When managing email infrastructure for multiple domains, the idea of using wildcard configurations for DKIM (DomainKeys Identified Mail) in OpenDKIM can be very appealing. It promises simplification and reduced overhead, especially if you're signing for a large number of domains, perhaps over a hundred, as some of us have experienced. The question that often arises is whether a wildcard in the KeyTable or SigningTable will actually work as intended.
My experience suggests that while OpenDKIM offers some flexibility with wildcards, particularly for specifying which domains should be signed, using them for key lookups in the KeyTable can be tricky and lead to unexpected behavior or security concerns. It's crucial to understand the nuances before implementing such a configuration across your email sending infrastructure.
OpenDKIM relies on several configuration files and directives to operate correctly. The primary configuration file, opendkim.conf, dictates the overall behavior of the DKIM signing and verification process. Within this, two key components for managing multiple domains are the KeyTable and SigningTable. These tables tell OpenDKIM which private keys to use for signing which domains.
The KeyTable maps a key name to its location on the filesystem. Each entry typically specifies a selector, domain, and the path to the private key file. For example, selector1._domainkey.example.com example.com:selector1:/etc/opendkim/keys/example.com.private would define a key for example.com using the selector1. The SigningTable, on the other hand, maps an email address or domain to a specific key defined in the KeyTable. This is where you specify which emails get signed by which keys.
Proper configuration of these tables is paramount for successful DKIM authentication. Misconfigurations can lead to DKIM failures, affecting your email deliverability and potentially landing your messages in spam folders. For a deeper dive into the configuration file, you can consult the official OpenDKIM documentation or the Linux man page.
Example KeyTable entriesini
mail._domainkey.example.com example.com:mail:/etc/opendkim/keys/example.com.private
mail._domainkey.another.net another.net:mail:/etc/opendkim/keys/another.net.private
# Proposed wildcard (typically not for KeyTable in this form)
mail._domainkey.* *:mail:/etc/opendkim/keys/mail.private
The nuance of wildcards in OpenDKIM
While the exact syntax and behavior can vary slightly with different versions of OpenDKIM, the general consensus is that wildcards are most effective and commonly supported within the SigningTable when specifying which domains to sign. For instance, using *.example.com to match all subdomains is a common and often successful pattern, especially when combined with the refile: prefix for the filename, allowing for regular expressions.
The example mail._domainkey.* *:mail:/etc/opendkim/keys/mail.private in your KeyTable aims to use a single key for all domains. While this might appear to simplify key management, it generally means that OpenDKIM will attempt to sign for every possible domain that passes through your Postfix server. This can include domains you don't own or intend to sign, leading to unnecessary processing and potential issues.
Many administrators find that while the wildcard might technically "work" to apply a single key, it's not the recommended approach for robust, secure, and manageable DKIM configurations, especially for multi-domain setups. A blog post on Spam Resource elaborates on signing DKIM for multiple domains and generally advises against wildcarding the domain setting in opendkim.conf due to this over-signing risk.
For situations with numerous domains, it's often more prudent to use a SigningTable that explicitly maps domains or users to specific keys, rather than relying on a single wildcard key for all. This approach offers better control, clearer accountability, and improved security. Understanding how OpenDKIM works with subdomains and which domain should be used for signing can further enhance your setup.
Best practices for OpenDKIM setup
Explicit configurations: Specify each domain and its corresponding key explicitly in KeyTable and SigningTable. This offers the most control.
Separate keys per domain: Ideally, generate unique DKIM keys for each sending domain. This isolates the impact if one key is compromised or needs rotation.
Use refile for SigningTable: If you must use wildcards, apply them primarily in SigningTable with the refile: option for regex matching of sending domains.
Practical implications of wildcard usage
The primary practical implication of using a wildcard for the key lookup in KeyTable, such as mail._domainkey.*, is that OpenDKIM may attempt to sign any email that passes through it, regardless of the domain in the From header. This "over-signing" can be problematic.
A significant security concern arises if you use a single private key for a vast number of domains. If this one key is compromised, it immediately jeopardizes the DKIM authentication for all your signed domains. This significantly amplifies the risk compared to a setup where each domain has its own unique key. Managing DKIM keys and domain reputation is a critical aspect of email security.
