Implementing DMARC effectively requires meticulous attention to the syntax and configuration of the DMARC record. Several common mistakes can lead to DMARC verification failures, undermining the intended security benefits. This guide is meant to serve as a reference for the most common errors in DMARC configuration.
Common DMARC Record Errors and How to Fix Them
| Error Type | Description | Example of Incorrect Syntax | Recommended Fix |
|---|---|---|---|
| Multiple DMARC Records | More than one DMARC record exists for the domain. | _dmarc.example.com TXT "v=DMARC1; p=none;" <br> _dmarc.example.com TXT "v=DMARC1; p=reject;" | Remove all but one DMARC record; merge the desired settings into a single record. |
| Missing or Incorrect v=DMARC1 Tag | The record does not start with the correct version tag. | p=none; | Ensure the record starts with v=DMARC1;, with “DMARC” in uppercase and no leading whitespace. |
| Incorrect Separators | Using the wrong characters to separate tags or values within tags. | v=DMARC1: p=none: | Use semicolons (;) to separate tags and commas (,) within rua, ruf, and fo tags. |
| Invalid Email Address URIs | Email addresses in rua or ruf do not have the mailto: prefix. | v=DMARC1; p=reject; [email protected]; | Precede all email addresses in rua and ruf with mailto:. |
| Missing p (Policy) Tag | The mandatory policy tag is absent from the record. | v=DMARC1; | Include the p tag with a valid policy value (none, quarantine, or reject). |
| Incorrect Policy Values | The value assigned to the p tag is not one of the valid options. | v=DMARC1; p=monitor; | Use only none, quarantine, or reject as valid policy values. |
| Out of Order Records | Mandatory tags are not placed in the recommended order. | v=DMARC1; rua=mailto:[email protected]; p=reject; | Ensure v=DMARC1 is the first tag, and p immediately follows. |
| Extra Characters and Bad Quoting | Unintended characters or incorrect use of quotation marks in the record. | v=DMARC1; p=none; rua=mailto:"[email protected]"; | Carefully review the record for any extraneous characters or incorrect quoting and remove them. |
| Omitting Reporting Address (rua tag) | The rua tag, used for receiving aggregate reports, is missing. | v=DMARC1; p=none; | Include the rua tag with at least one valid email address URI. |
| Formatting rua and ruf Incorrectly | Email addresses in reporting tags are not formatted as mailto: URIs. | v=DMARC1; p=reject; [email protected]; | Ensure all email addresses in rua and ruf are preceded by mailto:. |
| Not Configuring Third-Party Report Domain | The domain specified to receive reports via a third-party service is not verified. | v=DMARC1; p=none; rua=mailto:[email protected]; | Ensure the third-party domain is properly configured to receive DMARC reports for your domain. |
| Specifying Default or Implicit Tags | Including tags whose values are implied by the DMARC specification. | v=DMARC1; p=quarantine; pct=100; | Omit tags that have default values when the record is fully compliant (e.g., pct=100). |
The Interplay Between SPF and DMARC: How Incorrect SPF Setup Leads to Failures
Sender Policy Framework (SPF) is a foundational email authentication protocol that plays a crucial role in the success of DMARC. SPF works by allowing domain owners to specify which mail servers are authorized to send emails on behalf of their domain. Receiving mail servers can then check the SPF record to verify that an incoming email from that domain originated from an authorized source. However, several common errors in SPF record setup can lead to SPF authentication failures, which in turn can cause DMARC verification to fail.
One of the most prevalent issues is exceeding the SPF 10-DNS-lookup limit. SPF records often include directives like include, a, mx, ptr, and redirect, each of which can trigger DNS lookups. To prevent overloading receiving mail servers, the SPF protocol imposes a limit of 10 DNS lookups per SPF check. If an SPF record requires more than 10 lookups to resolve, it can result in a permerror, causing the SPF check to fail. This can lead to legitimate emails being rejected or marked as spam, and consequently, DMARC verification will also fail.
Similar to DMARC, a domain should have only one SPF record published in its DNS. The presence of multiple SPF records for the same domain creates confusion for receiving mail servers, as they cannot determine which record to trust for email authentication. This can lead to unpredictable SPF results and potential DMARC failures. If a domain uses multiple services that require SPF records, these should be merged into a single SPF record.
Incorrect syntax in the SPF record is another common cause of SPF failures. This can include missing spaces between mechanisms and values, the use of incorrect tags or unsupported mechanisms, and the improper placement of qualifiers like ~all, -all, or +all. Even seemingly minor errors, such as spaces where they do not belong, can render the entire SPF record invalid. Receiving mail servers rely on the correct syntax to parse the SPF record and validate sending sources.
