move

lib/ssh-connection/Cargo.toml

@@ -0,0 +1,11 @@
+[package]
+name = "ssh-connection"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+ssh-transport = { path = "../ssh-transport" }
+tracing.workspace = true
+
+[dev-dependencies]
+tracing-subscriber = { version = "0.3.18", features = ["env-filter"] }

lib/ssh-connection/README.md

@@ -0,0 +1,5 @@
+# ssh-connection
+
+Connection layer for SSH. This crate takes care of channel multiplexing.
+
+Based on [RFC 4254 The Secure Shell (SSH) Connection Protocol](https://datatracker.ietf.org/doc/html/rfc4254).

lib/ssh-connection/src/lib.rs

@@ -0,0 +1,732 @@
+use std::cmp;
+use std::collections::{HashMap, VecDeque};
+use tracing::{debug, info, trace, warn};
+
+use ssh_transport::packet::Packet;
+use ssh_transport::Result;
+use ssh_transport::{numbers, peer_error};
+
+/// A channel number (on our side).
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub struct ChannelNumber(pub u32);
+
+impl std::fmt::Display for ChannelNumber {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        std::fmt::Display::fmt(&self.0, f)
+    }
+}
+
+pub struct ChannelsState {
+    packets_to_send: VecDeque<Packet>,
+    channel_updates: VecDeque<ChannelUpdate>,
+
+    channels: HashMap<ChannelNumber, Channel>,
+    next_channel_id: ChannelNumber,
+
+    is_server: bool,
+}
+
+struct Channel {
+    /// Whether our side has closed this channel.
+    we_closed: bool,
+    /// The channel number for the other side.
+    peer_channel: u32,
+    /// The current max window size of our peer, controls how many bytes we can still send.
+    peer_window_size: u32,
+    /// The max packet size of the peer.
+    // We need to split our packets if the user requests more.
+    peer_max_packet_size: u32,
+
+    /// For validation only.
+    our_window_size: u32,
+    /// For validation only.
+    our_max_packet_size: u32,
+    /// By how much we want to increase the window when it gets small.
+    our_window_size_increase_step: u32,
+
+    /// Queued data that we want to send, but have not been able to because of the window limits.
+    /// Whenever we get more window space, we will send this data.
+    queued_data: Vec<u8>,
+}
+
+/// An update from a channel.
+/// The receiver-equivalent of [`ChannelOperation`].
+#[derive(Debug)]
+pub struct ChannelUpdate {
+    pub number: ChannelNumber,
+    pub kind: ChannelUpdateKind,
+}
+#[derive(Debug)]
+pub enum ChannelUpdateKind {
+    Open(ChannelOpen),
+    Request(ChannelRequest),
+    Data { data: Vec<u8> },
+    ExtendedData { code: u32, data: Vec<u8> },
+    Eof,
+    Closed,
+}
+#[derive(Debug)]
+pub enum ChannelOpen {
+    Session,
+}
+#[derive(Debug)]
+pub enum ChannelRequest {
+    PtyReq {
+        want_reply: bool,
+
+        term: String,
+        width_chars: u32,
+        height_rows: u32,
+        width_px: u32,
+        height_px: u32,
+        term_modes: Vec<u8>,
+    },
+    Shell {
+        want_reply: bool,
+    },
+    Exec {
+        want_reply: bool,
+
+        command: Vec<u8>,
+    },
+    Env {
+        want_reply: bool,
+
+        name: String,
+        value: Vec<u8>,
+    },
+    ExitStatus {
+        status: u32,
+    },
+}
+
+impl ChannelNumber {
+    #[must_use]
+    pub fn construct_op(self, kind: ChannelOperationKind) -> ChannelOperation {
+        ChannelOperation { number: self, kind }
+    }
+}
+
+/// An operation to do on a channel.
+/// The sender-equivalent of [`ChannelUpdate`].
