Will OpenDKIM wildcard configuration work?

Key findings

• Wildcard functionality: OpenDKIM can indeed utilize wildcard patterns in its configuration files, particularly for options like SigningTable and KeyTable. This is typically enabled by using the refile: prefix before the filename, allowing for regular expression matching.
• Broad application: A wildcard in the KeyTable, such as mail._domainkey.* *:mail:/etc/opendkim/keys/mail.private, may cause OpenDKIM to attempt to sign emails for every domain passing through your mail server (e.g., Postfix).
• Configuration files: Successful wildcard implementation relies on correctly configuring /etc/opendkim.conf, /etc/opendkim/KeyTable, and /etc/opendkim/SigningTable.

Key considerations

• Security implications: While convenient for many domains, using a single key (even with a wildcard) for a large number of domains, especially over 100, is generally not recommended due to increased risk if the key is compromised.
• Specificity vs. convenience: Although wildcards can simplify management, more specific configurations often provide better security, control, and easier troubleshooting of potential DKIM validation issues. Consider if the convenience outweighs the security trade-offs for your specific use case.
• Performance considerations: Signing every email that passes through the mail server could have performance implications, depending on traffic volume and server resources.
• Alternative methods: Explore options like using KeyFile instead of KeyTable for simplified setups, though this may not scale well for hundreds of domains. For detailed setup, consulting resources like EasyEngine's OpenDKIM guide can be helpful.

Key opinions

• Simplified management: Marketers frequently seek ways to streamline DKIM setup for multiple domains, seeing wildcards as a potential solution to avoid individual configurations for each domain.
• Scalability requirements: The need to sign emails for dozens or even hundreds of domains drives the interest in wildcard configurations, as manual setup becomes impractical at scale.
• Impact on deliverability: A primary concern for marketers is whether a wildcard DKIM setup will negatively affect their email deliverability or increase the risk of being placed on a blocklist.

Key considerations

• Practical implementation: Marketers need to assess the ease of implementing wildcard configurations in a live production environment, considering potential complications or unexpected behavior.
• Troubleshooting complexity: There is a valid concern about whether a broad wildcard setup will make diagnosing DKIM authentication failures more difficult, especially when managing multiple domains.
• Balancing security and convenience: Marketers must weigh the convenience of a shared key approach against the security implications and its potential impact on domain reputation. For example, users on the Proxmox Support Forum often discuss specific fixes for their outbound DKIM issues, suggesting a preference for precise configurations.

Key opinions

• Wildcard feasibility: Experts believe that wildcard configurations in OpenDKIM may work, drawing parallels from wildcard functionality observed in other areas of the software.
• Broad signing scope: A key point from experts is that a wildcard setup will likely attempt to sign every single domain that routes through the mail server, which may not always be desired.
• Key management choice: Experts discuss the practicality of using a single KeyFile and key across multiple domains as an alternative to KeyTable entries, weighing convenience against the security implications for very large domain counts.

Key considerations

• Security best practices: Using a single shared DKIM key for a very large number of domains (e.g., 100+) is generally advised against due to the magnified risk if the key is compromised. DKIM selectors can help manage different keys.
• Granular control: Experts recommend that while wildcards offer convenience, more specific OpenDKIM configurations provide better granular control over which domains are signed and with which keys, enhancing overall email authentication security.
• Scalability vs. risk: The decision to use wildcards versus explicit domain entries should balance the administrative overhead of managing many individual keys against the potential security vulnerabilities of a widespread key compromise. An expert perspective on this topic can be found on Spamresource.com.

Key findings

• Refile requirement: Documentation confirms that the wildcard symbol * will only be effective if the SigningTable option uses the refile: prefix, indicating that regular expression parsing is necessary.
• Core configuration files: The primary configuration for OpenDKIM involves editing essential files like /etc/opendkim.conf and /etc/opendkim/KeyTable, which are central to defining DKIM keys and their usage.
• Security recommendation: Documentation often advises specifying explicit usernames and domains instead of broad wildcards for increased security, particularly in the KeyTable configuration.

Key considerations

• Adhering to syntax: Correctly applying the refile: prefix is paramount for enabling and ensuring wildcard functionality in OpenDKIM configurations.
• Understanding file interactions: A thorough understanding of how opendkim.conf, KeyTable, and SigningTable interact is vital for effective DKIM configuration.
• Diagnosing issues: Being aware of common operational nuances, such as the need for reboots for socket file creation on some systems, can prevent OpenDKIM validation errors. For more specific advice on wildcard usage, see The FreeBSD Forums.