Arma 3: Remote Execution

Remote Execution is one of the cornerstones of Multiplayer Scripting in Arma 3. It is primarily needed to properly use commands that have local effect (LELocal) or only take local arguments (LALocal) in multiplayer.

Remote Execution Framework

The Remote Execution Framework currently consists of five commands:

• remoteExec
• remoteExecCall
• remoteExecutedOwner
• isRemoteExecuted
• isRemoteExecutedJIP

Furthermore, the framework encompasses an optional config entry to manage security settings: CfgRemoteExec (see Security to learn more).

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While primarily designed and used for MP, remoteExec and remoteExecCall also work in SP; the behaviour is the same as in MP.

Use Case Example

To understand why remote execution is needed and what it is used for, consider the following use case example: Let's say we want to use hint to display an announcement to all the players in our MP mission at the same time.

We already know how to use the hint command:

hint "You have 5 minutes left!"; // Normal use of hint.

The problem is that hint only has local effect. This means that only the machine where hint is executed will display our "You have 5 minutes left!" message. But we do not want to display our message to just a single player, we want to display our message to every player.

So in order to solve this problem, we need to make use of remote execution:

"You have 5 minutes left!" remoteExec ["hint", 0]; // Remote execution of hint.

Now all machines that are taking part in our MP session will execute hint "You have 5 minutes left!";, which means that our message will be displayed to every player at the same time - and we have achieved our goal.

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If it is available, information about the MP behaviour of a function / command can be found at the top of the documentation page of that function / command. Look for these icons: LALocal, GAGlobal, LELocal, GEGlobal and SEServer. You can always hover above the icons to view a tooltip explaining what each icon means.

History

Prior to 1.50 there was no engine based remote execution. The only officially supported way to execute code on a non-local machine was provided in the form of BIS_fnc_MP. This scripted framework suffered from several issues, mainly poor network traffic optimization and insufficient security control. The network traffic was optimized in 1.46, but the security issues of the scripted framework could not be fixed properly.

To fully address these issues, remote execution was implemented directly into the game engine, with 1.50 introducing two new script commands - remoteExec and remoteExecCall - as well as a new CfgRemoteExec, finally allowing for proper and secure remote execution from all machines.

As of 1.54, BIS_fnc_MP has been rewritten to internally use remoteExec and remoteExecCall, retaining full backwards compatibility and enabling it to work with the updated CfgRemoteExec. Despite this, BIS_fnc_MP is now deprecated and should no longer be used!

Security

The biggest issue of previous scripted remote execution was the lack of control over what can and cannot be executed from local clients. To address this we’ve created system where content authors can define through CfgRemoteExec config how the remote execution should operate on clients.

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Server doesn’t have any limitations at place, everything is enabled and opened for it. All limitations and rules apply only for clients.

Because remoteExec and remoteExecCall can be used to remotely execute script commands (like for e.g. setDamage) as well as scripted functions (e.g. BIS_fnc_setRank) we have separated the security settings into two groups: Functions and Commands. This allows people to quickly set different rules for functions and commands separately.

The security rules consist of 3 security settings - operation mode, allowed targets and jip. The operation mode is very important as it defines if functions/commands can be executed remotely from client or not and ev. allows you to whitelist specific functions/commands. The allowed targets and jip are very optional and you really do not need to use them, unless you want to be super safe.

To get more info about those settings and the CfgRemoteExec config, check the subsections below.

Operation modes

Operation mode is numeric value describing how functions or commands should be treated on server on client.

	0: remote execution is blocked
	1: only whitelisted functions/commands are allowed for remote execution
	2: remote execution fully opened

Allowed targets

In addition to the operation mode, in client subclass, allowed targets can be defined for individual whitelisted commands and functions. This adds another layer of security and control to the system, as it allows you to define not only who can send the execution request, but also where it can be executed.

	0: can target all machines (default)
	1: can target only clients, execution on server is denied
	2: can target only server, execution on clients is denied

JIP

To control who can add JIP message into JIP queue, we have added an optional parameter jip. This parameter can be defined in whitelisted function/command class, at the same place as allowed targets are defined or in the Functions and/or Commands classes. This parameter affects only clients (no effect if defined within Server subclass).

If it is defined on both levels, the more local definition takes precedence (which is the value defined in the whitelisted function/command).

	0: JIP flag cannot be set
	1: JIP flag can be set (default)

Config location

The CfgRemoteExec class can be defined in mission description.ext, campaign description.ext or global (addon) config. As usual the more local config takes precedence. In case of more global configs exist, the mode attribute will be overridden by the last parsed config and whitelisted commands and functions will be merged.

Sample CfgRemoteExec definition:

class CfgRemoteExec
{
	class Commands
	{
		mode = 1;

		class setFuel	{ allowedTargets = 2; };		// execute only on server
		class hint		{ jip = 0; };					// jip is not allowed for this command
	};

	class Functions
	{
		mode = 0;
		jip = 0;										// no functions can use jip

		class BIS_fnc_setRank { allowedTargets = 1; };	// execute only on clients, server execution denied
	};
};

Advanced Techniques & Functionality Insight

JIP Queue

When a command or function is executed through remoteExec or remoteExecCall, it can be flagged as persistent. If it is flagged, the statement is stored in the JIP queue on the server (under its unique JIP ID). When a player joins a multiplayer session that has already started (JIP stands for Join-In-Progress), all entries stored in the JIP queue are executed on that player's machine.

Overwriting JIP message in the queue

Every JIP message is stored in the queue. If you need to overwrite the JIP message with another message, use the same JIP id for the new message. This way the new message will be stored in the queue under the provided id and will overwrite the previously stored message that used the same id.

Deleting JIP message from the queue

To remove a specific JIP message from the queue call the remoteExec with function/command name set to an empty string, the JIP id of the message you want to remove and no other params.

remoteExec ["", "JIPid"];

Validity Verification

The validity of a remote execution request is verified in two steps:

1. When the request is initiated (issued from a client machine)
2. Before the server broadcasts the request

If the request is initialized directly from the server, step 1 is skipped (this also applies to hosted servers).

A remote execution request has to meet the following criteria to be considered valid:

1. The input parameters must be defined properly
2. The function / command must exist on the machine
3. The function / command must be allowed by CfgRemoteExec:
   • Remote execution must either be fully open (mode = 2) ...
   • ... or the particular function / command must be whitelisted.
4. If JIP is used, JIP must be allowed by CfgRemoteExec (jip = 1)

If any of these conditions is not met, the remote execution request is blocked.

See Also

• Multiplayer Scripting
• Multiplayer Scripting Commands