Diagnosing Intermitent DMARC Failures Due to DNS Resolver Behaviour

15 August 2026 · DMARC Engine · 36 min read

Intermittent DMARC Failures: A Real-World Conundrum

Intermittent DMARC failures can be a centre of frustration for email deliverability engineers, as they often manifest without a clear pattern or cause, making them difficult to diagnose and resolve. In our experience managing DMARC, SPF, DKIM, MTA-STS, and BIMI for customers, we have encountered numerous cases where intermittent failures have led to decreased email deliverability and increased complexity in troubleshooting. A common example of this issue arises when a customer's email authentication records, such as SPF or DKIM, are correctly configured, yet they still experience periodic DMARC failures.
For instance, consider a scenario where a customer has the following SPF record configured:

v=spf1 include:_spf.example.com -all

And the included record _spf.example.com contains:

v=spf1 ip4:192.0.2.1 ip4:192.0.2.2 include:thirdparty.example.net -all

In this case, even if the customer's SPF record is correctly set up, intermittent failures can still occur due to issues such as DNS resolver caching, network connectivity problems, or misconfigured third-party services.
To optimise the email deliverability and mitigate these issues, it is crucial to understand the intricacies of DNS resolver behaviour and its impact on DMARC. Hosted or managed DMARC setups, such as the one we provide at DMARC Engine, often include features to help centre and analyse these issues, such as aggregate report analysis and DNS record monitoring. However, even with these tools, diagnosing intermittent DMARC failures requires a deep understanding of the underlying DNS resolver behaviour and its trade-offs.
One of the primary challenges in diagnosing intermittent DMARC failures is the colour of randomness that often surrounds these issues. Failures may occur sporadically, without any apparent pattern or correlation to specific email campaigns or authentication records. This unpredictability makes it difficult to identify the root cause of the problem and implement effective mitigations.
In our experience, one of the most common causes of intermittent DMARC failures is DNS resolver caching. When a DNS resolver caches a record, it can lead to stale or outdated information being used for authentication, resulting in failures. For example, if a customer updates their SPF record to include a new IP address, but the DNS resolver has cached the old record, it can lead to authentication failures until the cache is updated.
To mitigate these issues, it is essential to understand the DNS resolver behaviour and configure the DNS records accordingly. This may involve adjusting the TTL (time to live) values for the records, implementing DNS record monitoring, or using features such as DNS resolver caching optimisation.
In addition to DNS resolver caching, other factors such as network connectivity issues, misconfigured third-party services, or email client quirks can also contribute to intermittent DMARC failures. For instance, if a customer is using a third-party email service that has a misconfigured SPF record, it can lead to authentication failures, even if the customer's own records are correctly set up.
To effectively diagnose and resolve intermittent DMARC failures, it is crucial to have a comprehensive understanding of the email authentication ecosystem, including the intricacies of DNS resolver behaviour, SPF, DKIM, and DMARC. By analysing aggregate reports, monitoring DNS records, and optimising DNS resolver behaviour, email deliverability engineers can identify and mitigate the root causes of these issues, ensuring optimal email deliverability and compliance.
In the next section, we will delve into the often-overlooked role of DNS resolver caching in intermittent DMARC failures, exploring the trade-offs and configuration options that can help mitigate these issues.

The Overlooked Role of DNS Resolver Caching

When diagnosing intermittent DMARC failures, it is easy to overlook the role of DNS resolver caching, as it is often assumed that DNS records are fetched in real-time. However, this is not always the case, and DNS resolver caching can play a significant part in causing DMARC failures. In our experience, managing DMARC for numerous customers, we have seen that DNS resolver caching can lead to a situation where a sender's DMARC record is not updated promptly, resulting in authentication failures.

For instance, consider a scenario where a customer updates their DMARC record to change the policy from none to quarantine. If the DNS resolver caching timeout, also known as the time to live (TTL), is set to a high value, such as 24 hours, it may take up to 24 hours for the updated record to be reflected in the DNS resolver's cache. During this time, any mail server that relies on the cached record will continue to see the old policy, leading to potential DMARC failures.

_dmarc.example.com. 86400 IN TXT "v=DMARC1; p=none; pct=100; rua=mailto:aggrep@example.com; ruf=mailto:forep@example.com; fo=1"

In this example, the TTL is set to 86400 seconds, which is equivalent to 24 hours. This means that any changes to the DMARC record will not be visible to mail servers that cache the record for up to 24 hours.

In a hosted or managed setup, such as the one we provide at DMARC Engine, we take steps to optimise DNS resolver behaviour and mitigate the effects of caching. For example, we use a combination of low TTL values and DNS record pre-loading to ensure that updates to DMARC records are propagated quickly. However, even with these measures in place, it is still possible for DNS resolver caching to cause issues, particularly if the mail server or receiving domain has a highly aggressive caching policy.

