mosh-go

Pure Go mosh. Client and server. No C dependencies, no CGO.

Wire-compatible with the standard C mosh implementation. You can use the Go client against a C mosh-server, or the Go server with a C mosh-client, or go pure Go on both ends.

Implements the full mosh wire protocol: AES-128-OCB3 authenticated encryption, SSP transport with sequencing and retransmission, datagram fragmentation, and the protobuf state sync layer.

unixshells.com uses mosh-go compiled to WASM for the web terminal

Install

go install github.com/unixshells/mosh-go/cmd/mosh@latest
go install github.com/unixshells/mosh-go/cmd/mosh-server@latest
go install github.com/unixshells/mosh-go/cmd/mosh-client@latest

Usage

The mosh command works like the C version -- SSHs to the host, starts mosh-server, connects over UDP:

mosh user@host
mosh -p 2222 user@host            # custom SSH port
mosh -i ~/.ssh/id_ed25519 user@host

Or use the server and client separately:

# On the server:
mosh-server                       # prints MOSH CONNECT <port> <key>
mosh-server -p 60001              # specific port
mosh-server -s /bin/zsh           # specific shell

# On the client:
MOSH_KEY=<key> mosh-client <host> <port>

Library

import mosh "github.com/unixshells/mosh-go"

// Server
srv, _ := mosh.NewServer("/bin/bash", 60000, 60999)
go srv.Serve()
fmt.Println(srv.ConnectLine())

// Client
c, _ := mosh.Dial("192.168.1.5", 60001, "base64key")
c.Resize(80, 24)
c.Send([]byte("ls\n"))
out := c.Recv(time.Second)

Embedding

ServeRW runs the server protocol over an io.ReadWriteCloser instead of a PTY. No shell is spawned -- the caller provides the I/O. This is how latch bridges mosh into its session model:

srv, _ := mosh.NewServer("", 0, 0)
fmt.Println(srv.ConnectLine())
srv.ServeRW(rw, func(cols, rows uint16) {
	// handle resize
})

Compatibility

Tested against the C reference implementation:

Client	Server	Status
Go	Go	always tested
Go	C mosh-server	tested if installed
C mosh-client	Go	tested if installed

All three pass. Run go test -race ./... to verify.

Latch extensions

The protobuf layer supports optional latch extension fields. Standard mosh clients and servers ignore them (forward-compatible).

• Capability negotiation (TransportInstruction field 8): bitfield exchanged on every datagram. Both sides AND their caps to get the intersection. No caps = classic mosh mode.
• Session control (Host/UserInstruction field 9): list, switch, and create latch sessions over the mosh connection.

WASM build

The mosh client compiles to WebAssembly for use in the web terminal:

GOOS=js GOARCH=wasm CGO_ENABLED=0 go build -tags osusergo -o mosh.wasm ./cmd/mosh-wasm/

The -tags osusergo flag is required to avoid os/user CGO issues with GOOS=js.

Files

• server.go -- server, PTY lifecycle, ServeRW bridge mode
• client.go -- client, send/recv loops
• transport.go -- SSP sequencing, timestamps, RTT, retransmission
• ocb.go -- AES-128-OCB3 (RFC 7253)
• fragment.go -- datagram fragmentation and reassembly
• pb.go -- protobuf encoding (transport, host, user, latch extensions)
• framebuffer.go -- terminal state tracking for diff protocol
• predict.go -- local echo prediction engine (speculative keystroke rendering)

Dependencies

creack/pty, golang.org/x/crypto, and the Go standard library.

License

Copyright (c) 2026 Unix Shells. MIT license. See LICENSE.