The evil bit is a fictional IPv4 packet header field proposed in RFC 3514, a humorous April Fools' Day RFC from 2003 authored by Steve Bellovin. The RFC recommended that the last remaining unused bit in the IPv4 packet header be used to indicate whether a packet had been sent with malicious intent, thus making computer security engineering an easy problem — simply ignore any messages with the evil bit set.
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.
The evil bit also became a noteworthy in-joke in Slashdot. News about the publication of this RFC was posted in Slashdot dozens of times, reworded each time, among other April Fools stories, poking humour at the common criticism of Slashdot often posting duplicate stories.
As a joke, FreeBSD implemented this on the same day but removed the changes on the following day. A Linux patch implementing the iptables module "ipt_evil" was posted the next year. Furthermore, a patch for FreeBSD 7 is available and is kept up-to-date.
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.