DKIM Failures

25 September 2026 · DMARC Engine · 39 min read

Troubleshooting DKIM Verification Failures Due to Mail Server Time Drift

Identifying Intermittent DKIM Verification Failures

When dealing with DKIM verification failures, it is crucial to identify the root cause of the issue, particularly if the failures are intermittent. Intermittent failures can be challenging to diagnose, as they may not occur consistently, making it difficult to reproduce the problem. In our experience, one of the primary causes of intermittent DKIM verification failures is mail server time drift.
To identify intermittent DKIM verification failures, we need to analyse the aggregate reports (RUA) from receivers, such as Gmail or Yahoo. These reports provide valuable information about the emails that failed DKIM verification. For instance, the report may contain a result field indicating the verification outcome, such as fail or temperror.
In a hosted or managed setup, such as the one we operate at DMARC Engine, we have the advantage of centralised logging and reporting, which simplifies the process of identifying intermittent DKIM verification failures. Our system collects and aggregates RUA reports from various receivers, allowing us to quickly pinpoint potential issues.
Let's consider an example of an aggregate report from Gmail, which indicates a DKIM verification failure:

{
 "report_metadata": {
 "org_name": "example.com",
 "email": "abuse@example.com",
 "extra_contact_info": "",
 "report_id": "1234567890",
 "date_range": {
 "begin": 1643723400,
 "end": 1646315200
 }
 },
 "policy": {
 "disposition": "none",
 "dkim": "fail",
 "spf": "pass",
 "reason": {
 "type": "dkim",
 "comment": "signature verification failed"
 }
 },
 "records": [
 {
 "row": {
 "source_ip": "192.0.2.1",
 "count": 10,
 "policy": {
 "disposition": "none",
 "dkim": "fail",
 "spf": "pass"
 }
 }
 }
 ]
}

In this example, the report indicates that 10 emails from the IP address 192.0.2.1 failed DKIM verification. To further investigate this issue, we can inspect the DKIM signature headers of the affected emails. The DKIM-Signature header contains a timestamp, which can help us identify potential time drift issues.
For instance, the DKIM-Signature header may look like this:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com; s=selector;
 t=1643723400; x=1646315200; bh=...; h=From:To:Subject; b=...

The t parameter in the DKIM-Signature header represents the timestamp when the signature was generated. By comparing this timestamp with the current time, we can determine if there is a significant time difference that could be causing the verification failure.
In our experience, a time difference of more than 5 minutes can cause DKIM verification failures. Therefore, it is essential to ensure that the mail server's clock is synchronised with a reliable time source, such as an NTP server.
To optimise the troubleshooting process, we recommend configuring the mail server to log DKIM verification failures, including the timestamp and the reason for the failure. This information can be invaluable in identifying the root cause of the issue.
In addition, we suggest implementing automated monitoring for DKIM time drift issues. This can be achieved by regularly checking the mail server's clock against a reliable time source and alerting the administrator if a significant time difference is detected.
By following these steps, administrators can quickly identify and resolve intermittent DKIM verification failures caused by mail server time drift, ensuring that their emails are properly authenticated and delivered to the intended recipients.
In the next section, we will delve deeper into the impact of mail server time drift on DKIM verification and explore ways to analyse aggregate reports for time-related DKIM failures.

Understanding Mail Server Time Drift and Its Impact on DKIM

Mail server time drift is a critical issue that can cause DKIM verification failures, and it is essential to understand its impact to troubleshoot these failures effectively. In a hosted or managed setup, such as the one we operate at DMARC Engine, we centre our attention on optimising the configuration of our customers' mail servers to prevent time drift issues. The colour of the problem, however, is not just about the technical aspects, but also about the organisational and operational challenges that come with managing a large number of mail servers.

When a mail server's clock is not synchronised with the correct time, it can lead to a mismatch between the timestamp in the DKIM signature and the current time. This mismatch can cause the DKIM verification to fail, resulting in emails being flagged as spam or rejected by the receiving mail server. For instance, if a mail server's clock is 10 minutes behind the correct time, and the DKIM signature has a timestamp that is 10 minutes in the future, the verification will fail because the signature is considered to be from the future.

To illustrate this issue, let's consider an example of a DKIM signature header:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
 s=selector1; t=1643723400; x=1643724000; bh=...; h=...

In this example, the t parameter represents the timestamp when the signature was generated, and the x parameter represents the expiration time of the signature. If the mail server's clock is not synchronised, the t parameter may be incorrect, causing the verification to fail.

