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How to Modify an STA Snort Rule

Once you install our Coralogix Security Traffic Analyzer (STA) it will automatically pull a set of snort rules from the Internet and will continue to update them on a daily basis.

However, in the security business, there’s no “one size fits all” solutions. Some of these rules might suit your organization while others might need a few tweaks to get them to work correctly in your environment. In the new version of the Coralogix Security Traffic Analyzer (STA), we added the support for synchronizing its configuration against an S3 bucket. By using this feature, you can easily fine-tune your snort alerts as explained here:

  1. Detect the problematic alert’s SID: By navigating to the NIDS dashboard in Kibana you should see a dashboard similar to this one:

    If you’ll scroll down a bit, you should be able to see the list of alerts by IP addresses, like the below example. The two columns marked on this screenshot are the group id (GID) and the snort id (SID) of the alert. We’ll need them in the next step to disable the relevant alert and create a new one.
  2. Get the original snort rule:  Connect to your STA instance via SSH by using the SSH key you provided during the installation of the STA and run the following command (no sudo permissions required):
    grep '<the snort sid of the alert>' /etc/nsm/rules/downloaded.rules

    You should get an output similar to this:

    alert tcp $HOME_NET any -> $EXTERNAL_NET $HTTP_PORTS (msg:"ET POLICY curl User-Agent Outbound"; flow:established,to_server; content:"User-Agent|3a| curl/"; nocase; http_header; reference:url,www.useragentstring.com/pages/useragentstring.php; classtype:attempted-recon; sid:2013028; rev:2; metadata:created_at 2011_06_14, updated_at 2011_06_14;)
  3. Disable the original snort rule: Copy the relevant config files from the S3 bucket you configured for the STA to use as its configuration bucket by using the following command:
    aws --region <your aws region> s3 cp s3://<s3 bucket used for STA configuration>/local.rules /tmp/
    aws --region <your aws region> s3 cp s3://<s3 bucket used for STA configuration>/disablesid.conf /tmp/

    Open the file /tmp/disablesid.conf with your favorite text editor (we highly recommend using either nano or vi/vim) and add lines similar to these at the bottom of the file:

    # Not relevant for my organization
    
    <alert group id>:<alert snort id>
  4. Create a modified version of the original rule (optional): If you only intend to modify an existing rule, copy the rule you found in step 2 above and paste it at the bottom of the local.rules file.
  5. Upload the files back to the S3 bucket: Use the following commands to upload the files to the config S3 bucket (if you only disabled a rule then the second command is not required)
    aws --region <your aws region> s3 cp /tmp/disablesid.conf s3://<s3 bucket used for STA configuration>/
    
    aws --region <your aws region> s3 cp /tmp/local.rules s3://<s3 bucket used for STA configuration>/

After you’ve completed the steps above, your STA will automatically detect the changes and apply them.

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