dosh - Dormant Shell

dosh is a low-latency remote terminal designed around fast attach and fast reconnect. It is mosh-shaped, but not a mosh clone: the server is a resident daemon, terminal sessions stay hot, and repeat connects try encrypted UDP before starting SSH.

The core target is simple:

• First secure trust establishment uses SSH.
• Existing sessions attach in one encrypted UDP exchange whenever cached credentials allow it.
• Reconnect after sleep, roaming, or network change resumes in one encrypted UDP exchange.
• Cold SSH fallback stays competitive with plain ssh by doing less after auth.

Why not just mosh?

mosh is excellent at roaming and high-latency interactivity. Its startup path still has work dosh can avoid:

1. SSH connects to the host.
2. SSH starts mosh-server.
3. The client receives connection material over SSH.
4. SSH exits and the mosh UDP session begins.

dosh keeps dosh-server running before the client arrives. Named PTY sessions can also be prewarmed, so attaching to default does not need to spawn a daemon, create a PTY, or start a shell on the user's critical path.

This is not an encryption argument against mosh. dosh also encrypts its UDP data channel; the speed difference comes from keeping the server and session hot.

Fast Path Order

The client always tries the cheapest valid path first:

1. UDP resume: existing ClientId and session key. No SSH. One encrypted UDP request, one encrypted UDP reply.
2. UDP attach ticket: cached server-issued attach ticket for the same host/user/session/mode. No SSH. One encrypted UDP request, one encrypted UDP reply.
3. SSH bootstrap: ssh -T user@host ~/.local/bin/dosh-auth ..., then one encrypted UDP attach.
4. New session: same as attach, but the server must create the PTY/shell unless the session was prewarmed.

The fastest path is not a custom SSH replacement. SSH remains the first trust root; dosh removes SSH from repeat attaches when the server has already issued valid credentials.

Attach tickets are implemented because they are the way a fresh client process can skip SSH after a recent successful bootstrap.

Connection Speed Contract

dosh is measured by terminal-ready time: elapsed time from running dosh host to the first usable terminal screen.

• UDP resume: <= one measured UDP RTT + local render time.
• UDP attach ticket: <= one measured UDP RTT + local render time.
• Warm attach with ControlMaster: <= ssh host true over the existing master + one measured UDP RTT.
• Cold attach without ControlMaster: <= cold ssh host terminal-ready time + one measured UDP RTT.
• New session: measured separately because it may need PTY and shell creation.

The client emits timing spans for credential lookup, SSH bootstrap, UDP resume, UDP ticket attach, and terminal-ready time.

Architecture

dosh-server
  UDP socket on one configurable port
  session table keyed by name
  one PTY per named session
  optional prewarmed sessions, default ["default"]
  terminal parser/screen state per session
  client table per session
  encrypted UDP protocol
  tiny SSH-invoked dosh-auth helper mode

dosh-client
  terminal raw mode
  local credential cache
  UDP resume/attach first
  SSH bootstrap fallback
  PTY input/output forwarding
  reconnect and roaming state machine

Install

Default UDP port: 50000. This is intentionally inside the common forwarded range 50000-52000/udp.

Install on each Linux server you want to attach to:

curl -fsSL https://git.palav.dev/Palav/dosh/raw/branch/main/install.sh \
  | DOSH_REPO=https://git.palav.dev/Palav/dosh.git DOSH_PORT=50000 sh -s -- server

Install the client on macOS:

curl -fsSL https://git.palav.dev/Palav/dosh/raw/branch/main/install.sh \
  | DOSH_REPO=https://git.palav.dev/Palav/dosh.git DOSH_SERVER=palav DOSH_HOST=git.palav.dev DOSH_PORT=50000 sh -s -- client

Update an installed client later:

dosh update

Install the client on Windows PowerShell:

$env:DOSH_REPO="https://git.palav.dev/Palav/dosh.git"; $env:DOSH_SERVER="palav"; $env:DOSH_HOST="git.palav.dev"; $env:DOSH_PORT="50000"; irm https://git.palav.dev/Palav/dosh/raw/branch/main/install.ps1 | iex

Attach:

dosh palav

Plain dosh palav opens a fresh terminal session. Use named sessions when you want to reattach to the same persistent terminal from multiple clients:

dosh --session work palav
dosh --session logs palav

Press Ctrl-] to detach the current client while leaving the server session alive. Typing exit in the remote shell closes that Dosh session and returns the client. If UDP packets stop arriving, Dosh shows a blue status bar at the top of the terminal with the disconnected duration and sends keepalive pings until traffic returns.

If SSH and UDP use different public names, specify the UDP address:

dosh-client --dosh-host public.example.com --dosh-port 50000 user@host

Develop

Build:

cargo build

Attach locally, using local bootstrap instead of SSH:

target/debug/dosh-client --local-auth --no-cache local

Benchmark local attach:

target/debug/dosh-bench --local-auth --server local --iterations 5

Benchmark a remote host over SSH bootstrap:

target/release/dosh-bench --server user@host --ssh-port 22 --iterations 3

Benchmark the ControlMaster-backed SSH bootstrap path:

target/release/dosh-bench --server user@host --controlmaster --iterations 3

Run the Docker OpenSSH benchmark gate used by CI. It checks both cold SSH bootstrap and ControlMaster-backed SSH bootstrap against a containerized sshd plus resident dosh-server:

make bench-docker-ssh

The CI workflow includes an optional remote benchmark job. It runs when DOSH_BENCH_HOST, DOSH_BENCH_USER, and DOSH_BENCH_SSH_KEY repository secrets are configured.

Install release binaries and the user systemd service:

make install

Performance Rules

The stack is performance-driven, not fixed by taste. Rust is the default because the likely bottlenecks are network RTT, SSH startup/auth, PTY/shell creation, packet size, and terminal rendering. Change language or runtime only if measurements show they are the bottleneck.

Hot-path rules:

• Custom UDP protocol with AEAD for v0; no QUIC handshake on attach.
• Fixed binary packet headers for terminal traffic; no JSON on the protocol path.
• Preallocated buffers; avoid per-packet heap churn.
• Single-thread event loop is preferred for the hot path.
• No PTY allocation, shell spawn, shell rc files, or MOTD on attach to an existing session.
• Initial snapshot should be sent in the first UDP reply when it fits under the packet budget.

Goals

• Connection speed as specified above.
• UDP roaming and reconnect.
• Encrypted terminal data.
• Reuse SSH pubkeys for first trust establishment.
• Named persistent sessions.
• Multiple clients attached to one session.
• Optional view-only clients.
• Single server port, not one port per session.
• Static server and client binaries where practical.

Non-Goals

• Replacing SSH as the first public-key trust mechanism.
• Multi-user access control.
• Windows support in v0.
• Full mosh compatibility.
• Perfect predictive local echo in the first MVP.

Status

Rust implementation is present in this repository. It contains dosh-server, dosh-client, dosh-auth, dosh-bench, shared auth/crypto/protocol modules, a resident PTY server, encrypted UDP bootstrap attach, UDP resume, sealed UDP attach tickets, client ACKs, server retransmit bookkeeping, sliding replay protection, server-side vt100 screen snapshots/diffs, a hardened user systemd unit, an install script, Docker SSH benchmark gates, CI, and protocol/integration tests.