2.1. Clients and Servers

PPP is strictly a peer to peer protocol; there is (technically) no difference between the machine that dials in and the machine that is dialed into. However, for clarity's sake, it is useful to think in terms of servers and clients.

When you dial into a site to establish a PPP connection, you are a client. The machine to which you connect is the server.

When you are setting up a Linux box to receive and handle dial-in PPP connections, you are setting up a PPP server.

Any Linux PC can be both a PPP server and client - even simultaneously if you have more than one serial port (and modem if necessary). As stated above, there is no real difference between clients and servers as far as PPP is concerned, once the connection is made.

This document refers to the machine that initiates the call (that dials in) as the CLIENT, whilst the machine that answers the telephone, checks the authentication of the dial in request (using user names, passwords and possibly other mechanisms) is referred to as the SERVER.

The use of PPP as a client to link one or more machines at a location into the Internet is, probably, the one in which most people are interested - that is using their Linux PC as a client.

The procedure described in this document will allow you to establish and automate your Internet connection.

This document will also give you guidance in setting up your Linux PC as a PPP server and in linking two LANs together (with full routing) using PPP (this is frequently characterized as establishing a WAN - wide area network - link).

2.2. Differences between Linux distributions

There are many different Linux distributions and they all have their own idiosyncrasies and ways of doing things.

In particular, there are two different ways a Linux (and Unix) computer actually starts up, configures its interfaces, and so forth.

These are BSD system initialization and System V system initialization. If you dip into some of the Unix news groups, you will find occasional religious wars between proponents of these two systems. If that sort of thing amuses you, have fun burning bandwidth and join in!

Possibly the most widely used distributions are

• Slackware which uses BSD style system initialization

• Red Hat (and its former associate Caldera) which use SysV system initialization (although in a slightly modified form)

• Debian which uses SysV system initialization

BSD style initialization typically keeps its initialization files in /etc/... and these files are:-

/etc/rc /etc/rc.local /etc/rc.serial (and possibly other files)

Of recent times, some BSD system initialization schemes use a /etc/rc.d... directory to hold the start up file rather than putting everything into /etc.

System V initialization keeps its initialization files in directories under /etc/... or /etc/rc.d/... and a number of subdirectories under there:-

drwxr-xr-x 2 root root 1024 Jul 6 15:12 init.d -rwxr-xr-x 1 root root 1776 Feb 9 05:01 rc -rwxr-xr-x 1 root root 820 Jan 2 1996 rc.local -rwxr-xr-x 1 root root 2567 Jul 5 20:30 rc.sysinit drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc0.d drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc1.d drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc2.d drwxr-xr-x 2 root root 1024 Jul 18 18:07 rc3.d drwxr-xr-x 2 root root 1024 May 27 1995 rc4.d drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc5.d drwxr-xr-x 2 root root 1024 Jul 6 15:12 rc6.d

If you are trying to track down where your Ethernet interface and associated network routes are actually configured, you will need to track through these files to actually find where the commands are that do this.

2.3. PPP configuration tools

There are many good tools available for configuring and using PPP on Linux. This document will try to cover some of the more popular ones, including:

• kppp A PPP setup and dialer tool for KDE.

• WvDial Another PPP driver for Linux, with an emphasis on simplicity.

• RP3...(short for RedHat PPP). It is a simple configuration program with a "wizard" interface- it asks a series of questions.

• Linuxconf A generalized tool for configuring and managing your Linux machine, it has a section that helps configure PPP.

4.1. Setting up a PPP Client

This document provides guidance to people who wish to use Linux and PPP to dial into a PPP server and set up an IP connection using PPP. It assumes that PPP has been compiled and installed on your Linux machine (but does briefly cover reconfiguring/recompiling your kernel to include PPP support).

Whilst DIP (the standard way of creating a SLIP connection) can be used to set up a PPP connection, DIP scripts are generally quite complex. For this reason, this document does NOT cover using DIP to set up a PPP connection.

Instead, this document describes the standard Linux PPP software (chat/pppd).

4.2. Linking two LANs or a LAN to the Internet using PPP

This document provides (basic) information on linking two LANs or a LAN to the Internet using PPP.

4.3. Using intuitive configuration tools to set up PPP

There are many tools used for configuring your PPP connection. This document will try to cover the most common and easiest to use of said programs.

4.4. Setting up a PPP server

This document provides guidance on how to configure your Linux PC as a PPP server (allowing other people to dial into your Linux PC and establish a PPP connection).

You should note that there are a myriad of ways of setting up Linux as a PPP server. This document gives one method - that used by the author to set up several small PPP servers (each of 16 modems).

This method is known to work well. However, it is not necessarily the best method.

4.5. Using PPP over a direct null modem connection

This document provides a brief overview of using PPP to link two Linux PCs via a null modem cable. It is possible to link other OS's to Linux this way as well. To do so, you will need to consult the documentation for the operating system you are interested in.

4.6. This document at present does NOT cover...

• Compiling the PPP daemon software See the documentation that comes with the version of pppd you are using.

• Connecting and configuring a modem to Linux (in detail) See the Serial-HOWTO and for modem specific initialization, see Modem Setup Information for information that may help you to configure your modem.

• Using DIP to make PPP connections. Use chat instead...

• Using socks or IP Masquerade. There are perfectly good documents already covering these two packages.

• Using EQL to gang together two modems into a single PPP link. This is covered in the Networking-HOWTO.

6.1. Useful Linux Mailing Lists

There are many Linux mailing lists that operate as a means of communication between users of many levels of ability. By all means subscribe to those that interest you and contribute your expertise and views.

A word to the wise: some lists are specifically aimed at "high powered" users and/or specific topics. Whilst no-one will complain if you 'lurk' (subscribe but don't post messages), you are likely to earn heated comments (if not outright flames) if you post 'newbie' questions to inappropriate lists.

This is not because guru level users hate new users, but because these lists are there to handle the specific issues at particular levels of difficulty.

By all means join the lists that offer open subscription, but keep your comments relevant to the subject of the list!

A good starting point for Linux mailing lists is Linux Mailing List Directory

7.1. KPPP - The KDE PPP Dialer

http://devel-home.kde.org/~kppp/index.html

KPPP is an easy tool for configuring your PPP links. It comes with the default installation of KDE, and is quite well integrated into that environment.

KPPP also has a good help system built in. If you just right click on the majority of the buttons in the application, a menu will pop up with an item called "Quickhelp". Clicking it will give a quick summary of whatever item you clicked on.

The opening screen of kppp will list any available connections, and allow you to specify a Login ID and password for the connection. There is also a "Show Log Window" option. Selecting this will show a log window. This can be handy if you are trying to debug a connection.

For now, click on the "Setup" button. This will bring up the setup window. It has a series of tabs across the top. Select the one called "Accounts". There will be a list of Accounts, if any.

Select "New" from the right hand side. Another menu will pop up.

First, specify a name for this connection in the "Connection Name" box. A good name would be the name of your ISP. Then enter the phone number. Select the type of authentication. If your ISP hasn't specified otherwise, leave it as "PAP". You can optionally store your password between sessions, but this may be a security risk.

Click on the tab marked "IP". Here you have the option to configure your IP address, either a Dynamic or Static address. You should know what kind you want, specified by your ISP. You can also optionally specify that you wish to configure the hostname for this IP, which means that kppp will name your machine according to the name given to it by your ISP.

Click on the tab marked "DNS". Here you can set up DNS. Fill in the appropriate fields with the information supplied from your ISP.

Click on the "Gateway" tab. Here you can configure your gateway, or router address. If a gateway or router address was specified by your ISP, select "Static Gateway" and enter it in the box below. Otherwise leave this alone. The box marked "Assign the Default Route" should be checked.

The "Login Script" tag is only useful if you specified "Login Script" authentication under the "Dial" tab. If you need to use a Login Script, you can enter it here, as a series of commands. Select the type of command you wish to use, followed by the argument, then click "Add". Click "Remove" to remove a selected command.

The "Accounting" tab allows you to track the number of bytes that you have sent and recieved for a given connection. This can be useful if your ISP tracks the amount of data that you use, and perhaps charges you based on that. If you wish to use accounting, click on the "Enable accounting" box. You can then select a country, if you live in a country that has taxes, tariffs, or other charges based on data transfer. Otherwise, just select either "Bytes in", "Bytes in and out", or "Bytes out". For more information, consult the KPPP manual.

Click OK to finish the creation of this new account.

Now we'll cover the other tabs in the kppp configuration window.

The next one is "Device". It contains configuration information about your modem. You can select the device, flow control, line termination, and connection speed. It is recommended that you leave these alone unless otherwise specified by your ISP. You can also specify values for using a lock file for your modem device(recommended) and a modem timeout. Modem timeout refers to the amount of time that kppp will wait for your modem to respond.

The "Modem" tab is mainly useful for debugging your modem. You can also adjust your modem volume. For more information on these commands, consult the kppp documentation.

Next is the "PPP" tag. It contains many useful items for making kppp tailor fit to your needs. The "pppd Timeout" field specifies how long kppp should wait for pppd to bring up a connection. The "Dock" option states that kppp should recede into the KDE dock when it is connected. The "Automatic redial" specifies that kppp should redial if it is disconnected. The "show clock" option will display how time you have been connected. The "Disconnect on X-server shutdown" means that if you leave KDE, PPP should shut down. The "Quit on Disconnect" option will close kppp if you disconnect from your ISP. Finally, the "Minimize window on connect" option will minimize kppp when it connects.

The "Graph" tab is useful for kppp's throughput graphing. For more information on this feature, see kppp's documentation.

Click Ok to leave this menu. Now ensure your connection is selected in the "Connect to:" box, and click "connect". You're all set!

