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Initial PPP Setup

by Bill Unruh

Table of contents:

No default route
PPPD module support
PPP options

The key problem in hooking up a PPP link to an ISP is that the ISPs seem to compete with each other as to who can find another obscure way of authenticating users. Thus we have login, PAP, CHAP 05, CHAP80, CHAP80-lanman, CHAP81, and combinations of these. The chief difficulty of connecting to an ISP is discovering which technique is actually being used by the ISP in an orderly way. Since few of them know anything about Linux, and since few of them even understand what technique they actually use, this procedure should allow you to set up without their help, and to understand what the ISP's help means if it is given.

The following has a number of steps to connecting to your ISP. The temptation is to skip steps. This is a bad idea, as it will almost certainly lead to grief. Do each step and you will succeed, and you will also learn something about your system in the process.

These instructions were developed with the 2.0.x versions of the Linux kernels, and also work with the 2.2.x series of kernels. Similarly, they work with versions of pppd at least from version 2.3.3 on (up to 2.4.1 by now). In fact, some of the features below only work with later versions of pppd. I would urge you to upgrade to the latest version. You can get the latest pppd from You may be able to get older versions from or are/ppp/ but these sites are not being updated.

Initial setup

These steps are essentially independent of your ISP. Many ISPs do not know their own systems, and their advice must often be taken with a grain of salt. Thus, the document below shows you how to find out what your ISP wants, independent of what your ISP says he wants. However, you cannot do the first step without your ISP. You must get a user name and a password from your ISP -- which usually means that you must open an account with them and pay money. It is worth asking them to make sure that they tell you exactly what form that user name must take when you sign on. For example, some of them demand that you use your full email address with them as your user name, rather than simply the name itself. Others demand additions to the bare user name. Try to get them to be as specific as possible about exactly what you need to use.

You also need to get the ISP's Domain Name Server (DNS-IP) address. (This is an address in Internet Protocol [IP] format, which is four numbers each less than 256 separated by dots [for example,].) Often they will give you more than one. Occasionally an ISP will refuse. In that case, they may supply the DNS dynamically instead. Using PPP version greater than 2.3.7 will allow you still to sign on with them as described below.

Having that information, log on as root, or, after you have logged on as a user, run su and enter your root password at the prompt.


Make sure that you know which port your modem is on. The two programs at modem-chk.html can help you to determine which port your modem is on. COM1 (serial port 1) is /dev/ttyS0. COM2 is /dev/ttyS1, etc. In the following, I will assume /dev/ttyS1. Change for your situation. It is preferable to use the ttyS ports rather than the cua ports or the modem port. The cua ports will disappear in future releases of Linux, and the /dev/modem port hides what you are actually doing and can lead to conflicts with other programs which use the serial ports.

[Note that if your modem is a winmodem, it will not work as-is under Linux. Some winmodems now have drivers for Linux. See It is probably a winmodem if it is a PCI modem, less so if it is an ISA and improbable if it is an external modem, and also improbably if it is not a 56-Kbps modem.

If you have the latest kernels with USB support (Kernels 2.2.17 with the USB patches or 2.4.x) you may also be able to use USB modems. (They must comply with the ACM standard.) See the file Documentation/usb/acm.txt included with the Linux source (/usr/src/linux* if included on your system, or here is a copy from kernel 2.2.17). Note that for USB modems, the correct port to use is not the serial port ttyS? series but probably the /dev/usb/acm/? series of ports, where ? is a USB number. I have never used a USB modem, so can provide no guidance on setting them up correctly.

See for a listing of known winmodems. For some advice on buying modems for Linux, see]

Message Logging

To figure out what is happening while you are trying to log on to your ISP, you must turn on debugging and log the debug messages to a file. The key programs are pppd, and a program it uses, called chat. We will store those debugging messages in a file called /var/log/ppp. (You can use any file you wish, including /var/log/messages. Some distributions are now installing a directory called /var/log/ppp (for example, the Corel distribution does this). If yours does, you must use a different name, such as /var/log/ppplog. Use that name instead everywhere this document talks about /var/log/ppp.)

To make your system record the debugging information from both these programs, do the following

cp /etc/syslog.conf /etc/syslog.conf.orig

to make sure you have a copy of a good version in case you mess up the next command. If you have trouble, copy back the original version.

Edit /etc/syslog.conf and add the lines

local2.*                                /var/log/ppp
daemon.*                                /var/log/ppp

(Some versions of syslogd (especially SunOS) demand that those be tabs -- not spaces -- between the two parts on each line. Some editors refuse to insert tabs and convert them to spaces. The syslogd shipped with most recent distributions of Linux do not seem to care, but be warned.)

Then, to tell syslogd to actually log the information, do

killall -1 syslogd

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