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Block invalid Email FROM Headers in Postfix: PCRE Regex Solution for Header Validation

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Introduction

Email spam continues to be a significant challenge for system administrators and email service providers. One particularly effective method of spam prevention involves validating the FROM header in incoming emails. In this comprehensive guide, we'll explore how to implement robust FROM header validation in Postfix using PCRE (Perl Compatible Regular Expressions) to block spam effectively.

Understanding the Problem

Spammers often send emails with malformed or missing FROM addresses in the email headers. These can take various forms, such as:

• Generic service names without email addresses (e.g., "From: Netflix Service Cancellation")
• Fake mailer daemon notifications (e.g., "From: Mailer Daemon Error")
• Financial service impersonation (e.g., "From: Paypal Accounting")

Legitimate email clients always include properly formatted email addresses in the FROM header. Therefore, blocking emails without valid FROM addresses can effectively reduce spam while maintaining legitimate email flow.

The Solution: Advanced PCRE Header Checks

We'll implement a sophisticated PCRE regex rule in Postfix's header checks to validate FROM headers. Our solution handles various edge cases, including multi-line headers and special formats.

The REGEX Pattern Explained

^From:(?!(?:(?:.*?\n)*)?.*?[a-zA-Z0-9._+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})

Let's break down this pattern:

• ^From: - Matches the start of the FROM header
• (?! - Negative lookahead to ensure what follows isn't present. Meaning: the REGEX will match if there's no Email address found.
• (?:.*?\n)*)? - Optionally matches any characters followed by a newline (for multi-line headers). Even though we use the /ims flags in postfix, in some cases, multi-line headers did break sometimes.
• [a-zA-Z0-9._+-]+@ - Matches the local part of an email address
• [a-zA-Z0-9.-]+\. - Matches the domain part before the TLD
• [a-zA-Z]{2,} - Matches the TLD (minimum 2 characters).

Implementation Guide

Step 1: Configure Postfix

First, we need to tell Postfix to use our header checks. Add the following line to your main.cf (or check if it already exists and point to an already existing file):

header_checks = pcre:/etc/postfix/header_checks.pcre

Step 2: Create the Header Checks File

Create and/or edit the header checks file:

nano /etc/postfix/header_checks.pcre

Add the following rule:

/^From:(?!(?:.*\n)?.*?[a-zA-Z0-9._+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,})/ims    REJECT    FROM without valid e-mail address

Step 3: Apply the Configuration

After adding the rule, process the file and reload Postfix:

postmap /etc/postfix/header_checks.pcre
systemctl reload postfix

Monitoring and Troubleshooting

You can monitor the effectiveness of your FROM header validation by checking the mail logs. To see rejected messages:

cat /var/log/mail.log |grep "FROM without valid e-mail address"

Edge Cases and Considerations

MAILER-DAEMON Messages

One known edge case involves certain MAILER-DAEMON responses. For example:

From: Mail Delivery System <MAILER-DAEMON@localhost>

While it's possible to extend the regex to allow such messages, many administrators choose to maintain strict validation since legitimate mailer-daemon messages typically include proper sender addresses. The trade-off between security and convenience should be evaluated based on your specific needs.

Multi-line Headers

Our regex pattern specifically accounts for multi-line FROM headers, which is a common format in email messages. The (?:.*?\n)*)? portion of the pattern ensures that we can match email addresses even when they span multiple lines. This should be solved by the /ims flag though but as it seems, it does not always.

Benefits and Impact

Implementing this FROM header validation provides several advantages:

• Reduced Spam: Effectively blocks a significant portion of spam attempts
• Low False Positives: Since legitimate email clients always include proper FROM addresses
• Minimal Resource Impact: Header checks are performed early in the mail processing chain
• Easy Maintenance: Simple configuration that requires minimal ongoing maintenance

Version Compatibility

This solution has been tested with Postfix 3.5.25 but should work with most modern Postfix versions, as PCRE header checks have been supported for many years. Always test in your specific environment before deploying to production.

Conclusion

FROM header validation is a powerful tool in the fight against spam. By implementing this PCRE-based solution, you can effectively block malformed emails while maintaining the flow of legitimate messages. The configuration is straightforward to implement and maintain, making it an excellent addition to any Postfix mail server's security arsenal.

Remember to monitor your logs after implementation and adjust the rules if needed based on your specific requirements and the pattern of spam attempts you encounter.