Mocks and breakpoints – EmuLabZip documentation

Rules let you change what an app gets back without touching the server: answer a request yourself, make it slow, make it fail, or stop it and edit it by hand. They live in the proxy, so they work on every device the Network tool is connected to.

Rules#

A rule has a matcher (which requests), and one action (what to do). The first enabled rule that matches wins.

Matcher field	Matches
`methods`	Any of these methods. Empty means any.
`host`	A pattern: `api.example.com`, `.example.com`. `` is any run of characters, `?` any one.
`path`	A pattern for the path without the query: `/v1/users/*`.
`urlContains`	Text the whole address (host, path and query) has to contain.

Every field that is set has to match.

Action	What happens
Answer (`mock`)	The proxy answers with your status, headers and body, after an optional delay. The server is never asked.
Delay	The request waits, then goes on. Use it to try a slow network.
Block	The app gets a gateway error, as if the network had failed. Nothing reaches the server.
Hold (`breakpoint`)	The request, the response, or both, stop and wait for you.

Four more actions send a request to a file or to another server, check the response against an OpenAPI file, or run a WebAssembly script, and a rule can let the ones below it apply too: see Network rules and Rule scripts.

Flows that a rule touched show a MOCK or RULE badge in the list and in the details.

A rule is for one thing you chose. To answer everything from a recording, see Replaying a recording. A rule still wins over the recording.

In the app#

Open Network, then Rules. Pick a preset (a JSON answer, a 500, a slow network, a block, a breakpoint) or start from a request: select it and press Mock to get a rule that answers the way the server did, ready to edit. Rules apply to the next request and are kept in net-rules.json in EmuLabZip's data folder.

Breakpoints#

A held request or response appears in a drawer at the bottom of the Network view. You can edit the headers (one name: value per line), the body (when it is text) and, for a response, the status. Continue sends it on with your edits, Abort fails it, Continue all lets everything through. A response is shown decoded and goes on uncompressed. If nobody answers, a held message continues unchanged after 2 minutes, so an app never hangs for good.

On the command line#

emulabzip net --rules rules.json

[
  {
    "name": "stub the profile",
    "matcher": { "host": "api.example.com", "path": "/v1/profile" },
    "action": { "type": "mock", "status": 200,
                "headers": [["content-type", "application/json"]],
                "body": "{\"name\": \"Ada\"}" }
  },
  { "name": "slow uploads", "matcher": { "methods": ["POST"] }, "action": { "type": "delay", "ms": 3000 } },
  { "name": "no analytics", "matcher": { "host": "*.analytics.example.com" }, "action": { "type": "block" } }
]

A mock can carry a binary body: set "base64": true and give the body as base64. Breakpoints need someone to continue them, so the command line skips such rules and says so.

Limits#

A mock body can be up to 5 MB, and a delay up to 60 seconds.
Rules see the decrypted request, so HTTPS needs the certificate installed. Hosts that are passed through (pinned apps) cannot be changed.
WebSocket upgrades are matched like any request, but only the handshake can be mocked or blocked.