+pub struct ChannelOperation {
+    pub number: ChannelNumber,
+    pub kind: ChannelOperationKind,
+}
+
+pub enum ChannelOperationKind {
+    Success,
+    Failure,
+    Data(Vec<u8>),
+    Request(ChannelRequest),
+    Eof,
+    Close,
+}
+
+impl ChannelsState {
+    pub fn new(is_server: bool) -> Self {
+        ChannelsState {
+            packets_to_send: VecDeque::new(),
+            channels: HashMap::new(),
+            channel_updates: VecDeque::new(),
+            next_channel_id: ChannelNumber(0),
+
+            is_server,
+        }
+    }
+
+    pub fn recv_packet(&mut self, packet: Packet) -> Result<()> {
+        // TODO: window
+
+        let mut packet = packet.payload_parser();
+        let packet_type = packet.u8()?;
+        match packet_type {
+            numbers::SSH_MSG_GLOBAL_REQUEST => {
+                let request_name = packet.utf8_string()?;
+                let want_reply = packet.bool()?;
+                debug!(%request_name, %want_reply, "Received global request");
+
+                self.packets_to_send
+                    .push_back(Packet::new_msg_request_failure());
+            }
+            numbers::SSH_MSG_CHANNEL_OPEN => {
+                // <https://datatracker.ietf.org/doc/html/rfc4254#section-5.1>
+                let channel_type = packet.utf8_string()?;
+                let sender_channel = packet.u32()?;
+                let initial_window_size = packet.u32()?;
+                let max_packet_size = packet.u32()?;
+
+                debug!(%channel_type, %sender_channel, "Opening channel");
+
+                let update_message = match channel_type {
+                    "session" => ChannelOpen::Session,
+                    _ => {
+                        self.packets_to_send
+                            .push_back(Packet::new_msg_channel_open_failure(
+                                sender_channel,
+                                numbers::SSH_OPEN_UNKNOWN_CHANNEL_TYPE,
+                                b"unknown channel type",
+                                b"",
+                            ));
+                        return Ok(());
+                    }
+                };
+
+                let our_number = self.next_channel_id;
+                self.next_channel_id =
+                    ChannelNumber(self.next_channel_id.0.checked_add(1).ok_or_else(|| {
+                        peer_error!("created too many channels, overflowed the counter")
+                    })?);
+
+                self.packets_to_send
+                    .push_back(Packet::new_msg_channel_open_confirmation(
+                        sender_channel,
+                        our_number.0,
+                        initial_window_size,
+                        max_packet_size,
+                    ));
+
+                self.channels.insert(
+                    our_number,
+                    Channel {
+                        we_closed: false,
+                        peer_channel: sender_channel,
+                        peer_max_packet_size: max_packet_size,
+                        peer_window_size: initial_window_size,
+                        our_max_packet_size: max_packet_size,
+                        our_window_size: initial_window_size,
+                        our_window_size_increase_step: initial_window_size,
+
+                        queued_data: Vec::new(),
+                    },
+                );
+
+                self.channel_updates.push_back(ChannelUpdate {
+                    number: our_number,
+                    kind: ChannelUpdateKind::Open(update_message),
+                });
+
+                debug!(%channel_type, %our_number, "Successfully opened channel");
+            }
+            numbers::SSH_MSG_CHANNEL_WINDOW_ADJUST => {
+                let our_channel = packet.u32()?;
+                let our_channel = self.validate_channel(our_channel)?;
+                let bytes_to_add = packet.u32()?;
+
+                let channel = self.channel(our_channel)?;
+                channel.peer_window_size = channel
+                    .peer_window_size
+                    .checked_add(bytes_to_add)
+                    .ok_or_else(|| peer_error!("window size larger than 2^32"))?;
+
+                if !channel.queued_data.is_empty() {
+                    let limit =
+                        cmp::min(channel.queued_data.len(), channel.peer_window_size as usize);
+                    let data_to_send = channel.queued_data.splice(..limit, []).collect::<Vec<_>>();
+                    self.send_data(our_channel, &data_to_send);
+                }
+            }
+            numbers::SSH_MSG_CHANNEL_DATA => {
+                let our_channel = packet.u32()?;
+                let our_channel = self.validate_channel(our_channel)?;
+                let data = packet.string()?;
+
+                let channel = self.channel(our_channel)?;
+                channel.our_window_size = channel
+                    .our_window_size
+                    .checked_sub(data.len() as u32)
+                    .ok_or_else(|| {
+                        peer_error!(
+                            "sent more data than the window allows: {} while the window is {}",
+                            data.len(),
+                            channel.our_window_size
+                        )
+                    })?;
+                if channel.our_max_packet_size < (data.len() as u32) {
+                    return Err(peer_error!(
+                        "data bigger than allowed packet size: {} while the max packet size is {}",
+                        data.len(),
+                        channel.our_max_packet_size
+                    ));
+                }
+
+                trace!(channel = %our_channel, window = %channel.our_window_size, "Remaining window on our side");
+
+                // We probably want to make this user-controllable in the future.
