HTTP Status Codes

Core Idea

HTTP status codes summarize what happened to a request.

Practice Alone

Run a header request and read the first line:

curl -I https://example.org

Read the first digit first:

• 2xx: success.
• 3xx: redirection or further action, such as using a cached representation.
• 4xx: the server cannot fulfil this request, for example a missing path or denied access.
• 5xx: the server failed while handling the request.

Common examples:

• 200: the request succeeded.
• 301 or 302: the server redirects the client to another URL.
• 304: the client can use its cached representation; not a move to another URL.
• 403: access is forbidden.
• 404: the server is reachable, but that path is missing.
• 500: the backend failed while handling the request.
• 502: a proxy could not get a valid response from its backend.

502 points to a gateway/upstream problem, not automatically to the client or the requested filename. Inspect the actual service configuration and logs before deciding on a repair.

An HTTP error response can still mean curl completed successfully. 000 is curl’s no-response placeholder, not an HTTP status code.

Done When

You can explain whether failure is missing content, server error, or bad proxying.

Docs Pointers

• Read curl -I, HTTP inspection, and reverse proxy.