1. Introduction

Most of the companies in USA and Europe provide "Remote Access Servers" (RAS), so that their employees can dial-in from remote locations either from home or from field. RAS servers are generally Linux RAS servers or MS Windows NT RAS servers.

Nowadays, Virtual Private Network (VPN) is becoming quite popular because it supports broadband and may eventually replace dial-up networking which is very slow running at 56KB/sec. See the VPN HOWTO and VPN-Masquerade howto and java based MindTerm-SSH-HOWTO.

Configuring a dial-up connection on Linux is about as simple as it is in Windows. We will use PPP.

Definition: PPP ('Point to Point Protocol') is a protocol, that makes it possible to make IP-Links (Internet Protocol) between two computers with a modem.

The operating system Linux has this protocol in the kernel (If you compiled it into the kernel). For a ppp-link you need to start a daemon called pppd.

With PPP you can make a link between two hosts which understand the protocol. A link between a Linux host, acting as client and a Windows NT server, acting as server, is called a (Microsoft term) 'Remote Access Service' (RAS) link.

First, see if your modem is supported by linux. Internal Modems (WinModems) do not work with linux. Some WinModems require special tweaking to make it work in linux.

1.1 Preparations

Check if you have all the needed hardware, software and data and hand:

• A modem supported by Linux. Some modems, like most internal PCI modems or some external USB modems are so-called WinModems. Which means that they rely on Windows-only software to work. Read Winmodems are not modems for more information and a compability list and check out Linmodems.org for (a few) drivers.

• The modem's init-string. GNU/Linux does not have an equivalent to the Windows hardware information file (.inf) mechanism. You may have to find out the init-string for your modem yourself. It should(TM) be listed in your hardware's documentation or in the modem's INF file, but there are also online sources like this list of modem init-strings . If you have no clue at all, use ATZ, that works with many modems.

• Provider data. You'll need your login name, password and the telephone number. DNS addresses would be fine, too, but are optional for recent versions of pppd.

• Check if your modem is connected correctly to the system or properly recognized by it. With an external modem, that's pretty easy. As 'root' do echo >/dev/ttyS0 and watch the modem LEDs. You should see them flicker for a moment. If not, try echo >/dev/ttyS1 in case your modem is connected to the second serial port. With an internal modem, use HardDrake to check if the system sees the modem. Or use this command to query the PCI list directly: cat /proc/pci. If it's an ISA bus modem, check '/etc/isapnp.conf' if it is listed there. If you don't have that file, you'll have to generate it. Read isapnpfaq.txt in the isapnptools doc directory or man isapnp or man isapnp.conf.