Beating Spam and Viruses with amavisd-new and Maia Mailguard
As a fail-safe measure, it often is a good idea to block mail containing executable attachments, even though your virus scanners may claim they're clean. Virus scanners aren't perfect, after all, and brand-new malware might reach your network before your antivirus vendor makes a new signature available to detect it. amavisd-new lets you define a list of file extensions, content classes and MIME-types that should be quarantined, rejected or discarded.
According to RFC 2822, mail headers are not supposed to contain any characters above 127 nor any NUL or bare carriage-return characters. Characters outside this range are supposed to be specially encoded, so that mail software around the world can parse them without confusion. When mail with invalid headers arrives, it could be the product of a poorly written mail client, but often this is a symptom of a specially designed program used by spammers to do their mass mailings. The authors of this so-called ratware often are English speakers, and they don't typically think about the fact that their software might be used by spammers who speak other languages. When those spammers try to use these tools to send their mail, the ratware does not encode the special characters, producing invalid mail headers. With amavisd-new, you can decide how to handle mail with invalid headers: quarantine it, reject it, discard it or let it through.
amavisd-new lets administrators define system-wide content-filtering policies, but these settings can be overridden at the domain and user levels. Some users may want to have their mail scanned for all four suspicious content types—viruses, spam, banned files and bad headers—while others might prefer to disable one or more of those checks. One user might want mail arriving with a spam score of 5.0 or higher to be quarantined, while another user might prefer to have the Subject: header prefixed with a special tag, such as ***SPAM***, if the score is 4.0 or higher but have it blocked only if the score is at least 8.0. This fine-grained control over the filtering process lets administrators accommodate a wide range of users with different needs.
Similarly, amavisd-new provides whitelists and blacklists at all three of these levels. This allows administrators to define system-wide lists; at the other end, users can maintain their own individual lists.
When amavisd-new blocks an e-mail, it can be configured to do a number of things to that mail. The mail can be stored in a quarantine directory or mailbox, including special per-user mailboxes, such as joe+spam. You also can configure amavisd-new to reject the mail, refusing to accept it from the upstream mail server or discard it quietly.
If your organization's policies require that you notify the senders of blocked mail, amavisd-new can be configured to do so. This is a controversial subject, however. A lot of people find virus alerts and spam complaint e-mails to be more of a nuisance than a help nowadays, particularly because the sender addresses of these items often are forged. If you must send virus notifications, amavisd-new provides a mechanism for listing the viruses known to fake the sender's address, so notices are not sent out when those viruses are detected. This list must be maintained by hand and must be matched to the names your particular virus scanners generate. If you find it easier to list the viruses that don't fake sender addresses, you can use an inverse list instead.
The Maia Mailguard Project began its life as a simple Web front end for amavisd-new, designed to let users adjust their content-filter settings and manage their quarantines from a convenient interface. The project proved quite popular with ISPs, Web-mail providers and companies offering off-site content filtering, however, and the needs of these larger-scale clients soon developed Maia Mailguard into something much more sophisticated.
Maia Mailguard is a complete spam and virus management system, consisting of PHP, SQL and Perl scripts, a MySQL or PostgreSQL database and, of course, amavisd-new, SpamAssassin and supported virus scanners. Arrays of content filters can be managed from a single Maia interface, all sharing the same SQL database. Designed to make content filtering, quarantine management and spam reporting easier, Maia Mailguard is in many ways a new kind of tool for mail users.
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