Chapter 9. Using SpamAssassin as a Proxy

In some environments, it makes no sense to install SpamAssassin on the mail server. For example, the mail server may be underpowered to perform content-checking. Or perhaps users have widely ranging preferences for how much (or indeed whether) spam-checking should be performed, and they may not have accounts on the mail server or any convenient way of configuring their preferences. In these environments, one way to provide those users who want the power of SpamAssassin with spam-checking is to help them install a SpamAssassin POP proxy.

Many more POP proxies are available than IMAP proxies, primarily because IMAP is a much more complex protocol and doesn't require that messages be downloaded to the client. At the time of writing, no freely distributed SpamAssassin IMAP proxies for Windows clients were available.

In addition, most extant proxies call SpamAssassin through the Perl API to avoid having to run the spamassassin shell script or a persistent spamd daemon. Because the Perl API will change in SpamAssassin 3.0, proxies written for SpamAssassin 2.63 are unlikely to continue to work until they are upgraded.

Proxy software is middleware. A proxy receives connections from a client and relays them to a server, intercepting all communication in each direction. Application proxies have been used to pierce smart holes in strong firewalls, to cache frequently accessed data, and to perform a variety of other functions.

Logically, POP proxies sit between a mail client and a POP server. Actually, these proxies typically run on the same computer as the mail client. The proxies discussed in this chapter not only relay data (email messages) between the client and server, but also invoke SpamAssassin to perform spam-checking on the email after it has been received from the server but before it is relayed to the client. Users continue to use their favorite POP client; no changes need be made at the POP server.

In this chapter, I review two SpamAssassin proxies. The first is the venerable Pop3proxy, a freely distributed command-line proxy script written in Perl and suitable for use on several operating systems. The second is the commercial proxy SAproxy Pro from Stata Labs.

POP proxies do not offer the complete functionality of POP servers; in particular, they may be limited in how they can perform authentication and secure the transaction. Using a POP proxy may result in sending your email password across the Internet in the clear.

Figure 9-1 illustrates the example topology for this chapter. pop.example.com is a POP mail server. win.example.com is a Windows-based user workstation that runs a POP mail client (e.g., Outlook Express, Eudora, Netscape Messenger). The SpamAssassin POP proxy will be installed on win.example.com , and the mail client will be configured to connect to the proxy rather than to the POP server. The proxy will be configured to connect to the POP server and to run SpamAssassin on messages as they are downloaded.

Figure 9-1. An example POP mail topology with a client-side proxy