+                if channel.our_window_size < 1000 {
+                    let peer = channel.peer_channel;
+                    let bytes_to_add = channel.our_window_size_increase_step;
+                    channel.our_window_size += bytes_to_add;
+                    self.packets_to_send
+                        .push_back(Packet::new_msg_channel_window_adjust(peer, bytes_to_add))
+                }
+
+                self.channel_updates.push_back(ChannelUpdate {
+                    number: our_channel,
+                    kind: ChannelUpdateKind::Data {
+                        data: data.to_owned(),
+                    },
+                });
+            }
+            numbers::SSH_MSG_CHANNEL_EOF => {
+                // <https://datatracker.ietf.org/doc/html/rfc4254#section-5.3>
+                let our_channel = packet.u32()?;
+                let our_channel = self.validate_channel(our_channel)?;
+
+                self.channel_updates.push_back(ChannelUpdate {
+                    number: our_channel,
+                    kind: ChannelUpdateKind::Eof,
+                });
+            }
+            numbers::SSH_MSG_CHANNEL_CLOSE => {
+                // <https://datatracker.ietf.org/doc/html/rfc4254#section-5.3>
+                let our_channel = packet.u32()?;
+                let our_channel = self.validate_channel(our_channel)?;
+                let channel = self.channel(our_channel)?;
+                if !channel.we_closed {
+                    info!("closeing here");
+                    let close = Packet::new_msg_channel_close(channel.peer_channel);
+                    self.packets_to_send.push_back(close);
+                }
+
+                self.channels.remove(&our_channel);
+
+                self.channel_updates.push_back(ChannelUpdate {
+                    number: our_channel,
+                    kind: ChannelUpdateKind::Closed,
+                });
+
+                debug!("Channel has been closed");
+            }
+            numbers::SSH_MSG_CHANNEL_REQUEST => {
+                let our_channel = packet.u32()?;
+                let our_channel = self.validate_channel(our_channel)?;
+                let request_type = packet.utf8_string()?;
+                let want_reply = packet.bool()?;
+
+                debug!(channel = %our_channel, %request_type, "Got channel request");
+
+                let channel = self.channel(our_channel)?;
+                let peer_channel = channel.peer_channel;
+
+                let channel_request = match request_type {
+                    "pty-req" => {
+                        if !self.is_server {
+                            return Err(peer_error!("server tried to open pty"));
+                        }
+
+                        let term = packet.utf8_string()?;
+                        let width_chars = packet.u32()?;
+                        let height_rows = packet.u32()?;
+                        let width_px = packet.u32()?;
+                        let height_px = packet.u32()?;
+                        let term_modes = packet.string()?;
+
+                        debug!(
+                            channel = %our_channel,
+                            %term,
+                            %width_chars,
+                            %height_rows,
+                            "Trying to open a terminal"
+                        );
+
+                        ChannelRequest::PtyReq {
+                            want_reply,
+                            term: term.to_owned(),
+                            width_chars,
+                            height_rows,
+                            width_px,
+                            height_px,
+                            term_modes: term_modes.to_owned(),
+                        }
+                    }
+                    "shell" => {
+                        if !self.is_server {
+                            return Err(peer_error!("server tried to open shell"));
+                        }
+
+                        info!(channel = %our_channel, "Opening shell");
+                        ChannelRequest::Shell { want_reply }
+                    }
+                    "exec" => {
+                        if !self.is_server {
+                            return Err(peer_error!("server tried to execute command"));
+                        }
+
+                        let command = packet.string()?;
+                        info!(channel = %our_channel, command = %String::from_utf8_lossy(command), "Executing command");
+                        ChannelRequest::Exec {
+                            want_reply,
+                            command: command.to_owned(),
+                        }
+                    }
+                    "env" => {
+                        if !self.is_server {
+                            return Err(peer_error!("server tried to set environment var"));
+                        }
+
+                        let name = packet.utf8_string()?;
+                        let value = packet.string()?;
+
+                        info!(channel = %our_channel, %name, value = %String::from_utf8_lossy(value), "Setting environment variable");
+
+                        ChannelRequest::Env {
+                            want_reply,
+                            name: name.to_owned(),
+                            value: value.to_owned(),
+                        }
+                    }
+                    "signal" => {
+                        if !self.is_server {
+                            return Err(peer_error!("server tried to send signal"));
+                        }
+
+                        debug!(channel = %our_channel, "Received signal");
+                        // Ignore signals, something we can do.