7.2. WvDial - A command-line pppd driver

The WvDial homepage is here

WvDial is a command-line pppd driver. It has two main components, wvdialconf and wvdial. Both must be run as root.

First you must run wvdialconf. wvdialconf is generates a configuration file containing information on your modem and ISP information. Running wvdialconf will probe your comm ports, looking for a modem, and determine the capabilities of any modems it finds.

wvdialconf is invoked with the name of your wvdial configuration file- which is always /etc/wvdial.conf. Here's what the output should look like.

[root@blah /root]# wvdialconf newconffile
Scanning your serial ports for a modem.

ttyS1<*1>: ATQ0 V1 E1 -- OK
ttyS1<*1>: ATQ0 V1 E1 Z -- OK
ttyS1<*1>: ATQ0 V1 E1 S0=0 -- OK
ttyS1<*1>: ATQ0 V1 E1 S0=0 &C1 -- OK
ttyS1<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 -- OK
ttyS1<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 S11=55 -- OK
ttyS1<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 S11=55 +FCLASS=0 -- OK
ttyS1<*1>: Modem Identifier: ATI -- 28800
ttyS1<*1>: Speed 2400: AT -- OK
ttyS1<*1>: Speed 4800: AT -- OK
ttyS1<*1>: Speed 9600: AT -- OK
ttyS1<*1>: Speed 19200: AT -- OK
ttyS1<*1>: Speed 38400: AT -- OK
ttyS1<*1>: Speed 57600: AT -- OK
ttyS1<*1>: Speed 115200: AT -- OK
ttyS1<*1>: Max speed is 115200; that should be safe.
ttyS1<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 S11=55 +FCLASS=0 -- OK
ttyS0<*1>: ATQ0 V1 E1 -- ATQ0 V1 E1 -- ATQ0 V1 E1 -- nothing.
Port Scan<*1>: S2   S3   

Found a modem on /dev/ttyS1.

If your output looks different, check that your modem is plugged in, turned on, and connected to a com port.

Once you have run wvdialconf, you need to edit the /etc/wvdial.conf file to reflect the phone number to dial, and your username and password. Open up /etc/wvdial.conf in your favorite text editor. It should look something like this:

[Dialer Defaults]
Modem = /dev/ttyS1
Baud = 115200
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 S11=55 +FCLASS=0
; Phone = 555-1212
; Username = my_login_name
; Password = my_login_password

Remove the ';' and fill in the appropriate fields- phone number, login name and password. Now you should be all set to run wvdial.

Run wvdial. Assuming that you filled in your phone number, username, and password correctly, wvdial will now dial your ISP. wvdial will not exit until the connection is terminated- you can do a Ctrl-C to terminate it.

wvdial is fairly new and untested software, so if you run into any problems along the way, you should contact the authors. You can find their contact info on the homepage listed above.

7.3. rp3 - RedHat PPP dialer

A good page about rp3 is located here.

rp3 is available on any RedHat system running RedHat 6.2 or later. It is located on your GNOME menu under Internet- called "Dialup Configuration Tool". It is perhaps the simplest to use of the tools this HOWTO covers. When you start it up, it presents you with a screen that looks like this:

Click "Next". If rp3 can't find a modem, it will try to confiugre one. Select your modem from the list and click next if this happens. You should see a screen asking for an "Account name" and a phone number. Enter your ISP's name and number in the appropriate fields. Click "Next".

rp3 will then ask you for your username and password. Enter them in the appropriate fields and click "Next". Next rp3 will ask you if your ISP is named in a list. If it is, select it and click next. If not, select "Normal ISP" and click next.

That's it! Click "Finish" to complete account creation.

To initiate a connection, go to the GNOME menu again, and look under Internet for "RH PPP Dialer". Click on it. When asked for an interface, select "ppp0". This should bring up a small window. Right click on the window, then on "Connect to ppp0". Wait a few seconds, and you're online! To disconnect, right click on the window again, and then click "Disconnect". That's it.

7.4. Linuxconf - Universal (almost) Linux PPP dialer

Linuxconf homepage

For help at any time while using Linuxconf, just hit the help key. Linuxconf has contextual help menus for most its functionality.

Linuxconf's PPP section is fairly simple. First of all, open up Linuxconf. This must be done as root. Here's a shot of Linuxconf in action:

Click on the box next to "Config", then the box on "Networking", then the box next to "Client tasks". You should see an entry called "PPP/SLIP/PLIP". Click on that. On the right hand side of Linuxconf, you should now see a box called "PPP/Slip/Plip configurations". There should be an area below that, listing existing PPP devices(if any) and three buttons below that.

Click "Add" to add a new connection. It will then ask you to select a type of interface. Make sure "PPP" is selected, then press Accept.

In this box, there are fields for phone number, modem, username and password. Enter the appropriate information into the appropriate fields, and you should be configured.

To bring up the PPP link, open the "Control" tab on the left hand side, then go to "Control Panels", then "Control PPP/SLIP/PLIP Link". You should see a list of PPP devices. Click on the one you just made. A question will pop up, asking you if you wish to activate the connection. Click "Yes".

There are also advanced configuration options, such as modem speed, connect scripts and etc. If you need to configure any of that, go back to the place where you originally configured the link, and click on the link you wish to customize. It should bring up a screen with four tabs: Hardware, Communication, Networking,and PAP.

It is advisable to not change any of these options unless you are specifically told to do so by your ISP or modem vendor. Most of them will work just fine without any additional configuration.

Linuxconf also has command line tools for use in bringing your PPP connection up and down:

• netconf --connect pppsetup [--fore] This will bring the link up. The optional --fore means that it remains in the foreground. Otherwise it will terminate either if the connection is successful or if it fails. It will return 0 if successful, higher if otherwise.

• netconf --dialctl This will display all the available connections and their status.

• netconf --disconnect pppsetup This will disconnect the link.

8.1. I have compiled PPP support into the kernel, but...

A very common problem is that people compile PPP support into the kernel and yet when they try to run pppd, the kernel complains that it does not support ppp! There are a variety of reasons this can occur.

8.2. My modem connects but ppp never starts up

There are innumerable variations on this (take a look in comp.os.linux...).

A VERY common mistake is that you have mistyped something in your scripts. The only thing to do here is to make sure you are logging the chat conversation between you Linux PC and the server into your syslog (/var/log/messages) and then go through this line by line to make sure there are no mistakes.You may need to dial into the ppp server manually to check things out again.

You need to check the log against the actual prompts very carefully - and bear in mind that we humans have a tendency to read what we THINK we have typed - not what is actually there!

8.3. The syslog says "serial line is not 8 bit clean"

There are variations on this too - such as serial line looped back etc., and the cause can be one (or a sequence) of a number of things.

To understand what is going on here, it is necessary to grasp a bit of what is going on behind the scenes in pppd itself.

When pppd starts up, it sends LCP (link control protocol) packets to the remote machine. If it receives a valid response it then goes on to the next stage (using IPCP - IP control protocol packets) and only when this negotiation completes is the actual IP layer started so that you can use the PPP link.

If there is no ppp server operating at the remote end when your PC sends lcp packets, these get reflected by the login process at the far end. As these packets use 8 bits, reflecting them strips the 8th bit (remember, ASCII is a 7 bit code). PPP sees this and complains accordingly.

There are several reasons this reflection can occur.

8.4. Default route not set

If pppd refuses to set up a default route, it is because (quite correctly) it refuses to remove/replace an existing default route.

The usual reason that this error occurs is that some distributions set up a default route via your Ethernet card as opposed to setting up a specific network route.

See the Linux NAG and the Net2/3 HOWTOs for information on correctly setting up your Ethernet card and associated routes.

An alternative to this is that your LAN uses a gateway/router already and your routing table has been set up to point the default route at this.

Fixing up this last situation can require a fair bit of IP networking knowledge and is beyond the scope of this HOWTO. It is suggested you obtain some expert advice (via the news groups of from someone locally you can ask).

8.5. Other Problems

There are many reasons apart from these that ppp fails to connect and/or operate properly.

Look in the PPP FAQ (which is really a series of questions and answers). This is a very comprehensive document and the answers ARE there! From my own (sad) experience, if the answer to your problems is not there, the problem is NOT ppp's fault! In my case I was using an ELF kernel that I had not upgraded to the appropriate kernel modules. I only wasted about 2 days (and most of one night) cursing what had been a perfect PPP server before the light dawned!

9.1. Obtaining/Installing the software

If your Linux distribution does not include the PPP software, you will need to obtain this from the Linux PPP daemon.

Select the latest possible version(2.4.0 at the time of this writing.)

The PPP package contains instructions on how to compile and install the software so this HOWTO does not!

9.2. Compiling PPP support into the kernel

Linux PPP operations come in two parts

• the PPP daemon mentioned above

• kernel support for PPP

Most distributions seem to provide PPP kernel support in their default installation kernels, but others do not.

If at boot your kernel reports messages like

PPP Dynamic channel allocation code copyright 1995 Caldera, Inc. PPP line discipline registered.

your kernel does have PPP support compiled in.

That said, you may want to compile your own kernel, whatever your distribution, to provide the most efficient use of system resources given your particular hardware configuration. It is worth remembering that the kernel cannot be swapped out of memory and so keeping the kernel as small as possible has advantages on a memory limited machine.

This document provides minimal kernel re-compilation instructions at section Chapter 10.

For greater detail, see the Kernel-HOWTO at The Linux Kernel HOWTO

9.3. Obtaining information from your ISP

There are an almost infinite number of ways in which a PPP server can be set up. In order to connect to your ISP (or corporate PPP server to access your intranet), you will need to obtain information on how the PPP server operates.

Because you are using Linux, you may have some difficulty with some ISP help desks (and work site based PPP intranet servers) which know only about MS Windows clients.

