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The CAPABILITIES command allows a client to determine the capabilities of the server at any given time.

This command MAY be issued at any time; the server MUST NOT require it to be issued in order to make use of any capability. The response generated by this command MAY change during a session because of other state information (which, in turn, may be changed by the effects of other commands or by external events). An NNTP client is only able to get the current and correct information concerning available capabilities at any point during a session by issuing a CAPABILITIES command at that point of that session and processing the response.

The capability list is returned as a multi-line data block following the 101 response code. Each capability is described by a separate capability line. The server MUST NOT list the same capability twice in the response, even with different arguments. Except that the VERSION capability MUST be the first line, the order in which the capability lines appears is not significant; the server need not even consistently return the same order.

While some capabilities are likely to be always available or never available, others (notably extensions) will appear and disappear depending on server state changes within the session or on external events between sessions. An NNTP client MAY cache the results of this command, but MUST NOT rely on the correctness of any cached results, whether from earlier in this session or from a previous session, MUST cope gracefully with the cached status being out of date, and SHOULD (if caching results) provide a way to force the cached information to be refreshed. Furthermore, a client MUST NOT use cached results in relation to security, privacy, and authentication extensions. See Section 12.6 for further discussion of this topic.

The keyword argument is not used by this specification. It is provided so that extensions or revisions to this specification can include extra features for this command without requiring the CAPABILITIES command to be used twice (once to determine if the extra features are available, and a second time to make use of them). If the server does not recognise the argument (and it is a keyword), it MUST respond with the 101 response code as if the argument had been omitted. If an argument is provided that the server does recognise, it MAY use the 101 response code or MAY use some other response code (which will be defined in the specification of that feature). If the argument is not a keyword, the 501 generic response code MUST be returned. The server MUST NOT generate any other response code to the CAPABILITIES command.

Indicating capability: READER

The GROUP command selects a newsgroup as the currently selected newsgroup and returns summary information about it.

The required argument is the name of the newsgroup to be selected (e.g., "news.software.nntp"). A list of valid newsgroups may be obtained by using the LIST ACTIVE command (see Section 7.6.3).

The successful selection response will return the article numbers of the first and last articles in the group at the moment of selection (these numbers are referred to as the "reported low water mark" and the "reported high water mark") and an estimate of the number of articles in the group currently available.

If the group is not empty, the estimate MUST be at least the actual number of articles available and MUST be no greater than one more than the difference between the reported low and high water marks. (Some implementations will actually count the number of articles currently stored. Others will just subtract the low water mark from the high water mark and add one to get an estimate.)

If the group is empty, one of the following three situations will occur. Clients MUST accept all three cases; servers MUST NOT represent an empty group in any other way.

• The high water mark will be one less than the low water mark, and the estimated article count will be zero. Servers SHOULD use this method to show an empty group. This is the only time that the high water mark can be less than the low water mark.
• All three numbers will be zero.
• The high water mark is greater than or equal to the low water mark. The estimated article count might be zero or non-zero; if it is non-zero, the same requirements apply as for a non-empty group.

The set of articles in a group may change after the GROUP command is carried out:

• Articles may be removed from the group.
• Articles may be reinstated in the group with the same article number, but those articles MUST have numbers no less than the reported low water mark (note that this is a reinstatement of the previous article, not a new article reusing the number).
• New articles may be added with article numbers greater than the reported high water mark. (If an article that was the one with the highest number has been removed and the high water mark has been adjusted accordingly, the next new article will not have the number one greater than the reported high water mark.)

Except when the group is empty and all three numbers are zero, whenever a subsequent GROUP command for the same newsgroup is issued, either by the same client or a different client, the reported low water mark in the response MUST be no less than that in any previous response for that newsgroup in this session, and it SHOULD be no less than that in any previous response for that newsgroup ever sent to any client. Any failure to meet the latter condition SHOULD be transient only. The client may make use of the low water mark to remove all remembered information about articles with lower numbers, as these will never recur. This includes the situation when the high water mark is one less than the low water mark. No similar assumption can be made about the high water mark, as this can decrease if an article is removed and then increase again if it is reinstated or if new articles arrive.

When a valid group is selected by means of this command, the currently selected newsgroup MUST be set to that group, and the current article number MUST be set to the first article in the group (this applies even if the group is already the currently selected newsgroup). If an empty newsgroup is selected, the current article number is made invalid. If an invalid group is specified, the currently selected newsgroup and current article number MUST NOT be changed.

The GROUP or LISTGROUP command (see Section 6.1.2) MUST be used by a client, and a successful response received, before any other command is used that depends on the value of the currently selected newsgroup or current article number.

If the group specified is not available on the server, a 411 response MUST be returned.

Indicating capability: READER

The LISTGROUP command selects a newsgroup in the same manner as the GROUP command (see Section 6.1.1) but also provides a list of article numbers in the newsgroup. If no group is specified, the currently selected newsgroup is used.

On success, a list of article numbers is returned as a multi-line data block following the 211 response code (the arguments on the initial response line are the same as for the GROUP command). The list contains one number per line and is in numerical order. It lists precisely those articles that exist in the group at the moment of selection (therefore, an empty group produces an empty list). If the optional range argument is specified, only articles within the range are included in the list (therefore, the list MAY be empty even if the group is not).

The range argument may be any of the following:

• An article number.
• An article number followed by a dash to indicate all following.
• An article number followed by a dash followed by another article number.

In the last case, if the second number is less than the first number, then the range contains no articles. Omitting the range is equivalent to the range 1- being specified.

If the group specified is not available on the server, a 411 response MUST be returned. If no group is specified and the currently selected newsgroup is invalid, a 412 response MUST be returned.

Except that the group argument is optional, that a range argument can be specified, and that a multi-line data block follows the 211 response code, the LISTGROUP command is identical to the GROUP command. In particular, when successful, the command sets the current article number to the first article in the group, if any, even if this is not within the range specified by the second argument.

Note that the range argument is a new feature in this specification and servers that do not support CAPABILITIES (and therefore do not conform to this specification) are unlikely to support it.

Indicating capability: READER

If the currently selected newsgroup is valid, the current article number MUST be set to the previous article in that newsgroup (that is, the highest existing article number less than the current article number). If successful, a response indicating the new current article number and the message-id of that article MUST be returned. No article text is sent in response to this command.

There MAY be no previous article in the group, although the current article number is not the reported low water mark. There MUST NOT be a previous article when the current article number is the reported low water mark.

Because articles can be removed and added, the results of multiple LAST and NEXT commands MAY not be consistent over the life of a particular NNTP session.

If the current article number is already the first article of the newsgroup, a 422 response MUST be returned. If the current article number is invalid, a 420 response MUST be returned. If the currently selected newsgroup is invalid, a 412 response MUST be returned. In all three cases, the currently selected newsgroup and current article number MUST NOT be altered.

Indicating capability: READER

If the currently selected newsgroup is valid, the current article number MUST be set to the next article in that newsgroup (that is, the lowest existing article number greater than the current article number). If successful, a response indicating the new current article number and the message-id of that article MUST be returned. No article text is sent in response to this command.

If the current article number is already the last article of the newsgroup, a 421 response MUST be returned. In all other aspects (apart, of course, from the lack of 422 response), this command is identical to the LAST command (Section 6.1.3).

Indicating capability: READER

The ARTICLE command selects an article according to the arguments and presents the entire article (that is, the headers and the body) to the client as a multi-line data block following the 220 response code.

If the argument is a message-id and no such article exists, a 430 response MUST be returned. If the argument is a number or is omitted and the currently selected newsgroup is invalid, a 412 response MUST be returned. If the argument is a number and that article does not exist in the currently selected newsgroup, a 423 response MUST be returned. If the argument is omitted and the current article number is invalid, a 420 response MUST be returned.

This command is mandatory.

The HEAD command behaves identically to the ARTICLE command except that, if the article exists, the response code is 221 instead of 220 and only the headers are presented (the empty line separating the headers and body MUST NOT be included).

Indicating capability: READER

The BODY command behaves identically to the ARTICLE command except that, if the article exists, the response code is 222 instead of 220 and only the body is presented (the empty line separating the headers and body MUST NOT be included).

This command is mandatory.

The STAT command behaves identically to the ARTICLE command except that, if the article exists, it is NOT presented to the client and the response code is 223 instead of 220. Note that the response is NOT multi-line.

This command allows the client to determine whether an article exists and, in the second and third forms, what its message-id is, without having to process an arbitrary amount of text.

Indicating capability: READER

This command returns a list of newsgroups created on the server since the specified date and time. The results are in the same format as the LIST ACTIVE command (see Section 7.6.3). However, they MAY include groups not available on the server (and so not returned by LIST ACTIVE) and MAY omit groups for which the creation date is not available.

Note that an empty list is a possible valid response and indicates that there are no new newsgroups since that date-time.

Indicating capability: NEWNEWS

This command returns a list of message-ids of articles posted or received on the server, in the newsgroups whose names match the wildmat, since the specified date and time. One message-id is sent on each line; the order of the response has no specific significance and may vary from response to response in the same session. A message-id MAY appear more than once; if it does, it has the same meaning as if it appeared only once.

Date and time are in the same format as the NEWGROUPS command.

Note that an empty list is a possible valid response and indicates that there is currently no new news in the relevant groups.

Clients SHOULD make all queries in Coordinated Universal Time (i.e., by using the "GMT" argument) when possible.