<sect1 id="configurations-dialup-usage">
<title>Usage Convenctions</title>
<para>
The infrastructure described in this chapter uses the
client/server model to provide a public mail service through
the telephone line. In this configuration, we (the poeple
building the infrastructure) provide the information you (the
person using the infrastructure) need to know in order to
establish a point-to-point connection from the client computer
to the server computer through the telephone line. </para>
<para>
The infrastructure described in this chapter is made available
to you free of charge, however, you should know that
maintaining it costs both money and time. For example, for
each hour the server computer is on production there is an
electrical consume that need to be paid every month.
Likewise, each call that you establish from the client
computer to the server computer will cost you money, based on
the location you made the call from and the time you spend
connected.
</para>
<para>
In this section we discuss usage convenctions we all must be
agree with, in order to achieve a practical and secure
interchange system.
</para>
<sect2 id="configurations-dialup-usage-connlimits">
<title>Administering Dial-Up Connections</title>
<para>
The lifetime of dial-up connections must be limitted based on
the number of users you expect to establish connection and the
kind of services you plan to provide. The mail service
provided by the server computer is conceived as a public
service so anyone with a modem attached to a computer would be
able to have access to it. However, due to hardware
limitations, only 100 users will be allowed to be registered
in the public mail service. Based on this information, the
lifetime of established connections will be of 15 minutes from
the established moment on. Once the connection has been
established, if the link is idle for 1 minute, the server
computer will close the established connection to free the
telephone line. This control can be implemented through the
<option>maxconnect</option> and <option>idle</option> options
inside the <application>pppd</application>'s configuration
file.
</para>
<para>
Only registered user profiles will be able to establish connections
to the server computer. This control can be implemented using
the <option>allow-number</option> option in the
<application>pppd</application>'s configuration file to define a
list of all telephone numbers that are allowed to establish
connection with the server computer, based on the list of
registered user profiles. By default, all telephone numbers
are denied from establishing access with the server computer,
except those ones explicitly set by
<option>allow-number</option> option. If the
<option>allow-number</option> option is not present in
<application>pppd</application>'s configuration file, all
telephone numbers are allowed to establish connection with the
server computer, so be sure to include the
<option>allow-number</option> option in
<application>pppd</application>'s configuration file if you
want to control who can/cannot establish connection with the
server computer.
</para>
<screen>
##### centos-pppd-config will overwrite this part!!! (begin) #####
allow-number 12345
allow-number 21345
allow-number 34567
##### centos-pppd-config will overwrite this part!!! (end) #####
</screen>
<para>
The <application>centos-pppd-admin</application> application
must be considered part of user profile registration process
inside the server computer. The
<application>centos-pppd-admin</application> application would
be used to control the list of allowed telephone numbers
inside the <application>pppd</application>'s configuration
file, based on the list of user profiles. The
<application>centos-pppd-admin</application> application
should be executed after any registration/deletion action
against the list of user profiles with <systemitem
class="username">root</systemitem> privilages in order to be
able of writing the settings on
<application>pppd</application>'s configuration file.
</para>
<para>
Redialing consecutive connections from the same telephone
number without any dealy between call retries must be avoided
from client computers. This would reduce the possibilities for
other client computers to establish connection with the server
computer. To prevent this issue from happening, it would be
necessary to provide more telephone lines than users
authorized to establish connection with the server computer.
Nevertheless, there is only one telephone line available for
the server computer to use.
</para>
</sect2>
<sect2 id="configurations-dialup-usage-users">
<title>Administering User Profiles</title>
<para>
In order for you to use any service provided by the server
computer it is required that you register yourself inside the
server computer creating a user profile. The user profile
provides the user information required by services inside the
server computer (e.g., username, password, e-mail address,
telephone number, etc.). To register new user profiles, you
need to use the web application provided by the server
computer. For example, assuming the domain name of the server
computer is <systemitem
class="domainname">example.com</systemitem>, the web
application would be accessable through the following URL:
<ulink url="https://example.com/people/?action=register" />.
</para>
<para>
To reach the web interface, the first thing you need to do is
establishing a dial-up connection to the server computer as
described in <xref
linkend="configurations-dialup-client-config-conn"/>. Once the
dial-up connection has been established, you need to open a
web browser (e.g., Firefox) and put the URL mentioned above in
the address space, and press Enter to go. This will present
you a web page with the instructions you need to follow in
order to register your user profile. Other actions like
updating or deleting your own user profile should be also
possible from this web interface.
</para>
<important>
<para>
The web interface used to manage user profiles inside the
server computer must be presented over an encrypted session in
order to protect all the information passing through.
