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15.2. Dial-up PPP

In order to communicate over TCP/IP using a modem (such as through a dial-up account to an Internet service provider) or through some other serial device (such as a "null modem" serial cable between two machines), Linux provides the Point-to-Point Protocol software suite, commonly known as PPP. PPP is a protocol that takes packets sent over a network (such as TCP/IP) and converts them to a format that can be easily sent over a modem or serial wire. Chances are, if you have an Internet account with an ISP, the ISP's server uses PPP to communicate with dialup accounts. By configuring PPP under Linux, you can directly connect to your ISP account in this way.

SLIP (Serial Line Internet Protocol) is an earlier protocol that has the same basic features as PPP. However, it lacks certain important qualities, such as the ability to negotiate IP addresses and packet sizes. These days SLIP has more or less been supplanted entirely by PPP; however, some older ISPs may still use SLIP rather than PPP. If this is the case, we refer you to other sources of information, such as the Linux Network Administrator's Guide.

In this section, we will cover configuration of a PPP client--that is, a system that will connect to an ISP (or other PPP server) in order to communicate with the Internet. Setting up a Linux machine as a PPP server itself is also possible but is somewhat more involved; this is covered in the Linux Network Administrator's Guide.

15.2.1. Basic PPP Configuration for Modems

In the United States and many parts of the world, people use traditional dial-up modems to send digital data over telephone lines. So we'll cover configuration for modems first. Then we'll show how to configure PPP for the faster and more convenient type of line called Integrated Services Digital Network (ISDN), which is especially popular in Europe and available but not very well marketed in most of the United States. Requirements

Most Linux systems come preinstalled with all of the software needed to run PPP. Essentially, you need a kernel compiled with PPP support and the pppd daemon and related tools, including the chat program.

Most Linux distributions include PPP support in the preconfigured kernel or as a kernel module that is loaded on demand. However, it may necessary to compile kernel PPP support yourself; this is a simple matter of enabling the PPP options during the kernel configuration process and rebuilding the kernel. PPP is usually compiled as a separate module, so it is sufficient to recompile only the kernel modules if this is the case. See "Section 7.4.2, "Building the Kernel"" in Chapter 7, "Upgrading Software and the Kernel" for information on compiling the kernel and modules.

The pppd and chat utilities are user-level applications that control the use of PPP on your system; they are included with nearly every Linux distribution. On Red Hat systems, these utilities are installed in /usr/sbin and are found in the ppp RPM package.

Also required for PPP usage is a modem that is compatible with both Linux and the type of modems used by your ISP's server. Most 14.4, 28.8, 56K and other standard modem types fit into this category; there are very few modem types not supported by Linux, and it would be unusual for an ISP to use anything so esoteric as to require you to buy something else.

One type of modem to watch out for is the so-called "Winmodem." This was originally a product sold by US Robotics but has now been produced in several varieties by other vendors. Winmodems use the host CPU to convert digital signals into analog signals so they can be sent over the phone line, unlike regular modems which have a special chip to perform this function. The problem with Winmodems is that, as of this writing, the programming details for these devices are proprietary, meaning that there are no Linux drivers for this class of devices. (Besides, some people scoff at the idea of wasting precious CPU cycles to generate modem signals, a job best left to specialized hardware. One perceived advantage of these so-called "software modems," on the other hand, is that upgrading their functionality is simply a matter of upgrading the operating system driver which controls them, rather than buying new hardware.) Serial device names

Under Windows 95/98 and MS-DOS, modems and other serial devices are named COM1 (for the first serial device), COM2 (for the second), and so forth, up to COM4. (Most systems support up to four serial devices, although multiport cards are available that can increase this number.) Under Linux, these same devices are referred to as /dev/ttyS0, /dev/ttyS1, on up to /dev/ttyS3.[60] On most systems, at installation time a symbolic link called /dev/modem will be created. This link points to the serial device on which the modem can be found, as shown in the following listing:

% ls -l /dev/modem
lrwxrwxrwx   1 root     root      10 May  4 12:41 /dev/modem -> /dev/ttyS0
If this link is incorrect for your system (say, because you know that your modem is not on /dev/ttyS0 but on /dev/ttyS2), you can easily fix it as root by entering:
# ln -sf /dev/ttyS2 /dev/modem

[60]Older versions of Linux also used special "callout" devices, called /dev/cua0 through /dev/cua3. These are obsolete as of Linux kernel Version 2.2. Setting up PPP

There are several steps involved in PPP configuration. The first is to write a so-called "chat script," which performs the "handshaking" necessary to set up a PPP connection between your machine and the ISP. During this handshaking phase, various pieces of information might be exchanged, such as your ISP username and password. The second step is to write a script that fires up the pppd daemon; running this script causes the modem to dial the ISP and start up PPP. The final step is to configure your system's /etc/resolv.conf file so it knows where to find a domain name server. We'll go through each of these steps in turn.

