Crate russh

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Expand description

Server and client SSH asynchronous library, based on tokio/futures.

The normal way to use this library, both for clients and for servers, is by creating handlers, i.e. types that implement client::Handler for clients and server::Handler for servers.

§Important crate features

  • RSA key support is gated behind the openssl feature (disabled by default).
  • Enabling that and disabling the rs-crypto feature (enabled by default) will leave you with a very basic, but pure-OpenSSL RSA+AES cipherset.

§Using non-socket IO / writing tunnels

The easy way to implement SSH tunnels, like ProxyCommand for OpenSSH, is to use the russh-config crate, and use the Stream::tcp_connect or Stream::proxy_command methods of that crate. That crate is a very lightweight layer above Russh, only implementing for external commands the traits used for sockets.

§The SSH protocol

If we exclude the key exchange and authentication phases, handled by Russh behind the scenes, the rest of the SSH protocol is relatively simple: clients and servers open channels, which are just integers used to handle multiple requests in parallel in a single connection. Once a client has obtained a ChannelId by calling one the many channel_open_… methods of client::Connection, the client may send exec requests and data to the server.

A simple client just asking the server to run one command will usually start by calling client::Connection::channel_open_session, then client::Connection::exec, then possibly client::Connection::data a number of times to send data to the command’s standard input, and finally Connection::channel_eof and Connection::channel_close.

§Design principles

The main goal of this library is conciseness, and reduced size and readability of the library’s code. Moreover, this library is split between Russh, which implements the main logic of SSH clients and servers, and Russh-keys, which implements calls to cryptographic primitives.

One non-goal is to implement all possible cryptographic algorithms published since the initial release of SSH. Technical debt is easily acquired, and we would need a very strong reason to go against this principle. If you are designing a system from scratch, we urge you to consider recent cryptographic primitives such as Ed25519 for public key cryptography, and Chacha20-Poly1305 for symmetric cryptography and MAC.

§Internal details of the event loop

It might seem a little odd that the read/write methods for server or client sessions often return neither Result nor Future. This is because the data sent to the remote side is buffered, because it needs to be encrypted first, and encryption works on buffers, and for many algorithms, not in place.

Hence, the event loop keeps waiting for incoming packets, reacts to them by calling the provided Handler, which fills some buffers. If the buffers are non-empty, the event loop then sends them to the socket, flushes the socket, empties the buffers and starts again. In the special case of the server, unsollicited messages sent through a server::Handle are processed when there is no incoming packet to read.

Modules§

  • cipher
    Cipher names
  • client
    Client side of this library.
  • kex
    Key exchange algorithm names
  • mac
    MAC algorithm names
  • server
    Server side of this library.

Structs§

  • Channel
    A handle to a session channel.
  • ChannelId
    The identifier of a channel.
  • ChannelStream
    AsyncRead/AsyncWrite wrapper for SSH Channels
  • CryptoVec
    A buffer which zeroes its memory on .clear(), .resize() and reallocations, to avoid copying secrets around.
  • Limits
    The number of bytes read/written, and the number of seconds before a key re-exchange is requested.
  • MethodSet
    Set of authentication methods, represented by bit flags.
  • Preferred
    Lists of preferred algorithms. This is normally hard-coded into implementations.
  • SendError

Enums§

Traits§