One common issue we see is when a customer's DMARC record is cached by a mail server or receiving domain for an extended period, even after the TTL has expired. This can happen if the mail server or receiving domain has a caching layer that is not properly configured or is overriding the TTL value. In such cases, it may be necessary to use tools such as dig or nslookup to verify the DNS record and caching behaviour. For example:

dig +short _dmarc.example.com TXT

This command will return the current DMARC record for the domain, along with any caching information. By analysing this information, we can determine if the issue is related to DNS resolver caching and take steps to mitigate it.

In addition to caching, another important consideration is the behaviour of DNS resolvers when it comes to handling multiple DNS records. In some cases, a DNS resolver may return multiple records for a single query, which can cause issues with DMARC authentication. For example, if a domain has multiple DMARC records with different policies, a mail server may not know which policy to apply, leading to authentication failures. To avoid this issue, it is essential to ensure that only one DMARC record is published for a domain, and that any duplicate records are removed.

To mitigate the effects of DNS resolver caching, we recommend using a low TTL value for DMARC records, such as 300-600 seconds. This will ensure that any changes to the record are propagated quickly and minimise the risk of authentication failures. Also, we recommend monitoring DNS record caching behaviour and taking steps to optimise it, such as using DNS record pre-loading or caching layers. By taking these steps, it is possible to reduce the risk of intermittent DMARC failures and ensure that email authentication works reliably.

It is also worth noting that some DNS providers and managed DNS services offer features such as automatic DNS record caching and propagation, which can help to mitigate the effects of DNS resolver caching. For example, some providers offer a "DNS caching" or "record propagation" feature that can help to ensure that DNS records are updated promptly across all DNS resolvers. In our experience, these features can be highly effective in reducing the risk of intermittent DMARC failures, and we recommend exploring them as part of a comprehensive DMARC implementation strategy.

Ultimately, the key to diagnosing and mitigating intermittent DMARC failures due to DNS resolver caching is to understand the complex interplay between DNS records, caching behaviour, and email authentication protocols. By taking a proactive and informed approach to managing DNS resolver caching, it is possible to reduce the risk of authentication failures and ensure that email authentication works reliably, which is centre to our work at DMARC Engine.

Case Study: Analysing Aggregate Reports for Clues

When dealing with intermittent DMARC failures, one of the centre pieces of our diagnostic toolkit is the aggregate report, which provides a wealth of information on authentication results, including those from DNS resolvers. At DMARC Engine, we organise these reports to identify patterns and clues that can help us pinpoint the root cause of failures. A key aspect of our analysis is understanding how DNS resolver behaviour can impact DMARC compliance.

To illustrate this, let's consider a real-world example. One of our customers, a large online retailer, was experiencing intermittent DMARC failures, with a failure rate of around 5%. Upon analysing their aggregate reports, we noticed a peculiar pattern. The reports showed that the failures were not consistent across all mail streams, but rather seemed to be concentrated around specific DNS resolvers. For instance, the report snippet below shows a sample of failed authentication attempts:

<record>
 <row>
 <source_ip>192.0.2.1</source_ip>
 <count>10</count>
 <policy_evaluated>
 <disposition>none</disposition>
 <dkim>fail</dkim>
 <spf>fail</spf>
 </policy_evaluated>
 </row>
 <identifiers>
 <header_from>example.com</header_from>
 </identifiers>
 <auth_results>
 <dkim>
 <domain>example.com</domain>
 <result>fail</result>
 <selector>selector1</selector>
 </dkim>
 <spf>
 <domain>example.com</domain>
 <result>fail</result>
 </spf>
 </auth_results>
</record>

In this example, the source_ip field indicates the IP address of the DNS resolver that performed the lookup, which in this case is 192.0.2.1. The policy_evaluated section shows that both DKIM and SPF authentication failed, resulting in a disposition of none. By analysing the auth_results section, we can see that the DKIM signature failed to verify, and the SPF check also failed.

To further investigate, we decided to dig deeper into the DNS resolver behaviour. We noticed that the DNS resolver in question was caching DNS records for an excessively long period, which was causing the authentication failures. The DNS resolver was caching the records for up to 24 hours, which meant that any changes to the DNS records would not be reflected in the cache for an extended period. This was causing the DMARC failures, as the cached records were not aligned with the current DNS configuration.

In a hosted or managed setup, such as the one we provide at DMARC Engine, we can optimise the DNS resolver behaviour to mitigate these issues. For instance, we can configure the DNS resolver to cache records for a shorter period, such as 1 hour, to ensure that changes to the DNS records are reflected in the cache more quickly. Also, we can implement techniques such as DNS record prefetching, which can help to reduce the latency associated with DNS lookups.

However, Notably, there are trade-offs to consider when optimising DNS resolver behaviour. For example, reducing the cache timeout can increase the load on the DNS resolver, which can impact performance. On the other hand, increasing the cache timeout can reduce the load on the DNS resolver, but may also increase the likelihood of authentication failures due to stale records.