In a real-world scenario, we encountered a customer who was experiencing intermittent DKIM verification failures. Upon investigating their mail server configuration, we found that their server's clock was drifting by several minutes every day. This drift was causing the DKIM signatures to be generated with incorrect timestamps, resulting in verification failures. To resolve this issue, we worked with the customer to configure their mail server to synchronise its clock with a reliable time source, such as a Network Time Protocol (NTP) server.

In a hosted or managed setup, it is crucial to monitor the time drift of mail servers and take corrective action to prevent DKIM verification failures. We use automated tools to monitor the time drift of our customers' mail servers and alert them if any issues are detected. This proactive approach helps to prevent DKIM verification failures and ensures that emails are delivered successfully.

Another critical aspect to consider is the trade-off between the frequency of NTP polling and the risk of DKIM verification failures. If the NTP polling interval is too long, the mail server's clock may drift significantly, causing DKIM verification failures. On the other hand, if the polling interval is too short, it may cause unnecessary network traffic and load on the NTP server. In our experience, a polling interval of 15-30 minutes is a reasonable compromise between accuracy and network efficiency.

To optimise the NTP polling interval, we recommend using a tiered approach, where the polling interval is adjusted based on the mail server's clock accuracy. For instance, if the mail server's clock is accurate to within 1 minute, the polling interval can be set to 30 minutes. However, if the clock accuracy is poor, the polling interval can be reduced to 15 minutes or even 5 minutes in extreme cases.

In addition to monitoring time drift, it is also essential to inspect the DKIM signature headers for timestamps and expiration times. This can help identify issues with the mail server's clock or the DKIM signing process. For example, if the x parameter is set too close to the current time, it may cause the signature to expire prematurely, resulting in verification failures.

To illustrate this, let's consider an example of a DKIM signature header with an expiration time that is too close to the current time:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
 s=selector1; t=1643723400; x=1643723405; bh=...; h=...

In this example, the expiration time is set to 5 seconds after the signature was generated, which is too short and may cause the signature to expire prematurely.

In conclusion to this section, understanding mail server time drift and its impact on DKIM verification is critical to troubleshooting intermittent DKIM verification failures. By monitoring time drift, adjusting the NTP polling interval, and inspecting DKIM signature headers, mail server administrators can prevent DKIM verification failures and ensure that emails are delivered successfully. In the next section, we will discuss how to analyse aggregate reports for time-related DKIM failures.

When troubleshooting DKIM verification failures due to mail server time drift, analysing aggregate reports is a crucial step in identifying the root cause of the issue. These reports, typically received via the Aggregate Report URI (RUA) specified in the DMARC record, provide valuable insights into the authentication results of emails sent from your domain. In a hosted or managed setup, such as the one we operate at DMARC Engine, these reports are collected and organised to facilitate easier analysis.

To start, it is essential to understand the structure of these reports. Aggregate reports are usually sent in XML format and contain information about the IP addresses from which emails were sent, the authentication results, including DKIM verification, and the timestamps of when these emails were received. A typical aggregate report snippet might look like this:

<?xml version="1.0" encoding="UTF-8" ?>
<feedback>
 <version>1</version>
 <report_metadata>
 <org_name>example.com</org_name>
 <email>abuse@example.com</email>
 <extra_contact_info>https://example.com/dmarc</extra_contact_info>
 <report_id>1234567890</report_id>
 <date_range>
 <begin>2023-02-15T00:00:00Z</begin>
 <end>2023-02-15T23:59:59Z</end>
 </date_range>
 </report_metadata>
 <policy_published>
 <domain>example.com</domain>
 <adkim>r</adkim>
 <aspf>r</aspf>
 <p>reject</p>
 <sp>reject</sp>
 <pct>100</pct>
 </policy_published>
 <record>
 <row>
 <source_ip>192.0.2.1</source_ip>
 <count>10</count>
 <policy_evaluated>
 <disposition>none</disposition>
 <dkim>fail</dkim>
 <spf>pass</spf>
 </policy_evaluated>
 </row>
 </record>
</feedback>

In this example, the report indicates that emails sent from 192.0.2.1 failed DKIM verification. To diagnose time-related issues, we need to delve deeper into the report, specifically looking for any indicators of time drift affecting DKIM verification. However, aggregate reports do not directly provide the specific reason for DKIM failures, such as time drift. Therefore, we often need to correlate this data with other sources, like the message headers of the emails in question, to identify time-related problems.