However, a rapidly growing number of ISPs use Linux to provide their service - and Linux is also penetrating the corporate environment as well, so you may be lucky if you do strike problems.

Section Chapter 11 tells you what you need to know about the PPP server to which you are going to connect - and how to find out the information you need to know.

9.4. Configuring your modem and serial port

In order to connect to a PPP server and to obtain the best possible data transfer rate, your modem needs to be configured correctly.

Similarly, the serial ports on your modem and computer need to be set up correctly.

Section Chapter 12 provides information on this.

9.5. Setting up Name to Address Resolution (DNS)

In addition to the files that run PPP and perform the automated log in to the PPP server, there are a number of text configuration files that have to be set up for your computer to be able to resolve names like www.interweft.com.au to the IP address that is actually used to contact that computer. These are:-

• /etc/resolv.conf

• /etc/host.conf

Section Chapter 13 for details on setting this up.

In particular, you do NOT need to run a name server on your Linux PC in order to connect to the Internet (although you may wish to). All you need is to know the IP number of at least one name server that you can use (preferably one at your ISPs site).

9.6. PPP and root Privileges

As establishing a PPP link between you Linux computer and another PPP server requires manipulation of network devices (the PPP interface is a network interface) and the kernel routing table, pppd requires root privileges.

For details on this, see section Chapter 14.

9.7. Checking your distribution PPP Files and setting up the PPP Options

There are a number of configuration and dialer files that need to be set up to get PPP operational. There are examples as part of the PPP distribution and this section shows what files you should have:-

/etc/ppp/options /etc/ppp/scripts/ppp-on /etc/ppp/scripts/ppp-on-dialer /etc/ppp/options.tpl

You may need to create some additional files depending on exactly what you are aiming to achieve with PPP:-

/etc/ppp/options.ttyXX /etc/ppp/ip-up /etc/ppp/pap-secrets /etc/ppp/chap-secrets

In addition, the PPP daemon can use a large number of command line options and it is important to use the right ones; so this section takes you through the standard PPP options and helps you choose the options you should use.

For details on this, see Chapter 15.

9.8. If your PPP server uses PAP (Password Authentication Protocol)

Many ISPs and corporate PPP servers use PAP. If your server does not require you to use PAP (if you can log in manually and receive the standard user name/password text based prompts it does not use PAP), you can safely ignore this section.

Instead of logging into such a server using a user name and password when prompted to enter them by the server, a PPP server using PAP does not require a text based login.

The user authentication information instead is exchanged as part of the link control protocol (LCP), which is the first part of establishing a PPP link.

Section Chapter 16 provides information on the files you need to set up to establish a PPP link using PAP.

9.9. Connecting to the PPP server by hand

Having set up the basic files, it is a good idea to test these by connecting (using minicom or seyon) and starting pppd on your Linux PC by hand.

See Section Chapter 17 for full details of setting this up.

9.10. Automating your PPP Connection

Once you are able to log in by hand, you can now move to setting up a set of scripts that will automate the establishment of the connection.

Section Chapter 18 covers setting up the necessary scripts, with considerable attention paid to chat and scripting the login process to the PPP server.

This section discusses scripts for user name/password authentication as well as scripts for PAP/CHAP authenticating servers.

9.11. Shutting down the link

Once your link is up and working, you need to be able to deactivate the link.

This is covered in Section Chapter 20.

9.12. If you have problems

Many people have problems getting PPP to work straight away. The variation in PPP servers and how they require you to set up the connection is enormous. Similarly, there are many options to PPP - and some combinations of these just do not work together, ever.

In addition to the problems of logging in and starting the PPP service, there are problems with the modems and the actual telephone lines as well!

Section Chapter 8 provides some basic information about common errors, how to isolate these and fix them.

This is NOT intended to provide more than just the basics. Al Longyear maintains the PPP-FAQ which contains much more information on this topic!

9.13. After the link comes up

Once a PPP link is operational (specifically, once the IP layer is operational), Linux PPP can automatically run (as the root user), a script to perform any function you can write a script to accomplish.

Section Chapter 25 provides information on the /etc/ppp/ip-up script, the parameters it receives from PPP, and how to use it to do things like, acquire your email from your ISP account, send any queued email waiting transmission on your machine and such.

9.14. Problems with standard IP services on a Dynamic IP number PPP link

As noted in the introduction, dynamic IP numbers affect the ability of your Linux PC to act as a server on the Internet.

Section Chapter 23 provides information on the (main) services affected and what you can do (if anything) to overcome this.

9.15. Maintaining a permanent connection to the net with pppd.

If you are fortunate enough to have a semi permanent connection to the net and would like to have your machine automatically redial your PPP connection if it is lost then here is a simple trick to do so.

Configure PPP such that it can be started by the root user by issuing the command:

# pppd

Be sure that you have the `-detach' option configured in your /etc/ppp/options file. Then, insert the following line into your /etc/inittab file, down with the getty definitions:

pd:23:respawn:/usr/sbin/pppd

This will cause the init program to spawn and monitor the pppd program and automatically restart it if it dies.

10.1. Installing the Linux Kernel source

The following are brief instructions for obtaining and installing the Linux kernel sources. Full information can be obtained from The Linux Kernel HOWTO.

In order to install and compile the Linux kernel, you need to be logged in as root.

1. Change directory to the /usr/src directory cd /usr/src.

2. Check in /usr/src/linux to see if you already have the sources installed.

3. If you don't have the sources, get them from Linux kernel source directory or your nearest mirror. Select the appropriate directory- v2.0 if you are running a 2.0.x kernel, or v2.2 if you are running a 2.2.x kernel. If you are looking for earlier versions of the kernel (such as 1.2.X), these are kept in Old Linux kernel source directory.

4. Choose the appropriate kernel - usually the most recent one available is what you are looking for. Retrieve this and put the source tar file in /usr/src. Note: a 'tar' file is an archive - possibly compressed (as are the Linux kernel source tar files) containing many files in a number of directories. It is the Linux equivalent of a DOS multi-directory zip file.

5. If you already have the Linux sources installed but are upgrading to a new kernel, you must remove the old sources. Use the command rm -rf /usr/src/linux.

6. Now uncompress and extract the sources using the command tar xzf linux-X.X.XX.tar.gz where "X.X.XX" is the version of the kernel you downloaded.

7. Now, cd /usr/src/linux and read the README file. This contains an excellent explanation of how to go about configuring and compiling a new kernel. Read this file, (it's a good idea to print it out and have a copy handy whilst you are compiling until you have done this enough times to know your way around).

10.2. Knowing your hardware

You MUST know what cards/devices you have inside your PC if you are going to recompile your kernel!!! For some devices (such as sound cards) you will also need to know various settings (such as IRQ's, I/O addresses and such).

10.3. Kernel compilation - the Linux 1.2.13 kernel

To start the configuration process, follow the instructions in the README file to properly install the sources. You start the kernel configuration process with...

make config

In order to use PPP, you must configure the kernel to include PPP support (PPP requires BOTH pppd AND kernel support for PPP).

PPP (point-to-point) support (CONFIG_PPP) [n] y

Answer the other make config questions according to the hardware in your PC and the features of the Linux operating system you want. Then continue to follow the README to compile and install your new kernel.

The 1.2.13 kernel creates only 4 PPP devices. For multi- port serial cards, you will need to edit the kernel PPP sources to obtain more ports. (See the README.linux file that comes as part of the PPP-2.1.2 distribution for full details of the simple edits you need to make).

Note: the 1.2.13 configuration dialogue does NOT allow you to go backwards - so if you make a mistake in answering one of the questions in the make config dialogue, exit by typing CTRL C and start again.

10.4. Kernel compilation - the Linux 1.3.x, 2.0.x, and 2.2.x kernels

For Linux 1.3.x, 2.0.x, and 2.2.x you can use a similar process as for Linux 1.2.13. Again, follow the instructions in the README file to properly install the sources. You start the kernel configuration process with...

make config

However, you also have the choice of...

make menuconfig

This provides a menu based configuration system with online help that allows you to move backwards and forwards in the configuration process.

There is also a highly recommended X windows based configuration interface...

make xconfig

You can compile PPP support directly into your kernel or as a loadable module.

If you only use PPP some of the time that your Linux machine is operating, then compiling PPP support as a loadable module is recommended. Using 'kerneld', your kernel will automatically load the module(s) required to provide PPP support when you start your PPP link process. This saves valuable memory space: no part of the kernel can be swapped out of memory, but loadable modules are automatically removed if they are not in use.

To do this, you need to enable loadable module support:-

Enable loadable module support (CONFIG_MODULES) [Y/n/?] y

To add PPP kernel support, answer the following question:-

PPP (point-to-point) support (CONFIG_PPP) [M/n/y/?]

For a PPP loadable module, answer M, otherwise for PPP compiled in as part of the kernel, answer Y.

Unlike kernel 1.2.13, kernel 2.0.x creates PPP devices on the fly as needed and it is not necessary to hack the sources to increase available PPP device numbers at all.

10.5. Note on PPP-2.x and /proc/net/dev

If you are using PPP-2.x, you will find that a side effect of the 'on the fly' creation of the PPP devices is that no devices show up if you look in the /proc/net file system until a device is created by starting up pppd:-

[hartr@archenland hartr]$ cat /proc/net/dev Inter-| Receive | Transmit face |packets errs drop fifo frame|packets errs drop fifo colls carrier lo: 92792 0 0 0 0 92792 0 0 0 0 0 eth0: 621737 13 13 0 23 501621 0 0 0 1309 0

Once you have one (or more) ppp services started, you will see entries such as this (from a ppp server):-

[root@kepler /root]# cat /proc/net/dev Inter-| Receive | Transmit face |packets errs drop fifo frame|packets errs drop fifo colls carrier lo: 428021 0 0 0 0 428021 0 0 0 0 0 eth0:4788257 648 648 319 650 1423836 0 0 0 4623 5 ppp0: 2103 3 3 0 0 2017 0 0 0 0 0 ppp1: 10008 0 0 0 0 8782 0 0 0 0 0 ppp2: 305 0 0 0 0 297 0 0 0 0 0 ppp3: 6720 7 7 0 0 7498 0 0 0 0 0 ppp4: 118231 725 725 0 0 117791 0 0 0 0 0 ppp5: 38915 5 5 0 0 28309 0 0 0 0 0

10.6. General kernel config considerations for PPP

If you are setting up your Linux PC as a PPP server, you must compile in IP forwarding support. This is also necessary if you want to use Linux to link to LANs together or your LAN to the Internet.

If you are linking a LAN to the Internet (or linking together two LANs), you should be concerned about security. Adding support for IP fire walls to the kernel is probably a MUST!

You will also need this if you want to use IP masquerade to connect a LAN that uses any of the above mentioned 'unconnected' IP network numbers.

To enable IP Masquerade and IP fire walling, you MUST answer yes to the first question in the make config process:-

Prompt for development and/or incomplete code/drivers (CONFIG_EXPERIMENTAL)?

Whilst this may sound a bit off-putting to new users, many users are actively using the IP Masquerade and IP fire walling features of the Linux 2.0.x or 2.2.x kernels with no problems.

Once you have installed and rebooted your new kernel, you can start configuring and testing your PPP link(s).

12.1. A note about serial ports and speed capabilities

If you are using a high speed (external) modem (14,400 Baud or above), your serial port needs to be capable of handling the throughput that such a modem is capable of producing, particularly when the modems are compressing the data.

This requires your serial port to use a modern UART (Universal Asynchronous Receiver Transmitter) such as a 16550(A). If you are using an old machine (or old serial card), it is quite possible that your serial port has only an 8250 UART, which will cause you considerable problems when used with a high speed modem.

Use the command...

setserial -a /dev/ttySx

to get Linux to report to you the type of UART you have. If you do not have a 16550A type UART, invest in a new serial card (available for under $50). When you purchase a new card, make sure you can move the IRQs around on it!

Note: the first versions of the 16550 UART chip had an error. This was rapidly discovered and a revision of the chip was released - the 16550A UART. A relatively small number of the faulty chips did however get into circulation. It is unlikely that you will encounter one of these but you should look for a response that says 16550A, particularly on serial cards of some vintage.

12.2. Serial Port Names

Historically, Linux used cuaX devices for dial-out and ttySx devices for dial-in.

The kernel code that required this was changed in kernel version 2.0.x and you should now use ttySx for both dial in and dial out. I understand that the cuaX device names may well disappear in future kernel versions.

12.3. Configuring your modem

You will need to configure your modem correctly for PPP - to do this READ YOUR MODEM MANUAL! Most modems come with a factory default setting that selects the options required for PPP. The minimum configuration specifies:-

• Hardware flow control (RTS/CTS) (&K3 on many Hayes modems)

Other settings (in standard Hayes commands) you should investigate are:

• E1 Command/usr/src/linux-2.0.27/include/linux/serial.h Echo ON (required for chat to operate).

• Q0 Report result codes (required for chat to operate).

• S0=0 Auto Answer OFF (unless you want your modem to answer the phone).

• &C1 Carrier Detect ON only after connect.

• &S0 Data Set Ready (DSR) always ON.

• (depends) Data Terminal Ready.

There is a site offering modem setups for a growing variety of modems, makes and models at Modem setup information which may assist you in this.

It is also worth while investigating how the modem's serial interface between your computer and modem operates. Most modern modems allow you to run the serial interface at a FIXED speed whilst allowing the telephone line interface to change its speed to the highest speed it and the remote modem can both handle.

This is known as split speed operation. If your modem supports this, lock the modem's serial interface to its highest available speed (usually 115,200 baud but maybe 38,400 baud for 14,400 baud modems).

Use your communications software (e.g. minicom or seyon) to find out about your modem configuration and set it to what is required for PPP. Many modems report their current settings in response to AT&V, but you should consult your modem manual.

If you completely mess up the settings, you can return to sanity (usually) by issuing an AT&F - return to factory settings. (For most modem modems I have encountered, the factory settings include all you need for PPP - but you should check).

Once you have worked out the modem setup string required, write it down. You now have a decision: you can store these settings in your modem non-volatile memory so they can be recalled by issuing the appropriate AT command, or you can pass the correct settings to your modem as part of the PPP dialing process.

If you only use your modem from Linux to call into your ISP or corporate server, the simplest set up will have you save your modem configuration in non-volatile RAM.

If on the other hand, you modem is used by other applications and operating systems, it is safest to pass this information to the modem as each call is made so that the modem is guaranteed to be in the correct state for the call. (This has the added advantage also of recording the modem setup string in case the modem looses the contents of its NV-RAM, which can indeed happen).

12.4. Note on Serial Flow Control

When data is traveling on serial communication lines, it can happen that data arrives faster than a computer can handle it (the computer may be busy doing something else - remember, Linux is a multi-user, multi- tasking operating system). In order to ensure that data is not lost (data does not over run in the input buffer and hence get lost), some method of controlling the flow of data is necessary.

There are two ways of doing this on serial lines:-

• Using hardware signals (Clear To Send/Request to Send - CTS/RTS).

• Using software signals (control S and control Q, also known as XON/XOFF).

Whilst the latter may be fine for a terminal (text) link, data on a PPP link uses all 8 bits - and it is quite probable that somewhere in the data there will be data bytes that translate as control S and control Q. So, if a modem is set up to use software flow control, things can rapidly go berserk!

For high speed links using PPP (which uses 8 bits of data) hardware flow control is vital and it is for this reason that you must use hardware flow control.

12.5. Testing your modem for dial out

Now that you have sorted out the serial port and modem settings it is a good idea to make sure that these setting do indeed work by dialing you ISP and seeing if you can connect.

Using you terminal communications package (such as minicom), set up the modem initialisation required for PPP and dial into the PPP server you want to connect to with a PPP session.

(Note: at this stage we are NOT trying to make a PPP connection - just establishing that we have the right phone number and also to find out exactly what the server sends to us in order to get logged in and start PPP).

During this process, either capture (log to a file) the entire login process or carefully (very carefully) write down exactly what prompts the server gives to let you know it is time to enter your user name and password (and any other commands needed to establish the PPP connection).

If your server uses PAP, you should not see a login prompt, but should instead see the (text representation) of the link control protocol (which looks like garbage) starting on your screen.

A few words of warning:-

• some servers are quite intelligent: you can log in using text based user name/passwords OR using PAP. So if your ISP or corporate site uses PAP but you do not see the garbage start up immediately, this may not mean you have done something wrong.

• some servers require you to enter some text initially and then start a standard PAP sequence.

• Some PPP servers are passive - that is they simply sit there sending nothing until the client that is dialing in sends them a valid lcp packet. If the ppp server you are connecting to operates in passive mode, you will never see the garbage!

• Some servers do not start PPP until you press ENTER - so it is worth trying this if you correctly log in and do not see the garbage!

It is worth dialing in at least twice - some servers change their prompts (e.g. with the time!) every time you log in. The two critical prompts your Linux box needs to be able to identify every time you dial in are:-

• the prompt that requests you to enter your user name;

• the prompt that requests you to enter your password;

If you have to issue a command to start PPP on the server, you will also need to find out the prompt the server gives you once you are logged in to tell you that you can now enter the command to start ppp.

If your server automatically starts PPP, once you have logged in, you will start to see garbage on your screen - this is the PPP server sending your machine information to start up and configure the PPP connection.

This should look something like this :-

~y}#.!}!}!} }8}!}$}%U}"}&} } } } }%}& ...}'}"}(}"} .~~y}

(and it just keeps on coming!)

On some systems PPP must be explicitly started on the server. This is usually because the server has been set up to allow PPP logins and shell logins using the same user name/password pair. If this is the case, issue this command once you have logged in. Again, you will see the garbage as the server end of the PPP connection starts up.

If you do not see this immediately after connecting (and logging in and starting the PPP server if required), press Enter to see if this starts the PPP server.

At this point, you can hang up your modem (usually, type +++ quickly and then issue the ATHO command once your modem responds with OK).

If you can't get your modem to work, read your modem manual, the man pages for your communications software and the Serial HOWTO! Once you have this sorted out, carry on as above.

13.1. The /etc/resolv.conf file

Your PPP server sysadmin/user support people should provide you with two DNS IP numbers (only one is necessary - but two gives some redundancy in the event of failure).

As previously mentioned, Linux cannot set its name server IP number in the way that MS Windows 95 does. So you must insist (politely) that your ISP provide you with this information!

Your /etc/resolv.conf should look something like :-

domain your.isp.domain.name nameserver 10.25.0.1 nameserver 10.25.1.2

Edit this file (creating it if necessary) to represent the information that your ISP has provided. It should have ownership and permissions as follows :-

-rw-r--r-- 1 root root 73 Feb 19 01:46 /etc/resolv.conf

If you have already set up a /etc/resolv.conf because you are on a LAN, simply add the IP numbers of the PPP DNS servers to your existing file.

13.2. The /etc/host.conf file

You should also check that your /etc/host.conf file is correctly set up. This should look like

order hosts,bind multi on

This tells the resolver to use information in the host file before it sends queries to the DNS for resolution.

15.1. The supplied options.tpl file

Some distributions of PPP seem to have lost the options.tpl file, so here is the complete file. I suggest that you do NOT edit this file to create your /etc/ppp/options file(s). Rather, copy this to a new file and then edit that. If you mess up your edits, you can then go back to the original and start again.