Before you start, you need to know the following pieces of information:

  • The ISP dialin account phone number

  • Your ISP username and password

  • The IP address of the ISP's domain name server

Your ISP should have told you this information when you established the account.

In addition, you might need to know the following:

  • The IP address of the ISP's server

  • The IP address of your system (if not dynamically assigned by the ISP)

  • The subnet mask you should use

These last three items can usually be determined automatically during the PPP connection setup; however, occasionally this negotiation does not work properly. It can't hurt to have this information in case you need it. Writing a chat script

chat is a program that can perform simple handshaking between a PPP client and server during connection setup, such as exchange usernames and passwords. chat is also responsible for causing your modem to dial the ISP's phone number and other simple tasks.

chat itself is automatically invoked by pppd when started (this is discussed later). All you need to do is write a simple shell script that invokes chat to handle the negotiation. A simple chat script is shown in the following example. Edit the file /etc/ppp/my-chat-script (as root) and place in it the following lines:

# my-chat-script: a program for dialing up your ISP
exec chat -v		\
     '' ATZ		\
     OK ATDT555-1212	\
     CONNECT ''		\
     ogin: mdw		\
     assword: my-password

Be sure that the file my-chat-script is executable; the command chmod 755 /etc/ppp/my-chat-script will accomplish this.

Note that each line ending in a backslash should not have any characters after the backslash; the backslash forces line-wrapping in the shell script.

The third line of this script runs chat itself with the options on the following lines. Each line contains two whitespace-delimited fields: an "expect" string and a "send" string. The idea is that the chat script will respond with the send string when it receives the expect string from the modem connection. For example, the last line of the script informs chat to respond with my-password when the prompt assword[61] is given by the ISP's server.

[61]This is not meant as an expletive. Rather, leaving off the first letter of the prompt admits the possibility of either Password: or password: to be used as the prompt!

The first line of the handshaking script instructs chat to send ATZ to the modem, which should cause the modem to reset itself. (Specifying an expect string as '' means that nothing is expected before ATZ is sent.) The second line waits for the modem to respond with OK, after which the number is dialed using the string ATDT555-1212. (If you use pulse dialing, rather than tone dialing, change this to ATDP555-1212. The phone number, of course, should be that of the remote system's modem line.)

When the modem responds with CONNECT, a newline is sent (indicated by '' as the send string). After this, chat waits for the prompt ogin: before sending the username and assword: before sending the password.

The various send strings starting with AT in the previous example are simply Hayes-modem-standard modem control strings. The manual that came with your modem should explain their usage; this is not specific to Linux or any other operating system. As one example, using a comma in a phone number indicates that the modem should pause before sending the following digits; one might use ATDT9,,,555-1212 if a special digit (9 in this case) must be dialed to reach an outside line.

Note that this is a very simple chat script that doesn't deal with timeouts, errors, or any other extraordinary cases that might arise while you're attempting to dial into the ISP. See the chat manual pages for information on how to spruce up your script to deal with these cases. Also, note that you need to know in advance what prompts the ISP's server will use (we assumed login and password). There are several ways of finding out this information; possibly, the ISP has told you this information in advance, or supplied a handshaking script for another system such as Windows 95 (which uses a mechanism very similar to chat). Otherwise, you can dial into the ISP server "by hand," using a simple terminal emulator, such as minicom or seyon. The man pages for those commands can help you to do this. Starting up pppd

Now, we're ready to configure the pppd daemon to initiate the PPP connection using the chat script we just wrote. Generally, this is done by writing another shell script that invokes pppd with a set of options.

The format of the pppd command is:

pppd device-name baudrate options

Table 15-1 shows the options supported by pppd. You almost certainly won't need all of them.

