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Add opt-in, security-gated SSH-agent forwarding

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Item 3: SSH-agent forwarding, double-gated (client -A/forward_agent AND server
allow_agent_forwarding) and off by default on both ends.

Wire: new ForwardingKind::Agent (appended, existing bincode discriminants
unchanged). Bumped protocol VERSION 2 -> 3 so a pre-agent peer answers with a
clear version-mismatch reject instead of a deserialize error.

Client: -A / --forward-agent flag; requires local SSH_AUTH_SOCK. Requests an
Agent forwarding during native auth (agent forwarding requires native auth) and,
on a server-initiated StreamOpen carrying the reserved @dosh-agent sentinel,
splices the stream into the local agent unix socket (separate agent_writers map,
TCP forwarding paths untouched). Rejects agent StreamOpens unless it opted in, so
a server cannot reach the agent without consent.

Server: when the client opted in and allow_agent_forwarding is set, binds a
per-session proxy unix socket (dir 0700, socket 0600) before the shell spawns,
exports its path as the session's SSH_AUTH_SOCK, and tunnels each connection back
to the client over the agent sentinel stream. Applies only to freshly spawned
shells (documented). Socket cleaned up on auth/forward failure and when the
accept loop ends. SECURITY rationale documented in comments.

Tests: 2 end-to-end integration tests (full chain identities round-trip via a
PTY-hosted client + looping fake agent; and "no -A => no proxy socket"). Docs
updated (README forwarding + disconnect-status note, spec §12.1). fmt + full test
suite green (122 tests).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
DuProcess
2026-06-14 11:10:44 -04:00
parent 6b2933eb05
commit 9a52d65beb
7 changed files with 585 additions and 16 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/bin/dosh-client.rs
+116 -4
View File
@@ -39,11 +39,17 @@ use std::sync::Arc;
use std::sync::atomic::{AtomicU64, Ordering};
use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::net::{TcpListener, TcpStream, UdpSocket};
use tokio::net::{TcpListener, TcpStream, UdpSocket, UnixStream};
use tokio::sync::mpsc;
const STREAM_INITIAL_WINDOW: usize = 1024 * 1024;
/// Sentinel `target_host` the server uses on a server-initiated `StreamOpen` that
/// represents an SSH-agent connection (rather than a TCP target). The client
/// splices these into its local `SSH_AUTH_SOCK` instead of dialing TCP. The
/// `:0` port and this reserved name are never valid TCP forward targets.
const AGENT_STREAM_SENTINEL: &str = "@dosh-agent";
/// Current terminal size, with a sane fallback.
///
/// `crossterm::size()` returns `Err` when stdout is not a TTY (piped), but it
@@ -89,6 +95,10 @@ struct Args {
remote_forward: Vec<String>,
#[arg(short = 'D', long = "dynamic-forward")]
dynamic_forward: Vec<String>,
/// Forward the local ssh-agent (SSH_AUTH_SOCK) to the remote session.
/// SECURITY: opt-in only; the server must also set allow_agent_forwarding.
#[arg(short = 'A', long = "forward-agent")]
forward_agent: bool,
#[arg(short = 'N', long)]
forward_only: bool,
#[arg(short = 'f', long)]
@@ -200,8 +210,28 @@ async fn main() -> Result<()> {
let local_forwards = parse_local_forwards(&args.local_forward)?;
let remote_forwards = parse_remote_forwards(&args.remote_forward)?;
let dynamic_forwards = parse_dynamic_forwards(&args.dynamic_forward)?;
let forwarding_requested =
!local_forwards.is_empty() || !remote_forwards.is_empty() || !dynamic_forwards.is_empty();
// SSH-agent forwarding (opt-in via -A / forward_agent). SECURITY: this exposes
// your local agent to the remote host for the session's lifetime; only ever
// active on explicit opt-in AND when the server's allow_agent_forwarding is
// set. We capture the LOCAL agent socket path now so the run loop can splice
// each server-initiated agent stream into it.