+                        return Ok(());
+                    }
+                    _ => {
+                        warn!(%request_type, channel = %our_channel, "Unknown channel request");
+                        self.send_channel_failure(peer_channel);
+                        return Ok(());
+                    }
+                };
+
+                self.channel_updates.push_back(ChannelUpdate {
+                    number: our_channel,
+                    kind: ChannelUpdateKind::Request(channel_request),
+                })
+            }
+            _ => {
+                todo!(
+                    "unsupported packet: {} ({packet_type})",
+                    numbers::packet_type_to_string(packet_type)
+                );
+            }
+        }
+
+        Ok(())
+    }
+
+    pub fn packets_to_send(&mut self) -> impl Iterator<Item = Packet> + '_ {
+        self.packets_to_send.drain(..)
+    }
+
+    pub fn next_channel_update(&mut self) -> Option<ChannelUpdate> {
+        self.channel_updates.pop_front()
+    }
+
+    /// Executes an operation on the channel.
+    /// If the channel has already been closed, the operation is dropped.
+    pub fn do_operation(&mut self, op: ChannelOperation) {
+        op.trace();
+
+        let Ok(channel) = self.channel(op.number) else {
+            debug!(number = %op.number, "Dropping operation as channel does not exist, probably because it has been closed");
+            return;
+        };
+        let peer = channel.peer_channel;
+
+        if channel.we_closed {
+            debug!(number = %op.number, "Dropping operation as channel has been closed already");
+            return;
+        }
+
+        match op.kind {
+            ChannelOperationKind::Success => self.send_channel_success(peer),
+            ChannelOperationKind::Failure => self.send_channel_failure(peer),
+            ChannelOperationKind::Data(data) => {
+                self.send_data(op.number, &data);
+            }
+            ChannelOperationKind::Request(req) => {
+                let packet = match req {
+                    ChannelRequest::PtyReq { .. } => todo!("pty-req"),
+                    ChannelRequest::Shell { .. } => todo!("shell"),
+                    ChannelRequest::Exec { .. } => todo!("exec"),
+                    ChannelRequest::Env { .. } => todo!("env"),
+                    ChannelRequest::ExitStatus { status } => {
+                        Packet::new_msg_channel_request_exit_status(
+                            peer,
+                            b"exit-status",
+                            false,
+                            status,
+                        )
+                    }
+                };
+                self.packets_to_send.push_back(packet);
+            }
+            ChannelOperationKind::Eof => {
+                self.packets_to_send
+                    .push_back(Packet::new_msg_channel_eof(peer));
+            }
+            ChannelOperationKind::Close => {
+                // <https://datatracker.ietf.org/doc/html/rfc4254#section-5.3>
+                self.packets_to_send
+                    .push_back(Packet::new_msg_channel_close(peer));
+
+                let channel = self.channel(op.number).unwrap();
+                channel.we_closed = true;
+            }
+        }
+    }
+
+    fn send_data(&mut self, channel_number: ChannelNumber, data: &[u8]) {
+        let channel = self.channel(channel_number).unwrap();
+
+        let mut chunks = data.chunks(channel.peer_max_packet_size as usize);
+
+        while let Some(data) = chunks.next() {
+            let channel = self.channel(channel_number).unwrap();
+            let remaining_window_space_after =
+                channel.peer_window_size.checked_sub(data.len() as u32);
+            match remaining_window_space_after {
+                None => {
+                    let rest = channel.peer_window_size;
+                    let (to_send, to_keep) = data.split_at(rest as usize);
+
+                    if !to_send.is_empty() {
+                        // Send everything we can, which empties the window.
+                        channel.peer_window_size -= rest;
+                        assert_eq!(channel.peer_window_size, 0);
+                        self.send_data_packet(channel_number, to_send);
+                    }
+
+                    // It's over, we have exhausted all window space.
+                    // Queue the rest of the bytes.
