eBlocker Environment Simulator

The component eblocker-icapserver interacts with the system via Redis channels (publish/subscribe) and via the script_wrapper program.

Redis channels are used to read and write network packets like ARP requests and responses via low-level C programs.

The script_wrapper program can call scripts in a defined directory and runs as root, so the eblocker-icapserver can start/stop services and configure the OS.

The purpose of the eBlocker Environment Simulator is to

• simulate the script_wrapper and the scripts it can run
• simulate the low-level C programs subscribing to and publishing into specific Redis channels
• simulate various failure cases that can occur on a real eBlocker system.

The simulator is a program running in the terminal that connects to Redis and subscribes to specific Redis channels at startup.

The simulator has internal state, for example it has a flag:

• is the eBlocker updating currently?

This state is stored in Redis in the hash simulator.

For example, the updating state is stored as

redis-cli> hset simulator updating true

Building the simulator

Go 1.25 is required for building the simulator.

Build:

make

Then move the simulator_script_wrapper to

/opt/eblocker-network/bin/script_wrapper

Running the simulator

You need a running Redis server that accepts connections at localhost:6379:

redis-server

Now you can run the simulator from any directory:

./simulator
2026/05/21 14:26:57 listening on simulator_script_wrapper:in
2026/05/21 14:26:57 listening on arp:out

Test the script_wrapper:

/opt/eblocker-network/bin/script_wrapper foo

The simulator should log this call:

2026/05/21 14:27:59 [11231] ⮕ foo args=[]
2026/05/21 14:27:59 [11231] ⬅︎ return=0

Configuring the simulator

Currently there is no UI, the simulator is configured via redis-cli.

Flags

Flags are stored in the Redis hash simulator.

They are set with

redis-cli hset simulator KEY VALUE

and removed with

redis-cli hdel simulator KEY

Updating

This flag indicates that eBlocker is updating:

redis-cli hset simulator updating true

Updates failed

This flag indicates that updating the eBlocker failed:

redis-cli hset simulator updates-failed true

Devices

Devices respond to ARP requests (mapping their IPv4 address to their MAC address).

To simulate this, you can set the key device/<IPv4 address>, for example:

hset simulator device/192.168.0.110 abcdef000110

To create 25 devices in this way you can run:

redis-cli < scripts/createManyDevices.redis

Implementation

The simulator is implemented in Go. It uses the go-redis client.

It has two components:

• the main simulator, running continuously
• the simulated script_wrapper that is run on demand by the eblocker-icapserver.

Communication between simulator components

The script_wrapper is called with various numbers of arguments:

• name of the script to run
• optional command line arguments.

It first generates a new integer from the Redis sequence (INCR) simulator_script_wrapper. This integer is the callID.

It subscribes to three Redis channels:

• simulator_script_wrapper:<callID>:stdout for strings to print to STDOUT,
• simulator_script_wrapper:<callID>:stderr for strings to print to STDERR,
• simulator_script_wrapper:<callID>:return for the return code of the script.

It then publishes the callID and the list of command line arguments to the channel simulator_script_wrapper:in as a JSON array, e.g.

[23, "myscript", "arg1", "arg2"]

When the wrapper receives the integer return code on the ...:return channel, it quits and returns the code.