Rustls FFI bindings (formerly "crustls")

This crate contains FFI bindings for the rustls TLS library, so you can use the library in C programs or any other language that supports FFI. It also contains a small demo C program that uses those bindings to make an HTTPS request.

Build

You'll need to install the Rust toolchain and a C compiler (gcc and clang should both work). Once you've got the Rust toolchain installed, run cargo install cbindgen. Then, to build in optimized mode:

make

To install:

make install

To build in debug mode:

make PROFILE=debug

Overview

Rustls doesn't do any I/O on its own. It provides the protocol handling, and leaves it up to the user to send and receive bytes on the network. Because of that it can be used equally well in a blockng or non-blocking I/O context. See the rustls documentation for a diagram of its input and output methods, along with a description of the TLS features it supports.

Conventions

This library defines an enum, rustls_result, to indicate success or failure of a function call. All fallible functions return a rustls_result. If a function has other outputs, it provides them using output parameters (pointers to caller-provided objects). For instance:

rustls_result rustls_connection_read(const rustls_connection *conn,
                                     uint8_t *buf,
                                     size_t count,
                                     size_t *out_n);

In this example, buf and out_n are output parameters.

Structs

For a given struct, all functions that start with the name of that struct are either associated functions or methods of that struct. For instance, rustls_connection_read is a method of rustls_connection. A function that takes a pointer to a struct as the first parameter is considered a method on that struct. Structs in this library are always created and destroyed by library code, so the header file only gives a declaration of the structs, not a definition.

As a result, structs are always handled using pointers. For each struct, there is generally a function ending in _new() to create that struct. Once you've got a pointer to a struct, it's your responsibility to (a) ensure no two threads are concurrently mutating that struct, and (b) free that struct's memory exactly once. Freeing a struct's memory will usually be accomplished with a function starting with the struct's name and ending in _free().

You can tell if a method will mutate a struct by looking at the first parameter. If it's a const*, the method is non-mutating. Otherwise, it's mutating.

Input and Output Parameters

Input parameters will always be either a const pointer or a primitive type (int, size_t, etc). Output parameters will always be a non-const pointer.

The caller is responsible for ensuring that the memory pointed to be output parameters is not being concurrently accessed by other threads. For primitive types and pointers-to-pointers this is most commonly accomplished by passing the address of a local variable on the stack that has no references elsewhere. For buffers, stack allocation is also a simple way to accomplish this, but if the buffer is allocated on heap and references to it are shared among threads, the caller will need to take additional steps to prevent concurrent access (for instance mutex locking, or single-threaded I/O).

When an output parameter is a pointer to a pointer (e.g. rustls_connection **conn_out, the function will set its argument to point to an appropriate object on success. The caller is considered to take ownership of that object and be responsible for the requirements above: preventing concurrent mutation, and freeing it exactly once.

For a method, the first parameter will always be a pointer to the struct being operated on. Next will come some number of input parameters, then some number of output parameters.

As a minor exception to the above: When an output parameter is a byte buffer (*uint8_t), the next parameter will always be a size_t denoting the size of the buffer. This is considered part of the output parameters even though it is not directly modified.

There are no in/out parameters. When an output buffer is passed, the library only writes to that buffer and does not read from it.

For fallible functions, values are only written to the output arguments if the function returns success. There are no partial successes or partial failures. Callers must check the return value before relying on the values pointed to by output arguments.

Callbacks and Userdata

Rustls supports various types of user customization via callbacks. All callbacks take a void *userdata parameter as their first parameter. Unless otherwise specified, this will receive a value that was associated with a rustls_connection via rustls_connection_set_userdata. If no such value was set, they will receive NULL. The read and write callbacks are a particular exception to this rule - they receive a userdata value passed through from the current call to rustls_connection_{read,write}_tls.

NULL

The library checks all pointers in arguments for NULL and will return an error rather than dereferencing a NULL pointer. For some methods that are infallible except for the possibility of NULL (for instance rustls_connection_is_handshaking), the library returns a convenient type (e.g. bool) and uses a suitable fallback value if an input is NULL.

Panics

In case of a bug (e.g. exceeding the bounds of an array), Rust code may emit a panic. Panics are treated like exceptions in C++, unwinding the stack. Unwinding past the FFI boundary is undefined behavior, so this library catches all unwinds and turns them into RUSTLS_RESULT_PANIC (when the function is fallible).

Functions that are theoretically infallible don't return rustls_result, so we can't return RUSTLS_RESULT_PANIC. In those cases, if there's a panic, we'll return a default value suitable to the return type: NULL for pointer types, false for bool types, and 0 for integer types.

Experimentals

Several features of the C bindings are marked as EXPERIMENTAL as they are need further evaluation and will most likely change significantly in the future.

Server Side Experimentals

The rustls_server_config_builder_set_hello_callback and its provided information in rustls_client_hello will change. The current design is a snapshot of the implementation efforts in mod_tls to provide rustls base TLS as module for the Apache webserver.

For a webserver hosting multiple domains on the same endpoint, it is highly desirable to have individual TLS settings, depending on the domain the client wants to talk to. Most domains have their own TLS certificates, some have configured restrictions on other features as well, such as TLS protocol versions, ciphers or client authentication.

The approach to this taken with the current rustls_client_hello is as follows:

One domain, one cert

If you have a single site and one certificate, you can preconfigure the rustls_server_config accordingly and do not need to register any callback.

Multiple domains/certs/settings

If you need to support multiple rustls_server_configs on the same connection endpoint, you can start the connection with a default rustls_server_config and register a client hello callback. The callback inspects the SNI/ALPN/cipher values announced by the client and selects the appropriate configuration to use.

When your callback returns, the handshake of rustls will fail, as no certificate was configured. This will be noticeable as an error returned from rustls_connection_write_tls(). You can then free this connection and create the one with the correct setting for the domain chosen.

For this to work, your connection needs to buffer the initial data from the client, so these bytes can be replayed to the second connection you use. Do not write any data back to the client while your are in the initial connection. The client hellos are usually only a few hundred bytes.

License

bagder/crustls

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