+                    let channel = self.channel(channel_number).unwrap();
+                    channel.queued_data.extend_from_slice(to_keep);
+                    for data in chunks {
+                        channel.queued_data.extend_from_slice(data);
+                    }
+                    debug!(channel = %channel_number, queue_len = %channel.queued_data.len(), "Exhausted window space, queueing the rest of the data");
+                    return;
+                }
+                Some(space) => channel.peer_window_size = space,
+            }
+            trace!(channel = %channel_number, window = %channel.peer_window_size, "Remaining window on their side");
+
+            self.send_data_packet(channel_number, data);
+        }
+    }
+
+    /// Send a single data packet.
+    /// The caller needs to ensure the windowing and packet size requirements are upheld.
+    fn send_data_packet(&mut self, channel_number: ChannelNumber, data: &[u8]) {
+        assert!(!data.is_empty(), "Trying to send empty data packet");
+
+        trace!(%channel_number, amount = %data.len(), "Sending channel data");
+        let channel = self.channel(channel_number).unwrap();
+        let peer = channel.peer_channel;
+        assert!(channel.peer_max_packet_size >= data.len() as u32);
+        self.packets_to_send
+            .push_back(Packet::new_msg_channel_data(peer, data));
+    }
+
+    fn send_channel_success(&mut self, recipient_channel: u32) {
+        self.packets_to_send
+            .push_back(Packet::new_msg_channel_success(recipient_channel));
+    }
+
+    fn send_channel_failure(&mut self, recipient_channel: u32) {
+        self.packets_to_send
+            .push_back(Packet::new_msg_channel_failure(recipient_channel));
+    }
+
+    fn validate_channel(&self, number: u32) -> Result<ChannelNumber> {
+        if !self.channels.contains_key(&ChannelNumber(number)) {
+            return Err(peer_error!("unknown channel: {number}"));
+        }
+        Ok(ChannelNumber(number))
+    }
+
+    fn channel(&mut self, number: ChannelNumber) -> Result<&mut Channel> {
+        self.channels
+            .get_mut(&number)
+            .ok_or_else(|| peer_error!("unknown channel: {number:?}"))
+    }
+}
+
+impl ChannelOperation {
+    /// Logs the attempted operation.
+    fn trace(&self) {
+        let kind = match &self.kind {
+            ChannelOperationKind::Success => "success",
+            ChannelOperationKind::Failure => "failure",
+            ChannelOperationKind::Data(_) => "data",
+            ChannelOperationKind::Request(req) => match req {
+                ChannelRequest::PtyReq { .. } => "pty-req",
+                ChannelRequest::Shell { .. } => "shell",
+                ChannelRequest::Exec { .. } => "exec",
+                ChannelRequest::Env { .. } => "env",
+                ChannelRequest::ExitStatus { .. } => "exit-status",
+            },
+            ChannelOperationKind::Eof => "eof",
+            ChannelOperationKind::Close => "close",
+        };
+        trace!(number = %self.number, %kind, "Attempt channel operation")
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use ssh_transport::{numbers, packet::Packet};
+
+    use crate::{ChannelNumber, ChannelOperation, ChannelOperationKind, ChannelsState};
+
+    /// If a test fails, add this to the test to get logs.
