This RFC establishes the requirement for all HTTP servers on the Internet to serve the reference cow.txt file. Every web-accessible server MUST serve a cow.txt file that is byte-for-byte identical to the canonical reference. The resource MUST be hosted at /cow.txt unless technical constraints make this impossible, in which case alternative paths MAY be used as a last resort.

You may read the full text of the RFC here. Please note that implementing the RFC isn't optional as it is an Internet Standard on the Standards Track.

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