Issues with DNS configuration can also lead to SPF failures. This can involve missing SPF records, incomplete entries, problems with DNS propagation across servers, or misconfigured DNS zones. If the receiving server cannot retrieve the SPF record corresponding to the sending domain due to DNS issues, the SPF authentication will fail, subsequently impacting DMARC verification.
The use of overly broad mechanisms in the SPF record, such as +all, can also be problematic. The +all mechanism allows any server to send emails on behalf of the domain, effectively negating the security benefits of SPF. Instead, domain owners should use -all for a hard fail (reject unauthorized emails) or ~all for a soft fail (mark unauthorized emails as suspicious) once their legitimate sending sources are properly defined.
Finally, SPF alignment issues are a frequent reason for DMARC failures, even when SPF authentication itself passes. For DMARC to pass based on SPF, the domain in the Return-Path (also known as MAIL FROM or envelope sender) must align with the domain in the From header of the email. This alignment often fails when using third-party email senders, as they typically use their own domain in the Return-Path for bounce handling. Email forwarding is another common scenario where SPF alignment breaks, as the forwarding server’s IP address is usually not included in the original sender’s SPF record, and the Return-Path may not match the From domain. DMARC offers two modes for SPF alignment: strict (aspf=s) which requires an exact domain match, and relaxed (aspf=r) which allows a parent/child domain relationship.
DKIM Signatures and DMARC: Addressing Invalid and Missing Signatures
DomainKeys Identified Mail (DKIM) is another critical email authentication protocol that works in conjunction with DMARC. DKIM provides a mechanism to verify the integrity of the email content and confirm that the email was indeed sent by the owner of the domain it claims to be from. This is achieved through the use of digital signatures added to the email headers, which are then validated by receiving mail servers using a public key published in the sender’s DNS records. Issues with DKIM signatures, whether they are missing or invalid, can lead to DMARC verification failures.
A missing DKIM signature can cause DMARC to fail, particularly if SPF alignment also fails. While DMARC only requires either SPF or DKIM to pass and align, the absence of DKIM means that the email’s authentication relies solely on SPF, making it more susceptible to forwarding issues and misalignment. In some cases, email service providers might add default DKIM signatures to outgoing emails, but these signatures may not align with the domain in the “From” header, still leading to DMARC failure.
An invalid DKIM signature can arise from various technical issues. These include syntax errors in the DKIM record itself, such as incorrect formatting or missing elements. DKIM signature alignment failure occurs when the domain specified in the d= tag of the DKIM signature does not match the domain in the “From” header of the email. This is a common issue with third-party services that might sign emails with their own domain. Problems with server communication, such as DNS resolution timeouts or general network connectivity issues, can also prevent the receiving server from properly verifying the DKIM signature. Furthermore, if the message body is modified in transit by Mail Transfer Agents (MTAs), the DKIM signature, which is based on the message content, can become invalid. A DNS outage or downtime can temporarily prevent DKIM signature verification. Finally, an incorrect public key published in the DNS, or a mismatch between the private and public keys, will also result in an invalid DKIM signature.
Issues related to the DKIM selector can also cause DMARC failures. The DKIM selector is a value specified in the DKIM signature that indicates which public key record in the DNS should be used for verification. If an incorrect selector is used in the signature, or if there are DNS issues preventing the receiving server from resolving the selector and retrieving the public key, DKIM verification will fail.
The Concept of SPF Alignment: Understanding and Resolving Failures
As previously mentioned, SPF alignment is a critical component of DMARC verification. It requires that the domain found in the Return-Path (MAIL FROM) of the email aligns with the domain present in the From header. Even if an email passes the basic SPF authentication check, it can still fail DMARC if this alignment is not met.
Several scenarios can lead to SPF failing to align with the From domain, resulting in DMARC failure. One common scenario involves third-party senders. These services often send emails on behalf of a domain but use their own domain for the Return-Path to manage bounces and feedback loops. While the third-party’s sending servers might be authorized in the SPF record of their own domain, this Return-Path domain will not align with the domain in the From header of the email, which belongs to the organization using the third-party service.
Email forwarding is another frequent cause of SPF alignment failure. When an email is forwarded, the IP address of the forwarding server becomes the apparent sending IP. If this IP is not included in the SPF record of the original sender’s domain, the SPF check will fail. Furthermore, the Return-Path is often rewritten by the forwarding server and may not align with the original From domain.
