Mosh

Mosh keeps a terminal session alive through network changes, weak signal, and your device sleeping. Switch from Wi-Fi to cellular mid-session and the shell is still there. SwiftServer speaks the Mosh protocol natively — the server just needs mosh-server installed.

Turn Mosh on for a machine

Mosh is a per-machine connection mode:

1. Edit the machine and set Connection to Mosh.
2. Optionally adjust Server Executable (defaults to mosh-server on the PATH — set an absolute path if yours lives elsewhere).
3. Save. The machine's terminal action becomes Start Mosh Terminal.

How a session starts

SwiftServer first connects over SSH exactly as usual (same authentication, same host key checks), starts mosh-server through that connection, then hands the session to UDP and closes the SSH channel. There is no password inside the Mosh protocol itself — the secret session key is exchanged over the encrypted SSH bootstrap, and all Mosh traffic is encrypted with it.

Test Connection in the machine editor verifies the SSH bootstrap; the UDP side is verified the first time you start the terminal.

Requirements

• mosh-server installed on the server (apt install mosh, dnf install mosh, brew install mosh, …).
• UDP reachable from your device to the server on the negotiated port. By default mosh-server picks a port in its standard range (60000–61000); open that range, or set a custom one.

To pin the port range, enable Use Custom UDP Port Range in the machine editor and set First UDP Port / Last UDP Port — SwiftServer passes them to mosh-server so you can open exactly one small range in your firewall.

Life of a Mosh session

• Change networks and the session follows — SwiftServer pins the UDP flow to a resolved address and rebuilds the connection as paths change. You may briefly see "Mosh is rebuilding its UDP connection", then "Mosh UDP connection restored".
• If the server stays silent, the status line counts it: "No Mosh server contact for N seconds".
• When the server ends the session (for example mosh-server is killed), the terminal reports "The Mosh server ended the session." Reconnecting starts a fresh session.

Troubleshooting

"mosh-server exited with status 127" (or similar with "command not found") — mosh-server is not installed or not on the PATH for non-interactive SSH. Install Mosh, or set the absolute path in Server Executable.

"mosh-server did not print MOSH CONNECT within 8 seconds" — the bootstrap ran but produced no session line. Usually a shell startup script printing noise or blocking; try running mosh-server manually over SSH to see its output.

Connects, then hangs at a blank terminal — the SSH bootstrap worked but UDP is blocked. Open the UDP port range in every firewall between you and the server (cloud security groups included), or set a custom range you can open.

"Could not resolve the Mosh UDP host" — the hostname doesn't resolve to an IPv4/IPv6 address from your device. Use an IP, or set UDP Host explicitly.

Mosh sessions include a Mosh Diagnostics section in the disconnect dialog with the full bootstrap and connection log — check it first, and include it if you write to support.