Anthropic This document defines SAIL (Source Address Identity List), a metadata format served at a well-known URI in which the operator of automated HTTP clients publishes the IP prefixes its services send from, together with two optional assertions: that a service sends from nowhere else, and that nothing else of the operator's sends from a given set of prefixes. A site that receives a request claiming to be one of those services can use that metadata to confirm the claim or, under those assertions, to refute it. The mechanism is intended to complement reverse-DNS verification and HTTP Message Signatures.