Table of contents:
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
users. Thus we have
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
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
You may be able to get older versions from
but these sites are not being updated.
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,
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
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
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
Change for your situation. It is preferable to use the ttyS ports rather
the cua ports or the modem port. The cua ports will disappear in future
of Linux, and the
/dev/modem port hides what you are actually doing and
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 www.linmodems.org--- 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
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.
http://www.o2.net/~gromitkc/winmodem.html for a listing of known
winmodems. For some advice on buying modems for Linux, see
To figure out what is happening while you are trying to log on to
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
We will store those debugging messages in a file called
(You can use any file you wish, including
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
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.
/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
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
The computers on the Internet are referenced by numbers called IP
However, as people are often bad at remembering numbers, names are used instead
axion.physics.ubc.ca instead of 184.108.40.206). The name resolver is a
that asks another computer, the nameserver, to translate Internet names
to IP numbers which your computer can actually use. Once
you are connected, you are able to actually resolve host names,
/etc/resolv.conf and add a line (or lines) like
Instead of 111.222.333.444 use the Domain Name Server IP address (DNS-IPaddress) which your ISP gave you. If they gave you two numbers enter another nameserver line with the second address.
It can also be useful to include a line like domain ISP.domain.name where ISP.domain.name is the domain name of your ISP. For example, my /etc/resolv.conf file looks like
domain physics.ubc.ca nameserver 220.127.116.11 nameserver 18.104.22.168
[Note: Do NOT put leading zeros on any of those numbers. They will not work. 137.082.43.9 is a different number from 22.214.171.124]
Edit /etc/host.conf and make sure the line
order hosts, bind
is in there. Also check /etc/nsswitch.conf and make sure a line like
hosts: files dns
is there. (that line may also contain other options like nis, which will do no harm)
Also make sure that all of these files are readable by everyone.
chmod a+r /etc/resolv.conf /etc/host.conf /etc/nsswitch.conf
Your computer passes messages for addresses it does not know to the default route. ppp assumes that there can only be one default route on any system. This is almost always the route to ISP's computer. However, if pppd finds that a default route already exists, it will not set up one to your ISP, causing problems (nothing gets through). In order that you not have any conflicts between the routing across ppp and onto some network you have, run
(or you can also run
which does the same thing) while not connected via ppp, and look for a default entry. (First entry in line is 0.0.0.0). This almost certainly should not be there. (Unless you already have a hookup to the internet via an ethernet connection, you do not want such a default route.) Unless you know that your computer has an ethernet connection to other computers already, the only thing there should be a route to 127.0.0.1 or 127.0.0.0 If there is such a default route (line starting with 0.0.0.0), and you want the ppp interface to be your default route, (the usual situation), run
/sbin/route del default
Also find out what was setting that wrong default route, and stop it from doing so. (If you cannot figure out what is doing so, put that line
/sbin/route del default
at the end of whichever of /etc/rc.local or /etc/rc.d/rc.local
on your system.)
PPPD module support
Now check to see if the module support for ppp is set up properly. Run
You should get a line of garbage which repeats about 10 times and then the command exits. This is good. Skip to the PPPD Options section.
If you get nothing and the command return to the prompt after a few seconds, this is probably OK as well, but try
where those are backward apostrophes (under the ~ on the US keyboard). This should give you the lines of garbage. If you do get this line of garbage, skip the next section. If you do not get that line of garbage, it is probable that the module support is not set up properly.
If you did not get that line of garbage, but instead get "permission denied", you are not running as root. Log on as root and try again.
If you get a message saying that your kernel does not support ppp, do not recompile the kernel. This error message is a grab bag error message which the pppd program produces almost anytime something goes wrong. What it almost certainly means in the 2.0.x series of kernels is that you are not running kerneld.
To test if kerneld is running, run the command
ps aux|grep kerneld|grep -v grep
It will produce output if kerneld is running. If kerneld is running (or you have the 2.2.x kernels which use kmod instead) and you get this error message, try doing
insmod slhc insmod ppp
and try again. ( these load the modules permanently and you may have to put those lines into the file rc.local (/etc/rc.d/rc.local on RedHat and in various places under /etc on other distributions)
(If none of this works, then there are more severe problems that this note cannot solve.)
Assuming that you find that kerneld is not running, then on a Redhat system, run
or even just
(which should work on any Linux system).
[ If kerneld was not running, you will need to make sure that it is set to run each time you switch on your system. Unfortunately every distribution has a slightly different way of starting up daemons at bootup. In Redhat, also make sure that there are lines like S01kerneld as links to /etc/rc.d/init.d/kerneld in each of the directories /etc/rc.d/rc1.d to /etc/rc.d/rc5.d in RedHat. On other systems, you may need to put a line like
into /etc/rc.local, or /etc/rc.d/rc.local, or ....]
One final item, especially for Redhat systems, make sure that your give your machine a name (any single word name you want). You can set it with
But you also want to change it permanently. In Redhat/Mandrake, put the line
into /etc/sysconfig/network. On other systems I am not sure, but you
could always put the hostname command at the end of your rc.local file.
This is not essential to running ppp, but if you do not do it, you will
find yourself unable to run any X application while running ppp.
The 2.4.x series of kernels.
The kernel and module support has changed significantly for the 2.4.x series of Linux kernels, and require a number of additional actions on your part.
First, you must make sure that you are use at least pppd 2.4.0b2.(Do not be confused with the fact that both the kernel numbering and the ppp version numbering are now almost the same. This is just an accident.) The older 2.3.x series of ppp will not work with the new kernel.
Now you must make sure that you have the device /dev/ppp on your system. Do
ls -lga /dev/ppp
which should show that this file is owned by root, and has only read and write permissions for root. If it does not exist, do
mknod /dev/ppp c 108 0 chmod 600 /dev/ppp
Now, you must also make sure that you have the correct entries in the /etc/modules.conf file. (Note that this file has changed name from conf.modules on the older kernels to modules.conf on the 2.4.x series of kernels.) Enter the lines
alias /dev/ppp ppp_generic alias char-major-108 ppp_generic alias tty-ldisc-3 ppp_async alias tty-ldisc-14 ppp_synctty
into /etc/modules.conf if they are not already there. At this point the the test above (just running /usr/sbin/pppd) should work and produce garbage.
You have now set up the critical files on your system and you know that you have a an operational pppd. You now need to set up the ppp options so that you can get debugging information when you try to connect, and so that it is minimally usable.
/etc/ppp/options so it contains
lock crtscts defaultroute noauth
[note that if you are in the unlikely situation that you already have a working default route to an Ethernet, omit the defaultroute option. If you are not sure, put that option in.]
[Also other options can be used. In general do not do so unless advised to do so. More harm is done by putting in too many un-understood options than by putting in too few. If you have a local Ethernet, and if your ISP demands that they assign your IP address for the PPP connection, then also use
You could also put in a line containing debug here but we will use it explicitly on the command line.]
Bill Unruh works for the Advanced Research Department of the Canadian Institute for Physics and Astronomy.
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