In our experience, the colour of the DNS resolver behaviour can vary significantly depending on the specific configuration and implementation. For instance, some DNS resolvers may be more aggressive in their caching behaviour, while others may be more conservative. By analysing the aggregate reports and understanding the DNS resolver behaviour, we can identify the root cause of intermittent DMARC failures and take targeted steps to mitigate them.

To illustrate this, let's consider another example. One of our customers, a small business, was experiencing intermittent DMARC failures due to issues with their SPF records. Upon analysing their aggregate reports, we noticed that the failures were concentrated around a specific set of IP addresses, which were associated with a particular DNS resolver. The report snippet below shows a sample of failed authentication attempts:

<record>
 <row>
 <source_ip>198.51.100.1</source_ip>
 <count>5</count>
 <policy_evaluated>
 <disposition>none</disposition>
 <dkim>pass</dkim>
 <spf>fail</spf>
 </policy_evaluated>
 </row>
 <identifiers>
 <header_from>example.net</header_from>
 </identifiers>
 <auth_results>
 <dkim>
 <domain>example.net</domain>
 <result>pass</result>
 <selector>selector2</selector>
 </dkim>
 <spf>
 <domain>example.net</domain>
 <result>fail</result>
 </spf>
 </auth_results>
</record>

In this example, the source_ip field indicates the IP address of the DNS resolver that performed the lookup, which in this case is 198.51.100.1. The policy_evaluated section shows that the DKIM signature passed, but the SPF check failed, resulting in a disposition of none. By analysing the auth_results section, we can see that the SPF check failed due to a misconfigured SPF record.

By understanding the DNS resolver behaviour and analysing the aggregate reports, we were able to identify the root cause of the intermittent DMARC failures and provide targeted recommendations to our customer. In this case, we recommended that the customer update their SPF records to include the missing IP addresses, which resolved the issue.

In short, analysing aggregate reports is a crucial step in diagnosing intermittent DMARC failures. By understanding the DNS resolver behaviour and identifying patterns and clues in the reports, we can pinpoint the root cause of failures and take targeted steps to mitigate them. Whether it's optimising DNS resolver behaviour or updating DNS records, our experience has shown that a thorough analysis of aggregate reports is essential for maintaining DMARC compliance and ensuring the integrity of email streams.

Understanding DNS Resolver Behaviour and DMARC

When diagnosing intermittent DMARC failures, it is crucial to centre our attention on the behaviour of DNS resolvers, as their caching mechanisms can significantly impact the outcome of DMARC evaluations. A key aspect to consider is how DNS resolvers handle the time to live (TTL) of DNS records, which can colour the results of DMARC checks. For instance, if a DNS resolver caches a DNS record with a low TTL, it may not refresh the record frequently enough, leading to stale data being used in DMARC evaluations.
To illustrate this point, let us consider an example of a DMARC record with a low TTL:

_dmarc.example.com. 300 IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:dmarc@example.com; ruf=mailto:dmarc@example.com; fo=1"

In this example, the TTL of the DMARC record is set to 300 seconds, which means that DNS resolvers should refresh the record every 5 minutes. However, in practice, we have observed that some DNS resolvers may not adhere to this TTL, leading to cached records being used for an extended period.
Hosted DMARC solutions, such as the one offered by DMARC Engine, can help mitigate this issue by optimising DNS record configuration and ensuring that DNS resolvers are properly configured to handle TTLs. For instance, our system automatically sets the TTL of DMARC records to a suitable value, taking into account the requirements of our customers and the behaviour of various DNS resolvers.
Another critical aspect of DNS resolver behaviour is the use of DNS resolver caches to store the results of previous DNS queries. While caching can improve the performance of DNS queries, it can also lead to issues with DMARC evaluations if not properly managed. For example, if a DNS resolver caches the result of a DNS query for an SPF record, it may not re-evaluate the record even if the underlying IP addresses have changed.
To demonstrate this issue, let us consider an example of an SPF record:

example.com. 3600 IN TXT "v=spf1 ip4:192.0.2.1 ip4:198.51.100.1 -all"

In this example, the SPF record includes two IP addresses, 192.0.2.1 and 198.51.100.1. If the DNS resolver caches the result of the SPF query, it may not re-evaluate the record even if one of the IP addresses is removed or modified. This can lead to DMARC failures if the modified SPF record is not properly evaluated.
In a hosted setup, our system ensures that DNS resolver caches are properly managed to prevent such issues. We achieve this by implementing a robust DNS record management system that takes into account the caching behaviour of various DNS resolvers.
The behaviour of DNS resolvers can also be influenced by the use of content delivery networks (CDNs) and load balancers, which can affect the evaluation of DMARC records. For instance, if a CDN or load balancer is used to distribute email traffic, it may modify the IP addresses used in SPF records, leading to DMARC failures if not properly configured.
To address this issue, it is essential to properly configure CDNs and load balancers to ensure that they do not interfere with DMARC evaluations. This can be achieved by implementing suitable DNS record configurations and ensuring that CDNs and load balancers are properly set up to handle email traffic.
In addition to these considerations, the use of DNSSEC can also impact the behaviour of DNS resolvers and DMARC evaluations. DNSSEC is a suite of extensions to DNS that provide authentication and integrity to DNS data, which can help prevent DNS spoofing attacks. However, DNSSEC can also introduce additional complexity to DMARC evaluations, particularly if not properly configured.
To illustrate this point, let us consider an example of a DNSSEC-enabled DNS record:

