SSH or mosh?

devmachine ssh and devmachine mosh both open a terminal on your workspace. They pick the same tmux session, so it makes no difference which one you used last time. This page is about which to use when, and the one setting mosh needs before it works.

You need: a workspace — see getting started.

Before you start: machine, skills, workspace

curl -fsSL https://mydevmachine.sh/install.sh | sh
devmachine setup
devmachine skills add
devmachine workspaces new acme
devmachine sync

Already have a machine? Skip setup. Already have the workspace? Skip the last two. See getting started for what each command does.

What each one is good for

ssh works everywhere, on any network, with no setup beyond what setup already did. It’s the default, and it’s enough most of the time.

mosh is worth it when your connection is not steady: it survives a change of Wi-Fi, your laptop going to sleep, roaming between networks, and a link that drops packets. It also shows what you type right away, even on a slow connection, instead of waiting for the server to echo it back.

Neither replaces tmux. mosh can’t scroll back through old output — tmux already gives you that inside the session both commands land in.

What has to be installed where

• On the server: the base package, part of essentials, installs mosh. A machine set up with devmachine setup already has it. A machine started with --no-essentials does not, until you add base.
• On your computer: devmachine machine setup installs ssh and mosh through Homebrew on a Mac (on Linux it tells you what to install instead). devmachine machine doctor says what’s missing — a missing mosh is only a warning, since ssh still works.
• devmachine does not bundle either program. ssh and mosh run the real system binaries, so your terminal, your SSH agent, and tmux all behave normally.

devmachine machine doctor
devmachine machine setup

Open the UDP range for mosh

Unlike ssh, mosh needs a UDP port range (60000–61000 by default), not just one TCP port. With the firewall package — part of essentials — that range is closed by default, on every interface. mosh will fail until you open it on one interface with firewall.mosh_interface.

The recommended way is over Tailscale: that keeps the range off the public internet, reachable only from your own devices.

devmachine packages add tailscale
devmachine login tailscale
devmachine sync

Then, in config.yml, set the interface under the machine’s settings::

machines:
  - name: main
    settings:
      firewall.mosh_interface: tailscale0

devmachine sync

By hand

1. Check your computer has both

devmachine machine doctor

If mosh is missing, install it:

devmachine machine setup

2. Connect with ssh

devmachine ssh acme

Always works, and needs nothing extra.

3. Connect with mosh

devmachine mosh acme

Needs firewall.mosh_interface set on the machine, as shown above. Both commands check <config>/known_hosts first, the same as any other login.

With your agent

Open a session on your own computer (devmachine skills add taught it the CLI) and say:

Open the mosh UDP range on my devmachine machine, over Tailscale only, then
sync.

The agent adds firewall.mosh_interface: tailscale0 to config.yml and runs devmachine sync --check first, then asks before applying it. Signing the server into Tailscale (devmachine login tailscale) is a browser step that’s yours to do.

Check it: run devmachine ssh acme and devmachine mosh acme from two different terminals — both land in the same tmux session, so anything you see in one shows up in the other.

Source: docs/reference/commands.md — ssh, mosh, docs/how-it-works/ssh.md, docs/concepts/reaching-your-server.md