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CrossNodes: Securing the Mail: Lock Spam and Viruses Out of Sendmail

Jul 18, 2002, 23:30 (4 Talkback[s])
(Other stories by Dee-Ann LeBlanc)

"Sendmail 8.9, the latest version, comes locked down against relaying right out of the box: you can find out more about previous versions by going to the Sendmail web site (Resources). So rather than focusing on initial configuration, let's look at a more insidious problem: taking over someone else's Sendmail setup and making sure they haven't punched any holes in it spammers might use.

"Don't roll your eyes. There are a lot of administrators who just kind of fell into their jobs, and rather than learning the proper way to do things will punch security holes to get their workarounds to function. Maybe you were even one of them once, but now you know better, but you've forgotten to go back and clean up all of your old messes.

"The key files for your Sendmail relaying setup are /etc/mail/access, and /etc/mail/relay-domains. /etc/mail/access is built in a series of single-line entries, and each of these entries consists of two components. The first half of an /etc/mail/access line contains information on whom the rule applies to; for example, 192.168.25.3 means this rule is meant for the machine with this IP address. The second half expresses what the rule is setting: for example, using RELAY here tells Sendmail to allow 192.168.25.3 to relay email through this server..."

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