example.com. 3600 IN DS 12345 8 2 1234567890abcdef1234567890abcdef1234567890abcdef1234567890abcdef

In this example, the DNS record includes a DS record that indicates the presence of DNSSEC. If the DNS resolver is not properly configured to handle DNSSEC, it may not correctly evaluate the DMARC record, leading to failures.
In a hosted setup, our system ensures that DNSSEC is properly configured and managed to prevent such issues. We achieve this by implementing a robust DNS record management system that takes into account the requirements of DNSSEC and DMARC.
In conclusion to this section, understanding DNS resolver behaviour is critical to diagnosing intermittent DMARC failures. By considering the impact of TTLs, caching, CDNs, load balancers, and DNSSEC on DMARC evaluations, organisations can better optimise their DNS configurations to prevent DMARC failures. In the next section, we will discuss practical steps to identify DNS resolver issues that may be contributing to DMARC failures.

Practical Steps to Identify DNS Resolver Issues

When dealing with intermittent DMARC failures, identifying DNS resolver issues can be a complex task, requiring a systematic approach to pinpoint the root cause. Our experience at DMARC Engine has shown that DNS resolver behaviour is often the culprit behind these failures, and understanding how to diagnose these issues is crucial for optimising DMARC compliance.

To start, it is essential to monitor your aggregate reports (RUA) closely, as they provide valuable insights into DMARC failures. For instance, a report may indicate a failure due to a missing SPF record, but upon further investigation, you may find that the record is indeed present, suggesting a DNS resolver caching issue. In a hosted setup like ours, we centre our efforts on organising these reports to highlight such discrepancies, allowing us to take corrective action promptly.

One practical step is to use DNS resolver testing tools to simulate queries from different resolvers and observe the responses. This can help identify if the issue is specific to a particular resolver or a more widespread problem. For example, using a tool like dig to query an SPF record:

dig +short txt example.com

If the response is inconsistent or missing, it may indicate a DNS resolver issue. We have seen cases where certain resolvers cache records for an excessively long time, leading to failures even after the records have been updated. In such cases, understanding the caching behaviour of the resolver in question is vital.

Another crucial step is to analyse the DNS resolver's behaviour in terms of caching and timeout values. Most resolvers cache DNS records to reduce the load on authoritative name servers, but overly aggressive caching can lead to issues with DMARC. The TTL (Time To Live) value of a record, which specifies how long a resolver should cache the record, plays a significant role here. A low TTL value can help mitigate caching issues but may increase the load on the name servers. In our managed setup, we work with customers to optimise these values, balancing the need for freshness with the potential impact on performance.

To illustrate this, consider a scenario where an SPF record has a TTL of 1 hour:

example.com. 3600 IN TXT "v=spf1 include:_spf.example.com -all"

If a DNS resolver caches this record for 2 hours, it may lead to DMARC failures if the record is updated within that timeframe. Adjusting the TTL to a lower value, such as 15 minutes, can help reduce the likelihood of such failures:

example.com. 900 IN TXT "v=spf1 include:_spf.example.com -all"

However, this approach requires careful consideration, as reducing TTL values can increase the query load on the name servers, potentially leading to other issues.

In addition to caching, understanding how different DNS resolvers handle record sizes and formatting is also important. For instance, some resolvers may have issues with large SPF records or those containing certain characters. In our experience, using a record like the following can sometimes cause issues:

example.com. 3600 IN TXT "v=spf1 include:_spf.example.com include:_spf2.example.com include:_spf3.example.com -all"

Splitting such records into smaller, more manageable pieces can help resolve these issues:

example.com. 3600 IN TXT "v=spf1 include:_spf.example.com -all"
_spf.example.com. 3600 IN TXT "v=spf1 include:_spf2.example.com include:_spf3.example.com -all"

This approach not only helps with resolver compatibility but also improves the overall organisation and maintainability of the SPF records.

Finally, it is crucial to test DMARC compliance regularly, using tools that simulate email sending from different sources and resolvers. This can help identify issues before they become critical and ensure that any changes made to mitigate DNS resolver issues do not introduce new problems. In our hosted environment, we automate this process, running regular tests to ensure DMARC compliance and promptly addressing any issues that arise.

