How To Build An Effective Mail Server Defense - A multi-stage procedure for securing your email communication When on the subject of mail server-related security, one will limit the problem to message applied security measures, and much more to Antivirus and Antispam protection. This is however just one stage within the more complex procedure for securing your server.
This article is aimed at identifying and explaining all security layers, very important when choosing a clear mail server and therefore when configuring and taking advantage of it. We have chosen a multi-stage way of your mail server securing procedure, each stage addressing among the security layers we consider relevant: connection-related layer, protocol security, email control parameters (including Antivirus and Antispam applications), and also the configuration and management layer (that appears to be affected by human errors).
How To Build An Effective Mail Server Defense
The sections below describe security measures adapted to every one layer of security:
1. Securing mail server connections
When utilizing a newly installed mail server, administrators should first be sure they use secure connections. There are two main chance to secure connections: encryption and firewall-like rules. Encoding methods have continuously been developed as the Internet is the preferred medium for data transfers. The most regularly used encryption methods are SSL (Secure Sockets Layer) and TLS (Transport Layer Security). However, incorrect using encryption often contributes to security breaches. Most common examples are internet pages containing both secured and unsecured information or communications secured only after login by way of a plain login page.
Firewall-like rules enforced at server level are recommended to backup a current Firewall or put it back when an example may be not available. They can impose limitations both on established connections as well as on hosted traffic. We recommend creating allow/deny rules both globally (put on all protocols and listeners) and especially for each listener in order to prevent attacks for instance DOS (Denial of service).
2. Securing mail server protocols
After securing the initial stage of the email transfer, your next action to take can be securing protocols. The recommended steps will be to use multiple listeners for every single interface and correlate all of them certain allow and deny rules. Also, limiting the quantity of connection and authentication errors, the absolute maximum number of commands or setting a time-out to your sessions will help protect your server from further DOS attacks.
To further enhance protocol security, we recommend client control rules, using the sender or receiver address and certain limitations with regards to the number and height and width of email messages.
Authentication is also vital at protocol level. By implementing several authentication methods, either simple (plain, login, CRAM-MD5), or complex (GSSAPI, Kerberos), the mail server enhances communication security and it is better equipped against attacks and unauthorized access. Other efficient protocol level solutions are earning sure your mail server is RFC compliant and preventing email looping (a brilliant method could well be setting a maximum amounts of "Received" headers per email).
3. Securing email control parameters
Apart while using different Antispam and Antivirus applications, you will discover further actions you need to keep in mind where email control based security is worried. One very handy option could well be using gray lists. Gray listing is essentially a request to own email resent, after temporarily rejecting the email. The server saves within a list the sender IP and also the recipient and returns a short lived error. All valid servers will resend the emails, unlike spamming scripts. Please note however a large number of servers cannot differentiate at the moment between a brief and a permanent error.
Host control is yet another easy way to ensure only valid emails are further processed because of your email server. Two popular methods are SPF (Sender Policy Framework) and DNS based black hole lists. SPF records are public details published by domains within DNS servers. Usually they examine and confirm the true addresses of domains. By using SPF checks, it is possible to successfully prevent spam and back-scatter emails.
Black lists can be either public (no cost) or private and, sometimes contain IP addresses of open-relay servers, open proxies and ISPs without having spam filtering. Your server ought to be set up like to request such lists but not to accept connections initiated by IP addresses built into them. If one of one's servers gets erroneously listed, to get removed from a real list, you will need to fill an internet form, contact their email list administrators or, in many severe situations, alter your IP. A more complicated authentication strategy is DKIM (Domain Keys Identified Mail Signature). Implemented by Yahoo and backed up by Google, Cisco, , PGP, DKIM has considerable probabilities of becoming the conventional authentication method.
The email header contains an encrypted signature and is particularly in its turn encrypted, pointing for an encrypted key, published on DNS servers through the sending domain. The server processing the email use this factor to decode the email body. If the decryption is productive, then this email is valid. Relay rules will often make the difference coming from a secured server along with an unsecured one. Our first recommendation is always to never accept open relaying, as it may easily allow you to get black listed. Therefore you need to implement several relay rules, according to sender address/recipient address, or relay for authenticated users only. When selecting your mail server, you should ensure it has these features: it allows creating relay rules, domain authentication is configurable, the sending interface is customizable, it supports SSL/TSL and different authentication methods and extensions.
4. Secure configuration and administration
Configuration and administration usually are not commonly viewed as a security layer. However, the configurability features offered because of the server along with the actual configuration made from the user play an essential part in securing your MTA. Firstly, the administrator should get acquainted to your solution, its features and all of its flaws, if any. The server executable file needs to support programming without memory leaks, dropping root privileges (on Unices systems only), and blocking all access requests except those for public files.
Access on the configuration file really should be granted on the administrator only. Further more, the file ought to be very specific, obvious to see and to modify, while all default values needs to be secure. For example, a default value allowing open relay would represent an essential security flaw.
Alternate administration modules (web interface, command line interface) needs to be provided for modifying the server configuration. It is also vital that all connections to those modules are designed through SSL. To ensure that you securely access these modules, we recommend employing a mail server with proprietary HTTP server and HTML-based scripting language. More detail please read :
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Our most complete security recommendation is implementing a "smart-hosting" system. Such a system contains several mail servers attached to different machines, each doing a specific task. The server providing the best connection and protocol security must be focused on firewall protection. The second you will need to run email control parameters (including Antispam and Antivirus applications). The third one really should be mainly aimed at domain management. However, smart hosting could possibly want more hardware and software resources than these available in your system.
Conclusions The most critical aspect you ought to keep in mind is the fact that there is no full proof security; therefore an optimal protection should substitute perfection. At each security layer, you can find possible flaws and breaches. The solution is usually to choose the best possible configuration and adapt it in your network's needs and topology.