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 Modems 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. 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: Only one connection (of 15 minutes) is possible at a time. 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 Denying of Service (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). 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. 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. 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). The implementation of services that required persistent connections (e.g., chats) will not be considered as a practical offer. Instead, only asynchronous services (e.g., e-mail) 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. 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. 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. 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 al@projects.centos.org. 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 ;). The projects.centos.org mentioned in this chapter must not be confused with the real infrastructure provided by &TCP; 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. 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). 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. 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: Establish a point-to-point connection to ISP, as described in . Register a new user profile through the web application provided by the ISP. 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.