how to send update notifications from a linux server to a slack channel

INSPIRATION

When I first set up my Ghost blog, I configured the host server to use the unattended-upgrades package to automatically install any security updates daily.

The unattended-upgrades package was configured to send email notifications to root@localhost whenever the update process resulted in a change. The Mutt email client was also installed on the server to allow me to read the emails. However, this meant that I needed to log in to the server in order to read the notifications. I needed another option.

IDEATION

What if I could configure the server to also send a short summary of the update report to a Slack channel? That way I would be instantly notified if any updates were applied and if the updates were successful, without me having to directly log in to the server each time.

As it turns out, the process of sending a short notification message to a Slack channel is fairly simple, if we use incoming webhooks. And, the process is outlined below.

IMPLEMENTATION

1. Create Slack Workspace and Channel

• See Create a Slack Workspace.

2. Create Incoming Webhook

• See Getting started with Incoming Webhooks, Steps #1 through #3.

3. Configure Linux Server to Send Notifications to Slack

• Create a new non-privileged Linux user with the adduser command and follow the prompts.

  adduser notifyuser
  

• Edit the postfix configuration file and set the default_privs parameter to your new user. This avoids the Permission Denied error when executing the bash script, since the command executes with the privileges of default_privs and by default the local delivery agent uses the nobody user for delivery to an external file or command.

  # open postfix configuration file
  sudo nano /etc/postfix/main.cf
  
  # set default_privs parameter
  default_privs = notifyuser
  
  # reload the configuration file
  sudo systemctl restart postfix
  

• Edit the aliases file to add a mail alias for the new user, and also add an external command to pipe any messages for the new alias to a script.

  # open the aliases file
  sudo nano /etc/aliases
  
  # add the below alias and external command
  notifyuser: "|/home/notifyuser/bin/notifyslack.sh"
  
  # rebuild the database for the mail aliases file
  sudo newaliases
  

• Update the unattended-upgrades package configuration file to also send updates to the new user (in addition to root).

  # open the configuration file 
  sudo nano /etc/apt/apt.conf.d/50unattended-upgrades
  
  # update the below line to include the new user
  Unattended-Upgrade::Mail "root@localhost,notifyuser@localhost";
  

• Create a bash script to handle posting messages to Slack using the incoming webhook created in Step #2 above. Ensure that the owner and group for both the user's private bin directory and the bash script file are also set to the new user.

  # create bash script file in user's private bin
  sudo mkdir /home/notifyuser/bin
  sudo nano /home/notifyuser/bin/notifyslack.sh
  
  # update owner and group for private bin and bash file
  sudo chown notifyuser:notifyuser /home/notifyuser/bin
  sudo chown notifyuser:notifyuser /home/notifyuser/bin/notifyslack.sh
  

  Add the below script to the bash file. These commands will format and send the notification. Replace https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX with your own incoming webhook URL from Slack.

  #!/bin/bash
  
  # Slack Message
  MESSAGE=$(sed -n -e '7'p -e '16,19'p $1)
  
  # Slack Webhook
  WEBHOOK_URL="https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX"
  
  # POST request to Slack webhook with subject from unattended-upgrade notification
  curl -X POST -H 'Content-type: application/json' --data '{"text":"'"${MESSAGE}"'"}' $WEBHOOK_URL
  

  Update the file permissions to allow execution of the bash script.

  sudo chmod +x /home/notifyuser/bin/notifyslack.sh
  

• Test the configuration to ensure it works.

  # perform a test run with the verbose and extra debug output options
  sudo unattended-upgrade -v -d
  

If everything is setup correctly then you should receive Slack messages after each change.

---

Sources:

M. Vogt. Ubuntu Manpage Repository: unattended-upgrade. (2019). Accessed: Feb. 19, 2023. [Online]. Available: https://manpages.ubuntu.com/manpages/jammy/man8/unattended-upgrade.8.html

"Postfix Configuration Parameters," postfix.org. https://www.postfix.org/postconf.5.html (accessed Feb. 19, 2023).

"Postfix: permission denied error while the incoming mail reading from python script," stackoverflow.com. https://stackoverflow.com/questions/61503015/postfix-permission-denied-error-while-the-incoming-mail-reading-from-python-scr (accessed Feb. 19, 2023).

X. Decuyper, Sending Fail2ban (and other) notifications to a Slack chat channel. savjee.be. https://savjee.be/blog/Sending-fail2ban-notifications-and-others-to-slack-chat-channel/ (accessed Feb. 19, 2023).