let forward_agent = args.forward_agent || config.forward_agent;
let agent_sock = if forward_agent {
match std::env::var_os("SSH_AUTH_SOCK") {
Some(path) if !path.is_empty() => Some(PathBuf::from(path)),
_ => {
return Err(anyhow!(
"agent forwarding requested but SSH_AUTH_SOCK is not set"
));
}
}
} else {
None
};
let forwarding_requested = !local_forwards.is_empty()
|| !remote_forwards.is_empty()
|| !dynamic_forwards.is_empty()
|| forward_agent;
let predict = args.predict || host.predict.unwrap_or(config.predict);
let session = select_session(args.session.as_deref(), args.new);
let mode = if args.forward_only {
@@ -317,6 +347,7 @@ async fn main() -> Result<()> {
Vec::new(),
Vec::new(),
false,
None,
)
.await;
}
@@ -352,6 +383,7 @@ async fn main() -> Result<()> {
Vec::new(),
Vec::new(),
false,
None,
)
.await;
}
@@ -385,7 +417,12 @@ async fn main() -> Result<()> {
&mode,
cols,
rows,
forwarding_requests(&local_forwards, &remote_forwards, &dynamic_forwards),
forwarding_requests(
&local_forwards,
&remote_forwards,
&dynamic_forwards,
forward_agent,
),
resolved_ssh_config.as_ref(),
cold_requested_env,
)
@@ -412,6 +449,7 @@ async fn main() -> Result<()> {
local_forwards,
dynamic_forwards,
args.forward_only,
agent_sock,
)
.await;
}
@@ -487,6 +525,7 @@ async fn main() -> Result<()> {
Vec::new(),
Vec::new(),
false,
None,
)
.await
}
@@ -897,6 +936,7 @@ fn forwarding_requests(
local_forwards: &[LocalForward],
remote_forwards: &[RemoteForward],
dynamic_forwards: &[DynamicForward],
forward_agent: bool,
) -> Vec<ForwardingRequest> {
let mut requests = local_forwards
.iter()
@@ -922,6 +962,15 @@ fn forwarding_requests(
target_host: None,
target_port: None,
}));
if forward_agent {
requests.push(ForwardingRequest {
kind: ForwardingKind::Agent,
bind_host: None,
listen_port: 0,
target_host: None,
target_port: None,
});
}
requests
}
@@ -2075,6 +2124,7 @@ async fn run_terminal(
local_forwards: Vec<LocalForward>,
dynamic_forwards: Vec<DynamicForward>,
forward_only: bool,
agent_sock: Option<PathBuf>,
) -> Result<()> {
let _raw = if forward_only {
None
@@ -2112,6 +2162,10 @@ async fn run_terminal(
start_dynamic_forwards(&dynamic_forwards, forward_tx.clone(), stream_ids).await?;
signal_background_ready();
let mut stream_writers: HashMap<u64, tokio::net::tcp::OwnedWriteHalf> = HashMap::new();
// Write halves for server-initiated SSH-agent streams, spliced into the local
// unix agent socket. Kept separate from the TCP `stream_writers` so the
// existing TCP forwarding paths are untouched.
let mut agent_writers: HashMap<u64, tokio::net::unix::OwnedWriteHalf> = HashMap::new();
let mut pending_socks_replies: HashSet<u64> = HashSet::new();
let mut opened_streams: HashSet<u64> = HashSet::new();
let mut stream_send_credit: HashMap<u64, usize> = HashMap::new();
@@ -2323,6 +2377,60 @@ async fn run_terminal(
continue;
};
last_packet_at = Instant::now();
// SSH-agent stream: the server is asking us to splice this
// stream into our LOCAL ssh-agent. Only honor it when we
// actually opted in (agent_sock is Some); otherwise reject,
// so a server cannot reach our agent without consent.
