3. Next Steps

After the installation you can test the service using one of the Thunderstorm collectors or the Python API client.

3.1. Configuration

The installation script for Linux system installs a service that passes the parameter -t /etc/thunderstorm/thunderstorm.yml to initialize the default config stored at that location.

The default configuration file on Linux looks like this:

# License path

license-path: /etc/thunderstorm

# Write all outputs to the following directory

logfile: /var/log/thunderstorm/thunderstorm.log

appendlog: True

# Listen on all possible network interfaces


server-port: 8080

# Pure YARA scanning

pure-yara: False


# SSL/TLS Server Certificate

#server-cert: /path/to/file

# SSL/TLS Server Certificate Private Key

#server-key: /path/to/file

# File Submissions

# Directory to which the samples get stored in asynchronous mode

server-upload-dir: /tmp/thunderstorm

# Permanently store the submitted samples (valied values: none/all/malicious)

server-store-samples: none

# Tuning

# Server Result Cache

# This is the number of cached results from asynchronous submission

# available for remote queries (default: 10000)

#server-result-cache-size: 10000

You can use all of THOR’s flags in that configuration. Be advised that you always have to use their long form.

This page lists all of THOR command line flags:


The following chapters list some of the most useful command line flags when using THOR Thunderstorm.

3.1.1. Forward Logs to SIEM or Analysis Cockpit


  • mysiem.local

Config entry to forward logs to a SIEM

We recommend reading chapter 10.2 Syslog or TCP/UDP Output in the THOR User Manual for details on the SYSLOG forwarding flags. You can find it in the folder /opt/nextron/thunderstorm/docs after a successful Thunderstorm installation on Linux or in the “Downloads” section in the customer portal. Keep Samples on the Thunderstorm Server

server-store-samples: malicious

Keep samples with findings

server-store-samples: all

Keep all samples

3.2. Log Output

The scan results and startup messages can be found in:


You could open another command line window and monitor new messages with:

tail -f /var/log/thunderstorm/thunderstorm.log

3.3. Thunderstorm API Documentation

An API documentation is integrated into the web service.

Simply visit the service URL, e.g.: http://my-server:8080/

Thunderstorm API Documentation

Thunderstorm API Documentation

3.4. Test Submission

To test the Thunderstorm service, you can create a tiny webshell sample and submit it to the service using the following commands.

echo "<%eval request(" > test.txt

curl -X POST "" -F "file=@test.txt"

This should produce the following output in the current command line.

        "level": "Alert",
        "module": "Filescan",
        "message": "Malware file found",
        "score": 350,
        "context": {
           "ext": ".txt",
           "file": "test.txt",
           "firstBytes": "3c256576616c2072657175657374280a / \\u003c%eval request(\\n"
           "md5": "2481bc6bb2d063522ef8b5d579fd97d7",
           "sha1": "4d40de75d7c8591d2ea59d3a000fb6cf58d97896",
           "sha256": "3b435df5076f6b1df7f2bc97cd86fbf7b479352e8c33960dfc4f1cbbe9b14fa7",
           "size": 16,
           "type": "JSP"

Output of test sample submission

Be aware that this has been a “synchronous” submission to the API endpoint “check”. The collection of high amounts of samples in collector scripts and tools uses the endpoint “checkAsync”, which doesn’t return a result to the submitting source.

3.4.1. Test Submission Using the API Documentation

The web GUI running on Port 8080 contains an interactive API documentation, which you can use to submit a first test sample.

Link to API Documentation on Start Page

Link to API Documentation on Start Page

Select the API function /api/check, then click “Try it out” and then select and submit a sample using the enabled form.

Test Sample Submission via API Documentation

Test Sample Submission via API Documentation

The result appears in a separate text field. Use the “pretty” flag to get a prettified JSON response.

3.5. Thunderstorm Collectors

You can find a Thunderstorm collector for numerous different operating systems and architecture in our Github repository.


See the README on Github for more information.

3.5.1. Performance Considerations for the Collection

In a THOR Thunderstorm setup, the system load moves from the end systems to the Thunderstorm server.

In cases in which you don’t use the default configuration file provided with the collectors (config.yml) and collect all files from an end system, the Thunderstorm server requires a much higher amount of time to process the samples.

E.g. A Thunderstorm server with 40 CPU Cores (40 threads) needs 1 hour to process all 400,000 files sent from a Windows 10 end system. Sending all files from 200 Windows 10 end systems to a Thunderstorm server with that specs would take up to 10 days to process all the samples.

As a rule of thumb, when using the hardware recommended in chapter 1.2 "Hardware Requirements", calculate with a processing speed of 250 samples per core per minute.

We highly recommend using the default configuration file named config.yml provided with the collectors. See the README on Github for more information.

3.6. Thunderstorm API Client

We provide a free and open source command line client written in Python to communicate with the Thunderstorm service.


It can be installed with:

pip install thunderstormAPI

3.7. Source Identification

The log file generated by THOR Thunderstorm doesn’t contain the current host as hostname in each line. By default, it contains the sending source’s FQDN or IP address if a name cannot be resolved using the locally configured DNS server.

However, every source can set a “source” value in the request and overwrite the automatically evaluated hostname. This way users can use custom values that are evaluated or set on the sending on the end system.

curl -X POST "http://myserver:8080/api/check?source=test" -F "file=@sample.exe"

3.8. Synchronous and Asynchronous Mode

It is also important to mention that THOR Thunderstorm supports two ways to submit samples, a synchronous and an asynchronous mode.

The default is synchronous submission. In this mode, the sender waits for the scan result, which can be empty in case of no detection or contains match elements in cases in which a threat could be identified.

In asynchronous mode, the submitter doesn’t wait for the scan result but always gets a send receipt with an id, which can just be discarded or used to query the service at a later point in time. This mode is best for use cases in which the submitter doesn’t need to know the scan results and batch submission should be as fast as possible.



Server API Endpoint



Client Parameter



Returns Scan Result

Faster Submission


Client waits for result of each sample

No immediate scan result
on the client side

In asynchronous mode, the Thunderstorm service keeps the samples in a queue on disk and processes them one by one as soon as a thread has time to scan them. The number of files in this queue can be queried at the status endpoint /api/status and checked on the landing page of the web GUI.

In environments in which the Thunderstorm service is used to handle synchronous and asynchronous requests at the same time, it is possible that all threads are busy processing cached asynchronous samples and not more synchronous requests are possible.

In this case use the --sync-only-threads flag to reserve a number of threads for synchronous requests. (e.g. --threads 40 --sync-only-threads 10)

3.9. Performance Tests

Performance tests showed the differences between the two submission modes.

In Synchronous mode, sample transmission and server processing take exactly the same time since the client always waits for the scan result. In asynchronous mode, the sample transmission takes much less time, but the processing on the server takes a bit longer, since the sever caches the samples on disk.



Client Transmission



Server Processing


Total Time



3.10. SSL/TLS

We do not recommend the use of SSL/TLS since it impacts the submission performance. In cases in which you transfer files through networks with IDS/IPS appliances, the submission in an SSL/TLS protected tunnel prevents IDS alerts and connection resets by the IPS.

Depending on the average size of the samples, the submission frequency and the number of different sources that submit samples, the transmission could take up to twice as much time.

Note: The thunderstormAPI client doesn’t verify the server’s certificate by default as in this special case, secrecy isn’t important. The main goal of the SSL/TLS encryption is an obscured method to transport potentially malicious samples over network segments that could be monitored by IDS/IPS systems. You can activate certificate checks with the --verify command line flag or verify parameter in API library’s method respectively.