The DMARC record includes the aspf tag to specify the alignment mode for SPF. In strict mode (aspf=s), there must be an exact match between the Return-Path domain and the From domain. In relaxed mode (aspf=r, which is the default), alignment passes if the organizational domains are the same, even if subdomains differ. Using a different subdomain in the Return-Path than in the From header will cause SPF alignment to fail under strict mode.
In some cases, an empty Return-Path can lead to SPF alignment failure. When the Return-Path is empty, receiving servers may fall back to using the domain provided in the HELO or EHLO SMTP command for the SPF check. This domain is often the hostname of the sending server and is unlikely to align with the From domain.
The Concept of DKIM Alignment: Understanding and Resolving Failures
DKIM alignment, similar to SPF alignment, is crucial for DMARC to effectively validate the sender’s identity. DKIM alignment occurs when the domain specified in the d= tag of the DKIM signature aligns with the domain in the From header.
One common scenario where DKIM fails to align is when third-party senders sign emails using their own domain instead of the domain of the organization they are sending on behalf of. While the DKIM signature might be valid, the domain in the d= tag will not match the domain in the From header, leading to DMARC failure. To achieve DKIM alignment with third-party senders, they need to be configured to sign emails using the domain of the organization, or the organization needs to delegate a subdomain for the vendor to sign with.
Similar to SPF, DMARC also offers strict (adkim=s) and relaxed (adkim=r) alignment modes for DKIM, specified using the adkim tag in the DMARC record. In strict mode, the domain in the d= tag must be an exact match for the From domain, while relaxed mode allows an organizational match, meaning subdomains can align with the parent domain. Using a subdomain in the d= tag that does not exactly match the From domain will cause DKIM alignment to fail under strict mode.
Mailing lists and forwarders can sometimes modify the From header of emails, which can lead to DKIM alignment failures, even if the DKIM signature itself remains valid. If the From domain is altered by the intermediary, it will no longer match the domain in the d= tag of the original DKIM signature.
Navigating Strict DMARC Policies (p=reject/quarantine) Without Proper SPF and DKIM
Implementing a DMARC policy of p=reject or p=quarantine signifies a strong commitment to protecting a domain from email spoofing. However, attempting to enforce these strict policies without first ensuring proper configuration and alignment of SPF and DKIM can have unintended and detrimental consequences. Legitimate emails that fail SPF or DKIM checks, or fail to align, will be either blocked entirely (p=reject) or sent to the spam/junk folder (p=quarantine), potentially disrupting critical business communications and impacting email deliverability.
The recommended best practice is to adopt a phased approach to DMARC implementation. Organizations should begin with a policy of p=none. This monitoring mode allows domain owners to gather reports on email authentication status and identify all legitimate sending sources without impacting email delivery. Once domain owners are confident that all their legitimate email sources are properly authenticated and aligned, they can gradually increase the enforcement level to p=quarantine. This policy treats emails that fail DMARC checks as suspicious and typically places them in the spam or junk folder. After a period of monitoring under the quarantine policy and ensuring minimal false positives, organizations can then move to the strictest policy, p=reject. This policy instructs receiving servers to reject emails that fail DMARC checks, preventing them from being delivered to the recipient’s inbox at all. The pct tag in the DMARC record can also be used to gradually apply the quarantine or reject policy to a percentage of failing emails, allowing for further monitoring and fine-tuning.
The Impact of Email Forwarding on DMARC Verification and Common Failure Scenarios.
Email forwarding is a common practice, but it can significantly impact DMARC verification, often leading to failures. The primary reason for this is that email forwarding often causes SPF to fail. SPF is a path-based authentication protocol that verifies the sending server’s IP address against a list of authorized IPs for the domain specified in the Return-Path. When an email is forwarded, the IP address of the forwarding server becomes the apparent sending IP, and this IP is unlikely to be included in the SPF record of the original sender’s domain. Additionally, the Return-Path itself might be rewritten by the forwarding server, further contributing to SPF alignment failure.
DKIM signatures, on the other hand, are generally more resilient to email forwarding. As long as the content of the email and certain key headers are not modified during the forwarding process, the DKIM signature should remain valid. However, if a forwarding service or a mailing list modifies the email content or adds headers (e.g., for unsubscribe links or list information), it can invalidate the DKIM signature.
If an email relies solely on SPF for DMARC alignment and is then forwarded, DMARC verification is likely to fail. While DMARC only requires either SPF or DKIM to pass and align, the susceptibility of SPF to forwarding makes it crucial to have DKIM properly configured as a backup authentication mechanism. Automatic email forwarding, often set up through email client rules or server-side configurations, is more prone to causing DMARC failures compared to manual forwarding, where the user typically sends a new email.