When analysing these reports in the context of a hosted or managed DMARC setup, it's beneficial to utilise the tools and interfaces provided by the service to filter and sort the reports. For instance, at DMARC Engine, our platform allows customers to easily navigate through their aggregate reports, filtering by disposition, DKIM result, or source IP, which can help pinpoint potential time drift issues more efficiently.

A common challenge in analysing aggregate reports for time-related DKIM failures is the lack of explicit timestamps for when the DKIM signature was generated versus when it was verified. This information is crucial because the validity of a DKIM signature is time-sensitive, relying on the alignment of the signing and verifying systems' clocks. While the aggregate reports provide date ranges for the report period, they do not offer granular detail on the timing of individual email transmissions and verifications.

To overcome this limitation, inspecting the DKIM signature headers in the email messages themselves can provide the necessary timestamps. The DKIM-Signature header includes a t parameter that specifies the timestamp when the signature was generated. For example:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
 s=selector; t=1644884400;
 bh=...; h=From:To:Subject;
 b=...

In this header, the t parameter value 1644884400 represents the Unix timestamp (seconds since January 1, 1970, 00:00:00 UTC) when the DKIM signature was generated. By comparing this timestamp with the time the email was received and verified, one can infer if time drift was a factor in any DKIM verification failures.

In practice, the process of analysing aggregate reports for time-related DKIM failures involves a combination of automated tools and manual inspection. Automated scripts can parse aggregate reports and flag potential issues based on predefined criteria, such as an unusual number of DKIM failures from a specific IP address. However, human analysis is necessary to interpret these findings in the context of the email ecosystem, considering factors like recent changes in mail server configurations, network issues, or spikes in email volume that could contribute to time drift.

In a managed setup, this analysis can be optimised by leveraging the service's expertise and tools. For instance, at DMARC Engine, our team uses custom-built tools to monitor aggregate reports and alert customers to potential issues, including those that might indicate time drift problems. This proactive approach helps in identifying and addressing DKIM verification failures early, minimising the impact on email deliverability.

Ultimately, the key to successfully troubleshooting DKIM verification failures due to mail server time drift lies in meticulous analysis of aggregate reports, coupled with a deep understanding of the DKIM protocol and its dependencies on system time accuracy. By focusing on the specifics of time-related failures within these reports and leveraging the capabilities of hosted or managed DMARC services, organisations can better optimise their email authentication setups to ensure reliable deliverability.

Inspecting DKIM Signature Headers for Timestamps

When troubleshooting DKIM verification failures due to mail server time drift, inspecting the DKIM signature headers for timestamps is a crucial step. This involves analysing the DKIM-Signature header in the email message to identify the timestamp when the signature was generated. The DKIM-Signature header typically contains several parameters, including t and x, which represent the signature timestamp and expiration time, respectively.
For instance, consider the following DKIM-Signature header:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com; s=selector1;
 t=1643723400; x=1643724000; bh=...; h=from:to:subject; b=...

In this example, the t parameter indicates that the signature was generated at Unix timestamp 1643723400, which corresponds to 2022-02-01 12:30:00 GMT. The x parameter specifies that the signature expires at Unix timestamp 1643724000, which is 3600 seconds (or 1 hour) after the signature was generated.
To optimise the troubleshooting process, it is essential to understand the colour of the timestamps in the DKIM-Signature header. A hosted or managed setup, such as the one we use at DMARC Engine, can centre the analysis on the timestamp parameters to quickly identify potential time drift issues.
In our experience, a common issue is that the mail server's clock is not synchronised with a reliable time source, resulting in a significant time drift. This can cause the DKIM signature to expire prematurely or be considered invalid due to an outdated timestamp.
To inspect the DKIM-Signature header, you can use various tools, such as openssl or a DKIM verification library. For example, you can use the openssl command to verify the DKIM signature and extract the timestamp parameters:

openssl dgst -sha256 -verify pubkey.pem -signature signature.bin email_message.eml

This command will output the verification result, including the timestamp parameters. Alternatively, you can use a DKIM verification library, such as OpenDKIM, to parse the DKIM-Signature header and extract the timestamp parameters.
When inspecting the DKIM-Signature header, it is crucial to consider the trade-offs between the signature timestamp and expiration time. A shorter expiration time can reduce the risk of replay attacks but may also increase the likelihood of false negatives due to time drift issues.
In our experience, a reasonable compromise is to set the expiration time to 1 hour, as shown in the example above. However, this value may need to be adjusted depending on the specific use case and requirements.
To illustrate the importance of inspecting the DKIM-Signature header, consider the following example. Suppose we receive a report of intermittent DKIM verification failures from a customer. After analysing the aggregate reports, we identify a pattern of failures that suggests a time drift issue. We then inspect the DKIM-Signature header of a failed email message and find the following:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com; s=selector1;
 t=1643723000; x=1643723400; bh=...; h=from:to:subject; b=...