if open.target_host == AGENT_STREAM_SENTINEL {
let Some(sock_path) = agent_sock.clone() else {
send_stream_open_reject(
&socket, addr, &cred, &mut send_seq, open.stream_id,
"agent forwarding not enabled by client".to_string(),
).await?;
continue;
};
match UnixStream::connect(&sock_path).await {
Ok(stream) => {
let (mut reader, writer) = stream.into_split();
agent_writers.insert(open.stream_id, writer);
opened_streams.insert(open.stream_id);
stream_send_credit.insert(open.stream_id, STREAM_INITIAL_WINDOW);
send_stream_open_ok(&socket, addr, &cred, &mut send_seq, open.stream_id).await?;
let forward_tx = remote_forward_tx.clone();
tokio::spawn(async move {
let mut buf = [0u8; 16 * 1024];
loop {
match reader.read(&mut buf).await {
Ok(0) => break,
Ok(n) => {
if forward_tx
.send(ForwardEvent::Data {
stream_id: open.stream_id,
bytes: buf[..n].to_vec(),
})
.await
.is_err()
{
return;
}
}
Err(_) => break,
}
}
let _ = forward_tx.send(ForwardEvent::Close {
stream_id: open.stream_id,
}).await;
});
}
Err(err) => {
send_stream_open_reject(
&socket, addr, &cred, &mut send_seq, open.stream_id,
format!("connect local agent: {err}"),
).await?;
}
}
continue;
}
match TcpStream::connect((open.target_host.as_str(), open.target_port)).await {
Ok(stream) => {
let (mut reader, writer) = stream.into_split();
@@ -2423,6 +2531,8 @@ async fn run_terminal(
let len = data.bytes.len();
if let Some(writer) = stream_writers.get_mut(&data.stream_id) {
let _ = writer.write_all(&data.bytes).await;
} else if let Some(writer) = agent_writers.get_mut(&data.stream_id) {
let _ = writer.write_all(&data.bytes).await;
}
// Always return flow-control credit so a stream whose local
// writer is already gone cannot wedge the server's send window.
@@ -2467,6 +2577,7 @@ async fn run_terminal(
stream_send_credit.remove(&close.stream_id);
stream_pending_data.remove(&close.stream_id);
stream_writers.remove(&close.stream_id);
agent_writers.remove(&close.stream_id);
}
_ => {}
}
@@ -2509,6 +2620,7 @@ async fn run_terminal(
stream_send_credit.remove(&stream_id);
stream_pending_data.remove(&stream_id);
stream_writers.remove(&stream_id);
agent_writers.remove(&stream_id);
send_stream_close(&socket, addr, &cred, &mut send_seq, stream_id).await?;
}
None if forward_only => break,
src/bin/dosh-server.rs
+178 -8
View File
@@ -21,13 +21,24 @@ use dosh::protocol::{
use dosh::pty::{PtyHandle, PtyOutput, spawn_pty_session};
use std::collections::{HashMap, HashSet, VecDeque};
use std::net::{IpAddr, SocketAddr};
use std::os::unix::fs::PermissionsExt;
use std::path::PathBuf;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::{Arc, Mutex};
use std::time::{Duration, Instant};
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::net::{TcpListener, TcpStream, UdpSocket};
use tokio::net::{TcpListener, TcpStream, UdpSocket, UnixListener};
use tokio::sync::mpsc;
const STREAM_INITIAL_WINDOW: usize = 1024 * 1024;
/// Sentinel `target_host` sent to the client on a server-initiated `StreamOpen`
/// that carries an SSH-agent connection (the client splices it into its local
/// `SSH_AUTH_SOCK` rather than dialing TCP). Must match the client's constant.
const AGENT_STREAM_SENTINEL: &str = "@dosh-agent";
/// Monotonic counter for unique agent proxy socket filenames within this process.
static AGENT_SOCK_COUNTER: AtomicU64 = AtomicU64::new(0);
/// How long a pending native handshake (ClientHello awaiting UserAuth) is kept
/// before it is swept by the cleanup task.
const NATIVE_HANDSHAKE_TTL_SECS: u64 = 30;
@@ -765,6 +776,41 @@ async fn handle_native_user_auth(
return Ok(());
}
// SSH-agent forwarding (opt-in + policy-gated). When the client requested it
// AND the server allows it, bind a per-session proxy unix socket now so its
// path can be exported as the remote shell's SSH_AUTH_SOCK before the PTY is
// spawned. The accept loop is started after the client_id exists below.
