Control message

Control message


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 [1] [2] and forbidden according to RFC 5537. [3] 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.

Types of control messages


A cancel message requests the deletion of a specific article. The body of the Control field contains one argument, the Message-ID of the article to delete.

According to RFC 1036 only the author of the target message or the local news administrator is allowed to send a cancel. To verify authorization the From: line (or Sender: line, if it exists) of the cancel message must match the target article. This verification does not work well in modern day Usenet and is rarely used.[4]

Anyway, additional hierarchy specific rules (see Breidbart Index) allow cancelbots to send third-party cancels to remove spam.

Control: cancel <>
Subject: cmsg cancel <>


A newgroup message is issued to create a new Usenet newsgroup. The body of the Control field contains one mandatory argument, the name of the new group. The second argument is optional. If present it consists of the keyword moderated. The body of the message typically contains tagline, charter and rationale.

If the group already exists, only the status of the group is changed, i.e. whether it is moderated or nor not.[5]

Typically newgroup messages having a correct digital signature are executed automatically. In some hierarchies (alt.*, free.*, de.alt.*) unsigned newgroup messages just serve as formalized proposal to create a new group. Objections to the proposal are then expressed with a rmgroup.[6] [7]

In many cases newgroup messages are archived by the Internet Systems Consortium.[8]

Control: newgroup comp.object.moderated moderated
Subject: cmsg newgroup comp.object.moderated moderated


A rmgroup message is issued to remove a newsgroup. The body of the Control field contains one mandatory argument, the name of the group to remove.

Typically rmgroup messages having a correct digital signature are executed automatically. In some hierarchies unsigned rmgroup messages are used to veto a preceding newgroup.

In the hierarchy de.alt.* removal and creation of groups is handled symmetrically, i.e. an unsigned rmgroup message is used as formalized proposal. Objections to the proposal are then expressed with a newgroup.[7]

Control: rmgroup comp.object.moderated
Subject: cmsg rmgroup comp.object.moderated


A checkgroups message lists all groups of a hierarchy.

Control: checkgroups
Subject: cmsg checkgroups
Example conforming to RFC 5537
Control: checkgroups de !de.alt #2009021301

Obsolete message types

Name Definition Description
Ihave RFC 850, RFC 1036, RFC 5537 Announce arrival of particular message
Sendme RFC 850, RFC 1036, RFC 5537 Request transmission of particular message
Sendsys RFC 850, RFC 1036, RFC 5537 Request email with list of newsgroups sent to each neighbor
Senduuname RFC 850, RFC 5537 Request email with list of all uucp neighbors
Version RFC 850, RFC 1036, RFC 5537 Request email with name and version of Usenet software
Whogets RFC 5537 No description, just declared obsolete

The ihave/sendme protocol was obsoleted by NNTP.

Answering control messages with large emails can be exploited for a Denial of service attack. Thus news servers stopped implementing sendsys long before it was declared obsolete by RFC 5537. [9]

Security considerations

Header field "Approved:"

Messages of type newgroup and rmgroup are ignored unless there is an "Approved" line in the same message header.[10] Newsservers traditionally allow only selected users to send articles with these lines. As long as there were only a handful of Usenet sites this provided sufficient protection against abuse.

Digital signature

The format of "Arpa Internet Text Messages"[11] is the common base for Usenet[12] and E-mail.[13] The format provides no means of authentication. Various extensions adding a digital signature were developed to prevent forgeries.

Signature format Covered data Usage
PGP/INLINE arbitrary text NoCeM
PGP/MIME MIME body parts E-mail
S/MIME MIME body parts E-mail
pgpcontrol body and selected header fields newgroup, rmgroup, checkgroups

For control message a special format is required since the essential information is in the header fields. Pgpcontrol was originally designed for PGP but also works with OpenPGP.[14]

Hierarchy keys

Newsgroup maintenance of the main Usenet hierarchies (Big 8 and regional hierarchies) is done through signed control messages. Each hierarchy has unique key that is guarded by the hierarchy founders (or their successors). Most newsservers are configured to both automatically execute controls signed with the right key and ignore anything else.

Theoretically this system is also applicable to cancel messages. However, it would not only require a key pair for every Usenet user but also that the respective public key is known to every news server. Cancel-lock is much simpler, but neither commonly accepted, nor implemented in popular news servers and newsreaders.[15]


Control messages are typically not shown in the target Newsgroup. Instead many servers put them into pseudo newsgroups like control.[16]

Google Groups provides no means to read or write control messages. It does not even execute cancels.

The Internet Systems Consortium archives newgroup and rmgroup together as a single file per group[8] and checkgroups as one file per year.[17]


  1. ^ InterNetNews never supported all.all.ctl. INN version 1.0 was released in 1990.
  2. ^ "Subject: cmsg" is described only in RFC 1036 (published in December 1987) but not in RFC 850 (published in June 1983). INN dropped this feature with version 2.3. See INN Changes and Upgrade Information (Internet Archive).
  3. ^ 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 <newsgroup-name> 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.
  4. ^ RFC 850 uses the term "local super user" instead of "local news administrator". Son-of-RFC-1036 (this is the colloquial name of an Internet Draft written by Henry Spencer) drops the administrator's cancel altogether. The problem with the verification scheme is that the From: line is trivial to forge and with cancelbots the cancel message often arrives before the target article. Option verifycancels of INN defaults to false.
  5. ^ Section 5.2.1. of RFC 5537 says: The newgroup control message requests that the specified group be created or, if already existing, that its moderation status or description be changed. The feature is much older, though. For example the manual page of the ctlinnd utility (part of INN) says: If the newsgroup already exists, this is equivalent to the changegroup command.
  6. ^ *How To Write a Good Newgroup Message
  7. ^ a b Regeln für die Einrichtung, Änderung und Entfernung von Usenet-Gruppen
  8. ^ a b
  9. ^ Section "3.5. Sendsys" of RFC 1036 includes the following clause: This information is considered public information, and it is a requirement of membership in USENET that this information be provided on request, either automatically in response to this control message, or manually, by mailing the requested information to the author of the message.
  10. ^ RFC 1036, sections "3.3. Newgroup" and "3.4. Rmgroup". Section "3.7. Checkgroups" does not contain this clause.
  11. ^ RFC 822, published in August 1982
  12. ^ RFC 1036, section "2. Message Format"
  13. ^ RFC 2822, obsoleted RFC 822 in April 2001
  14. ^ INN already ships with pgpcontrol. The project site additionally provides instructions on how to set up PGP/OpenPGP and a huge archive of hierarchy keys.
  15. ^ Cancel-Locks in Usenet articles: draft-ietf-usefor-cancel-lock-01.txt, published in November 1998, expired in May 1999
  16. ^ INN normally files control messages to the pseudo newsgroup control. However, if a subgroup of control exists that matches the control command, the control message will be filed into that group instead.
  17. ^

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