Table 15-1. Common pppd Options




Locks the serial device to restrict access to pppd.


Uses hardware flow control.


Doesn't try to determine the local IP address from the hostname. The IP is assigned by the remote system.

user username

Specifies the hostname or username for PAP or CHAP identification.

netmask mask

Specifies the netmask for the connection.


Adds a default route to the local system's routing table, using the remote IP address as the gateway.

connect command

Uses the given command to initiate the connection. pppd assumes this script is in /etc/ppp. If not, specify the full path of the script.

local_IP_address: remote_IP_address

Specifies the local and/or remote IP addresses. Either or both of these could be to indicate that the address should be assigned by the remote system.


Logs connection information through the syslog daemon.

It is common to invoke the pppd command from a shell script. Edit the file /etc/ppp/ppp-on and add the following lines:

# the ppp-on script

exec /usr/sbin/pppd /dev/modem 38400 lock crtscts noipdefault \
     defaultroute connect my-chat-script

As with the my-chat-script file in the earlier example, be sure this is executable and watch out for extra characters after a backslash at the end of a line.

With this script in place, it should be possible to connect to the ISP using the command:

% /etc/ppp/ppp-on
You need not be root to execute this command. Upon running this script, you should hear your modem dialing, and if all goes well, after a minute PPP should be happily connected. The ifconfig command should report an entry for ppp0 if PPP is up and running:

# ifconfig
lo        Link encap:Local Loopback
          inet addr:  Bcast:  Mask:

          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0

ppp0      Link encap:Point-to-Point Protocol
          inet addr:  P-t-P:  Mask:
          UP POINTOPOINT RUNNING  MTU:1500  Metric:1
          RX packets:1862 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1288 errors:0 dropped:0 overruns:0 carrier:0

Here, we can see that PPP is up, the local IP address is, and the remote server IP address is

If you wish to be notified when the PPP connection is established (the ppp-on script returns immediately), add the following line to /etc/ppp/ip-up :

/usr/bin/wall "PPP is up!"
/etc/ppp/ip-up is executed when PPP establishes an IP connection, so you can use this script to trigger the wall command when the connection is complete.

Another simple shell script can be used to kill the PPP session. Edit the file /etc/ppp/ppp-off as follows:

# A simple ppp-off script

kill `cat /var/run/ppp0.pid`

Running /etc/ppp/ppp-off now kills the PPP daemon and shuts down the modem connection. PAP and CHAP

Some ISPs may require you to use a special authentication protocol, such as PAP (Password Authentication Protocol) or CHAP (Challenge Handshake Authentication Protocol). The protocols rely on some form of "shared secret" known to both the client and the server; in most cases, this is just your ISP account password.

If PAP or CHAP is required by your ISP, they are configured by adding information to the files /etc/ppp/pap-secrets and /etc/ppp/chap-secrets, respectively. Each file has four fields separated by spaces or tabs. Here is an example of a pap-secrets file:

# Secrets for authentication using PAP
# client      server	     secret		IP or Domain
mdw	      *		     my-password

The first field is your system's name as expected by the remote system, usually your ISP username. The second field specifies the ISP's server name; an asterisk allows this entry to match all ISP servers to which you might connect. The third field specifies the shared secret provided by your ISP; as stated earlier, this is usually your ISP password. The fourth field is primarily used by PPP servers to limit the IP addresses to which users dialing in have access. These addresses can be specified as either IP addresses or domain names. For most PPP client configurations, however, this field is not required.

The chap-secrets file has the same four fields, but you need to include an entry other than * for the service provider's system; this is a secret the ISP shares with you when you establish the account.

If PAP or CHAP is being used, it's not necessary for the chat script to include handshaking information after CONNECT is received; pppd will take care of the rest. Therefore, you can edit /etc/ppp/my-chat-script to contain only the lines:

# my-chat-script: a program for dialing up your ISP
exec chat -v            \
     '' ATZ             \
     OK ATDT555-1212    \
     CONNECT ''

You will also need to add the user option to the pppd command line in /etc/ppp/ppp-on, as so:

# the ppp-on script

exec /usr/sbin/pppd /dev/modem 38400 lock crtscts noipdefault \
     user mdw defaultroute connect my-chat-script

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