In this example, the signature timestamp is 1643723000, which corresponds to 2022-02-01 12:20:00 GMT. However, the current time is 2022-02-01 12:35:00 GMT, which is outside the expiration time specified in the x parameter. This suggests that the mail server's clock is not synchronised, resulting in a time drift that causes the DKIM signature to expire prematurely.
By inspecting the DKIM-Signature header and analysing the timestamp parameters, we can quickly identify the root cause of the issue and take corrective action to synchronise the mail server's clock. This highlights the importance of inspecting the DKIM-Signature header as part of the troubleshooting process for DKIM verification failures due to mail server time drift.
In a hosted or managed setup, such as the one we use at DMARC Engine, we can automate the process of inspecting the DKIM-Signature header and analysing the timestamp parameters to quickly identify potential time drift issues. This allows us to optimise the troubleshooting process and reduce the time to resolve issues.
In short, inspecting the DKIM-Signature header for timestamps is a critical step in troubleshooting DKIM verification failures due to mail server time drift. By analysing the timestamp parameters and considering the trade-offs between signature timestamp and expiration time, we can quickly identify the root cause of the issue and take corrective action to synchronise the mail server's clock.
In our experience, a combination of manual inspection and automated analysis is the most effective approach to troubleshooting DKIM verification failures due to mail server time drift. By using tools such as openssl or a DKIM verification library, we can efficiently inspect the DKIM-Signature header and extract the timestamp parameters.
Also, we can use automated monitoring tools to detect potential time drift issues and alert our team to take corrective action. This approach allows us to centre our efforts on resolving the issue quickly and efficiently, rather than spending time on manual analysis.
To illustrate this approach, consider the following example. Suppose we have a customer who is experiencing intermittent DKIM verification failures due to mail server time drift. We can use automated monitoring tools to detect the issue and alert our team to take corrective action.
Once alerted, we can inspect the DKIM-Signature header of a failed email message and analyse the timestamp parameters to identify the root cause of the issue. We can then take corrective action to synchronise the mail server's clock and prevent future DKIM verification failures.
By using a combination of manual inspection and automated analysis, we can optimise the troubleshooting process and reduce the time to resolve issues. This approach allows us to provide a better service to our customers and ensure that their email messages are delivered reliably and securely.
In our experience, the key to successful troubleshooting

Configuring Mail Servers to Synchronise Time

To prevent DKIM verification failures due to mail server time drift, it is essential to configure mail servers to synchronise their time with a reliable time source. This can be achieved by using the Network Time Protocol (NTP). In a hosted or managed setup, such as the one we operate at DMARC Engine, time synchronisation is typically handled automatically by the hosting provider, but for on-premise mail servers, this needs to be configured manually.

The first step is to choose a reliable NTP time source. There are several public NTP pools available, such as the NTP Pool Project, which provides a list of time servers that can be used for synchronisation. For example, the following NTP servers can be used:

0.pool.ntp.org
1.pool.ntp.org
2.pool.ntp.org
3.pool.ntp.org

These time servers can be added to the NTP configuration file, usually located at /etc/ntp.conf. The exact configuration will depend on the operating system and NTP implementation being used. For example, on a Ubuntu-based system using the ntpd service, the configuration file would contain the following lines:

server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst

It is also important to ensure that the NTP service is enabled and running on the mail server. This can be done by running the following command:

sudo service ntp start

To verify that the NTP service is synchronising the time correctly, the ntpq command can be used:

sudo ntpq -p

This will display a list of time servers and their current status. The * symbol indicates the current time source.

In addition to configuring the NTP service, it is also important to ensure that the mail server's clock is not drifting over time. This can be done by adjusting the maxclock parameter in the NTP configuration file. For example:

maxclock 100

This sets the maximum allowed clock drift to 100 parts per million (ppm).

It is also worth noting that some mail servers, such as Sendmail and Postfix, have their own built-in time synchronisation mechanisms. For example, Sendmail has a Timeout option that can be used to specify the time synchronisation interval. However, these mechanisms are not always reliable and should be used in conjunction with a dedicated NTP service.

In our experience, configuring mail servers to synchronise time with a reliable time source is crucial to preventing DKIM verification failures due to time drift. By using a public NTP pool and configuring the NTP service correctly, mail servers can ensure that their time is accurate and consistent, which is essential for DKIM verification. In the next section, we will discuss the practical steps to fix time drift issues on Sendmail and Postfix.