By following these practical steps and considering the specific trade-offs and recommendations outlined, organisations can effectively identify and mitigate DNS resolver issues affecting their DMARC compliance. Whether operating a hosted or managed setup, understanding the intricacies of DNS resolver behaviour and its impact on DMARC is key to maintaining optimal email deliverability and security.

Mitigating DMARC Failures: DNS Resolver Configuration Options

When dealing with intermittent DMARC failures, one of the centre points of focus should be on the DNS resolver configuration, as it plays a crucial role in the resolution of DNS records, including those used by DMARC, such as SPF and DKIM. The behaviour of DNS resolvers can significantly impact the performance and reliability of DMARC, particularly in terms of caching, which can lead to failures if not properly managed.

To mitigate DMARC failures stemming from DNS resolver behaviour, several configuration options can be explored. Firstly, it is essential to understand the caching behaviour of the DNS resolver in use. Most DNS resolvers cache DNS records to optimise performance and reduce the load on authoritative name servers. However, this caching can sometimes lead to stale records being served, causing DMARC failures.

One approach to managing this issue is to adjust the TTL (Time To Live) of the DNS records. The TTL value determines how long a DNS record is cached by a resolver before it is refreshed from the authoritative name server. For DMARC-related records, such as SPF and DKIM, it is often recommended to keep the TTL relatively low to ensure that changes to these records are propagated quickly. For example, a TTL of 300 to 600 seconds (5 to 10 minutes) can be a reasonable setting, depending on the specific requirements and the frequency of changes to these records.

; Example of a SPF record with a TTL of 300 seconds
example.com. 300 IN TXT "v=spf1 a mx ip4:192.0.2.1 -all"

In a hosted or managed DMARC setup, such as the one provided by DMARC Engine, the management of TTLs for SPF and DKIM records is typically handled as part of the service, with default settings that aim to balance the need for quick propagation of changes with the performance benefits of caching. However, for organisations managing their own DNS infrastructure, careful consideration of TTL settings is crucial to avoid intermittent DMARC failures due to stale records.

Another crucial aspect of DNS resolver configuration is the use of DNSSEC. DNSSEC (Domain Name System Security Extensions) is a suite of extensions that add an additional layer of security to the DNS by providing authentication and integrity of DNS data. While DNSSEC does not directly prevent DMARC failures, it can help in ensuring that the DNS records used by DMARC are authentic and have not been tampered with, which can indirectly contribute to reducing failures by preventing man-in-the-middle attacks that could alter DNS records.

; Example of a DNSKEY record for DNSSEC
example.com. 3600 IN DNSKEY 256 3 8 AwEAAddt2b...

The configuration of DNS resolvers to validate DNSSEC can vary depending on the resolver software in use. For instance, Unbound and Knot Resolver are examples of DNS resolvers that support DNSSEC validation. In a managed setup, the configuration and maintenance of DNSSEC are typically handled by the service provider, which can simplify the process for organisations but also means they have less direct control over the specifics of the configuration.

Also, the choice of DNS resolver itself can impact DMARC performance. Different DNS resolvers have varying capabilities and behaviours, especially regarding caching, DNSSEC validation, and handling of certain types of DNS records. For example, some resolvers might have better support for EDNS0 (Extension Mechanisms for DNS) which can be relevant for handling larger DNS responses that might be seen with certain DMARC configurations.

In terms of practical recommendations, organisations should consider using a DNS resolver that supports both DNSSEC validation and has configurable caching behaviour. This allows for a balance between security, performance, and the need to quickly propagate changes to DMARC-related records. Also, monitoring the DNS resolver's behaviour and the health of DMARC can help in quickly identifying and addressing any issues that arise.

For organisations with complex DNS setups or those who manage multiple domains, a comprehensive approach to managing DNS resolvers and DMARC configurations is essential. This might involve using automated tools to monitor DNS records and DMARC reports, as well as implementing a structured process for updating DNS records to ensure consistency and minimise errors.

In conclusion to this section, while the specifics of DNS resolver configuration can be complex and depend heavily on the organisational setup and requirements, careful management of TTLs, consideration of DNSSEC, and choice of DNS resolver can significantly mitigate intermittent DMARC failures. By understanding the intricacies of DNS resolver behaviour and its impact on DMARC, organisations can better optimise their configurations to achieve higher deliverability rates and improved email security.

The colour of DNS resolver configuration options can seem daunting at first, but with a systematic approach to understanding and managing these configurations, the process can be streamlined. Organisations should centre their efforts on finding a balance that optimises performance, security, and the ability to quickly adapt to changes in DMARC configurations. By doing so, they can reduce the incidence of intermittent DMARC failures and improve the overall reliability of their email infrastructure.

Ultimately, the goal is to ensure that DMARC configurations are aligned with the organisational email strategy, and that DNS resolver configurations support rather than hinder this strategy. With the right approach to DNS resolver configuration, organisations can better navigate the complexities of DMARC and achieve their email deliverability and security objectives.