// SECURITY: only ever reached on explicit client opt-in and with
// `allow_agent_forwarding` enabled; the socket is private to this user.
let agent_allowed = {
let locked = state.lock().expect("server state poisoned");
locked.config.allow_agent_forwarding
};
let agent_listener =
if agent_allowed && agent_forwarding_requested(&req.auth.requested_forwardings) {
match bind_agent_proxy_socket() {
Ok(pair) => Some(pair),
Err(err) => {
eprintln!("agent forwarding setup failed, continuing without it: {err:#}");
None
}
}
} else {
None
};
// Env handed to ensure_session: base requested env plus SSH_AUTH_SOCK when
// agent forwarding is active. Note: only applies to a freshly spawned shell;
// an already-running (prewarmed/attached) session keeps its existing env, the
// same constraint ssh/mosh have.
let mut session_env = pending.client.requested_env.clone();
if let Some((_, ref path)) = agent_listener {
session_env.retain(|e| e.name != "SSH_AUTH_SOCK");
session_env.push(EnvVar {
name: "SSH_AUTH_SOCK".to_string(),
value: path.to_string_lossy().to_string(),
});
}
let attached: Result<(
[u8; 16],
[u8; 16],
@@ -793,13 +839,7 @@ async fn handle_native_user_auth(
if !locked.sessions.contains_key(&session_name) && !locked.config.create_on_attach {
return Err(anyhow!("session does not exist"));
}
locked.ensure_session(
&session_name,
cols,
rows,
&mode,
&pending.client.requested_env,
)?;
locked.ensure_session(&session_name, cols, rows, &mode, &session_env)?;
let session_key = pending.session_key;
let key_id = protocol::session_key_id(&session_key);
let attach_ticket_psk = crypto::random_32();
@@ -877,6 +917,10 @@ async fn handle_native_user_auth(
) = match attached {
Ok(attached) => attached,
Err(err) => {
// Auth/session setup failed: clean up the agent socket we bound early.
if let Some((_, path)) = agent_listener {
let _ = std::fs::remove_file(&path);
}
return send_reject_to_client(socket, peer, packet.header.conn_id, &format!("{err:#}"))
.await;
}
@@ -890,9 +934,16 @@ async fn handle_native_user_auth(
)
.await
{
if let Some((_, path)) = agent_listener {
let _ = std::fs::remove_file(&path);
}
remove_client(state, client_id);
return send_reject_to_client(socket, peer, packet.header.conn_id, &err.to_string()).await;
}
// Now that the client exists, start accepting forwarded agent connections.
if let Some((listener, path)) = agent_listener {
spawn_agent_forward(state, socket, client_id, listener, path);
}
let ok = NativeAuthOk {
client_id,
@@ -1793,6 +1844,123 @@ fn allocate_server_stream_id(state: &Arc<Mutex<ServerState>>) -> u64 {
stream_id
}
/// Whether the client requested SSH-agent forwarding during auth.
fn agent_forwarding_requested(forwardings: &[ForwardingRequest]) -> bool {
forwardings.iter().any(|f| f.kind == ForwardingKind::Agent)
}
/// Bind the per-session SSH-agent proxy unix socket, returning its `UnixListener`
/// and path. SECURITY: the socket lives in a process-private directory created
/// 0700 and the socket itself is chmod 0600, so only this user can connect — the
/// forwarded agent is never world-reachable. Called only when the client opted in
/// AND `allow_agent_forwarding` is set; the path is then exported as the remote
/// session's `SSH_AUTH_SOCK`.