When implementing time synchronisation, it is essential to consider the trade-offs between NTP polling intervals and DKIM verification. A shorter NTP polling interval can provide more accurate time synchronisation, but it can also increase the load on the NTP service and the mail server. A longer polling interval can reduce the load, but it can also increase the risk of time drift. In our experience, a polling interval of 1-5 minutes is a good compromise between accuracy and load.

By configuring mail servers to synchronise time with a reliable time source and considering the trade-offs between NTP polling intervals and DKIM verification, organisations can prevent DKIM verification failures due to time drift and ensure that their email authentication mechanisms are working correctly. This is particularly important in a hosted or managed setup, where time synchronisation is critical to ensuring the integrity of email authentication mechanisms.

Practical Steps to Fix Time Drift Issues on Sendmail and Postfix

To address time drift issues on Sendmail and Postfix, which are common mail servers used in various email environments, it is crucial to understand the specific configurations and commands that can help synchronise the server time, thus preventing DKIM verification failures. The first step is to ensure that the server time is accurately set and synchronised with a reliable time source.

For Sendmail, this can be achieved by configuring the ntpd service to synchronise the system time with a trusted NTP server. The following example shows how to configure ntpd on a CentOS system:

# Configure ntpd to synchronise with a trusted NTP server
ntpdate -q 0.pool.ntp.org

Then, to ensure ntpd starts automatically on boot, you can use:

# Enable ntpd to start automatically on boot
systemctl enable ntpd

On systems using Postfix, similar steps can be taken to configure ntpd. However, Postfix also provides a configuration option to specify the time-to-live (TTL) for DKIM signatures, which can help mitigate the impact of minor time drifts. This can be configured in the main.cf file:

# Configure Postfix to set a TTL for DKIM signatures
dkim_milter_default_privacy = yes
dkim_milter_ttl = 3600

In this example, the dkim_milter_ttl parameter is set to 3600 seconds (1 hour), allowing for some flexibility in case of minor time discrepancies.

When configuring these settings, it is essential to consider the trade-offs between security and usability. A shorter TTL provides better security but may cause more verification failures due to time drift, while a longer TTL is more forgiving but may reduce the security benefits of DKIM.

In a hosted or managed setup, such as the one provided by DMARC Engine, these configurations are typically handled by the service provider, and customers are often not required to manually configure ntpd or adjust DKIM settings. However, understanding these configurations can still be beneficial for troubleshooting and optimising email deliverability.

To inspect the current time settings on a Sendmail or Postfix server, you can use the date command:

# Check the current system time
date

And to verify that the ntpd service is running and synchronising the time correctly, you can use:

# Check the ntpd service status
systemctl status ntpd

On some systems, especially those with limited network connectivity or specific security requirements, it may be necessary to use alternative time synchronisation methods or to configure ntpd to use specific NTP servers. For example, to configure ntpd to use the Google public NTP servers, you can add the following lines to the ntp.conf file:

# Configure ntpd to use the Google public NTP servers
server time.google.com
server time1.google.com
server time2.google.com
server time3.google.com

After updating the ntp.conf file, you need to restart the ntpd service to apply the changes:

# Restart the ntpd service to apply changes
systemctl restart ntpd

In addition to configuring ntpd, it is also important to ensure that the mail server's clock is not drifting significantly over time. This can be achieved by regularly checking the system time and adjusting it as necessary. Some systems may also benefit from using a more precise timekeeping mechanism, such as a hardware clock or a specialised time synchronisation service.

To illustrate the impact of time drift on DKIM verification, consider a scenario where a mail server's clock is 30 minutes fast compared to the recipient's server. If the sender's DKIM signature has a timestamp that is 30 minutes in the future, the recipient's server may reject the email due to an invalid signature. By synchronising the mail server's time using ntpd and configuring a suitable TTL for DKIM signatures, such issues can be mitigated, and email deliverability can be improved.

In real-world scenarios, the effects of time drift on DKIM verification can be seen in aggregate reports, such as those provided by DMARC Engine. These reports often include details about the verification failures, including the specific error messages and timestamps. By analysing these reports, email administrators can identify potential time drift issues and take corrective action to optimise their mail server configurations.

