Writing a new driver

Adding a vendor to rcfg-sim is intentionally small: one runtime file for the driver and one template plus a registry entry for the generator. Nothing in the hot path needs to know your vendor exists.

1. Implement the Driver

Create internal/sshsrv/driver_<vendor>.go and implement the four-method Driver interface. Register it from init():

package sshsrv

func init() { registerDriver(myVendor{}) }

type myVendor struct{}

func (myVendor) Name() string            { return "myvendor_os" }
func (myVendor) RequiresSSHAuth() bool   { return true }
func (myVendor) Commands() []string      { return []string{ /* Cmd* label strings */ } }
func (myVendor) Serve(ctx *sessionCtx)   { /* the interactive loop */ }

2. Route responses through the shared helpers

In Serve, never sleep or write to the channel directly for command responses. Use the session-context machinery so timing, faults, and metrics stay uniform:

• Call ctx.applyResponseDelay() between resolving a command and dispatching it.
• Build a Response and pass it to ctx.emit(cmd, cmdStart, resp); check its return value to know when to close the session.
• Stream large payloads via Response.ConfigOutput (the mmap slice) to preserve the zero-copy hot path. Use Response.Trailer for any closing bytes (e.g. a terminator).

This is what gives your driver fault injection (disconnect_mid, malformed, slow_response) and the command-duration metric for free.

3. Add command labels

Add any new Cmd* constants and their String() cases, and return them from Commands(). The server unions every driver’s Commands() and pre-registers them, keeping the rcfgsim_command_duration_seconds cardinality bounded. Never use raw user input as a label value.

4. Add a generator model

To produce devices for your driver, add one registry entry in internal/configs/generator.go (vendor string, the template id matching your driver’s Name(), the template file, and a data builder) and a templates/<name>.tmpl. The model name becomes a valid --distribution key.

5. Test it

• Add unit tests for command resolution and response shape.
• Add an integration test (build tag integration) that connects over real SSH.
• If your output is meant to be byte-stable for a seed, add a determinism test like TestCienaDeterministic.

Run the gate before opening a PR:

make vet fmt test
make integration

See Contributing for the full PR checklist, and the shipped Ciena TL1 driver as a complete, self-contained worked example.