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mosh

Mosh: The Mobile Shell

Mosh (mobile shell) is a remote terminal application that allows roaming, supports intermittent connectivity, and provides intelligent local echo. Unlike SSH, Mosh keeps your session alive even when your IP changes, you switch networks, or your connection drops for a moment.

Why Mosh?

  • Roaming: Move between Wi-Fi, cellular, and wired networks without losing your session.
  • Intermittent connectivity: If your connection drops, Mosh patiently waits and resumes automatically when the network returns.
  • Local echo: Mosh predicts your typing and displays it instantly, even on high-latency links, making it feel snappy.
  • No root required on the client: Only the server side needs mosh-server installed.

Installation

On the Server (Debian/Ubuntu)

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sudo apt update
sudo apt install mosh

On the Client

macOS:

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brew install mosh

Debian/Ubuntu:

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sudo apt install mosh

Arch Linux:

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sudo pacman -S mosh

Using the Mosh Client

The simplest usage is very similar to SSH:

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mosh user@hostname

You can also pass SSH options or specify a command:

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mosh user@hostname --ssh="ssh -p 2222"
mosh user@hostname -- tmux new -A -s main

Mosh uses UDP ports in the range 60000–61000 by default. If your server is behind a firewall, make sure to open the required range:

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sudo ufw allow 60000:61000/udp

Or for iptables:

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sudo iptables -A INPUT -p udp --dport 60000:61000 -j ACCEPT

Booting mosh-server with systemd in Debian

By default, mosh starts mosh-server on demand via SSH. However, if you want mosh-server to be available immediately after boot (for example, on a headless server or to reduce connection latency), you can create a systemd user service.

Step 1: Create the systemd user service file

Create the directory and file:

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mkdir -p ~/.config/systemd/user/

Edit ~/.config/systemd/user/mosh-server.service:

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[Unit]
Description=Mosh Server
Documentation=man:mosh-server(1)

[Service]
Type=simple
ExecStart=/usr/bin/mosh-server new -s -l LANG=en_US.UTF-8
Environment="MOSH_SERVER_NETWORK_TIMEOUT=60"
StandardInput=socket
StandardOutput=journal
StandardError=journal

[Install]
WantedBy=default.target

Note: The StandardInput=socket line allows systemd to allocate a UDP socket for mosh-server and pass it in. This is optional; without it, mosh-server will bind to a dynamic port as usual.

Step 2: Enable and start the service

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systemctl --user daemon-reload
systemctl --user enable mosh-server.service
systemctl --user start mosh-server.service

Check the status:

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systemctl --user status mosh-server.service

Step 3: Ensure user services persist after logout (Debian default)

Debian enables lingering for user services by default via systemd-logind, but if your session is terminated when you log out of SSH, enable lingering explicitly:

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sudo loginctl enable-linger $USER

Step 4: Connect from the client

If you started mosh-server manually via systemd, it prints a connection command. Read the journal:

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journalctl --user -u mosh-server.service

Look for a line like:

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MOSH CONNECT 60001 XXXXXXXXXXXXXXXX

Then connect from the client:

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mosh-client 192.168.1.100 60001

Or simply use the usual mosh user@host and let SSH spawn a new server instance if you prefer the on-demand model.

Tips

  • Restrict port range: If you want to limit the UDP port range, set the environment variable MOSH_SERVER_CMD or use --server:
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    mosh user@host --server="mosh-server new -p 60000:60010"
  • Tmux/screen integration: Mosh works great with tmux or screen. Start a persistent session and reattach anytime:
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    mosh user@host -- tmux new -A -s remote
  • IPv6: Mosh fully supports IPv6. Just connect to an IPv6 address or hostname with AAAA records.

Conclusion

Mosh is an excellent complement (or replacement) to SSH for mobile or unreliable network conditions. Combined with a systemd user service on Debian, you can have a fast-resuming remote shell ready at boot time. Give it a try and enjoy truly mobile terminal sessions!

References: