Editing News server (section)

==Types of Servers==
A ''reader server'' provides an interface to read and post articles, generally with the assistance of a [[news client]]. A ''transit server'' exchanges articles with other servers. Most servers can provide both functions.

===Transit server===
Modern transit servers usually use [[Network News Transfer Protocol|NNTP]] to exchange news continually over the [[Internet]] and similar always-on connections. In the past, servers normally employed the [[UUCP]] protocol, which was designed for intermittent dial-up connections.  Other ''ad hoc'' protocols, including [[e-mail]], are less commonly seen.  News servers normally connect with multiple peers, with the redundancy helping to spread loads and ensure that articles are not lost.  Smaller sites, called ''leaf nodes'', are connected to one other major server.<ref name="Administering Usenet" />

Articles are routed based on information found in the header lines defined in RFC 1036.{{citation needed|date=August 2020}}  Of particular interest to a transit server are:

*''[[Message-ID]]'' – a [[globally unique key]]
*''Newsgroups'' – a list of one or more [[newsgroup]]s where the article is intended to appear
*''Distribution'' – (optional) a supplement to Newsgroups, used to restrict circulation of articles.
*''Date'' – the time when the article was created
*''Path'' – a list of the servers an article passed through on its way to the local server
*''Expires'' – (optional) the time when it is requested that the article be deleted
*''Approved'' – (optional) indicates an article that has been accepted for a [[moderated newsgroup]]
*''Control'' – (optional) contains [[Control message|command requests]]

In most cases, the sending server controls the article transfer process.  It compares the Newsgroups and Distribution of each newly arrived article against a set of patterns called ''newsfeeds'', listing each remote server and the newsgroups its operator wishes to receive. Some senders also examine the Path; if the receiving server appears in this line, it is not offered. Other local rules may also be added.  The sender transmits matching articles' Message-IDs to the receiving server.  The receiver indicates which Message-IDs it has not yet stored locally, and those articles are sent.<ref name="Administering Usenet" />

The receiving server examines the incoming articles.  A message is normally discarded if the Message-ID is duplicated by an article already received (i.e., another server sent it in the meantime), the Date or Expires lines indicate that the article is too old, the header syntax appears to be invalid, the Approved header is missing for a moderated newsgroup, or additional local rules disallow it.{{citation needed|date=August 2020}} Most servers also maintain a list of active newsgroups. If the Newsgroups header of a new article does not match the active list, it may be discarded or placed in a special "junk" newsgroup. Once the article is stored, the server attempts to retransmit it to any servers in its own newsfeed list.<ref name="Administering Usenet" />

Articles with Control lines are given special handling.  They are typically filed in special "control" newsgroups and may cause the server to automatically carry out exceptional actions.  The ''<code>newgroup</code>'' and ''<code>rmgroup</code>'' commands can cause newsgroups to be created or removed; ''<code>checkgroups</code>'' can be used to reconcile the local active list with a commonly accepted set; and ''<code>cancel</code>'' commands are used to request the deletion of a specific article.  ''<code>ihave</code>'' and ''<code>sendme</code>'' are sometimes used with UUCP to transmit lists of offered and wanted Message-IDs. Other commands (''<code>version</code>'', ''<code>sendsys</code>'', and ''<code>uuname</code>'') are requests for server configuration details. Once used to create network maps, they now are generally obsolete.<ref name="Administering Usenet" />

===Reader server===
A reader server is one that makes the articles available in the hierarchical [[hard disk|disk]] [[directory (file systems)|directory]] format originated by [[B News]] 2.10, or offers the NNTP or [[Internet Message Access Protocol|IMAP]] commands, for use by newsreaders. A reader server typically also works as a transit server, but it may operate independently or serve as an alternative interface to an [[Internet forum]].  When receiving news, this type of server must perform the additional steps of filing articles into newsgroups and assigning sequential numbers within each group.  An ''Xref'' line is usually added, listing all the groups where the message appears and the sequence numbers.  Unlike Message IDs, the numbers and ordering of articles will differ on each server; but related servers may force agreement by operating in a slave mode, re-using their siblings' Xref lines. Reader servers typically also maintain a [[NOV (computers)|News Overview]] (NOV) database that allows newsreaders to quickly obtain message summaries and present messages in threaded form.<ref name="Administering Usenet" />

Most reader servers support posting, either through NNTP or a special ''inews'' program.{{citation needed|date=August 2020}}  When an article is posted, the process is much the same as when a transit server receives news, but with additional checks. For posting, the server will normally fill in missing Path and Message-ID lines and check the syntax of headers intended for human readers, such as ''From'' and ''Subject''.  If the article is posted to a moderated group, the server will attempt to mail it to the newsgroup moderator if the Approved header is absent. Additional identity checks and filters are also typically applied at this point.<ref name="Administering Usenet" />

===Hybrid or cache server===
Smaller sites with limited network bandwidth may operate "sucking" or [[cache (computing)|cache]] servers.  These perform the same reader server role as conventional news servers, but themselves act as newsreaders to exchange articles with other reader servers.{{citation needed|date=August 2020}}  Hybrid servers allow greater flexibility for the server operator in that received groups can be adjusted without manual intervention by operators.  They may also be the only available means to obtain articles from remote servers that do not offer conventional feeding.

Because hybrid servers usually use the posting function to send news, article headers are reformatted by the posting function and tracing information can be lost. Also, the delayed sucking process can result in excess activity on the remote reader servers. For these reasons, the use of hybrid servers is often discouraged or disallowed without prior agreement.<ref name="Administering Usenet" />