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This section covers some ways to protect your MTA from penetration and reduce the risk of an attacker gaining root through your MTA. I will cover a bit about chrooting your MTA, hiding your MTA s identity, disabling some dangerous SMTP commands, protecting your MTA from DoS attacks, and providing some general security. One of the biggest issues with MTA security is the need for many MTAs to utilize root, utilize setuid, or require quite high privileges to correctly function. Older Sendmail versions are particular culprits of this. Both Postfix and more recent versions of Sendmail, from version 8.12 onward, run without setuid root, which reduces the potential risk of an attacker using your MTA as a conduit to root privileges on your system. This is another reason, if you are running Sendmail, to update to a more recent version. So how does Sendmail achieve this Sendmail is split into two operational modes: an MTA function and a Mail Submission Program (MSP) function. How you start Sendmail depends on which function is called. So, effectively now you have two running Sendmail modes: one is an SMTP daemon that performs your MTA functions, and the other is an MSP daemon that handles the submission and queuing of e-mail. To accommodate for this, an additional configuration file has been created, submit.cf, which controls the mail submission functions.
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CHAPTER 7 s SECURING YOUR MAIL SERVER
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In version 8.12, Sendmail still needs root privileges to perform a few actions, such as binding to port 25, reading .forward files, performing local delivery of mail, and writing e-mail submitted via the command line to the queue directory. The last option is what Sendmail principally had used a setuid root binary for. The new version downgrades the requirements for root privileges by changing the sendmail binary to a setgid binary and writing to a group-writable queue directory. Sendmail still needs to be started as root, but then it drops privileges once it has performed the required root tasks. This is a fairly simplistic explanation, and I recommend you carefully read the changes articulated in the Sendmail README and SECURITY documents that come with the source distribution to fully understand how the structure and running of Sendmail has changed. You need to consider a few caveats and warnings, though. You can also find these documents on the Sendmail Web site.
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s Note Both Sendmail and Postfix still use the less dangerous setgid for several functions. Postfix uses
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setgid as part of the postdrop program for mail submission whilst Sendmail uses it to setgid to a different user and group, called smmsp, as part of the new separate mail submission process.
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So how do you further limit the risk to your system from a penetration of your MTA s daemons Well, one of the possible methods is chrooting. I will cover how both MTAs I am discussing can be chrooted and under what circumstances you may choose to do this. Sendmail can be highly complicated to completely chroot, and because of its monolithic nature, the benefits derived from chrooting are potentially much more limited. Postfix consists of many small daemons, so you can therefore be selective about which you chroot; however, Sendmail is one binary, which means you have to attempt to chroot all its functions. Since Sendmail requires write access to objects that are almost certainly going to be outside your chroot jail, the value of the jail is negated. The security changes that have been made to the way Sendmail runs in version 8.12 reduce the risk of a root penetration. This does not mean you should not look at chroot for Sendmail. I still think, though, you may want to run Sendmail chrooted in some important instances, such as if you are running an SMTP gateway, so I will take you through chrooting that type of Sendmail installation in the next section. Postfix by default is designed to have most of its daemons running chrooted with fixed low privileges. Additionally, adjusting its configuration to enable chroot is simple and quick to achieve. I will cover the configuration of Postfix as a chrooted system in the Chrooting Postfix section.
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