The unique fully-qualified domain name
    This document will introduce you to the Message-ID and FQDN difficulties
    and help you out.

What is this fully-qualified domain name or FQDN?
    The unique fully-qualified domain name (or host name, FQDN or FQHN for
    short) is used by leafnode to generate the Message-ID.

    What is the Message-ID?

    Every message (mail or news) has a unique piece of text in its header,
    the so-called Message-ID. It is not usually visible in your software
    unless you choose "display all headers" or "display raw" or "display
    source".

    The Message-ID looks roughly like a strange E-Mail address,
    <12345.67890@host.example.com>: it has two parts. Left of the @ is the
    "local" part, right of the "@" is the domain part. The local part of the
    Message-ID is different for different pieces of mail or news, while the
    domain part of the mail address is constant. The domain part is usually
    the name of the host that generated the Message-ID: your computer's name
    when you are sending mail or posting news.

    Whenever a news server is offered a new news article for redistribution,
    it looks at the Message-ID to determine if it already has the article,
    to avoid double work, and to avoid that new articles run in circles.

    Therefore, each message sent, mail or news, must have a unique
    Message-ID. If the Message-ID is not unique, because you use the same
    host name as somebody else, and you and the other person write an
    article at the same time, the article that arrives later will be
    considered a *duplicate* or *Message-ID collision* and be discarded by
    the server - and as messages take a few seconds from one end of the
    world to another, two servers at different ends of the net may see the
    messages arrive in reverse order.

    Leafnode will tell you that the Message-ID of the article it is about to
    post is already in use upstream.

    Please note that this problem is not leafnode-specific. Any mail or news
    software may suffer from collisions, but it's less prominent in mail,
    and it's difficult to see at all, because the messages disappear
    silently, no message will be coming back to tell you another message was
    lost.

How to get a unique fully-qualified domain name
    There are several ways to obtain one. When you got yours, see below on
    how to configure your domain name.

    If you have a domain registered:
        Assume you are the rightful owner of example.com. You can now
        reserve any sub domain you wish, say mid.example.com, and a host
        name for your leafnode computer, say, abacus.mid.example.com, or for
        a friend. How you track that only one machine has the same name at
        the same time, is up to you. Writing *gave abacus.mid.example.com to
        Joe at 2002-07-11* on a sheet of paper is sufficient - if you can
        find this sheet later on.

        Again: This host name need not be entered into your DNS data base,
        just make sure only one computer uses this name at the same time.

    If you have an account at "news.individual.de" or "news.individual.net":
        You have been assigned a user ID. To find it out, type in your
        shell:

         telnet news.individual.de 119

        (wait until connected)

         authinfo user sixpack
         authinfo pass joe
         quit

        Replace "sixpack" and "joe" by your login and password. After the
        "authinfo pass" line, you should see a line that reads:

         281 Authentication accepted. (UID=00000)

        If you get a 481 line, please retry, you may have mistyped user name
        or password. Correcting these lines with Backspace or Delete may
        also lead to failed logins. Retry with careful typing so that you do
        not need to correct your input.

        The server would have printed your user ID where my example shows
        00000.

        Your hostname then is ID-00000.user.uni-berlin.de. DO MAKE SURE TO
        REPLACE THE NUMBER IN ID-00000 by the number the server told you in
        the UID= LINE.

    Specific providers:

        T-Online
            T-Online customers, your hostname is NNNNN.dialin.t-online.de,
            where NNNNN is your T-Online number. If your T-Online number
            contains your telephone number, contact T-Online support to have
            a new T-Online number assigned. I'm unaware if they charge you
            for this change.

    Ask your network administrator or your Internet service provider.
        Your local network administrator can assign you a domain to use for
        Message-IDs.

        Your Internet service provider may have reserved a special sub
        domain for the sole purpose of letting users create their own unique
        Message-IDs.

  When leafnode ignores your host name
    Well, it is probably the default name or domain that your OS vendor
    chose, like "localhost.localdomain". As such, it is not unique, but used
    on many computers, and can therefore cause collisions and in severe
    cases make your articles disappear.

  Why localhost.ANYTHING will not work
    Many sites run resolvers that are based on ISC Bind code. And many sites
    configure their name servers so that they will resolve
    localhost.example.com. Therefore, localhost.example.com will not
    designate a single computer, but any computer that has "localhost" as a
    name. These resolvers are problematic because they will first see the
    domain as unqualified and append the domain or searchlist, so assuming
    that your domain is example.com, these resolvers will try
    localhost.example.com first, which will resolve to 127.0.0.1 at many
    sites.

    (It is usually a mistake to add localhost to the name server for a
    domain, the clients had better be fixed instead. As a workaround,
    removing all domain and searchlist lines from "/etc/resolv.conf" will
    usually work at the expense of not being able to use short names unless
    they are listed in "/etc/hosts".)

How to configure the fully-qualified domain name
  System-wide
    Preferably, the host name is entered into your system configuration so
    that it is available globally, to your mailers and news readers should
    they desire to create a FQDN.

    How exactly the hostname is configured, depends on your system, it is
    usually a two-step approach, but your system installation/configuration
    software may simplify things for you.

    1. Configure the bare hostname. Every system has its own favorite
    configurator, but you'll find this in the index. SuSE Linux 8.0 and
    newer read the hostname from the file "/etc/HOSTNAME". FreeBSD 4 and
    newer reads it from the "hostname" variable in "/etc/rc.conf".

    2. On many systems, you will have to put the fully qualified host name
    into "/etc/hosts", too, so the system can find out the full name with
    domain if given just its bare name without a domain. (On networked
    systems, using NIS, DNS or LDAP is also feasible if the client is
    configured to use the respective system to resolve host names.) Usually,
    a computer that is to resolve a hostname will look at "/etc/hosts" first
    and then at DNS.

    An "/etc/hosts" line might look like this:

     192.168.0.1 abacus.mid.example.com abacus oldname

    Keep the original name of the computer as an alias in case you
    configured some other software to use the old name.

  Local to leafnode
    You can also write a line like

     hostname = abacus.mid.example.com

    into your "/etc/leafnode/config". But I recommend against doing that,
    see the next section why:

  Should I configure the FQDN system-wide or local to leafnode?
    You should configure the FQDN system-wide. Your news reader may generate
    a Message-ID itself, and it is not aware of leafnode's configuration and
    will generate an invalid Message-ID -- leafnode will then reject the
    posting because the Message-ID is invalid.

Author, Copyright
    This document was written and is (C) Copyright 2002 - 2004, 2006, 2008
    by Matthias Andree.

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