The Impact of DNS Resolver Caching on SPF and DKIM

When dealing with intermittent DMARC failures, one crucial aspect to consider is the impact of DNS resolver caching on SPF and DKIM. In our experience at DMARC Engine, we have seen numerous cases where caching issues led to authentication failures, resulting in DMARC failures. To understand this, let's dive into the specifics of how DNS resolver caching affects SPF and DKIM.

SPF, or Sender Policy Framework, relies on DNS records to specify which IP addresses are authorised to send emails on behalf of a domain. These records are typically cached by DNS resolvers to reduce the load on DNS servers and improve performance. However, this caching can sometimes lead to issues. For instance, if a domain's SPF record is updated, the changes may not be immediately visible to all mail servers, as some resolvers may still be caching the old record.

Here is an example of an SPF record:

v=spf1 a mx ip4:192.0.2.1 include:_spf.example.com -all

In this example, the record specifies that the domain's A and MX records, as well as the IP address 192.0.2.1, are authorised to send emails. The include directive also authorises any IP addresses specified in the _spf.example.com record. The -all directive at the end indicates that any IP addresses not explicitly authorised should be rejected.

DKIM, or DomainKeys Identified Mail, uses a similar approach, relying on DNS records to store public keys used for email authentication. Like SPF records, DKIM records are also cached by DNS resolvers. However, DKIM records are typically larger and more complex than SPF records, which can lead to issues with caching. For example, some DNS resolvers may have size limits for cached records, which can cause issues with larger DKIM records.

Here is an example of a DKIM record:

k1._domainkey.example.com. 300 IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQC+yt+4BhvG5F0db+0sP6R3r2uRQJ0x8jXeVYh9y5QxS3aSjCt8nZ0oGwF6eTy6Z4k3n6Yt8hKuZ7FqH7xq1X8RtF+8r2iH1Y1nJPW9n5i5rNzFz7x4G6k3iR1nR1nR1iR1iR1iR1iR1jR1jR1jR1jR1jR1jR1jR1iR1i"

In this example, the record specifies the public key used for DKIM authentication. The k1 selector is used to specify the key, and the v parameter specifies the DKIM version. The p parameter contains the public key itself.

In a hosted or managed setup, such as the one provided by DMARC Engine, these issues can be mitigated through careful configuration and monitoring of DNS resolver caching. For example, we use a combination of DNS resolver caching and periodic record refreshes to ensure that the latest SPF and DKIM records are always available. However, in a self-managed setup, it is crucial to understand the specifics of DNS resolver caching and how it affects SPF and DKIM.

One common issue we see is that some DNS resolvers cache records for a longer period than the specified TTL (time to live). This can lead to authentication failures if the records are updated, as the resolver may still be caching the old record. To mitigate this, it is essential to set a reasonable TTL for SPF and DKIM records, taking into account the expected update frequency and the desired level of caching.

Another issue is that some DNS resolvers may not properly handle record updates, leading to authentication failures. For example, if a domain's SPF record is updated to include a new IP address, some resolvers may not immediately reflect this change, leading to emails being rejected. To address this, it is crucial to test and verify that record updates are properly propagated to all DNS resolvers.

In terms of concrete recommendations, we advise setting a TTL of at least 1 hour for SPF and DKIM records, to ensure that updates are properly propagated. Also, it is essential to monitor DNS resolver caching and update behaviour, to quickly identify and address any issues that may arise. By understanding the impact of DNS resolver caching on SPF and DKIM, and taking steps to mitigate these issues, organisations can improve the reliability and authenticity of their email communications.

To centre our efforts on mitigating these issues, we need to consider the trade-offs between caching and authentication. While caching can improve performance, it can also lead to authentication failures if not properly managed. By carefully configuring and monitoring DNS resolver caching, organisations can strike a balance between performance and authentication, ensuring that their emails are properly authenticated and delivered.

In our experience, the colour of the DNS resolver caching issue can vary depending on the specific setup and configuration. However, by following best practices and carefully monitoring DNS resolver behaviour, organisations can optimise their setup to minimise the risk of authentication failures. By doing so, they can ensure that their emails are properly authenticated and delivered, improving the overall reliability and trustworthiness of their email communications.

To organise our approach to mitigating these issues, we need to consider the specifics of our setup and configuration. This includes setting reasonable TTLs, monitoring DNS resolver caching and update behaviour, and testing and verifying that record updates are properly propagated. By taking a proactive and organised approach, organisations can ensure that their emails are properly authenticated and delivered, improving the overall reliability and trustworthiness of their email communications.

In short, the impact of DNS resolver caching on SPF and DKIM is a critical aspect of email authentication, and organisations need to carefully consider the trade-offs between caching and authentication. By understanding the specifics of DNS resolver caching and taking steps to mitigate these issues, organisations can improve the reliability and authenticity of their email communications.