For instance, a DMARC aggregate report might contain the following record snippet, indicating a DKIM verification failure due to a timestamp issue:

{
 "org_name": "example.com",
 "date_range": {
 "start": "2022-01-01T00:00:00Z",
 "end": "2022-01-01T23:59:59Z"
 },
 "records": [
 {
 "row": {
 "source_ip": "192.0.2.1",
 "count": 10,
 "disposition": "none",
 "dkim": {
 "domain": "example.com",
 "result": "fail",
 "reason": "timestamp"
 }
 }
 }
 ]
}

In this example, the report indicates that 10 emails from the IP address 192.0.2.1 failed DKIM verification due to a timestamp issue. By investigating this further and adjusting the mail server's time configuration, the email administrator can resolve the issue and improve the overall deliverability of emails from their domain.

By following these practical steps and considering the specific requirements of their email environment, administrators can effectively troubleshoot and fix time drift issues on Sendmail and Postfix, ensuring that their DKIM signatures are valid and their emails are delivered successfully. Regular monitoring and analysis of aggregate reports can also help identify potential issues before they become major problems, allowing for proactive optimisation of mail server configurations to centre around the goal of optimal email deliverability.

Trade-offs Between NTP Polling Intervals and DKIM Verification

When configuring mail servers to synchronise time, a critical aspect to consider is the trade-off between NTP polling intervals and DKIM verification. The NTP polling interval determines how frequently the mail server checks and updates its time with a reference time source. A shorter polling interval can help minimise time drift, but it also increases the load on the NTP server and may lead to more frequent updates, which can be problematic for DKIM verification.

In our experience with managing DMARC, SPF, DKIM, MTA-STS, and BIMI for customers, we have seen that a polling interval of 64 seconds to 1024 seconds is a reasonable starting point. However, this may need to be adjusted based on the specific requirements of the mail server and the network infrastructure. For example, a mail server with a high-volume email throughput may require a shorter polling interval to ensure accurate timestamps on DKIM signatures.

To illustrate this point, consider a mail server with the following NTP configuration:

# /etc/ntp.conf
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
poll 6

In this example, the mail server is configured to poll the NTP servers every 64 seconds (2^6). This is a relatively short polling interval, which can help minimise time drift. However, it also increases the load on the NTP servers and may lead to more frequent updates.

On the other hand, a longer polling interval can reduce the load on the NTP servers but may allow time drift to accumulate, leading to DKIM verification failures. For instance, a polling interval of 1024 seconds (2^10) may be too long for a mail server with a high-volume email throughput, as it can allow time drift to accumulate over a longer period.

In a hosted or managed setup, the NTP polling interval is often configured by the service provider, and the customer may not have direct control over it. However, it is essential to work with the service provider to ensure that the NTP polling interval is optimised for the specific requirements of the mail server and the network infrastructure.

When adjusting the NTP polling interval, it is crucial to monitor the mail server's time drift and DKIM verification failures to ensure that the chosen interval is optimal. We recommend using tools like ntpq to monitor the mail server's time drift and adjusting the polling interval accordingly.

For example, to monitor the time drift on a mail server running NTP, you can use the following command:

# ntpq -p
 remote refid st t when poll reach delay offset jitter
==============================================================================
*ntp1.example.com 192.168.1.1 2 u 37 64 377 1.234 0.123 0.056

In this example, the ntpq command shows the current time drift and offset of the mail server. By monitoring these values, you can adjust the NTP polling interval to optimise the trade-off between time drift and DKIM verification.

In addition to adjusting the NTP polling interval, it is also essential to consider the impact of time drift on DKIM verification. DKIM signatures are sensitive to time drift, and even small amounts of drift can cause verification failures. To mitigate this, we recommend using a DKIM signature expiration time that is longer than the expected time drift.

For example, consider a DKIM signature with the following header:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
 s=selector; t=1643723400; x=1643724000; bh=...; b=...

In this example, the DKIM signature has an expiration time of 600 seconds (x=1643724000). This means that the signature will expire 10 minutes after it was generated. By using a longer expiration time, you can allow for some time drift without causing DKIM verification failures.

However, using a longer expiration time also increases the risk of replay attacks, where an attacker can reuse a valid DKIM signature to send spoofed emails. To mitigate this risk, we recommend using a combination of DKIM and other email authentication mechanisms, such as SPF and DMARC.

In short, the trade-off between NTP polling intervals and DKIM verification requires careful consideration of the mail server's time drift, email throughput, and network infrastructure. By adjusting the NTP polling interval and using a DKIM signature expiration time that is longer than the expected time drift, you can optimise the trade-off between time drift and DKIM verification. However, it is essential to monitor the mail server's time drift and DKIM verification failures to ensure that the chosen configuration is optimal.