# /etc/ppp/options -*- sh -*- general options for pppd # created 13-Jul-1995 jmk # autodate: 01-Aug-1995 # autotime: 19:45 # Use the executable or shell command specified to set up the serial # line. This script would typically use the "chat" program to dial the # modem and start the remote ppp session. #connect "echo You need to install a connect command." # Run the executable or shell command specified after pppd has # terminated the link. This script could, for example, issue commands # to the modem to cause it to hang up if hardware modem control signals # were not available. #disconnect "chat -- \d+++\d\c OK ath0 OK" # async character map -- 32-bit hex; each bit is a character # that needs to be escaped for pppd to receive it. 0x00000001 # represents '\x01', and 0x80000000 represents '\x1f'. #asyncmap 0 # Require the peer to authenticate itself before allowing network # packets to be sent or received. #auth # Use hardware flow control (i.e. RTS/CTS) to control the flow of data # on the serial port. #crtscts # Use software flow control (i.e. XON/XOFF) to control the flow of data # on the serial port. #xonxoff # Add a default route to the system routing tables, using the peer as # the gateway, when IPCP negotiation is successfully completed. This # entry is removed when the PPP connection is broken. #defaultroute # Specifies that certain characters should be escaped on transmission # (regardless of whether the peer requests them to be escaped with its # async control character map). The characters to be escaped are # specified as a list of hex numbers separated by commas. Note that # almost any character can be specified for the escape option, unlike # the asyncmap option which only allows control characters to be # specified. The characters which may not be escaped are those with hex # values 0x20 - 0x3f or 0x5e. #escape 11,13,ff # Don't use the modem control lines. #local # Specifies that pppd should use a UUCP-style lock on the serial device # to ensure exclusive access to the device. #lock # Use the modem control lines. On Ultrix, this option implies hardware # flow control, as for the crtscts option. (This option is not fully # implemented.) #modem # Set the MRU [Maximum Receive Unit] value to <n> for negotiation. pppd # will ask the peer to send packets of no more than <n> bytes. The # minimum MRU value is 128. The default MRU value is 1500. A value of # 296 is recommended for slow links (40 bytes for TCP/IP header + 256 # bytes of data). #mru 542 # Set the interface netmask to <n>, a 32 bit netmask in "decimal dot" # notation (e.g. 255.255.255.0). #netmask 255.255.255.0 # Disables the default behaviour when no local IP address is specified, # which is to determine (if possible) the local IP address from the # hostname. With this option, the peer will have to supply the local IP # address during IPCP negotiation (unless it specified explicitly on the # command line or in an options file). #noipdefault # Enables the "passive" option in the LCP. With this option, pppd will # attempt to initiate a connection; if no reply is received from the # peer, pppd will then just wait passively for a valid LCP packet from # the peer (instead of exiting, as it does without this option). #passive # With this option, pppd will not transmit LCP packets to initiate a # connection until a valid LCP packet is received from the peer (as for # the "passive" option with old versions of pppd). #silent # Don't request or allow negotiation of any options for LCP and IPCP # (use default values). #-all # Disable Address/Control compression negotiation (use default, i.e. # address/control field disabled). #-ac # Disable asyncmap negotiation (use the default asyncmap, i.e. escape # all control characters). #-am # Don't fork to become a background process (otherwise pppd will do so # if a serial device is specified). #-detach # Disable IP address negotiation (with this option, the remote IP # address must be specified with an option on the command line or in an # options file). #-ip # Disable magic number negotiation. With this option, pppd cannot # detect a looped-back line. #-mn # Disable MRU [Maximum Receive Unit] negotiation (use default, i.e. # 1500). #-mru # Disable protocol field compression negotiation (use default, i.e. # protocol field compression disabled). #-pc # Require the peer to authenticate itself using PAP. # This requires TWO WAY authentication - do NOT use this for a standard # PAP authenticated link to an ISP as this will require the ISP machine # to authenticate itself to your machine (and it will not be able to). #+pap # Don't agree to authenticate using PAP. #-pap # Require the peer to authenticate itself using CHAP [Cryptographic # Handshake Authentication Protocol] authentication. # This requires TWO WAY authentication - do NOT use this for a standard # CHAP authenticated link to an ISP as this will require the ISP machine # to authenticate itself to your machine (and it will not be able to). #+chap # Don't agree to authenticate using CHAP. #-chap # Disable negotiation of Van Jacobson style IP header compression (use # default, i.e. no compression). #-vj # Increase debugging level (same as -d). If this option is given, pppd # will log the contents of all control packets sent or received in a # readable form. The packets are logged through syslog with facility # daemon and level debug. This information can be directed to a file by # setting up /etc/syslog.conf appropriately (see syslog.conf(5)). (If # pppd is compiled with extra debugging enabled, it will log messages # using facility local2 instead of daemon). #debug # Append the domain name <d> to the local host name for authentication # purposes. For example, if gethostname() returns the name porsche, # but the fully qualified domain name is porsche.Quotron.COM, you would # use the domain option to set the domain name to Quotron.COM. #domain <d> # Enable debugging code in the kernel-level PPP driver. The argument n # is a number which is the sum of the following values: 1 to enable # general debug messages, 2 to request that the contents of received # packets be printed, and 4 to request that the contents of transmitted # packets be printed. #kdebug n # Set the MTU [Maximum Transmit Unit] value to <n>. Unless the peer # requests a smaller value via MRU negotiation, pppd will request that # the kernel networking code send data packets of no more than n bytes # through the PPP network interface. #mtu <n> # Set the name of the local system for authentication purposes to <n>. # This will probably have to be set to your ISP user name if you are # using PAP/CHAP. #name <n> # Set the user name to use for authenticating this machine with the peer # using PAP to <u>. # Do NOT use this if you are using 'name' above! #user <u> # Enforce the use of the host name as the name of the local system for # authentication purposes (overrides the name option). #usehostname # Set the assumed name of the remote system for authentication purposes # to <n>. #remotename <n> # Add an entry to this system's ARP [Address Resolution Protocol] # table with the IP address of the peer and the Ethernet address of this # system. #proxyarp # Use the system password database for authenticating the peer using # PAP. #login # If this option is given, pppd will send an LCP echo-request frame to # the peer every n seconds. Under Linux, the echo-request is sent when # no packets have been received from the peer for n seconds. Normally # the peer should respond to the echo-request by sending an echo-reply. # This option can be used with the lcp-echo-failure option to detect # that the peer is no longer connected. #lcp-echo-interval <n> # If this option is given, pppd will presume the peer to be dead if n # LCP echo-requests are sent without receiving a valid LCP echo-reply. # If this happens, pppd will terminate the connection. Use of this # option requires a non-zero value for the lcp-echo-interval parameter. # This option can be used to enable pppd to terminate after the physical # connection has been broken (e.g., the modem has hung up) in # situations where no hardware modem control lines are available. #lcp-echo-failure <n> # Set the LCP restart interval (retransmission timeout) to <n> seconds # (default 3). #lcp-restart <n> # Set the maximum number of LCP terminate-request transmissions to <n> # (default 3). #lcp-max-terminate <n> # Set the maximum number of LCP configure-request transmissions to <n> # (default 10). # Some PPP servers are slow to start up. You may need to increase this # if you keep getting 'serial line looped back' errors and your are SURE # that you have logged in correctly and PPP should be starting on the server. #lcp-max-configure <n> # Set the maximum number of LCP configure-NAKs returned before starting # to send configure-Rejects instead to <n> (default 10). #lcp-max-failure <n> # Set the IPCP restart interval (retransmission timeout) to <n> # seconds (default 3). #ipcp-restart <n> # Set the maximum number of IPCP terminate-request transmissions to <n> # (default 3). #ipcp-max-terminate <n> # Set the maximum number of IPCP configure-request transmissions to <n> # (default 10). #ipcp-max-configure <n> # Set the maximum number of IPCP configure-NAKs returned before starting # to send configure-Rejects instead to <n> (default 10). #ipcp-max-failure <n> # Set the PAP restart interval (retransmission timeout) to <n> seconds # (default 3). #pap-restart <n> # Set the maximum number of PAP authenticate-request transmissions to # <n> (default 10). #pap-max-authreq <n> # Set the CHAP restart interval (retransmission timeout for # challenges) to <n> seconds (default 3). #chap-restart <n> # Set the maximum number of CHAP challenge transmissions to <n> # (default 10). #chap-max-challenge # If this option is given, pppd will re-challenge the peer every <n> # seconds. #chap-interval <n> # With this option, pppd will accept the peer's idea of our local IP # address, even if the local IP address was specified in an option. #ipcp-accept-local # With this option, pppd will accept the peer's idea of its (remote) IP # address, even if the remote IP address was specified in an option. #ipcp-accept-remote

15.2. What options should I use? (No PAP/CHAP)

Well, as in all things, that depends, (sigh). The options specified here should work with most servers.

However, if it does NOT work, READ THE TEMPLATE FILE (/etc/ppp/options.tpl) and the pppd man pages and speak to the sysadmin/user support people who run the server to which you are connecting.

You should also note that the connect scripts presented here also use some command line options to pppd to make things a bit easier to change.

# /etc/ppp/options (NO PAP/CHAP) # # Prevent pppd from forking into the background -detach # # use the modem control lines modem # use uucp style locks to ensure exclusive access to the serial device lock # use hardware flow control crtscts # create a default route for this connection in the routing table defaultroute # do NOT set up any "escaped" control sequences asyncmap 0 # use a maximum transmission packet size of 552 bytes mtu 552 # use a maximum receive packet size of 552 bytes mru 552 # #-------END OF SAMPLE /etc/ppp/options (no PAP/CHAP)

15.3. Other options to consider adding

There are a couple useful things you might want to add to the /etc/ppp/options file.