When DKIM is misconfigured, whether through problematic wildcard use or other errors, it can severely impact your email deliverability. Recipients' mail servers often check for valid DKIM signatures as part of their anti-spam measures. If the signature is invalid or missing, your emails are much more likely to be flagged as spam or outright rejected. This means your legitimate messages might not reach the inbox, potentially leading to your domain being put on an email blacklist or blocklist (or blocklist).
Therefore, even if a wildcard setup for KeyTable technically processes messages, its implications for security, maintainability, and deliverability often outweigh the perceived simplicity. It's generally better to prioritize explicit and secure configurations over a broad wildcard approach.
Scalability and maintainability
Per-domain keys: Each domain has its own unique private/public key pair. This allows for isolated key rotation and management. For more details, see our article on wildcard, DKIM, and DMARC best practices.
Clear mapping:KeyTable and SigningTable entries explicitly state which key signs which domain, simplifying troubleshooting.
Security and deliverability
Reduced blast radius: A compromised key only affects a single domain, minimizing broader impact.
Better reputation management: Individual keys allow for granular control over domain reputation, which is key to inbox placement.
Scalability and maintainability
Simpler initial setup: Potentially fewer entries in configuration files if a single key signs many domains.
Increased risk: A single compromised key can impact all signed domains, leading to widespread email authentication failures and potential blocklisting.
Over-signing: Risk of signing emails from unintended domains that pass through the MTA, which can be an unexpected side effect and harm your sender reputation. Monitoring your email domain reputation becomes even more vital.
Considerations for robust DKIM configurations
My general advice would be to avoid relying on a single wildcard entry for key lookups in KeyTable if you're managing a large number of diverse domains. While the OpenDKIM documentation does mention wildcard capabilities for certain options, its implementation and desired effect need careful consideration, especially for SigningTable with the refile prefix. For explicit keys, an explicit approach is safer.
If you're dealing with hundreds of domains, automated key generation and management might be a more scalable solution than attempting to force a single wildcard entry in the KeyTable. This allows you to maintain separate keys for each domain while still managing them efficiently. Ensuring your MAIL FROM address is signed using OpenDKIM is essential for DMARC alignment.
Ultimately, thorough testing of any configuration changes is critical to ensure proper DKIM signing and prevent deliverability issues. Always verify your DKIM records and email headers after making changes to confirm that emails are being signed correctly and that verification passes with major email providers.
Views from the trenches
Best practices
Always test new OpenDKIM configurations thoroughly before deploying them to production environments.
Use separate DKIM keys for different domains to enhance security and simplify key rotation and management.
Leverage the `refile:` prefix with `SigningTable` for wildcard matching where appropriate, rather than relying on wildcarding in `KeyTable` entries for key lookups.
Monitor your DMARC reports to identify any DKIM authentication failures or over-signing issues that may arise.
Common pitfalls
Using a single wildcard KeyTable entry that results in signing for unintended domains (over-signing).
Compromising a single key that affects the DKIM authentication of many domains simultaneously.
Assuming wildcard functionality for key lookups is as robust as for domain matching in `SigningTable`.
Neglecting to test after configuration changes, leading to undetected DKIM failures and deliverability issues.
Expert tips
If you need to sign for many domains, a common approach is to use a specific `KeyFile` (singular) and let the `SigningTable` (plural) define which keys are used for which domains, allowing for greater control and specificity.
While wildcards in the `SigningTable` can be efficient for subdomains, always be cautious with how broadly they apply, especially in high-volume environments.
For complex multi-domain setups, consider using a DKIM proxy or a sending platform that handles DKIM key management, simplifying your OpenDKIM configuration.
Regularly review your OpenDKIM logs to spot any warnings or errors related to signing attempts, which can indicate misconfiguration.
Expert view
Expert from Email Geeks says that using a wildcard in the OpenDKIM KeyTable will likely work, but it will attempt to sign for every possible domain that passes through your Postfix server.
2021-11-04 - Email Geeks
Expert view
Expert from Email Geeks states that their own configuration does not use a KeyTable and instead relies on a single KeyFile for all domains, suggesting it as an alternative for certain scenarios.
