What is the rua tag in my DMARC record

I kept getting emails from noreply-dmarc-support@mydomain.com every day. After finishing a cool bucket hat design I wondered What is the rua tag in my DMARC record? Then, I did a little investigation and I remembered I set this myself when I was setting up my DMARC records in DNS.

The rua tag in a DMARC (Domain-based Message Authentication, Reporting & Conformance) record specifies the email address or addresses to which aggregate reports should be sent. These reports provide insights into how email from your domain is being handled and help you monitor the effectiveness of your DMARC policy.

Example of a DMARC Record with rua Tag

v=DMARC1; p=none; rua=mailto:dmarc-aggregate@yourdomain.com; ruf=mailto:dmarc-forensic@yourdomain.com; fo=1;

What is the rua tag used For?

Aggregate Reports: The rua tag tells receiving mail servers where to send aggregate reports. These reports include information about the email messages sent from your domain and whether they passed or failed SPF (Sender Policy Framework) and DKIM (DomainKeys Identified Mail) checks.

Monitoring: By analyzing these reports, you can identify unauthorized use of your domain, configure your SPF and DKIM settings properly, and eventually set a more strict DMARC policy (p=quarantine or p=reject).

Is it Safe to Remove the rua Tag?

Monitoring Loss: If you remove the rua tag, you will no longer receive aggregate reports. This means you won’t have visibility into how your domain’s email is being handled, which can make it difficult to detect misuse or misconfiguration.

Recommendation: It’s generally not recommended to remove the rua tag unless you have another mechanism in place to monitor your email authentication results. The insights provided by aggregate reports are valuable for maintaining and improving email security.

What is the rua tag conclusion

The rua tag is an important component of a DMARC record that helps you monitor email activity for your domain. Removing it would mean losing access to aggregate reports, which can be detrimental to maintaining proper email security. It is advisable to keep the rua tag in your DMARC record to continue receiving these valuable reports.

Hope this helps somebody out there. You can always contact me if you have any questions.

How to use Telnet in Linux

Telnet is a network protocol used to provide a bidirectional interactive text-oriented communication facility using a virtual terminal connection. Here’s how you can install and use Telnet on a Linux system. I had to take a long walk in NYC before taking on this one. I assumed Telnet was no longer in use by anybody.

Installing Telnet

The process of installing Telnet depends on the Linux distribution you are using. Here are the commands for some common distributions:

For Debian/Ubuntu-based distributions:

sudo apt update
sudo apt install telnet

For Red Hat/CentOS-based distributions:

sudo yum install telnet

For Arch Linux:

sudo pacman -S inetutils

Using Telnet in Linux

After installing Telnet, you can use it to connect to remote servers. Here are some common uses:

Basic Telnet Command

To connect to a remote server, use the following syntax:

telnet [hostname or IP address] [port]

For example, to connect to example.com on port 80:

telnet example.com 80

Example Usage

  1. Connecting to a Remote Server:
   telnet example.com 80

If the connection is successful, you will see a blank screen or some welcome message from the server.

  1. Testing a Mail Server:
    You can use Telnet to test an SMTP mail server:
   telnet smtp.example.com 25

Once connected, you can type SMTP commands such as HELO, MAIL FROM, RCPT TO, and DATA to interact with the mail server.

  1. Checking an Open Port:
    Telnet can be used to check if a particular port is open on a server:
   telnet yourserver.com 22

If the port is open, you will connect to it, otherwise, you will get a connection error.

Closing a Linux Telnet Session

To close a Telnet session, you can type exit or quit, or you can simply press Ctrl+] to escape to the Telnet prompt and then type quit to exit.

Ctrl + ]
telnet> quit

Telnet Commands and Options

  • open [hostname] [port]: Open a connection to the specified hostname and port.
  • close: Close the current Telnet session.
  • quit: Exit the Telnet client.
  • status: Print status information.
  • ? or help: Display help information.

Security Note

Telnet is not a secure protocol as it transmits data in plaintext, including usernames and passwords. For secure remote access, it is recommended to use SSH (Secure Shell) instead of Telnet.