One is an idle time. Pppd can terminate the connection if it has been idle for too long. This could be nice if your isp charges by time, or if you don't want to keep your phone line tied up. To use this feature, simply add the line:

# Idle after X seconds
idle X

Replace 'X' with the number of seconds you wish the connection to terminate after.

The next feature is "dial on demand". This means that every time you attempt to open an interent connection, pppd will try to open a PPP connection to your ISP. In order to do this, add the following lines to /etc/ppp/options

# dial on demand
demand

This should make pppd dial out when necessary.

16.1. Using MSCHAP

Microsoft Windows NT RAS can be set up to use a variation on CHAP (Challenge/Handshake Authentication Protocol). In your PPP sources tar ball, you will find a file called README.MSCHAP80 that discusses this.

You can determine if the server is requesting authentication using this protocol by enabling debugging for pppd. If the server is requesting MS CHAP authentication, you will see lines like:-

rcvd [LCP ConfReq id=0x2 <asyncmap 0x0> <auth chap 80> <magic 0x46a3>]

The critical information here is auth chap 80.

In order to use MS CHAP, you will need to recompile pppd to support this. Please see the instructions in the README.MSCHAP80 file in the PPP source file for instructions on how to compile and use this variation.

You should note that at present this code supports only Linux PPP clients connecting to an MS Windows NT server. It does NOT support setting up a Linux PPP server to use MSCHAP80 authentication from clients.

16.2. The PAP/CHAP secrets file

If you are using pap or chap authentication, then you also need to create the secrets file. These are:

/etc/ppp/pap-secrets /etc/ppp/chap-secrets

They must be owned by user root, group root and have file permissions 740 for security.

The first point to note about PAP and CHAP is that they are designed to authenticate computer systems not users.

Huh? What's the difference? I hear you ask.

Well now, once your computer has made its PPP connection to the server, ANY user on your system can use that connection - not just you. This is why you can set up a WAN (wide area network) link that joins two LANs (local area networks) using PPP.

PAP can (and for CHAP DOES) require bidirectional authentication - that is a valid name and secret is required on each computer for the other computer involved. However, this is NOT the way most PPP servers offering dial-up PPP PAP-authenticated connections operate.

That being said, your ISP will probably have given you a user name and password to allow you to connect to their system and thence the Internet. Your ISP is not interested in your computer's name at all, so you will probably need to use the user name at your ISP as the name for your computer.

This is done using the name user name option to pppd. So, if you are to use the user name given you by your ISP, add the line

name your_user name_at_your_ISP

to your /etc/ppp/options file.

Technically, you should really use user our_user name_at_your_ISP for PAP, but pppd is sufficiently intelligent to interpret name as user if it is required to use PAP. The advantage of using the name option is that this is also valid for CHAP.

As PAP is for authenticating computers, technically you need also to specify a remote computer name. However, as most people only have one ISP, you can use a wild card (*) for the remote host name in the secrets file.

It is also worth noting that many ISPs operate multiple modem banks connected to different terminal servers - each with a different name, but ACCESSED from a single (rotary) dial in number. It can therefore be quite difficult in some circumstances to know ahead of time what the name of the remote computer is, as this depends on which terminal server you connect to!

16.3. The PAP secrets file

The /etc/ppp/pap-secrets file looks like

# Secrets for authentication using PAP # client server secret acceptable_local_IP_addresses

The four fields are white space delimited and the last one can be blank (which is what you want for a dynamic and probably static IP allocation from your ISP).

Suppose your ISP gave you a user name of fred and a password of flintstone you would set the name fred option in /etc/ppp/options[.ttySx] and set up your /etc/ppp/pap-secrets file as follows

# Secrets for authentication using PAP # client server secret acceptable local IP addresses fred * flintstone

This says for the local machine name fred (which we have told pppd to use even though it is not our local machine name) and for ANY server, use the password (secret) of flintstone.

Note that we do not need to specify a local IP address, unless we are required to FORCE a particular local, static IP address. Even if you try this, it is unlikely to work as most PPP servers (for security reasons) do not allow the remote system to set the IP number they are to be given.

16.4. The CHAP secrets file

This requires that you have mutual authentication methods - that is you must allow for both your machine to authenticate the remote server AND the remote server to authenticate your machine.

So, if your machine is fred and the remote is barney, your machine would set name fred remotename barney and the remote machine would set name barney remotename fred in their respective /etc/ppp/options.ttySx files.

The /etc/chap-secrets file for fred would look like

# Secrets for authentication using CHAP # client server secret acceptable local IP addresses fred barney flintstone barney fred wilma

and for barney

# Secrets for authentication using CHAP # client server secret acceptable local IP addresses barney fred flintstone fred barney wilma

Note in particular that both machines must have entries for bidirectional authentication. This allows the local machine to authenticate itself to the remote AND the remote machine to authenticate itself to the local machine.

16.5. Handling multiple PAP-authenticated connections

Some users have more than one server to which they connect that use PAP. Provided that your user name is different on each machine to which you want to connect, this is not a problem.

However, many users have the same user name on two (or more - even all) systems to which they connect. This then presents a problem in correctly selecting the appropriate line from /etc/ppp/pap-secrets.

As you might expect, PPP provides a mechanism for overcoming this. PPP allows you to set an 'assumed name' for the remote (server) end of the connection using the remotename option to pppd.

Let us suppose that you connect to two PPP servers using the username fred. You set up your /etc/ppp/pap-secrets something like

fred pppserver1 barney fred pppserver2 wilma

Now, to set connect to pppserver1 you would use name fred remotename pppserver1 in your ppp-options and for pppserver2 name fred remotename pppserver2.

As you can select the ppp options file to use with pppd using the file filename option, you can set up a script to connect to each of your PPP servers, correctly picking the options file to use and hence selecting the right remotename option.

18.1. Connection scripts for User name/Password Authentication

If your ISP does NOT require the use of PAP/CHAP, these are the scripts for you!

If the ppp package installed correctly, you should have two example files. For PPP 2.1.2 they are in /usr/sbin and for PPP 2.2 they are in /etc/ppp/scripts. They are called

for PPP-2.1.2

ppp-on ppp-off

and for PPP-2.2

ppp-off ppp-on ppp-on-dialer

Now, if you are using PPP 2.1.2, I strongly urge you to delete the sample files. There are potential problems with these - and don't tell me they work fine - I used them for ages too (and recommended them in the first version of this HOWTO)!

For the benefit of PPP 2.1.2 users, here are BETTER template versions, taken from the PPP 2.2 distribution. I suggest you copy and use these scripts instead of the old PPP-2.1.2 scripts.

18.2. The ppp-on script

This is the first of a PAIR of scripts that actually fire up the connection.

#!/bin/sh # # Script to initiate a PPP connection. This is the first part of the # pair of scripts. This is not a secure pair of scripts as the codes # are visible with the 'ps' command. However, it is simple. # # These are the parameters. Change as needed. TELEPHONE=555-1212 # The telephone number for the connection ACCOUNT=george # The account name for logon (as in 'George Burns') PASSWORD=gracie # The password for this account (and 'Gracie Allen') LOCAL_IP=0.0.0.0 # Local IP address if known. Dynamic = 0.0.0.0 REMOTE_IP=0.0.0.0 # Remote IP address if desired. Normally 0.0.0.0 NETMASK=255.255.255.0 # The proper netmask if needed # # Export them so that they will be available to 'ppp-on-dialer' export TELEPHONE ACCOUNT PASSWORD # # This is the location of the script which dials the phone and logs # in. Please use the absolute file name as the $PATH variable is not # used on the connect option. (To do so on a 'root' account would be # a security hole so don't ask.) # DIALER_SCRIPT=/etc/ppp/ppp-on-dialer # # Initiate the connection # # exec /usr/sbin/pppd debug /dev/ttySx 38400 \ $LOCAL_IP:$REMOTE_IP \ connect $DIALER_SCRIPT

Here is the ppp-on-dialer script:-

#!/bin/sh # # This is part 2 of the ppp-on script. It will perform the connection # protocol for the desired connection. # /usr/sbin/chat -v \ TIMEOUT 3 \ ABORT '\nBUSY\r' \ ABORT '\nNO ANSWER\r' \ ABORT '\nRINGING\r\n\r\nRINGING\r' \ '' \rAT \ 'OK-+++\c-OK' ATH0 \ TIMEOUT 30 \ OK ATDT$TELEPHONE \ CONNECT '' \ ogin:--ogin: $ACCOUNT \ assword: $PASSWORD

For PPP-2.2, the ppp-off script looks like:-

#!/bin/sh ###################################################################### # # Determine the device to be terminated. # if [ "$1" = "" ]; then DEVICE=ppp0 else DEVICE=$1 fi ###################################################################### # # If the ppp0 pid file is present then the program is running. Stop it. if [ -r /var/run/$DEVICE.pid ]; then kill -INT `cat /var/run/$DEVICE.pid` # # If the kill did not work then there is no process running for this # pid. It may also mean that the lock file will be left. You may wish # to delete the lock file at the same time. if [ ! "$?" = "0" ]; then rm -f /var/run/$DEVICE.pid echo "ERROR: Removed stale pid file" exit 1 fi # # Success. Let pppd clean up its own junk. echo "PPP link to $DEVICE terminated." exit 0 fi # # The ppp process is not running for ppp0 echo "ERROR: PPP link is not active on $DEVICE" exit 1

18.3. Editing the supplied PPP startup scripts

As the new scripts come in two parts, we will edit them in turn.

18.4. What a Chat script means...

A chat script is a sequence of expect string, send string pairs. In particular, note that we ALWAYS expect something before we send something.

If we are to send something WITHOUT receiving anything first, we must use an empty expect string (indicated by " ") and similarly for expecting something without sending anything! Also, if a string consists of several words, (e.g. NO CARRIER), you must quote the string so that it is seen as a single entity by chat.

