rustdllproxy

Installation

cargo install rustdllproxy

Compatibility

This crate currently only supports the standard DLL PE format.

Current Limitations

• When hooking functions with custom code, the function signature must be known
  • This can be found using disassemblers and reverse engineering tools like Ghidra

Commands

Rustdllproxy ships two subcommands:

Command	Purpose
rustdllproxy new	Generate a new proxy cdylib crate from one or more existing DLLs.
rustdllproxy build	Sync the .def file with src/lib.rs and build the crate.

rustdllproxy --help        # top-level help
rustdllproxy new --help    # generation flags
rustdllproxy build --help  # build flags

Creating a New Crate

A Quick Note on Strategy

Before generating your crate, decide how you would like your proxy to work. A typical pattern is search order hijacking, where you would first rename your target DLL to something like target_.dll, and then use the compiled proxy as target.dll. This creates a flow resembling binary -> target.dll -> target_.dll

There are multiple paths forward depending on your use case. If however you need to rename the underlying DLL being proxied, update the generated .def file accordingly.

---

rustdllproxy new -p path/to/target_.dll -n my_proxy

Tip: rustdllproxy is built as a CLI with clap. Run rustdllproxy --help to see all options and flags.

Writing Hooks

The macro library supports 3 main hook types: prehook, posthook, and fullhook.

Hook Implementation Steps

1. Replace the #[no_mangle] directive with the hook macro (leave the //<dllname>.dll trailing comment in place)

   #[prehook("dllbeingproxied.dll", "function_name")] //dllbeingproxied.dll
   

2. Fill out the function signature (declare inputs as mut to modify them)

3. Build with rustdllproxy build.

Hook Types

prehook

Executes code before the original function. Allows you to add functionality or modify input variables.

#[prehook("target.dll", "my_function")] //target.dll
fn my_function(mut param1: i32, mut param2: &str) {
    // Your code here - executes before original function
    param1 *= 2;  // Modify parameters if needed
}

posthook

Executes code after the original function. View and edit the return value using the magic ret variable.

#[posthook("target.dll", "calculate")] //target.dll
fn calculate(input: i32) -> i32 {
    // Original function executes first
    // Then your code runs with access to 'ret'
    ret = ret * 2;  // Modify return value
}

Note: The ret variable is automatically defined as mutable. You don't need to reference it if not needed.

fullhook

Provides complete control over function execution. Manually manage the return value and function calling.

#[fullhook("target.dll", "do_multi_add")] //target.dll
fn do_multi_add(mut a: i32, mut b: i32, mut c: i32) -> i32 {
    // Pre-processing
    a += 10;
    b += 20;

    // Call original function with magic func()
    let mut return_value: i32 = func(a, b, c);

    // Post-processing
    return_value *= 2;

    // Must explicitly return the value
    return_value
}

Building the Crate

Run from the proxy crate directory (or pass it as the first argument):

rustdllproxy build [PATH] [--profile <name>] [--no-build] [-- <extra cargo args>]

Flag	Default	Effect
PATH	.	Path to the proxy crate root.
--profile <name>	release	Cargo build profile (release, dev, custom).
--no-build	off	Regenerate the .def file but skip cargo build.
-- <args>	—	Forwarded verbatim to cargo build.

Caveats

• The .def file is fully regenerated on every build, manual changes will be overwritten. If you need to make manual changes against how rustdllproxy builds, cargo can be used to accomplish this.
• The build system relies on generated comments, .def exports, and hook names to retrieve the name of the underlying DLL before building. If there is not enough information, and error will be thrown to explain how this can be recovered.

Example Workflow

Let's say you want to modify office.dll used in office software via DLL search order hijacking:

Step 1: Prepare the Original DLL

# Rename the original DLL
mv office.dll office_.dll

Step 2: Generate Proxy Crate

rustdllproxy new -p office_.dll -n office_proxy

Step 3: Implement Hooks

#[prehook("office_.dll", "open_window")] //office_.dll
fn open_window() {
    // Your custom code here...
    println!("Window is about to open!");
}

Step 4: Build and Deploy

cd office_proxy
rustdllproxy build

Build files are located under /target

DLL Bundling Considerations

It is possible to proxy several target DLLs with a single crate. This feature is rarely used and comes with some important caveats.

When bundling multiple DLLs:

• Function ordinals may change due to export ordering
• This is rarely problematic since modern software uses export names for compatibility
• Primarily useful for analysis and custom application development

Changelog

Release notes live in CHANGELOG.md.

Contributing

Contributions are welcome! Please feel free to submit issues and pull requests.