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Control messages are a special kind of Usenet post that are used to control news servers. They differ from ordinary posts by a header field named Control. The body of the field contains control name and arguments. There are two historical alternatives to header field Control. They are not supported by contemporary software InterNetNews never supported all.all.ctl. INN version 1.0 was released in 1990. and forbidden according to RFC 5537.RFC 5537, 5. Control Messages: […] The presence of a Subject header field starting with the string “cmsg ” MUST NOT cause an article to be interpreted as a control message. […] Likewise, the presence of a ending in “.ctl” in the Newsgroups header field or the presence of an Also-Control header field MUST NOT cause the article to be interpreted as a control message. However, the traditional format of the subject line is widely used in addition to the Control header: the subject line consists of the word “cmsg” followed by control name and arguments.