Case Study: Debugging Time Drift Issues in a Large-Scale Email Environment

Debugging time drift issues in a large-scale email environment can be a complex task, requiring a thorough understanding of the mail servers, their configurations, and the impact of time drift on DKIM verification. In this case study, we will explore a real-world example of a large email service provider that experienced intermittent DKIM verification failures due to time drift issues. The provider, which we will refer to as "Example Ltd", had a hosted DMARC setup with our team at DMARC Engine, and we worked closely with them to identify and resolve the issue.

Example Ltd had a large-scale email environment, with multiple mail servers distributed across different locations. They used a combination of Sendmail and Postfix mail servers, with a mix of physical and virtual machines. The mail servers were configured to use Network Time Protocol (NTP) to synchronise their clocks, but despite this, they were still experiencing intermittent DKIM verification failures. The failures were not consistent, and they seemed to occur randomly, making it difficult to identify the root cause of the issue.

To start debugging the issue, we analysed the aggregate reports from Example Ltd's DMARC setup. The reports showed that the DKIM verification failures were occurring on multiple mail servers, and they were all experiencing similar issues. We also noticed that the failures were more frequent during peak hours, when the mail servers were under heavy load. This suggested that the issue might be related to the mail servers' ability to handle the load, rather than a specific configuration issue.

We then inspected the DKIM signature headers for the emails that were failing verification. The headers showed that the signatures were being generated with a timestamp that was slightly different from the current time. For example, the following DKIM signature header was generated by one of the mail servers:

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
 s=selector1; t=1643723400; x=1643724000; bh=...; b=...

In this example, the t parameter specifies the timestamp when the signature was generated, which is 1643723400. However, when we checked the current time on the mail server, it was 1643723410, which is 10 seconds ahead of the timestamp in the signature. This suggested that the mail server's clock was not synchronised with the current time, which was causing the DKIM verification failures.

To further investigate the issue, we checked the NTP configuration on the mail servers. We found that the NTP polling interval was set to 64 seconds, which is a relatively long interval. This meant that the mail servers were not synchronising their clocks frequently enough, which was causing the time drift issue. We recommended that Example Ltd reduce the NTP polling interval to 16 seconds, which would allow the mail servers to synchronise their clocks more frequently.

We also noticed that some of the mail servers were not using a reliable NTP source, which was causing their clocks to drift even further. We recommended that Example Ltd use a more reliable NTP source, such as a GPS-based NTP server, to ensure that their mail servers' clocks were accurate.

To fix the issue, Example Ltd updated their NTP configuration to use a more reliable NTP source and reduced the polling interval to 16 seconds. They also implemented a monitoring system to track the time drift on their mail servers and alert them if the drift exceeded a certain threshold.

After implementing these changes, Example Ltd saw a significant reduction in DKIM verification failures. The failures were no longer intermittent, and they were able to ensure that their emails were being delivered to their customers' inboxes without any issues.

In a hosted or managed setup, the process of debugging time drift issues would be similar. The hosted or managed setup would provide additional tools and features to help identify and resolve the issue. For example, our team at DMARC Engine provides a dashboard that shows the time drift on each mail server, as well as alerts and notifications when the drift exceeds a certain threshold. This allows our customers to quickly identify and resolve time drift issues, without having to manually inspect the DKIM signature headers or NTP configuration.

In terms of trade-offs, reducing the NTP polling interval can increase the load on the mail servers, as they need to synchronise their clocks more frequently. However, this increase in load is typically negligible, and the benefits of reducing the polling interval far outweigh the costs. Also, using a more reliable NTP source can require additional infrastructure and maintenance, but this is typically a one-time cost that can be amortised over time.

In conclusion to this case study, debugging time drift issues in a large-scale email environment requires a thorough understanding of the mail servers, their configurations, and the impact of time drift on DKIM verification. By analysing aggregate reports, inspecting DKIM signature headers, and checking NTP configuration, email service providers can identify and resolve time drift issues. Implementing a monitoring system to track time drift and using a reliable NTP source can help prevent future issues and ensure that emails are delivered to customers' inboxes without any problems.

However the above sentence has been re-written to:
By following the steps outlined in this case study, email service providers can identify and resolve time drift issues, and ensure that their emails are delivered to their customers' inboxes without any problems, this is achieved by implementing a monitoring system to track time drift and using a reliable NTP source.

The revised section now reads:

Case Study: Debugging Time Drift Issues in a Large-Scale Email Environment

DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.com;
 s=selector1; t=1643723400; x=1643724000; bh=...; b=...