The chat line in our template is:-

exec /usr/sbin/chat -v

Invoke chat, the -v tells chat to copy ALL its I/O into the system log (usually /var/log/messages). Once you are happy that the chat script is working reliably, edit this line to remove the -v to save unnecessary clutter in your syslog.

TIMEOUT 3

This sets the timeout for the receipt of expected input to three seconds. You may need to increase this to say 5 or 10 seconds if you are using a really slow modem!

ABORT '\nBUSY\r'

If the string BUSY is received, abort the operation.

ABORT '\nNO ANSWER\r'

If the string NO ANSWER is received, abort the operation

ABORT '\nRINGING\r\n\r\nRINGING\r'

If the (repeated) string RINGING is received, abort the operation. This is because someone is ringing your phone line!

\rAT

Expect nothing from the modem and send the string AT.

OK-+++\c-OK ATH0

This one is a bit more complicated as it uses some of chat's error recovery capabilities.

What is says is...Expect OK, if it is NOT received (because the modem is not in command mode) then send +++ (the standard Hayes-compatible modem string that returns the modem to command mode) and expect OK. Then send ATH0 (the modem hang up string). This allows your script to cope with the situation of your modem being stuck on-line!

TIMEOUT 30

Set the timeout to 30 seconds for the remainder of the script. If you experience trouble with the chat script aborting due to timeouts, increase this to 45 seconds or more.

OK ATDT$TELEPHONE

Expect OK (the modem's response to the ATH0 command) and dial the number we want to call.

CONNECT ''

Expect CONNECT (which our modem sends when the remote modem answers) and send nothing in reply.

ogin:--ogin: $ACCOUNT

Again, we have some error recovery built in here. Expect the login prompt (...ogin:) but if we don't receive it by the timeout, send a return and then look for the login prompt again. When the prompt is received, send the username (stored in the shell variable $ACCOUNT).

assword: $PASSWORD

Expect the password prompt and send our password (again, stored in a shell variable).

This chat script has reasonable error recovery capability. Chat has considerably more features than demonstrated here. For more information consult the chat manual page (man 8 chat).

18.5. A chat script for PAP/CHAP authenticated connections

If your ISP is using PAP/CHAP, then your chat script is much simpler. All your chat script needs to do is dial the telephone, wait for a connect and then let pppd handle the logging in!

#!/bin/sh # # This is part 2 of the ppp-on script. It will perform the connection # protocol for the desired connection. # exec /usr/sbin/chat -v \ TIMEOUT 3 \ ABORT '\nBUSY\r' \ ABORT '\nNO ANSWER\r' \ ABORT '\nRINGING\r\n\r\nRINGING\r' \ '' \rAT \ 'OK-+++\c-OK' ATH0 \ TIMEOUT 30 \ OK ATDT$TELEPHONE \ CONNECT '' \

18.6. The pppd debug and file option_file options

As we have already seen, you can turn on debug information logging with the -d option to pppd. The 'debug' option is equivalent to this.

As we are establishing a new connection with a new script, leave in the debug option for now. (Warning: if your disk space is tight, logging pppd exchanges can rapidly extend your syslog file and run you into trouble - but to do this you must fail to connect and keep on trying for quite a few minutes).

Once you are happy that all is working properly, then you can remove this option.

If you have called your ppp options file anything other than /etc/ppp/options or /etc/ppp/options.ttySx, specify the file name with the file option to pppd - e.g.

exec /usr/sbin/pppd debug file options.myserver /dev/ttyS0 38400 \

22.1. I can't see beyond the PPP server I connect to

OK - your PPP connection is up and running and you can ping the PPP server by IP number (the second or "remote" IP number shown by ifconfig ppp0), but you can't reach anything beyond this.

First of all, try pinging the IP numbers you have specified in /etc/resolv.conf as name servers. If this works, you can see beyond your PPP server (unless this has the same IP number as the "remote" IP number of your connection). So now try pinging the full Internet name of your service provider - eg

ping my.provider.net.au

If this does NOT work, you have a problem with the name resolution. This is probably because of a typo in your /etc/resolv.conf file. Check this carefully against the information you acquired by ringing your service provider. If all looks OK, ring your service provider and check that you wrote down the IP numbers correctly.

If it STILL doesn't work (and your service provider confirms that his name servers are up and running), you have a problem somewhere else - and I suggest you check carefully through your Linux installation (looking particularly for file permissions).

If you STILL can't ping your service provider's IP name servers by IP number, either they are down (give them a voice call and check) or there is a routing problem at your service provider's end. Again, ring them and check this out.

One possibility is that the "remote end" is a Linux PPP server where the IP forwarding option has not been specified in the kernel!

A good general test is to try connecting to your service provider using the software that most supply for (gulp) Microsoft Windows. If everything works from another operating system to exactly the same account, then the problem is with your Linux system and NOT your service provider.

22.2. I can send email, but not receive it

If you are using dynamic IP numbers, this is perfectly normal. See "Setting up Services" below.

22.3. Why can't people finger, WWW, gopher, talk, etc. to my machine?

Again, if you are using dynamic IP numbers, this is perfectly normal. See "Setting up Services" below.

23.1. Setting up email

Even for dynamic IP numbers, you can certainly configure sendmail on your machine to send out any email that you compose locally. Configuration of sendmail can be obscure and difficult - so this document does not attempt to tell you how to do this. However, you should probably configure sendmail so that your Internet service provider is designated as your "smart relay" host (the sendmail.cf DS option). (For more sendmail configuration info, see the sendmail documents - and look at the m4 configurations that come with sendmail. There is almost certain to be one there that will meet your needs).

There are also excellent books on Sendmail (notably the 'bible' from O'Reilly and Associates), but these are almost certainly overkill for most users!

Once you have sendmail configured, you will probably want to have sendmail dispatch any messages that have been sitting in the outbound mail queue as soon as the PPP connection comes up. To do this, add the command

sendmail -q &

to your /etc/ppp/ip-up script (see below).

Inbound email is a problem for dynamic IP numbers. The way to handle this is to:-

• configure your mail user agent so that all mail is sent out with a "reply to" header giving your email address at your Internet Service provider. If you can, you should also set your FROM address to be your email address at your ISP as well.

• use the popclient, fetchmail programs to retrieve your email from your service provider. Alternatively, if your ISP is using IMAP, use an IMAP enabled mail user agent, (such as pine).

You can automate this process at dial up time by putting the necessary commands in the /etc/ppp/ip-up script (see below).

23.2. Setting Up a local Name server

Whilst you can quite happily use the domain name servers located at your ISP, you can also set up a local caching only (secondary) name server that is brought up by the ip-up script. The advantage of running a local (caching only) name server is that it will save you time (and bandwidth) if you frequently contact the same sites during a long on-line session.

DNS configuration for a caching only nameserver (that uses a "forwarders' line in the named.boot file pointing at your ISPs DNS) is relatively simple. The O'Reilly book (DNS and Bind) explains all you want to know about this.

There is also a DNS-HOWTO available.

If you are running a small LAN that can access the Internet through you Linux PC (using IP Masquerade for example), it is probably a good idea to run a local name server (with a forwarders directive) whilst the link is up as this will minimise the bandwidth and delays associated with name resolution.

One point of Nettiquette: ask permission from your ISP before you start using a secondary, caching only name server in your ISP's domain. Properly configured, your DNS will not cause any problems to your ISP at all, but if you get things wrong, it can cause problems.

24.1. Setting up the IP numbers

Arrange with the network administrator of the other LAN the IP numbers that will be used for each end of the PPP interface. If you are using static IP numbers, this will also probably require you to dial into a specific telephone number.

Now edit the appropriate /etc/ppp/options[.ttyXX] file - it's a good idea to have a specific modem and port at your end for this connection. This may well require you to change your /etc/ppp/options file - and create appropriate options.ttyXX files for any other connections!

Specify the IP numbers for your end of the PPP link in the appropriate options file exactly as shown above for static IP numbers.

24.2. Setting up the routing

You must arrange that packets on your local LAN are routed across the interface that the PPP link establishes. This is a two stage process.

First of all, you need to establish a route from the machine running the PPP link to the network(s) at the far end of the link. If the link is to the Internet, this can be handled by a default route established by pppd itself at your end of the connection using the 'defaultroute' option to pppd.

If however, the link is only linking two LANs, then a specific network route must be added for each network that is accessible across the link. This is done using a 'route' command for each network in the /etc/ppp/ip-up script (see "After the link comes up"...) for instructions on doing this.

The second thing you need to do is to tell the other computers on your LAN that your Linux computer is actually the 'gateway' for the network(s) at the far end of the ppp link.

Of course, the network administrator at the other end of the link has to do all this too! However, as s/he will be routing packets to your specific networks, a specific network route will be required, not a default route (unless the LANs at the far and of the link are linking into you to access the Internet across your connection).

24.3. Network security

If you are linking you LAN to the Internet using PPP - or even just to a "foreign" LAN, you need to think about security issues. I strongly urge you to think about setting up a firewall!

You should also speak to the LAN administrator at your site BEFORE you start linking to foreign LANs or the Internet this way. Failure to do so could earn you anything from no reaction to really serious trouble!

25.1. Special routing

If you are linking two LANs, you will need to set up specific routes to the 'foreign' LANs. This is easily done using the /etc/ppp/ip-up script. The only difficulty arises if your machine handles multiple PPP links.

This is because the /etc/ppp/ip-up is executed for EVERY ppp connection that comes up, so you need to carefully execute the correct routing commands for the particular link that comes up - and not when any other link comes up!

25.2. Handling email queues

When the link between two LANs comes up, you may well want to make sure that email that is queued at either end is flushed - sent out to its destination. This is done by adding the appropriate sendmail invocation.

