Setting Anti-Spam Policy for Linux Mail Server (1)

The mail system is an important part of Linux network applications. The complete mail system includes the underlying operating system, the mail sending proxy MTA, the mail distribution proxy MDA, and The Mail User proxy MUA.

Currently, the main danger facing the Linux email system is spam, Linux virus, and DoS attacks. This article focuses on Anti-Spam policies for Linux mail servers.

Prevention of Spam

Currently, the following anti-spam technologies are widely used:

1) SMTP User Authentication: a common and very effective method for SMTP authentication on the email sending proxy MTA) for mail users from the internet outside the local network, only Authenticated Users are allowed to perform remote forwarding. This not only effectively avoids the use of the email sending proxy server for spam senders, but also facilitates employees who work on a business trip or at home. Without SMTP authentication, it is also feasible to set up an Internet-Oriented Web Mail Gateway without sacrificing security. In addition, if the SMTP service and POP3 Service are integrated on the same server, it is safer to perform POP3 access verification for POP before SMTP before the user attempts to send a mail.

2) Disable Open Relay: there are still not a few Open Relay servers. Therefore, disabling Open Relay has a significant effect on anti-spam.

3) Real-time blacklist Filtering: the preventive measures described earlier do not apply to spam emails using valid domain names. In this case, you can use the blacklist service list. For each incoming email message, the MTA program obtains the address of the remote server and queries the remote Internet server to authenticate the address. If the address is in the spam host list, the MTA rejects the message. The following figure shows the process of using the PCIe authentication.

Figure 1

4) content filtering: MTA, MUA, and MDA filtering have their own characteristics. These methods are usually used at the same time.

◆ MTA Filtering: Most MTA provides some filtering, because they are at the front end of the email, it is usually easier to control the arrival of the email.

◆ MDA Filtering: Most MTA does not filter mail content, and the filtering of mail content is done by MDA. Many complex Filters use the MDA filter.

◆ MUA Filtering: The MDA is located on the mail server, and many users want to manage filtering rules from the mail interface, so they need MUA filtering. Mainstream MUA, such as Outlook, Foxmail in Windows, and Evolution in Linux, all have filtering functions.

◆ Dedicated tools: such as SpamAssassin.

◆ Commercial software: for example, Trend Micro IMSS 5.5 integrates spam Control Service (SPS ).