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$&{client_addr} is a macro that inserts the client address from which the message was sent.
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The last option present, 127.0.0.2. (note the trailing dot, which you need that for the syntax to be correct), is the response code that Sendmail expects from the RBL in order to reject an e-mail. You can specify more than one response by adding response codes. You can also use rule operators to make up specific response codes. The next two lines show both these capabilities: FEATURE(enhdnsbl,`sbl.spamhaus.org',`"550 Mail from" $&{client_addr} "rejected by sbl.spamhaus.org"',`t',`127.0.0.2.', `127.0.0.3.', `127.0.0.4.') FEATURE(enhdnsbl,`bl.spamcop.net',`"550 Mail from" $&{client_addr}
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You can see in the first line that the RBL feature will reject e-mail as spam when it receives the response 127.0.0.2., 127.0.0.3., or 127.0.0.4. from the RBL. In the second option I have used the rule operator, $-, which tells the feature to reject any e-mail as spam when the response code matches 127.0.0.anynumber. You could also use a class to specify all the possible response codes you want to match against. Sendmail Header Checks Using header checks allows you to filter e-mail using Sendmail rule sets. You can filter using normal text or via a regular expression. I will show you how to filter using both methods, focusing on the checking the content of the Subject: line. Using rule sets in Sendmail is a complicated undertaking. I will introduce you to the basics as they relate to my particular requirements to do antispam filtering. I recommend you do further reading on the topic to fully understand the scope and usage of these rule sets. The first thing to consider is that your rule sets are going to be quite long. You need a lot of code to do filtering using rule sets, and as you add additional items to be checked, this will increase the content of your rule sets. Rather than clutter your sendmail.mc file with a large number of rule sets, I recommend you use the include function to include additional mc files. The next line shows how to include an additional file to your sendmail.mc file: include(`/etc/mail/subject_rulesets.mc') Here I have added another mc file called subject_rulesets.mc located in the /etc/mail directory. I usually divide my different rule sets into separate files and include each of them individually. I have separate files for To:, From:, and other major header fields. This keeps my sendmail.mc file neat and reduces the risk of confusion and errors. So how do you filter on a particular subject Listing 7-36 shows header checking, and I will break it down to explain it. Listing 7-36. Sample Subject Header Check in Sendmail HSubject: $>Check_Subject_Spam
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D{SMsg}This email has been rejected as spam D{Subj001}Test our Internet pharmacy D{Subj002}Low Interest Rate SCheck_Subject_Spam R${Subj001} $* $#error $: 550 RRe: ${Subj001} $* $#error $: R${Subj002} $* $#error $: 550 RRe: ${Subj002} $* $#error $:
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${SMsg} 550 ${SMsg} ${SMsg} 550 ${SMsg}
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The first line declares the header. It is structured like this: Hheaderfield: $>ruleset where headerfield: and ruleset are replaced with the relevant header you want to act on; in this case in Listing 7-36 I am using the Subject: header and the name of the rule set that I want to use to process this header field. (You should not include spaces or special characters in your
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rule set name.) Overall, the line sends the content of the header field, Subject:, into the rule set Check_Subject_Spam to be processed for matches.
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s If you want to include any RFC 2822 comments in the data outputted from the Subject: field, then Tip
replace the $> with $>+ in the H line.
The next lines declare configuration file macros. The macros are structured like this: D{macroname}macro content The first line declares a macro called SMsg with the content of This email has been rejected as spam. I will use this macro to provide a message that Sendmail will send to the sending SMTP server if the e-mail is rejected because of the Subject: content. The next two lines are subject lines that I am testing for in my incoming mail. I have named the first macro Subj001 and the second Subj002. It is important to note that when you test against the subject that it is not a regular expression. The test will try to match the exact content of the Subj001 macro. So the subject Test our Internet Pharmacy will be picked up by the rule set, but the subject Test our Internet Pharmacy!!! will not be matched. This limits the functionality of this sort of rule set. The next line declares the start of the particular rule set. The name after the S must match the rule set you specified in the H line. Following this S line are R lines that are the actual rules being used. The R lines are divided into three sections. RLHS RHS comment
The R line starts the rule. Then you will see the left-and side (LHS) of the rule, the right-hand side (RHS) of the rule, and an optional comment. The LHS of the rule does not need to be separated from the R line. But the LHS, RHS, and comments should all be separated by a tab character; otherwise, Sendmail will fail to parse the rule. (You cannot use space characters the separator must be a tab). In the case of the Subject: checking antispam rule, the LHS content is going to be the macro ${subj001}. I tell the rule set that it is a macro by prefixing $ to the front of the macro name (which is still enclosed in brackets). It is then followed by the rule operator $*, which is a wildcard operator that tries to match zero or more tokens (in the case the tokens being the content of the Subject: header field). R${Subj001} $* $#error $: 550 ${SMsg}
The RHS side starts with the operator $#, which indicates an action. When you are testing for a match on your headers, these rule sets can return two possible values: $#error and $#discard. The first response tells Sendmail to reject the message, and the second tells Sendmail to silently discard it. Following the action returned by the rule is the operator $:, which defines the default value to return. So if the Subject: field matches the ${subj001} macro, then Sendmail generates an $#error and specifies the value to return to Sendmail, which in this case is: 550 This email has been rejected as spam, which is the content of the first macro I defined as ${SMsg}. The second line that matches against the macro ${subj001} adds Re: in front of the macro to match any case where this subject appears with a reply appended to the subject. This pattern is repeated for the next macro ${subj002} in Listing 7-36.
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