Using the bash 'case' statement on an appropriate parameter that pppd passes into the script accomplishes this. For example, this is the /etc/ppp/ip-up script I use to handle our WAN links and the link to my home Ethernet (also handled on the same ppp server).

25.3. A sample /etc/ppp/ip-up script

The example below provides a variety of example uses.

#!/bin/bash # # Script which handles the routing issues as necessary for pppd # Only the link to Newman requires this handling. # # When the ppp link comes up, this script is called with the following # parameters # $1 the interface name used by pppd (e.g. ppp3) # $2 the tty device name # $3 the tty device speed # $4 the local IP address for the interface # $5 the remote IP address # $6 the parameter specified by the 'ipparam' option to pppd # case "$5" in # Handle the routing to the Newman Campus server 202.12.126.1) /sbin/route add -net 202.12.126.0 gw 202.12.126.1 # and flush the mail queue to get their email there asap! /usr/sbin/sendmail -q & ;; 139.130.177.2) # Our Internet link # When the link comes up, start the time server and synchronise to the world # provided it is not already running if [ ! -f /var/lock/subsys/xntpd ]; then /etc/rc.d/init.d/xntpd.init start & fi # Start the news server (if not already running) if [ ! -f /var/lock/subsys/news ]; then /etc/rc.d/init.d/news start & fi ;; 203.18.8.104) # Get the email down to my home machine as soon as the link comes up # No routing is required as my home Ethernet is handled by IP # masquerade and proxyarp routing. /usr/sbin/sendmail -q & ;; *) esac exit 0

As a result of bringing up the ppp link to our Newman campus and this script, we end up with the following routing table entries (this machine also is our general dial up PPP server AND handles our Internet link). I have interspersed comments in the output to help explain what each entry is) :-

[root@kepler /root]# route -n Kernel routing table Destination Gateway Genmask Flags MSS Window Use Iface # the HOST route to our remote internet gateway 139.130.177.2 * 255.255.255.255 UH 1500 0 134 ppp4 # the HOST route to our Newman campus server 202.12.126.1 * 255.255.255.255 UH 1500 0 82 ppp5 # the HOST route to my home ethernet 203.18.8.104 * 255.255.255.255 UH 1500 0 74 ppp3 # two of our general dial up PPP lines 203.18.8.64 * 255.255.255.255 UH 552 0 0 ppp2 203.18.8.62 * 255.255.255.255 UH 552 0 1 ppp1 # the specific network route to the Newman campus LAN 202.12.126.0 202.12.126.1 255.255.255.0 UG 1500 0 0 ppp5 # the route to our local Ethernet (super-netting two adjacent C classes) 203.18.8.0 * 255.255.254.0 U 1500 0 1683 eth0 # the route to the loop back device 127.0.0.0 * 255.0.0.0 U 3584 0 483 lo # the default route to the Internet default 139.130.177.2 * UG 1500 0 3633 ppp4

25.4. Handling email

The previous section shows how to handle the outgoing mail - simply by flushing the mail queue once the link is up.

If you are running a WAN link, you can arrange with the network administrator of the remote LAN to do exactly the same thing. For example, at the Newman Campus end of our WAN link, the /etc/ppp/ip-up script looks like :-

#!/bin/bash # # Script which handles the routing issues as necessary for pppd # Only the link to Hedland requires this handling. # # When the ppp link comes up, this script is called with the following # parameters # $1 the interface name used by pppd (e.g. ppp3) # $2 the tty device name # $3 the tty device speed # $4 the local IP address for the interface # $5 the remote IP address # $6 the parameter specified by the 'ipparam' option to pppd # case "$5" in 203.18.8.4) /usr/sbin/sendmail -q ;; *) esac exit 0

If however you have only a dynamic IP PPP link to your ISP, you need to get your email from the account on your ISPs machine. This is usually done using the POP (Post Office Protocol). This process can be handled using the 'popclient' program - and the ip-up script can automate this process for you too!

Simply create a /etc/ppp/ip-up script that contains the appropriate invocation of popclient. For my laptop that runs Red Hat Linux (which I take on any travels), this is...

popclient -3 -c -u hartr -p <password> kepler.hedland.edu.au |formail -s procmail

You could use slurp or whatever to do the same for news, and so forth. Remember, the ip-up script is just a standard bash script and so can be used to automate ANY function that needs to be accomplished every time the appropriate PPP link comes up.

27.1. Note on Security

When you set up a Linux box on an existing LAN to link into the Internet, you are potentially opening your entire LAN to the Internet - and the hackers that reside there. Before you do this, I strongly urge you to consult your network administrator and site security policy. If your PPP connection to the Internet is used to successfully attack your site, you will at the very least earn the intense anger of your fellow users, network and system administrators. You may also find yourself in very much more serious trouble!

Before you connect a LAN to the Internet, you should consider the security implications of even a DYNAMIC connection - hence the earlier reference to the O'Reilly "Building Internet Firewalls"!

28.1. Kernel compilation

All the earlier comments regarding kernel compilation and kernel versions versus pppd versions apply. This section assumes that you have read the earlier sections of this document!

For a PPP server, you MUST include IP forwarding in your kernel. You may also wish to include other capabilities (such as IP fire walls, accounting etc etc).

If you are using a multi-port serial card, then you must obviously include the necessary drivers in your kernel too!

28.2. Overview of the server system

We offer dial up PPP (and SLIP) accounts and shell accounts using the same user name/password pair. This has the advantages (for us) that a user requires only one account and can use it for all types of connectivity.

As we are an educational organization, we do not charge our staff and students for access, and so do not have to worry about accounting and charging issues.

We operate a firewall between our site and the Internet, and this restricts some user access as the dial up lines are inside our (Internet) firewall (for fairly obvious reasons, details of our other internal fire walls are not presented here and are irrelevant in any case).

The process a user goes through to establish a PPP link to our site (once they have a valid account of course) is :-

• Dial into our rotary dialer (this is a single phone number that connects to a bank of modems - the first free modem is then used).

• Log in using a valid user name and password pair.

• At the shell prompt, issue the command ppp to start PPP on the server.

• Start PPP on their PC (be it running Windows, DOS, Linux MAC OS or whatever - that is their problem).

The server uses individual /etc/ppp/options.ttyXX files for each dial in port that set the remote IP number for dynamic IP allocation. The server users proxyarp routing for the remote clients (set via the appropriate option to pppd). This obviates the need for routed or gated.

When the user hangs up at their end, pppd detects this and tells the modem to hang up, bringing down the PPP link at the same time.

28.3. Getting the software together

You will need the following software:-

• Linux, properly compiled to include the necessary options.

• The appropriate version of pppd for your kernel.

• A 'getty' program that intelligently handles modem communications. We use getty_ps2.0.7h, but mgetty is highly thought of. I understand that mgetty can detect a call that is using pap/chap (pap is the standard for Windows95) and invoke pppd automatically, but I have yet to explore this.

• An operational domain name server (DNS) that is accessible to your dial up users. You should really be running your own DNS if possible...

28.4. Setting up standard (shell access) dialup.

Before you can set up your PPP server, your Linux box must be capable of handling standard dial up access.

This howto does NOT cover setting this up. Please see the documentation of the getty of your choice and serial HOWTO for information on this.

28.5. Setting up the PPP options files

You will need to set up the overall /etc/ppp/options with the common options for all dial up ports. The options we use are:-

asyncmap 0 netmask 255.255.254.0 proxyarp lock crtscts modem

Note - we do NOT use any (obvious) routing - and in particular there is no defaultroute option. The reason for this is that all you (as a PPP server) are required to do is to route packets from the ppp client out across your LAN/Internet and route packets to the client from your LAN and beyond.

All that is necessary for this is a host route to the client machine and the use of the 'proxyarp' option to pppd.

The 'proxyarp' option sets up (surprise) a proxy ARP entry in the PPP server's ARP table that basically says 'send all packets destined for the PPP client to me'. This is the easiest way to set up routing to a single PPP client - but you cannot use this if you are routing between two LANs - you must add proper network routes which can't use proxy ARP.

You will almost certainly wish to provide dynamic IP number allocation to your dial up users. You can accomplish this by allocating an IP number to each dial up port. Now, create a /etc/ppp/options.ttyXX for each dial up port.

In this, simply put the local (server) IP number and the IP number that is to be used for that port. For example

kepler:slip01

In particular, note that you can use valid host names in this file (I find that I only remember the IP numbers of critical machines and devices on my networks - names are more meaningful)!

28.6. Setting pppd up to allow users to (successfully) run it

As starting a ppp link implies configuring a kernel device (a network interface) and manipulating the kernel routing tables, special privileges are required - in fact full root privileges.

Fortunately, pppd has been designed to be 'safe' to run set uid to root. So you will need to...

chmod u+s /usr/sbin/pppd

When you list the file, it should then appear as...

-rwsr-xr-x 1 root root 74224 Apr 28 07:17 /usr/sbin/pppd

If you do not do this, users will be unable to set up their ppp link.

28.7. Setting up the global alias for pppd

In order to simplify things for our dial up PPP users, we create a global alias (in /etc/bashrc) so that one simple command will start ppp on the server once they are logged in.

This looks like...

alias ppp="exec /usr/sbin/pppd -detach"

What this does is

• exec : this means replace the running program (in this case the shell) with the program that is run.

• pppd -detach : start up pppd and do NOT fork into the background. This ensures that when pppd exits there is no process hanging around.

When a user logs in like this, they will appear in the output of 'w' as...

6:24pm up 3 days, 7:00, 4 users, load average: 0.05, 0.03, 0.00 User tty login@ idle JCPU PCPU what hartr ttyC0 3:05am 9:14 -

And that is it...I told you this was a simple, basic PPP server system!