To install SSH on a Linux system:

For Debian/Ubuntu-based distributions:

sudo apt update
sudo apt install openssh-client

For Red Hat/CentOS-based distributions:

sudo yum install openssh-clients

For Fedora:

sudo dnf install openssh-clients

For Arch Linux:

sudo pacman -S openssh

To connect to a remote server using SSH:

ssh user@hostname

By following these instructions, you should be able to install and use Telnet on your Linux system. Remember to consider using more secure alternatives like SSH for sensitive communications.

Flask Application Skeleton Example

A friend called me the other day wanting to learn more about putting together a Flask application. After brewing a good Catimor coffee variety from Dominican Republic I decided to get a sample Flask code for starters and share it with everybody.

This Flask application includes:

  1. Importing the Flask class from the flask module.
  2. Creating a Flask application instance with app = Flask(__name__).
  3. Defining a route / using the @app.route() decorator and a corresponding view function (index()) that returns a simple message.
  4. Running the Flask application with app.run().

You can save this code to a Python file (e.g., app.py) and run it using Python. The Flask development server will start, and you can access the application in your web browser at http://localhost:5000/.

from flask import Flask
# Create a Flask application instance
app = Flask(__name__)
# Define a route and its corresponding view function
def index():
    return 'Hello, World!'
# Run the Flask application
if __name__ == '__main__':

As usual, you can contact me if you have any questions or want to collaborate.

Docker CLI basic commands

Docker is like a virtual box that lets you put your app inside and run it without worrying about messing up your computer. It’s like having a bunch of separate little rooms where you can run different apps at the same time without them bothering each other. Each room (or container) is lightweight and has everything your app needs to work, so you don’t have to worry about what’s already on your computer. Plus, you can easily share these containers with others, making sure everyone has the same setup.

Install Docker

Get the latest version from Docker official site.

Some sample projects



#Build an Image from a Dockerfile
docker build -t
#Build an Image from a Dockerfile without the cache
docker build -t . –no-cache
#List local images
docker images
#Delete an Image
docker rmi
#Remove all unused images
docker image prune

Docker hub

Service platform provided by Docker to store and share container images.

#Login into Docker
docker login -u <username>
#Publish an image to Docker Hub
docker push <username>/<image_name>
#Search Hub for an image
docker search <image_name>
#Pull an image from a Docker Hub
docker pull <image_name>

Help commands

#Start the docker daemon
docker -d
#Get help with Docker. Can also use –help on all subcommands
docker --help

This one is very cool. It tells you all details about your system:

#Display system-wide information
docker info


A container is like a special environment created from an image. It always behaves consistently, no matter where it’s running. Containers keep your software separate from its surroundings, making sure it works the same way whether you’re developing, testing, or running it live.

#Create and run a container from an image, with a custom name:
docker run --name <container_name> <image_name>
#Run a container with and publish a container’s port(s) to the host.
docker run -p <host_port>:<container_port> <image_name>
#Run a container in the background
docker run -d <image_name>
#Start or stop an existing container:
docker start|stop <container_name> (or <container-id>)
#Remove a stopped container:
docker rm <container_name>
#Open a shell inside a running container:
docker exec -it <container_name> sh
#Fetch and follow the logs of a container:
docker logs -f <container_name>
#To inspect a running container:
docker inspect <container_name> (or <container_id>)
#To list currently running containers:
docker ps
#List all docker containers (running and stopped):
docker ps --all
#View resource usage stats
docker container stats

I like to keep these commands handy. Sometimes you tend to forget. Contact me if you have any questions or want to collaborate.

How to check users group in Linux

After I got back from my trip to Puebla, Mexico to taste and learn about Pulque. My friend called me looking to find out how to check users group in Linux. There are some steps I took to find users and groups in Linux.

Steps to check users on a Linux server

Use the cat /etc/passwd command to view the contents of the /etc/passwd file, which contains information about all the user accounts on the system.

Alternatively, you can use the getent passwd command to query the system’s user database and list all the users.