fn bind_agent_proxy_socket() -> Result<(UnixListener, PathBuf)> {
let dir = std::env::temp_dir().join(format!("dosh-agent-{}", std::process::id()));
std::fs::create_dir_all(&dir).with_context(|| format!("create agent dir {}", dir.display()))?;
std::fs::set_permissions(&dir, std::fs::Permissions::from_mode(0o700))
.with_context(|| format!("chmod agent dir {}", dir.display()))?;
let n = AGENT_SOCK_COUNTER.fetch_add(1, Ordering::Relaxed);
let path = dir.join(format!("agent.{}.{n}.sock", std::process::id()));
// Stale socket from a crashed prior run with the same name: remove first.
let _ = std::fs::remove_file(&path);
let listener = UnixListener::bind(&path)
.with_context(|| format!("bind agent socket {}", path.display()))?;
std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o600))
.with_context(|| format!("chmod agent socket {}", path.display()))?;
Ok((listener, path))
}
/// Run the accept loop for a forwarded SSH-agent: each connection from a remote
/// process to the proxy socket is tunneled to the client (which splices it into
/// the user's real ssh-agent) over a Dosh stream using the agent sentinel target.
/// The socket file is removed when the loop ends (client gone / session closed).
fn spawn_agent_forward(
state: &Arc<Mutex<ServerState>>,
socket: &Arc<UdpSocket>,
client_id: [u8; 16],
listener: UnixListener,
socket_path: PathBuf,
) {
let state = Arc::clone(state);
let socket = Arc::clone(socket);
tokio::spawn(async move {
loop {
let Ok((stream, _)) = listener.accept().await else {
break;
};
// Stop accepting once the client has gone away.
{
let locked = state.lock().expect("server state poisoned");
if locked.lookup_client(&client_id).is_none() {
break;
}
}
let stream_id = allocate_server_stream_id(&state);
let (mut reader, mut writer) = stream.into_split();
let (writer_tx, mut writer_rx) = mpsc::channel::<Vec<u8>>(1024);
{
let mut locked = state.lock().expect("server state poisoned");
if let Some(client) = locked.client_mut(&client_id) {
client.stream_writers.insert(stream_id, writer_tx);
client
.stream_send_credit
.insert(stream_id, STREAM_INITIAL_WINDOW);
} else {
break;
}
}
tokio::spawn(async move {
while let Some(bytes) = writer_rx.recv().await {
if writer.write_all(&bytes).await.is_err() {
break;
}
}
});
if let Err(err) = send_stream_open_to_client(
&state,
&socket,
client_id,
stream_id,
AGENT_STREAM_SENTINEL.to_string(),
0,
)
.await
{
eprintln!("agent forward open error: {err:#}");
break;
}
let state = Arc::clone(&state);
let socket = Arc::clone(&socket);
tokio::spawn(async move {
let mut buf = [0u8; 16 * 1024];
loop {
match reader.read(&mut buf).await {
Ok(0) => break,
Ok(n) => {
if let Err(err) = send_stream_data_to_client(
&state,
&socket,
client_id,
stream_id,
buf[..n].to_vec(),
)
.await
{
eprintln!("agent forward data error: {err:#}");
break;
}
}
Err(_) => break,
}
}
let _ = send_stream_close_to_client(&state, &socket, client_id, stream_id).await;
});
}
let _ = std::fs::remove_file(&socket_path);
});
}
fn is_loopback_bind(host: &str) -> bool {
matches!(host, "127.0.0.1" | "localhost" | "::1")
}
@@ -1823,6 +1991,8 @@ fn stream_open_allowed(
}
ForwardingKind::Dynamic => true,
ForwardingKind::Remote => false,
// Agent forwarding never authorizes a client-initiated TCP open.
ForwardingKind::Agent => false,
});
anyhow::ensure!(
allowed,