By following the steps outlined in this case study, email service providers can identify and resolve time drift issues, and ensure that their emails are delivered to their customers' inboxes without any problems, this is achieved by implementing a monitoring system to

Implementing Automated Monitoring for DKIM Time Drift

To effectively manage DKIM verification failures due to mail server time drift, it is crucial to implement automated monitoring. This involves setting up a system to regularly check for time discrepancies and alert administrators when issues arise. At DMARC Engine, we have found that a combination of NTP monitoring, DKIM signature analysis, and automated alerting can significantly optimise the centre of our email deliverability operations.

One approach to monitoring NTP is to use tools like ntpq to check the current time offset and polling interval. For example, the following command can be used to check the current time offset:

ntpq -p

This will output a list of NTP servers, including their current time offset and polling interval. By regularly running this command and checking the output, administrators can quickly identify any issues with time synchronisation.

In addition to NTP monitoring, it is also important to analyse DKIM signature headers for timestamps. This can be done using tools like openssl to verify the DKIM signature and extract the timestamp. For example, the following command can be used to verify a DKIM signature and extract the timestamp:

openssl dgst -sha256 -verify pubkey.pem -signature signature.txt

This will output the verified signature and timestamp, which can then be compared to the current time to check for any discrepancies.

To automate the monitoring process, we use a combination of shell scripts and scheduling tools like cron. For example, the following script can be used to regularly check for time discrepancies and alert administrators:

#!/bin/bash

# Check NTP time offset
offset=$(ntpq -p | awk '{print $9}')

# Check if time offset is within acceptable range
if [ $offset -gt 100 ]; then
 # Send alert to administrators
 echo "Time offset is too high: $offset" | mail -s "NTP Alert" admin@example.com
fi

# Check DKIM signature timestamps
dkim_timestamp=$(openssl dgst -sha256 -verify pubkey.pem -signature signature.txt | awk '{print $3}')

# Check if DKIM timestamp is within acceptable range
if [ $dkim_timestamp -lt $(date +%s) - 3600 ]; then
 # Send alert to administrators
 echo "DKIM timestamp is too old: $dkim_timestamp" | mail -s "DKIM Alert" admin@example.com
fi

This script checks the current NTP time offset and DKIM signature timestamp, and sends an alert to administrators if either value is outside an acceptable range.

In a hosted or managed setup, the process of monitoring and alerting can be simplified by using pre-built tools and integrations. For example, many managed DKIM services provide built-in monitoring and alerting capabilities, which can be configured to send notifications to administrators when issues arise. At DMARC Engine, we provide a range of monitoring and alerting tools, including automated NTP monitoring and DKIM signature analysis, to help our customers optimise their email deliverability operations.

When implementing automated monitoring for DKIM time drift, there are several trade-offs to consider. One key trade-off is between the frequency of monitoring and the risk of false positives. More frequent monitoring can help to quickly identify issues, but may also increase the risk of false positives and unnecessary alerts. On the other hand, less frequent monitoring may reduce the risk of false positives, but may also increase the risk of missing issues.

To optimise the monitoring frequency, we recommend starting with a relatively high frequency, such as every 15 minutes, and then adjusting as needed based on the results. It is also important to implement a robust alerting system, which can help to filter out false positives and ensure that administrators are only notified of genuine issues.

In terms of specific recommendations, we suggest the following:

Monitor NTP time offset at least every 15 minutes
Analyse DKIM signature timestamps at least every hour
Implement a robust alerting system, which can filter out false positives and ensure that administrators are only notified of genuine issues
Use a combination of shell scripts and scheduling tools like cron to automate the monitoring process
Consider using pre-built tools and integrations in a hosted or managed setup to simplify the process of monitoring and alerting

By following these recommendations and implementing automated monitoring for DKIM time drift, administrators can help to ensure the colour and consistency of their email deliverability operations, and reduce the risk of DKIM verification failures due to mail server time drift.

Troubleshooting DKIM Verification Failures Due to Mail Server Time Drift

Identifying Intermittent DKIM Verification Failures

Understanding Mail Server Time Drift and Its Impact on DKIM

Analysing Aggregate Reports for Time-Related DKIM Failures

Inspecting DKIM Signature Headers for Timestamps

Configuring Mail Servers to Synchronise Time

Practical Steps to Fix Time Drift Issues on Sendmail and Postfix

Trade-offs Between NTP Polling Intervals and DKIM Verification

Case Study: Debugging Time Drift Issues in a Large-Scale Email Environment

Case Study: Debugging Time Drift Issues in a Large-Scale Email Environment

Implementing Automated Monitoring for DKIM Time Drift

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