To get a count of the total number of users, you can pipe the output of the cat /etc/passwd or getent passwd commands to the wc -l command to count the number of lines, which corresponds to the number of users.

If you only want to list the normal user accounts (excluding system accounts), you can use the getent passwd {1000..60000} command to list users with UIDs (User IDs) in the typical range for normal user accounts.

You can also use the awk command to extract just the usernames from the /etc/passwd file, for example: awk -F':' '{ print $1 }' /etc/passwd

How to check a user’s group?

You can use the groups command:


This will list all the groups the current user is a member of.

To check the groups for a different user, use the following syntax:

groups <username>

Use the id command to get more details about a user:

id <username>

This will show the user’s primary group, as well as any secondary groups they belong to.

Another option is to use the getent command to query the system’s user database and list the groups a user belongs to:

getent group | grep <username>

The above will show all groups the user is a member of. Usually will highlight the username.

If you want to see all the users that belong to a specific group, you can use the following command:

getent group <groupname> | cut -d: -f4

To check users group Linux is quite easy, but if you don’t do this on a daily basis is easy to forget. This is my case. You can always contact me if you have any questions or want to collaborate. Thanks!

Connect to Exchange Online using PowerShell

This one was a new experience for me. After brewing a good cup of Caturra variety coffee at home, a friend contacted me wanting to connect to Exchange online using PowerShell. This was due to some strange email activity with one of her users. Connecting to Exchange Online using PowerShell allows administrators to manage Exchange Online settings, mailboxes, and other features remotely.

First you need to check if the Exchange Online module is installed:

Get-Module -ListAvailable -Name ExchangeOnlineManagement

If the module is not installed, open a Windows Powershell and run as Administrator:

Install-Module -Name ExchangeOnlineManagement -Force

Follow the prompts. Yes, you can install from untrusted repositories. Press Y to confirm. If you already have the module installed and want to update it:

Update-Module ExchangeOnlineManagement

Now, connect to Exchange online:


Enter your credentials in the pop up window. Usually using MFA (Multi-factor Authentication).

After you connect you can run the desired command against any inbox. For example we wanted to check all rules on a shared inbox:

get-inboxrule -mailbox sharedbox@hola.com  | fl 

After you’re done, you can disconnect:

Disconnect-ExchangeOnline -Confirm:$False

Remember to contact me if you have any questions or like to collaborate. Thanks!

Configure log rotation for Postfix on Linux

A client running a postfix server called me the other day wanting to configure log rotation for Postfix on Linux. This was a Redhat box and I didn’t have much experience configuring log rotation. After my trip to Coyoacan in Mexico City I was very relaxed and started putting things together. Below is what I found.

To configure log rotation for Postfix on a Linux server, you can create a custom logrotate configuration file specifically for Postfix log files. Postfix logs are usually located at /var/log/maillog, /var/log/mail.log or something like that. You can create a logrotate configuration file at /etc/logrotate.d/postfix Here’s a sample file:

rotate 14

Explanation log rotation

  • rotate 14: Keep up to 14 rotated log files.
  • daily: Rotate the log files daily.
  • missingok: Do not generate an error if the log file is missing.
  • notifempty: Do not rotate the log file if it’s empty.
  • delaycompress: Compress the rotated log files one day after rotation.
  • compress: Compress the rotated log files using gzip.
  • postrotate/endscript: Execute a custom command after log rotation. This one is very cool! I didn’t use it on this example. Good for running scripts before or after.

Save the Configuration File: Save the changes to the “postfix” configuration file and exit the text editor.

Test Log Rotation: Test the log rotation configuration using the logrotate command with the -d or --debug option to simulate rotation without actually rotating files:

sudo logrotate -d /etc/logrotate.d/postfix

You can add -v option for verbose output. Use it! Verify that log rotation works as expected and that no errors occur.

Automatic Rotation: Logrotate is typically run as a cron job. It will automatically rotate log files based on the schedule defined in the configuration file.