Real-World Record Examples and Troubleshooting Tips

When dealing with intermittent DMARC failures, it is essential to examine the DNS records that underpin the DMARC, SPF, and DKIM protocols. A thorough understanding of these records, including their syntax, structure, and potential pitfalls, can significantly aid in diagnosing and resolving issues related to DNS resolver behaviour. In this section, we will delve into real-world examples of DNS records, highlighting common mistakes, and providing troubleshooting tips to optimise your setup for better DMARC compliance.

Examining DMARC Records

DMARC records are crucial for defining the policy and preferences for handling emails that fail DMARC validation. A well-crafted DMARC record can help mitigate issues stemming from DNS resolver caching and behaviour. Consider the following example of a DMARC record:

_dmarc.example.com. IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:aggrep@example.com; ruf=mailto:forensics@example.com; fo=1"

In this record, p=reject specifies that emails failing DMARC validation should be rejected, and pct=100 indicates that this policy applies to 100% of emails. The rua and ruf tags define the email addresses for aggregate and forensic reports, respectively. It is vital to ensure that these addresses are correctly configured and monitored, as they provide invaluable insights into DMARC failures and potential DNS resolver issues.

Understanding SPF Records

SPF records are used to define which IP addresses are authorised to send emails on behalf of a domain. Incorrect or overly restrictive SPF records can lead to intermittent DMARC failures, especially when combined with DNS resolver caching issues. The following is an example of an SPF record:

example.com. IN TXT "v=spf1 ip4:192.0.2.1 ip4:198.51.100.1 include:_spf.example.net -all"

This record authorises the IP addresses 192.0.2.1 and 198.51.100.1 to send emails, includes the SPF record from _spf.example.net, and specifies that all other IP addresses should be rejected. When troubleshooting SPF-related issues, it is essential to verify that all authorised sending IP addresses are included in the SPF record and that the record is not too restrictive, which could inadvertently block legitimate email sources.

DKIM Records and Selector Configuration

DKIM records are used to publish the public keys that correspond to the private keys used for signing emails. The selector configuration is critical, as it defines which public key should be used for verifying the DKIM signature of an incoming email. A misconfigured selector can lead to DKIM verification failures, contributing to intermittent DMARC failures. Consider the following example of a DKIM record:

selector._domainkey.example.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCq4xuwU+YHb6W9e3xgP6J7J3Q3L4Nn6G6Z2F0JL8J9V0G9S0L9K8J7J3Q3L4Nn6G6Z2F0JL8J9V0G9S0L9K8J7J3Q3L4"

In this example, selector is the selector name, and the p tag contains the public key. It is crucial to ensure that the selector name matches the one used in the DKIM signature and that the public key is correctly published in the DNS.

Troubleshooting Tips

When troubleshooting intermittent DMARC failures related to DNS resolver behaviour, consider the following tips:
- Monitor Aggregate Reports: Regularly review aggregate reports (RUA) to identify trends and patterns in DMARC failures. This can help pinpoint issues related to DNS resolver caching and behaviour.
- Verify DNS Record Propagation: Ensure that all DNS records, including DMARC, SPF, and DKIM records, are correctly propagated across all DNS servers. Tools like dig or online DNS propagation checkers can be invaluable for this purpose.
- Check for Record Syntax Errors: A single syntax error in a DNS record can render it ineffective. Use tools or services that validate DNS record syntax to catch and correct any errors.
- Optimise DNS Resolver Configuration: Depending on your setup, optimising the DNS resolver configuration can help mitigate issues related to DNS resolver caching. This might involve adjusting TTL values, using a different DNS resolver, or implementing a local DNS cache.
- Leverage Hosted or Managed Services: For domains with complex email setups or those experiencing persistent issues, consider leveraging hosted or managed DMARC, SPF, and DKIM services. These services often provide automated record management, real-time monitoring, and expert support, which can significantly simplify the troubleshooting process.

Edge Cases and Considerations

Several edge cases and considerations can impact the effectiveness of your DMARC setup and its interaction with DNS resolver behaviour:
- Subdomain Handling: When implementing DMARC, it is essential to consider how subdomains are handled. A subdomain with its own DMARC record can override the parent domain's policy, potentially leading to unexpected behaviour.
- Third-Party Senders: If your domain uses third-party services to send emails (e.g., marketing platforms, CRM systems), ensure that these services are correctly configured in your SPF record and that their DKIM signatures are aligned with your domain's DMARC policy.
- DNSSEC and DANE: Implementing DNSSEC and DANE (DNS-based Authentication of Named Entities) can provide an additional layer of security and help mitigate certain types of DNS resolver issues. However, these protocols require careful configuration and may introduce new complexities.

By carefully examining your DNS records, understanding the intricacies of DMARC, SPF, and DKIM, and applying the troubleshooting tips and considerations outlined above, you can more effectively diagnose and resolve intermittent DMARC failures related to DNS resolver behaviour. This proactive approach not only enhances your domain's email security but also improves deliverability and reduces the risk of spam filtering issues.

