We ♥ forums.
Lurker indexes forum threads on behalf of the people who ask it to. Here's what that means in practice.
Nothing gets crawled unless a signed-in user explicitly points Lurker at a thread. There's no independent discovery of whole boards - Lurker is a tool acting on a specific request, the same way a browser or a read-it-later extension is.
Re-checking a thread for new posts is capped to once an hour per thread, and automatic refreshes only happen for threads a user has explicitly starred to watch - not every thread ever indexed. Lurker identifies itself with a descriptive User-Agent rather than pretending to be a browser.
By default Lurker only reads threads that are publicly viewable, no login required. If a thread sits behind a login, indexing it uses that user's own cookie for that site - never shared with other users, and never reused for scheduled re-crawls.
Every post Lurker surfaces in search results or a summary links back to the original author, post, and forum - credit stays with the community that wrote it.