</para>
</important>
<para>
Inside the server computer, all related subsystems in need of
user information (e.g., Postix, Cyrus-Imapd and Saslauthd)
retrive user information from one single (LDAP) source. The
web application provided by the server computer manages all
these subsystems' configuration files in order to provide a
pleasant experience for end users. The web interface must be
as simple as possible in order to achieve all administration
tasks in the range of time permitted by the server computer
before it closes the connection established from the client
computer.
</para>
<para>
More information about the web interface you need to use to
manage your user profile inside the server computer can be
found in <xref linkend="administration-mail" />.
</para>
</sect2>
<sect2 id="configurations-dialup-usage-scope">
<title>Administering Services</title>
<para>
The information generated inside the server computer is
isolated from Internet. This way, any information generated
inside the server computer will be available only to people
registered inside the server computer. For example, don't ever
expect to send/receive e-mails to/from Internet e-mail
accounts like Gmail or Yahoo, nor visiting web sites like
<ulink url="http://www.google.com/">Google</ulink> or <ulink
url="http://www.wikipedia.org/">Wikipedia</ulink> either. For
this to happen, it is required an established connection
between the server computer you are establishing connection
through and the Internet network those services are available
in. Without that link, it is not possible to direct your
requests to those sites.
</para>
<para>
The implementation of services that required persistent
connections (e.g., <application>chats</application>) will not
be considered as a practical offer inside the server computer.
Instead, only asynchronous services (e.g.,
<application>e-mail</application>) will be supported. This
restriction is required to reduce the amount of time demanded
by services. For example, consider an environment where you
connect to the server computer for sending/receiving e-mails
messages and then quickly disconnect from it to free the
telephone line for others to use. In this environment, there
is no need for you and other person to be both connected at
the same time to send/receive e-mail messages to/from each
other. The e-mails sent from other person to you will be
available in your mailbox the next time you get connected to
the server computer and use your e-mail client to send/receive
e-mail messages. Likewise, you don't need to be connected to
the server computer in order to write your e-mail messages.
You can write down your messages off-line and then establish
connection once you've finished writing, just to send them
out and receive new messages that could have been probably
sent to you.
</para>
<para>
Another issue related to e-mail exchange is the protocol used
to receive messages. Presently, there are two popular ways to
do this, one is through IMAP and another through POP3. When
you use IMAP protocol, e-mail messages are retained in the
server computer and aren't downloaded to client computer.
Otherwise, when you use POP3 protocol, e-mail messages are
downloaded to the client computer and removed from server
computer. Based on the resources we have and the kind of link
used by the client computer to connect the server computer,
using POP3 is rather prefered than IMAP. However both are made
available.
</para>
<para>
Assuming you use IMAP protocol to read your mailbox, be aware
that you need to be connected to the server computer. Once
the connection is lost you won't be able to read your messages
(unless your e-mail client possesses a feature that let you
reading messages off-line). Morover, you run the risk of
getting your mailbox out of space. If your mailbox gets out of
space, new messages sent to you will not be deliver to your
mailbox. Instead, they will be deferred for a period of time
(e.g., about 5 days when using
<application>Postfix</application> defaults) hoping you to
free the space in your mailbox to deliver them. If you don't
free space within this period of time, the deferred e-mails
will be bounced back to their senders and you will never see
them. On the other hand, assuming you are using POP3 protocol
to read your mailbox, you always keep your mailbox free to
receive new e-mails messages and keep them for you until the
next time you establish connection with the server computer
and download them to your client computer using your e-mail
client.
</para>
</sect2>
<sect2 id="configurations-dialup-usage-diskspace">
<title>Administering Disk Space</title>
<para>
The maximum number of registered user profiles is limited
inside the server computer, based on the maximum disk space
the server computer confines to such purpose. For example,
consider an environment where users can get registered
themselves using a web interface. In this case the web
interface must know how much disk space is available before
proceeding to register new mail accounts inside the server
computer and this way preventing any disk writing when there
isn't enough free space on disk to perform a new user
registration. Considering the server computer has confined
1GB of disk space to handle the mail service (e.g., mail
queues, mailboxes, etc.) and each user mailbox is 10MB, it
will be possible to provide self-registration through the web
interface for 100 users in total.
</para>
<para>
Another measure related to save disk space might be to remove
unused user accounts and their related files (e.g., mailboxes)
from the server computer. For example, consider an environment
where user accounts are automatically removed from the server
computer when they don't establish a connection with the
server computer in a period greater than 7 days since the last
valid connection established to the server computer. Once the
user account is removed, it is no longer functional of course,
and the person whom lost the account will need to create a new
one, assuming it want to have access to the mail service
again.
</para>
</sect2>
</sect1>