Mailing lists present a particular challenge as they frequently modify email content by adding footers, headers, or unsubscribe links, which can invalidate DKIM signatures. Additionally, mailing lists might alter the Return-Path, leading to SPF alignment issues.
To mitigate DMARC failures due to email forwarding, it is essential to ensure that all outgoing emails are DKIM signed using the domain of the organization. Implementing Authenticated Received Chain (ARC) can also help, as it allows intermediaries to sign messages and preserve authentication results across forwarding hops. In the case of mailing lists, configuring the list to rewrite the From header to use the list’s domain can help maintain DMARC compliance. As a general strategy, starting with a DMARC policy of p=quarantine can be a more forgiving approach when dealing with forwarding issues, as it directs failing emails to the spam folder rather than rejecting them outright.
Third-Party Email Senders and DMARC: Configuration Challenges and Solutions
Many organizations rely on third-party email senders for various communication needs, including marketing campaigns, customer relationship management (CRM), and transactional emails. Ensuring that these third-party senders are properly configured to align with a domain’s DMARC policy is crucial for preventing DMARC verification failures.
A common oversight is failing to identify all legitimate third-party senders and authorize them in both SPF and DKIM records. Organizations should conduct a thorough inventory of all services that send emails on their behalf and ensure that the necessary authentication mechanisms are in place for each.
One of the primary challenges with third-party senders is that they often use their own domain for the Return-Path (MAIL FROM), which leads to SPF alignment failures with the sender’s From domain. While SPF authentication might pass for the third-party’s domain, it does not satisfy DMARC’s alignment requirement with the organization’s domain in the From header. Similarly, third-party vendors might sign emails with their own DKIM keys, causing DKIM alignment failures unless they are specifically configured to use the sender’s domain for signing.
Several solutions can be implemented to address these challenges. For SPF, the SPF record of the sender’s domain needs to be updated to include the IP addresses or the domain name of the third-party sender’s mail servers, typically using the include: mechanism. For DKIM, the third-party sender should be configured to sign emails using the domain of the organization. This often involves generating a DKIM key pair, providing the public key to the organization to publish in their DNS as a TXT record, and the third-party sender using the private key to sign outgoing emails. Another approach for DKIM is to use CNAME records to delegate the DKIM signing process to the third-party vendor’s domain. Organizations also need to ensure that they choose the appropriate DMARC alignment mode (relaxed or strict) based on how their third-party senders are configured.
Conclusion: Comprehensive Strategies for Preventing DMARC Verification Failures
Preventing DMARC verification failures requires a comprehensive strategy that addresses various potential pitfalls. Common reasons for failure include errors in DMARC record syntax and configuration, incorrect setup of SPF and DKIM, alignment issues between the authenticated domains and the From domain, the impact of strict DMARC policies, email forwarding, and misconfigurations with third-party email senders.
A crucial first step in implementing DMARC is to begin with a policy of p=none. This allows domain owners to monitor their email traffic and identify all legitimate sending sources. Continuous monitoring of DMARC reports, facilitated by the rua tag, is essential for gaining visibility into authentication results and detecting any unauthorized use of the domain.
Ensuring proper SPF and DKIM setup is fundamental. This includes adhering to syntax rules, respecting DNS lookup limits for SPF, and correctly configuring DKIM keys and selectors. Crucially, domain owners must understand and address the concept of alignment. Ensuring that the domain used for SPF and DKIM authentication aligns with the domain in the From header is crucial for DMARC to pass.
Domain owners should exercise caution when moving to stricter DMARC policies like p=quarantine or p=reject. A gradual transition, starting with p=none, followed by p=quarantine, and finally p=reject, is the safest approach to minimize disruptions to legitimate email flow.
Email forwarding can introduce complexities, primarily by causing SPF alignment failures. Domain owners should prioritize DKIM signing for all emails, as it is more resilient to forwarding. Considering mechanisms like ARC can also help preserve authentication results during forwarding.
Finally, when using third-party email senders, it is crucial to properly configure SPF to include their sending sources and to establish DKIM signing using the organization’s domain. Regular communication and coordination with these vendors are necessary to ensure ongoing DMARC compliance.
By addressing these common pitfalls and implementing a well-planned and monitored DMARC strategy, organizations can significantly reduce the risk of email spoofing and enhance the security and trustworthiness of their email communications.
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