How to prevent spam emails in Linux

For those who frequently access the Internet, they will always receive spam emails, which is a headache. Microsoft has suffered a lot from spam in Windows. Do not think that we can avoid spam harassment on the Linux operating system platform, it is very important to counter and filter junk emails. The following describes some anti-spam technologies widely used in Linux.

1) SMTP User Authentication

Currently, the common and effective method is to perform SMTP authentication on the Mail proxy Mail Transport Agent (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. If SMTP authentication is not adopted, it is 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, however, there are not many email client programs that currently support this authentication method in the application.

2) reverse name resolution

No matter which type of authentication, the purpose is to prevent the mail sending proxy server from being used by spammers, but it is still helpless for the Spam Sent to the local. To solve this problem, the simplest and most effective method is to reverse name the sender's IP address. The DNS query is used to determine whether the sender's IP address is the same as the name it claims. For example, if the sender's name is mx.hotmail.com and its connection address is 255.200.200.200, it is rejected if it does not match the DNS record. This method can effectively filter out spam from dynamic IP addresses. For some senders using dynamic domain names, it can also be blocked based on the actual situation. However, the above method is still ineffective for spam via Open Relay. In this regard, the further technique is to assume that valid users only use the email sending proxy server with a valid Internet name in the domain to send emails. For example, if the sender's mail address is a someone@yahoo.com, the Internet Name of the mail proxy server it uses should have a suffix of yahoo.com. This restriction does not comply with the SMTP protocol, but is effective in most cases. It should be noted that reverse name resolution requires a large number of DNS queries.

3) Real-time blacklist Filtering

The preceding preventive measures are still invalid for spams using their own valid domain names. The more effective method is to use the blacklist service. The blacklist service is a database composed of domain names or IP addresses established based on user complaints and samples. The most famous database is RBL, DCC, and Razor, these databases store host names or IP addresses that frequently send spam messages for the MTA to perform real-time queries to determine whether to reject the emails. However, it is difficult to ensure the correctness and timeliness of various blacklist databases. For example, RBL and DCC in North America contain a large number of host names and IP addresses in China, some of which are caused by early Open Relay and some by false positives. However, these latencies have not been corrected, which has hindered China's contact with emails in North America to some extent and the use of these blacklist services by Chinese users.

4) Content Filtering

Even with the technology in the previous sections, there will still be a considerable number of spam leaks. In this case, the most effective method is to filter the content based on the mail title or body. A simple method is to use the content scanning engine to filter information such as the common title language of spam, the name, phone number, and Web address of the spam beneficiaries. The more complex but more intelligent method is the content filtering based on Bayesian probability theory. This algorithm was first proposed by Paul Graham for http://www.paulgraham.com/spam.html ), and use the self-designed Arc language for implementation. The theoretical basis of this method is to analyze the common keywords in a large number of spam mails to obtain the statistical model of their distribution, and then calculate the possibility that the target email is a spam email. This method has certain adaptive and self-learning capabilities and has been widely used. Spamassassin is the most famous spam content filtering method. It is implemented in Perl and integrates the above two filtering methods, which can be integrated with various mainstream MTA. Content filtering is the most computing resource-consuming among all of the above methods. When the mail traffic is large, it must be used with high-performance servers.

The above is the Anti-Spam technology in the Linux operating system.

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