Integrating wildcard in your SPF record minimizes the risk of impersonation of your domain or subdomain. A wildcard DNS record with * acts as a catch-all for any subdomain that is not explicitly defined in DNS. It basically includes all possible subdomains of a domain.
How does wildcarding work in SPF?
Wildcarding can simplify SPF management for organizations with numerous subdomains by avoiding explicitly listing each subdomain.
Example of a wildcard DNS record in SPF-
v=spf1 include:*.example.com -all
This configuration ensures that all subdomains of example.com, such as mail.example.com or info.example.com, are treated as authorized senders.
Practical use of wildcarding in SPF
Since wildcarding streamlines management, it is particularly beneficial for organizations with complicated domain structures or multiple subdomains. Instead of explicitly listing each subdomain in the SPF record, wildcarding allows a single configuration to cover all subdomains. Doing so helps domain owners save money on administration while ensuring that legitimate emails from any subdomain are authenticated correctly.
For example, consider an organization that uses multiple subdomains for different purposes, such as:
- marketing.example.com for promotional emails
- support.example.com for customer service communication
- notifications.example.com for system alerts
Without wildcarding, the organization would need to explicitly list each subdomain in the SPF record as follows:
v=spf1 include:marketing.example.com include:support.example.com include:notifications.example.com -all
However, with wildcarding, the organization can simplify this configuration:
v=spf1 include:*.example.com -all
This single record automatically includes all subdomains under example.com, ensuring that emails from any legitimate subdomain are authenticated without requiring individual entries.
Apart from making it easier to maintain multiple domains for authentication, using wildcards in SPF increases scalability. This means the wildcard will automatically cover any subdomain you will add in the future. You don’t have to manually update your SPF record each time you create a new subdomain.
Moreover, it brings consistency by ensuring uniform SPF coverage across all subdomains. This surely minimizes the risks of misconfigurations or omitting a subdomain by mistake.
Caution with wildcards
While wildcard entries in SPF records make it easy to manage domains, they pose some risks-
Non-existent domain spoofing
Wildcards can unintentionally validate non-existent subdomains, making it easier for attackers to spoof email addresses from fake subdomains.
Confusion with domain existence
Wildcards may create uncertainty for email servers about whether a domain exists, which could lead to the acceptance of emails from invalid subdomains.
Reduced control
By broadening the scope of authorized senders, wildcard entries reduce the ability to manage SPF authorization precisely, potentially including unintended or unauthorized sources.
Although some authentication systems are designed to handle unexpected records, relying solely on them can be inconsistent and risky.
Final words
The blog discusses both the risks and benefits of using wildcard entries in SPF. Now, if we consider both, then it’s suggested that you should only apply this concept to domains that are never used for sending emails. If you want to enhance email security, it’s better to go for targeted and selective SPF configurations. When it comes to email security, you have to go for a precise and focused approach to build stronger defenses against phishing and spoofing.
