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How to Rebuild access.db in Sendmail

Did you know that the access.db file in Sendmail is a powerful tool for controlling email traffic and preventing spam? Understanding how it works can help you manage your mail server more efficiently.

It allows administrators to define rules that control whether emails from specific domains or IP addresses are accepted or rejected. This database is generated from the plain text /etc/mail/access file, making it easier to manage filtering rules in a structured format.

Why is access.db Important?

1. The access.db file enables administrators to create rules that determine which emails should be accepted or rejected. For instance, an entry like the one below ensures that emails from this sender are blocked, effectively reducing unwanted messages.

   spammer@aol.com REJECT
2. With access.db, Sendmail can block known spam sources, mitigating phishing attempts and malicious emails before they reach users.
3. This file is also used to manage relaying permissions. Trusted networks can be allowed to relay mail while others are denied, helping to prevent unauthorized use of the mail server.
4. Administrators can implement custom rules tailored to their organization’s needs, considering factors such as sender addresses and connection sources.
5. Sendmail’s access.db works alongside other security tools, like DNS Blackhole Lists (DNSBL), to block spam sources dynamically in real time.
6. If an incoming email does not match any rule in access.db, Sendmail typically accepts it unless otherwise configured. This ensures a balance between security and functionality.

How to Rebuild the access.db File in Sendmail

Whenever changes are made to /etc/mail/access, the access.db file must be updated. Follow these steps to rebuild it:

1. First, we must edit the /etc/mail/access file. This plain text file contains rules that dictate whether to accept or reject emails. Open the file using a text editor:

   sudo vim /etc/mail/access
   Then, modify entries as needed. Examples include:

   • Block a domain:

     spammer.com REJECT
   • Allow relaying for a specific IP:

     Connect:X.X.X.X RELAY
   • Accept all emails from a domain:

     gooddomain.com OK

   Finally, save and exit the editor.
2. After editing, convert the plain text file into the database format:

   
sudo makemap hash /etc/mail/access.db < /etc/mail/access

3. To apply changes, restart Sendmail with:

   sudo service sendmail restart

   Or ,

   sudo /etc/init.d/sendmail restart
4. Instead of restarting Sendmail, we can refresh its configuration:
   • If started using startsrc:

     refresh -s sendmail
   • If started directly via /usr/sbin/sendmail:

     kill -1 `cat /etc/mail/sendmail.pid`
5. Also, ensure FEATURE(‘access_db’) is enabled in /etc/mail/sendmail.mc. If changes are made to sendmail.mc, rebuild the main configuration file:

   cd /etc/mail
sudo make

6. To avoid permission errors, set the correct ownership and access rights:

   sudo chown root /etc/mail/access.db
sudo chmod g-w /etc/mail/access.db