The evil bit is a fictional IPv4 packet header field proposed in a humorous April Fools' Day RFC from 2003,[1] authored by Steve Bellovin.
A 2015 research done by network engineer Ben Cartwright-Cox revealed that a number of popular websites (436 websites out of Alexa 20k at the time), such as those belonging to several universities and banks, to antivirus provider Kaspersky and to remote desktop software provider Teamviewer respect the "evil bit" by dropping the inbound request, making them compliant with RFC 3514.
[3] The evil bit has become a synonym for all attempts to seek simple technical solutions for difficult human social problems which require the willing participation of malicious actors, in particular efforts to implement Internet censorship using simple technical solutions.
[7] This RFC has also been quoted in the otherwise completely serious RFC 3675, ".sex Considered Dangerous", which may have caused the proponents of .xxx to wonder whether the Internet Engineering Task Force (IETF) was commenting on their application for a top-level domain (TLD) – the document was not related to their application.
[8] For April Fool's 2010, Google added an &evil=true parameter to requests through the Ajax APIs.