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 your client computer to the server computer through the telephone line. 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 your 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. In this section we discuss usage convenctions we all must be agree with, in order to achieve a practical and secure interchange system. To establish a dial-up connection to the server computer you need to install and configure a Modem device in your client computer. Each operating system has its own way of doing this, but if you are using &TCD;, you can use the wvdialconf and system-config-network commands, as described in . In the configuration process you need to enter the following information: ISP Name: server.example.com ISP Phone: +53043515094 Username: client.example.com Password: mail4u 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. Using the information described in as reference, the lifetime of dial-up connections will be 15 minutes from the moment they were established on. Likewise, once the connection has been established, if the link is idle for 1 minute, the server computer will close the connection to free the telephone line for others to use. The number of consecutive connections realized from the same telephone number in a fixed period of time must be also controlled in order to reduce Denial of Service (DoS) attacks. This way, you can consider an environment where: more than 3 consecutive connections (that last 15 or less minutes each) from the same telephone number in a time range of 60 minutes will be taken as a DoS attack from the client computer. In such cases, once the client computer is disconnected from server computer, the telephone number originating the call won't be able to establish any further connection to the server computer in the next 15 minutes since the last it was disconnected on. In order to achieve an acceptable degree of efficiency when controlling consecutive connections from the same telephone number, it is required that both the client's telephone number and connection times (e.g., when the connection was opened, and when it was closed) be registered somehow in the server computer (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. Another issue to consider here is that, in order to realize any control over incoming telephone calls, it is required that the client computer realizes a telepohne call into the server computer to provide the telephone number information and that certainly occupies the telephone line until the access control actions take place. This could be used by evil users to generate DoS attacks (e.g., by configuring a client computer to redial the server computer telephone number forever), since there is no way to control access at a Modem level without occupying the telephone line for a few seconds at least. The only change legitimate users have against such evil users' attacks would be establish connection before them (e.g., in the exact range of time between disconnection and redial). In order for a you to use any service provided by the server computer it is required that you get registered a user profile first. 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 example.com, the URL of the web application would be: . 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 . 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 list of instructions that will guide you through the self-registration process. Other actions like updating or deleting your user profile can be also achieved from this web interface. 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. 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. More information about the web interface you need to use to manage your user profile inside the server computer can be found in . 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 Google or Wikipedia either. For this to happen, it is required an established connection between the server computer we are configuring and the Internet network we want those services in, but such established connection isn't possible in the current environment. The implementation of services that required persistent connections (e.g., chats) will not be considered as a practical offer inside the server computer. Instead, only asynchronous services (e.g., e-mail) will be supported. This restriction is required to reduce the amount 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. 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 prefered than IMAP. However both are made available. 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 get 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 about 5 days hoping you free the space in your mailbox to deliver them. If you don't free space within this period of time, e-mail messages sent to you will be bounced back to their senders. Otherwise, if you use 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. Assuming you are providing a public service, it is required to limit the maximum number of users registered 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 which requires the web application to know how much free space is available before proceeding to register new mail accounts inside the server computer; this, to prevent user registrations when there isn't enough free space to perform a new user registration. Considering the computer server has confined 5GB of disk space to handle the mail service (e.g., mail queues, mailboxes, etc.), if we set 10MB for each user account, it will be possible to provide self-registration through the web interface for 500 users in total. Another measure related to disk space saving 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 back to the mail service inside the server computer.