<sect1 id="configurations-ppp-intro">

    <title>Introduction</title>

    <para>

        This chapter describes two computers configuration, one acting

        as server and other as client. The server computer will be

        configured to provide internet services and the client to make

        use of internet services provided by the server computer. The

        connection medium both client and server computer use is the

        telelphone line (i.e., the same medium you use to realize

        phone calls).  In this configuration, both client and server

        computers use special devices named <quote>Modems</quote> to

        transmit data in form of sound across the telephone line. The

        configuration described in this chapter could be a good choise

        when the only communication medium you have access to is the

        telephone system.

    </para>

    <para>

        Even this configuration tries to reduce the lack of

        communication, there are limitations around it that we cannot

        take off, yet. The following list shows what these limitations

        are:

    </para>

    <itemizedlist>

    <listitem>

    <para>

        Only one connection (of 15 minutes) is possible at a time.

    </para>

    </listitem>

    <listitem>

    <para>

        More than 3 consecutive connections from the same phone number

        in a time range of 60 minutes means that that number is

        attacking the ISP to provoke a <quote>Denying of

        Service</quote> (DoS) attacks. In such cases, the phone number

        originating the phone call will be denyed from realizing

        further phone calls onto the ISP in the next 15 minutes. If

        after 15 mintes, 3 new consecutive connections are detected

        from the same phone number than before, the delay time will be

        duplicated on each consecutive interval (e.g., 15*1 for the

        first time, 15*2 for the second time, 15*3 for the third time,

        and so on).

    </para>

    <note>

    <para>

        In order to achieve an acceptable degree of efficiency when

        controlling consecutive connections from the same phone

        number, it is required that both the client's phone number and

        connection time be registered somehow in the server (e.g., Is

        it on pppd's log file?). Without such information it would be

        very difficult to achieve any prevention against DoS attacks

        originated from incoming calls.

    </para>

    </note>

    </listitem>

    <listitem>

    <para>

        The ISP is isolated from Internet, so it is not possible to

        provide Internet access through the ISP. For example, don't

        ever think you will be able to send international e-mail to

        Gmail or Yahoo, nor visit web sites like Google or Wikipedia.

        I really would like to provide such accesses, but without a

        link to Internet I don't have where to send your requests.

    </para>

    </listitem>

    <listitem>

    <para>

        The information generated inside the ISP is jailed to it. This

        way, it will be available to people registered inside the ISP

        only (e.g., through the web interface).

    </para>

    </listitem>

    <listitem>

    <para>

        The implementation of services that required persistent

        connections (e.g., <application>chats</application>) will not

        be considered as a practical offer.  Instead, only

        asynchronous services (e.g.,

        <application>e-mail</application>) will be supported. This

        restriction is required to reduce the connection effective

        times. For example, consider an environment where you connect

        the ISP to send/receive e-mails only and then quickly

        disconnect from ISP to release the line for others to use.

        There is no need for you to be connected at the same time

        someone else sends you an e-mail, this in order for you to

        receive it.  E-mail messages sent to you will be available in

        your mailbox the next time you establish a point-to-point

        connection with the ISP and use your mail client to send and

        receive new messages. Likewise, you don't need to be connected

        to the ISP in order to write your e-mail messages.  You can

        write your messages off-line and then establish connection to

        send it whe it be ready.

    </para>

    </listitem>

    <listitem>

    <para>

        Your user profile will be automatically removed from the ISP

        when no effective point-to-point connection be established by

        you in a period greater than 7 days since the last effective

        point-to-point connection you established to the ISP. When

        your user profile is removed, you will need to get registered

        again (i.e., create a new user profile) using the web

        interface provided by the ISP.  </para>

    </listitem>

    <listitem>

    <para>

        When a user receive messages, the user's e-mail client must be

        configure to move the e-mail messages from server to client.

        This is forced in the ISP computer by denying user's from

        accessing the IMAP service. Only POP service will be

        available. This restriction is required to save disk space on

        ISP computer.

    </para>

    </listitem>

    </itemizedlist>

    <para>

        I'm very sorry about these limitations, but this is the best I

        can offer with one PC, one modem, and one single telephone

        line. If you think this configuration can be improved somehow,

        please send me an e-mail to

        <email>al@projects.centos.org</email>.  Notice that, in order

        for you to be able to send e-mails to this address you need to

        do it using the Mail Transfer Agent provided in the server

        computer. I don't answer phone calls personally, the phone is

        very busy answering point-to-point connections ;).

    </para>

    <warning>

    <para>

        The 

        class="domainname">projects.centos.org</systemitem> mentioned in

        this chapter must not be confused with the real infrastructure

        provided by &TC;; on Internet.  The domain name mentioned in

        this chapter is not available on Internet and was created to

        illustrate the real infrastructure inside an isolated

        environment.

    </para>

    </warning>

    <para>

        In order for you to share information with others, it is

        required that both you and the person you want to share

        information with, have an e-mail address registered inside

        ISP. This registration process is realized through a secured

        web interface accessable through an encrypted connection.  The

        web interface provided should permit everyone to update or

        delete their personal profiles. All actions realized through

        this web interface must be simple enough to be achieved in

        less than 15 minutes (the time you have before the

        point-to-point connection be closed by the ISP).

    </para>

    <para>

        Inside the ISP, user information is stored inside an LDAP

        server. The web application manipulates LDAP records and all

        related files inside the operating system that make possible a

        user to establish a point-to-point connection to the ISP, as

        well as registering, updating or deleting its profile inside

        the ISP.  Care should be taken to prevent one user to

        modify/delete profiles from other users. The user's profile

        administration is individual to each user using the user's

        identity as reference. The user's identity is determined by a

        username (e.g., the e-mail address) and a password.  The LDAP

        server will be available for everyone to consult from their

        mail clients.  Inside the web application, verifications must

        be included to avoid duplicated values, invalid characters and

        similar stuff.

    </para>

    <para>

        Inside the ISP, all related subsystems (e.g., Postix,

        Cyrus-Imapd and Saslauthd) must retrive user information from

        LDAP server.  Likewise, the mailbox administration must be

        automated based on the users in the LDAP server. The web

        application must be able to be aware of all files related

        inside the infrastructure in a way that administration tasks

        can be automated and presented friendly to end users (this

        will required the web application to run some program that

        needs root privileges =:-|). The whole process would be as

        follows:

    </para>

    <orderedlist>

    <listitem>

    <para>

        Establish a point-to-point connection to ISP, as described in

        <xref linkend="configurations-ppp-modem-client" />.

    </para>

    </listitem>

    <listitem>

    <para>

        Register a new user profile through the web application

        provided by the ISP.

    </para>

    </listitem>

    <listitem>

    <para>

        Configure your workstation using the information provided as

        result of a successful registration in order to start using

        the services provided by the ISP you recently get registered

        in. 

    </para>

    </listitem>

    </orderedlist>

</sect1>