", HttpProxy, router=DirectedRouter(SockAddrInet('192.168.1.242', 80), forge_addr=TRUE)) Listener(SockAddrInet('169.254.1.254', 50080), "red_http")
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.
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.
The English-language home for Balabit, creators of Zorp: www.balabit.com.
The root download directory for ZorpOS contains some tools that make using Zorp GPL much easier, including iptables-utils, a TPROXY-enabled Linux kernel and iptables command. In fact, these are the free parts of the Debian distribution included with Zorp Pro, which is why everything in ZorpOS is in the form of Debian packages. If you aren't a Debian user, everything you want is in the subdirectories of pool; at the top of each package's subdirectory are tar.gz files containing source code. If you are a Debian user, you can use the URL as an apt-get source: www.balabit.com/downloads/zorp/zorp-os.
The Zorp Users' Mailing List is an amazingly quick and easy way to get help using Zorp, whether Pro or GPL. This URL is the site for subscribing to it or browsing its archives. Note that Balabit is a Hungarian company and its engineers (and some of the most helpful Zorp users) operate in the CET (GMT+1) time zone: https://lists.balabit.hu/mailman/listinfo/zorp.
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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