Olaf Kirch - Linux Network Administrator Guide, Second Edition

NNTP also provides for an active and a passive way to transfer news, colloquially called "pushing" and "pulling." Pushing is basically the same as the ihave/sendme protocol used by C News (described in Chapter 21, C News). The client offers an article to the server through the IHAVE msgid command, and the server returns a response code that indicates whether it already has the article or if it wants it. If the server wants the article, the client sends the article, terminated by a single dot on a separate line.

Pushing news has the single disadvantage that it places a heavy load on the server system, since the system has to search its history database for every single article.

The opposite technique is pulling news, in which the client requests a list of all (available) articles from a group that have arrived after a specified date. This query is performed by the NEWNEWS command. From the returned list of message IDs, the client selects those articles it does not yet have, using the ARTICLE command for each of them in turn.

Pulling news needs tight control by the server over which groups and distributions it allows a client to request. For example, it has to make sure that no confidential material from newsgroups local to the site is sent to unauthorized clients.

There are also a number of convenience commands for newsreaders that permit them to retrieve the article header and body separately, or even single header lines from a range of articles. This lets you keep all news on a central host, with all users on the (presumably local) network using NNTP-based client programs for reading and posting. This is an alternative to exporting the news directories via NFS, as described in Chapter 21.

An overall problem of NNTP is that it allows a knowledgeable person to insert articles into the news stream with false sender specification. This is called news faking or spoofing . [133] The same problem exists with the Simple Mail Transfer Protocol (SMTP), although most mail transport agents now provide mechanisms to prevent spoofing. An extension to NNTP allows you to require user authentication for certain commands, providing some measure of protection against people abusing your news server in this way.

There are a number of NNTP packages. One of the more widely known is the NNTP daemon, also known as the reference implementation . Originally, it was written by Stan Barber and Phil Lapsley to illustrate the details of RFC-977. As with much of the good software available today, you may find it prepackaged for your Linux distribution, or you can obtain the source and compile it yourself. If you choose to compile it yourself, you will need to be quite familiar with your distribution to ensure you configure all of the file paths correctly.

The nntpd package has a server, two clients for pulling and pushing news, and an inews replacement. They live in a B News environment, but with a little tweaking, they will be happy with C News, too. However, if you plan to use NNTP for more than offering newsreaders access to your news server, the reference implementation is not really an option. We will therefore discuss only the NNTP daemon contained in the nntpd package and leave out the client programs.

If you wish to run a large news site, you should look at the InterNet News package, or INN, that was written by Rich Salz. It provides both NNTP and UUCP-based news transport. News transport is definitely better than nntpd. We discuss INN in detail in Chapter 23, Internet News.

We've mentioned two NNTP commands that are key to how news articles are pushed or pulled between servers. Now we'll look at these in the context of an actual NNTP session to show you how simple the protocol is. For the purposes of our illustration, we'll use a simple telnet client to connect to an INN-based news server at the Virtual Brewery called news.vbrew.com. The server is running a minimal configuration to keep the examples short. We'll look at how to complete the configuration of this server in Chapter 23. In our testing we'll be very careful to generate articles in the junk newsgroup only, to avoid disturbing anyone else.

Connecting to the news server is a simple as opening a TCP connection to its NNTP port. When you are connected, you will be greeted with a welcome banner. One of the first commands you might try is help . The response you get generally depends upon whether the server believes you are a remote NNTP server or a newsreader, as there are different command sets required. You can change your operating mode using the mode command; we'll look at that in a moment:

$ telnet news.vbrew.com nntp

Trying 172.16.1.1...

Connected to localhost.

Escape character is '^]'.

200 news.vbrew.com InterNetNews server INN 1.7.2 08-Dec-1997 ready

help

100 Legal commands

authinfo

help

ihave

check

takethis

list

mode

xmode

quit

head

stat

xbatch

xpath

xreplic

For more information, contact "usenet" at this machine.

.

The responses to NNTP commands always end with a period (.) on a line by itself. The numbers you see in the output listing are response codes and are used by the server to indicate success or failure of a command. The response codes are described in RFC-977; we'll talk about the most important ones as we proceed.

We mentioned the IHAVE command when we talked about pushing news articles onto a news server. Let's now have a look at how the IHAVE command actually works:

ihave ‹123456@gw.vk2ktj.ampr.org›

335

From: terry@gw.vk2ktj.ampr.org

Subject: test message sent with ihave

Newsgroups: junk

Distribution: world

Path: gw.vk2ktj.ampr.org

Date: 26 April 1999

Message-ID: ‹123456@gw.vk2ktj.ampr.org›

Body:

This is a test message sent using the NNTP IHAVE command.

.

235

All NNTP commands are case insensitive, so you may enter them in either upper- or lowercase. The IHAVE command takes one mandatory argument, it being the Message ID of the article that is being pushed. Every news article is assigned a unique message ID when it is created. The IHAVE command provides a way of the NNTP server to say which articles it has when it wants to push articles to another server. The sending server will issue an IHAVE command for each article it wishes to push. If the command response code generated by the receiving NNTP server is in the "3xx" range, the sending NNTP server will transmit the complete article, including it's full header, terminating the article with a period on a line by itself. If the response code was in the "4xx" range, the receiving server has chosen not to accept this article, possibly because it already has it, or because of some problem, such as running out of disk space.

When the article has been transmitted, the receiving serve issues another response code indicating whether the article transmission was successful.