+    #[allow(dead_code)]
+    fn init_test_log() {
+        tracing_subscriber::fmt()
+            .with_env_filter(tracing_subscriber::EnvFilter::from_default_env())
+            .init();
+    }
+
+    #[track_caller]
+    fn assert_response_types(state: &mut ChannelsState, types: &[u8]) {
+        let response = state
+            .packets_to_send()
+            .map(|p| numbers::packet_type_to_string(p.packet_type()))
+            .collect::<Vec<_>>();
+
+        let expected = types
+            .iter()
+            .map(|p| numbers::packet_type_to_string(*p))
+            .collect::<Vec<_>>();
+        assert_eq!(expected, response);
+    }
+
+    fn open_session_channel(state: &mut ChannelsState) {
+        state
+            .recv_packet(Packet::new_msg_channel_open_session(
+                b"session", 0, 2048, 1024,
+            ))
+            .unwrap();
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_OPEN_CONFIRMATION]);
+    }
+
+    #[test]
+    fn interactive_pty() {
+        let state = &mut ChannelsState::new(true);
+        open_session_channel(state);
+
+        state
+            .recv_packet(Packet::new_msg_channel_request_pty_req(
+                0, b"pty-req", true, b"xterm", 80, 24, 0, 0, b"",
+            ))
+            .unwrap();
+        state.do_operation(ChannelNumber(0).construct_op(ChannelOperationKind::Success));
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_SUCCESS]);
+
+        state
+            .recv_packet(Packet::new_msg_channel_request_shell(0, b"shell", true))
+            .unwrap();
+        state.do_operation(ChannelNumber(0).construct_op(ChannelOperationKind::Success));
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_SUCCESS]);
+
+        state
+            .recv_packet(Packet::new_msg_channel_data(0, b"hello, world"))
+            .unwrap();
+        assert_response_types(state, &[]);
+
+        state.recv_packet(Packet::new_msg_channel_eof(0)).unwrap();
+        assert_response_types(state, &[]);
+
+        state.recv_packet(Packet::new_msg_channel_close(0)).unwrap();
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_CLOSE]);
+    }
+
+    #[test]
+    fn only_single_close_for_double_close_operation() {
+        let state = &mut ChannelsState::new(true);
+        open_session_channel(state);
+        state.do_operation(ChannelOperation {
+            number: ChannelNumber(0),
+            kind: ChannelOperationKind::Close,
+        });
+        state.do_operation(ChannelOperation {
+            number: ChannelNumber(0),
+            kind: ChannelOperationKind::Close,
+        });
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_CLOSE]);
+    }
+
+    #[test]
+    fn ignore_operation_after_close() {
+        let mut state = &mut ChannelsState::new(true);
+        open_session_channel(state);
+        state.recv_packet(Packet::new_msg_channel_close(0)).unwrap();
+        assert_response_types(&mut state, &[numbers::SSH_MSG_CHANNEL_CLOSE]);
+        state.do_operation(ChannelOperation {
+            number: ChannelNumber(0),
+            kind: ChannelOperationKind::Data(vec![0]),
+        });
+        assert_response_types(state, &[]);
+    }
+
+    #[test]
+    fn respect_peer_windowing() {
+        let state = &mut ChannelsState::new(true);
+        state
+            .recv_packet(Packet::new_msg_channel_open_session(b"session", 0, 10, 50))
+            .unwrap();
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_OPEN_CONFIRMATION]);
+
+        // Send 100 bytes.
+        state.do_operation(
+            ChannelNumber(0)
+                .construct_op(ChannelOperationKind::Data((0_u8..200).collect::<Vec<_>>())),
+        );
+
+        // 0..10
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_DATA]);
+
+        state
+            .recv_packet(Packet::new_msg_channel_window_adjust(0, 90))
+            .unwrap();
+        // 10..60, 60..100
+        assert_response_types(
+            state,
+            &[numbers::SSH_MSG_CHANNEL_DATA, numbers::SSH_MSG_CHANNEL_DATA],
+        );
+
+        state
+            .recv_packet(Packet::new_msg_channel_window_adjust(0, 100))
+            .unwrap();
+        // 100..150, 150..20
+        assert_response_types(
+            state,
+            &[numbers::SSH_MSG_CHANNEL_DATA, numbers::SSH_MSG_CHANNEL_DATA],
+        );
+
+        state
+            .recv_packet(Packet::new_msg_channel_window_adjust(0, 100))
+            .unwrap();
+        assert_response_types(state, &[]);
+    }
+
+    #[test]
+    fn send_windowing_adjustments() {
+        let state = &mut ChannelsState::new(true);
+        state
+            .recv_packet(Packet::new_msg_channel_open_session(
+                b"session", 0, 2000, 2000,
+            ))
+            .unwrap();
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_OPEN_CONFIRMATION]);
+
+        state
+            .recv_packet(Packet::new_msg_channel_data(0, &vec![0; 2000]))
+            .unwrap();
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_WINDOW_ADJUST]);
+
+        // We currently hardcode <1000 for when to send window size adjustments.
+        state
+            .recv_packet(Packet::new_msg_channel_data(0, &vec![0; 1000]))
+            .unwrap();
+        assert_response_types(state, &[]);
+        state
+            .recv_packet(Packet::new_msg_channel_data(0, &vec![0; 1]))
+            .unwrap();
+        assert_response_types(state, &[numbers::SSH_MSG_CHANNEL_WINDOW_ADJUST]);
+    }
+}