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httpmock Tutorial

FAQ

Can I start multiple mock servers in one Rust test function?

Yes, you can start multiple mock servers. However, httpmock uses a server pool with a default limit of 25 servers. You can adjust this limit by setting the HTTPMOCK_MAX_SERVERS environment variable.

In which order are mocks evaluated?

When the httpmock server receives a request that matches multiple mocks, they are evaluated in reverse order of their definition. This means the most recently defined mock takes precedence and will be used.

How does Proxy Mode work?

A mock server acts as a “man-in-the-middle” (MITM) proxy. Here’s how it works:

  • Intercepting HTTPS Traffic: When you configure your System Under Test to use mock server as a proxy, the mock server intercepts the HTTPS traffic. Normally, in a CONNECT tunnel, the traffic would be encrypted end-to-end. However, a mock server performs a MITM attack (with your consent, of course) to decrypt this traffic.
  • SSL/TLS Termination: A mock server creates its own SSL/TLS connection with the client and another with the target server. When the client sends an HTTPS request, it is intercepted by the mock server. The mock server terminates the original SSL/TLS connection and establishes a new connection with the actual destination server.
  • Certificate Authority (CA): To intercept and decrypt HTTPS traffic, mock server uses a self-signed or a custom Certificate Authority (CA). This CA certificate is used to generate SSL certificates on the fly for the destination servers. You need to configure the client (or system under test) to trust this CA, which allows the mock server to decrypt the traffic. The client sees the proxy server as the legitimate endpoint because it trusts the certificate issued by mock server.
  • Request/Response Handling: Once the traffic is decrypted, the mock server can read, modify, or log the HTTPS requests and responses. After processing the request, the mock server forwards it to the actual destination server, receives the response, and then re-encrypts it before sending it back to the client.