As usual, you can contact me if you have any questions or like to collaborate. I’m working on fixing my online shop for now. Stay tuned.

Find uptime on Windows

The other day a client wanted to find out if his Windows machines were rebooting over the weekend. They had a scheduled task to apply updates and reboot. He wanted to quickly find uptime on Windows. I found some options:

Command line to find uptime on Windows

  1. Open your command line
  2. Type Systeminfo | find “Boot Time”

You should see something like:

Days: 9 Hours: 11 Minutes: 20 Seconds: 12

You can also query a remote Windows machine, but it seems to take longer to get results:

systemsinfo /S "machineName" | find "Boot Time"

Use the Task Manager to find uptime on Windows

  1. Open your task manager
  2. Expand ‘More Details’
  3. Go to Performance
  4. CPU
  5. See on the bottom ‘Up time’

Another way tricky way

This command is a bit tricky and I still need to find out how to read the results:

C:\Users\Tom> wmic path Win32_OperatingSystem get LastBootUpTime

After putting this brief tutorial together I decided to brew a good cup of Caturra coffee at home and head out for a walk.

As usual, you can contact me if you have questions or like to collaborate.

SEO Ready Website for Maximum Visibility

Making your website SEO ready involves optimizing various aspects of your site to improve its visibility and ranking in search engine results. I have to admit this topic is not my favorite, but is important if you want your website to be found by our good friend Google and other search engines.

After brewing my favorite coffee from Peru I decided to put a brief key steps to make your website SEO ready:

SEO analysis

  1. SEO title width: The SEO title has a viewable limit. Under 48 characters should be OK. Don’t make it too short either. Have at least four words. Include the focus keyphrase at the beginning.
  2. Text length: Write at least 300 words for your blog content. Include the focus keyphrase at least 2 times depending on the length of your text.
  3. Meta description length: The length should be from 120 to 156 characters long. Always include the focus keyphrase here.
  4. Key phrase length: Use the recommended maximum of 4 content words.
  5. Internal links: Have at least one internal link. Point it to a relevant content in your website.
  6. Outbound links: Have at least one external link.
  7. Images: Include at least one image with ALT tag using your focus keyphrase.

By following these steps and staying up-to-date with SEO best practices, you can make your website more SEO friendly and improve its visibility in search engine results.

SEO Website Readability

To enhance the ranking of your content, ensure it’s engaging and enjoyable to read. When your audience appreciates your writing, search engines will also do. Here are some steps to follow and make your website readability optimized:

  1. Passive voice: Use enough active voice. The recommended passive voice is 10%.
  2. Consecutive sentences: Use enough variety for your content. Do not start more than two sentences with the same word.
  3. Subheading distribution: Use subheading to divide your content. Probably use a subheading for every 150 words. Try to use your focus keyphrase in it.
  4. Paragraph length: Should be more than two sentences and less than 200 words.
  5. Sentence length: Do not write sentences longer than 20 words.
  6. Transition words: Use transition words for about. The recommended is 30% of your content should use transition words.

Print the email addresses with status=”bounced” in postfix mail log

To print the email addresses with status “bounced” only from the provided log file, you can use grep to filter lines containing “status=bounced” and then use awk to extract the email addresses. Here’s how you can do it:

grep "status=bounced" /var/log/mail.log | grep -oE 'to=<[^>]+>' | awk -F'<' '{print $2}' | awk -F'>' '{print $1}'


  • grep "status=bounced" /var/log/mail.log: This grep command filters lines from the mail log file containing “status=bounced”.
  • grep -oE 'to=<[^>]+>': This grep command uses a regular expression (-oE) to extract occurrences of “to=<…>” from the filtered lines and only outputs the matching text.
  • awk -F'<' '{print $2}': This awk command uses “<” as the field separator (-F'<') and prints the second field (the part after “<“).
  • awk -F'>' '{print $1}': This awk command uses “>” as the field separator (-F'>') and prints the first field (the part before “>”).

This series of commands will extract and print only the email addresses from lines with status “bounced” in the log file, removing any surrounding characters or additional information.

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