Optimising DNS Resolver Behaviour for DMARC Compliance

To optimise DNS resolver behaviour for DMARC compliance, it is crucial to centre your strategy around the nuances of DNS caching, query patterns, and the specific requirements of DMARC, SPF, and DKIM records. A key aspect often overlooked is the impact of DNS resolver caching on the availability and freshness of DNS records. For instance, consider a scenario where a customer updates their SPF record to include a new sending IP, but due to aggressive caching by a DNS resolver, the update takes an excessively long time to propagate, leading to intermittent DMARC failures.

In a hosted or managed setup, such as the one we operate at DMARC Engine, we have the advantage of controlling the DNS infrastructure and can thus optimise DNS resolver behaviour to minimise caching issues. However, for organisations managing their own DNS, understanding how to configure their DNS resolvers or work with their DNS providers to optimise behaviour is vital.

One approach to mitigate caching issues is to adjust the TTL (Time To Live) of DNS records. The TTL dictates how long a DNS resolver should cache a record before refreshing it. While a lower TTL can help ensure records are updated more frequently, it also increases the load on DNS servers and can lead to higher latency for users. For DMARC, SPF, and DKIM records, which do not typically change frequently, a balanced approach is necessary. We recommend a TTL of 3600 seconds (1 hour) for these records as a starting point, allowing for a reasonable balance between update frequency and query efficiency.

; Example of a DMARC record with a 1-hour TTL
_dmarc.example.com. 3600 IN TXT "v=DMARC1; p=reject; pct=100; rua=mailto:aggrep@example.com; ruf=mailto:forensics@example.com; fo=1"

Another critical aspect is the colour and depth of DNS resolver logging. Detailed logs can provide invaluable insights into query patterns, cache hits, and misses, helping identify potential issues before they lead to DMARC failures. In our experience, resolvers that provide logs with query response times, cache status, and error codes are particularly useful for troubleshooting. For example, analysing logs from a resolver like Unbound can reveal patterns of cache exhaustion or misconfiguration that might affect DMARC record availability.

; Example log snippet from Unbound showing a cache hit for a DMARC record query
15:34:45 | 192.168.1.100 | example.com | A | NOERROR | udp | 1234 | 0.000200 | hit

Also, the choice of DNS resolver software and its configuration can significantly impact DMARC compliance. Some resolvers, like Knot Resolver, offer advanced features such as automatic DNSSEC validation and built-in support for DNS over TLS (DoT) and DNS over HTTPS (DoH), which can enhance the security and privacy of DNS queries. However, these features must be carefully configured to ensure they do not inadvertently interfere with DMARC record queries. For instance, overly aggressive DNSSEC validation can lead to failures if the DNS infrastructure of a sender's domain is not fully compliant.

In addition to resolver choice and configuration, network architecture plays a crucial role. The placement of DNS resolvers within an organisation's network can affect the performance and reliability of DMARC record queries. Centralising DNS resolver services can simplify management and reduce the complexity of troubleshooting but may also introduce single points of failure. Distributing resolvers across different network segments can improve resilience but requires careful management to ensure consistency and avoid configuration discrepancies that could lead to DMARC failures.

To illustrate this point, consider a scenario where an organisation has multiple branches, each with its own DNS resolver. If the configuration of these resolvers is not synchronised, differences in caching behaviour or DNSSEC validation could lead to inconsistent DMARC compliance across different locations. We have seen cases where a single misconfigured resolver in a remote branch caused intermittent DMARC failures for emails sent from that location, highlighting the need for a centralised management approach.

; Example configuration snippet for a distributed DNS resolver setup
server:
 # Centralised configuration for all resolvers
 log-level: 6
 log-queries: yes
 cache-min-ttl: 300
 cache-max-ttl: 3600

remote-control:
 # Control port for centralised management
 control-enable: yes
 control-port: 8952

Finally, regular monitoring and analysis of aggregate DMARC reports are essential for identifying and addressing DNS resolver behaviour issues. These reports can provide insights into DMARC compliance rates, failure reasons, and the sources of emails that are failing DMARC checks. By integrating this feedback into the optimisation process, organisations can refine their DNS resolver configurations and DMARC policies to achieve higher compliance rates and better email deliverability.

In our operational experience, a combination of careful DNS resolver configuration, balanced TTL settings for DMARC and related records, and ongoing monitoring of DMARC reports has proven effective in minimising intermittent failures due to DNS resolver behaviour. By focusing on these specifics and adapting to the unique requirements of their email ecosystem, organisations can significantly improve their DMARC compliance and protect their domain from spam and phishing attacks.

Related reading

• DKIM Key Management for Organisations with Frequent Staff Turnover
• Email Authentication for Load Balancers and Content Delivery Networks
• DMARC Record Management for Domain Portfolio Owners