DNS calculator
A Reverse Polish calculator accepts operands in a DNS name and returns a dynamically computed answer, a small service you can query with dig.
The Postel.org example is a Reverse Polish notation calculator. A query name carries the operands and operation; the authoritative server calculates the result and returns it dynamically.
The input and output are small enough to inspect with dig, but the design is best understood as a protocol experiment rather than a general-purpose calculator.
Primary source: https://www.cambus.net/interesting-dns-hacks/