The PPP Daemon

• The PPP daemon - pppd - is the program that handles the PPP connection, even if you use a graphical front-end, such as kppp.
  
  
• pppd manages the dialing process (often using a separate program) and then negotiates the connection with your ISP.
  
  
• The negotiation process includes a set of steps, using a set of protocols such as LCP (Link Control Protocol), authentication protocols, NCP (Network Control Protocol) and IPCP (Internet Protocol Control Protocol).
  
  
• Each of these "curses" denotes a step in the PPP connection setup. If the connection fails, determining which sub-protocol caused the failure is half the way to locating the configuration problem.
  
  

• pppd is usually invoked with two command line parameters:
  
  pppd /dev/ttyS0 57600
  
  
• The first parameter denotes the name of the modem device. 'ttyS0' stands for COM1, 'ttyS1' stands for COM2, etc ('S' is short for 'serial', for the 'serial port').
  
  
• The second parameter is the speed of the communications between your computer and your modem. Note that the connection speed between your modem and the modem of your ISP might be different.
  
  
• By supplying only these two parameters, we assume that the rest of the parameters are supplied via standard pppd configuration files.
  
  

• The configuration files of pppd are stored under directory /etc/ppp/.
  
  
• The 'options' file contains commands-line options to be used when pppd runs. These complement the options actually supplied to pppd on the command line.
  
  
• The 'pap-secrets' file contains information used to authenticate with an ISP that uses the PAP method. This information contains the user's password in clear text, and thus this file should be made readable only by root.
  
  
• The 'chap-secrets' file contains similar information for authenticating using the CHAP method, and should be protected in a similar manner.
  
  
• The 'ip-up' file may contain a script that will be executed by pppd, after it establishes a connection. The 'ip-down' file may contain a script that will be invoked by pppd after it closes the connection.
  
  

A typical pppd 'options' file might look like the following. Lines beginning with '#' are comments.

# lock the modem device, to avoid another process trying
# to use it.
#
lock
#
#
# after connecting to the ISP, make the default route of
# the system go via this PPP connection. This means all
# packets not meant for the local network, will go out
# via the modem.
#
defaultroute
#
#
# use the given chat script in order to perform the dialing
# and possibly the login script as well.
#
connect '/usr/sbin/chat -v -f /etc/ppp/chat-script'
#
#
# use hardware flow-control between the computer and the
# _local_ modem, to avoid data loss.
#
crtscts
#
#
# the name of the user with which we intend to login to
# the remote system. This will be used when looking up
# the password in the 'pap-secrets' or 'chap-secrets'
# file.
#
user choo
#
#
# do not require the remote side to authenticate to us.
#
noauth
#

The chat script described above looks like this:

# define inputs after which we should abort the connection.
#
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
ABORT "ERROR"
ABORT "NO ANSWER"
ABORT "BUSY"
#
# the following is a set of 'expect' and 'send' sequences.
# chat waits for the first string, and if received, sends
# the second string.
#
# expect - nothing. send - modem reset command (atz).
"" "atz"
# expect - OK. send - modem initialization string.
OK "at&d0&c1"
# expect - OK. end dial command with ISP's modems number.
OK "atdt8319000"
# expect a connection string (CONNECT) and then we're done.
"CONNECT"

The 'pap-secrets' file would contain lines, each containing a user name, the '*' sign, and a password (in clear text). Here is an example:

# Secrets for authentication using PAP
# client        server  secret                  IP addresses
choo            *       mypasswd

• When we launch pppd, it immediately sets itself in to the background, and only then starts dialing.
  
  
• pppd logs all its operations to the system log file, /var/log/messages .
  
  
• chat also logs all the dial sequence into the same file, due to the usage of the '-v' flag.
  
  
• Running the command 'tail -f /var/log/messages' in a terminal, right after launching pppd, would show us the connection's progress.
  
  
• If the connection is not made properly - look at the log messages and try to understand where the problem is - and what the problem is about.
  
  
• Note that using the log messages could be useful also for figuring out problems when dialing out using kppp.
  
  
• In the worst case, pppd supports an option called 'debug', that causes much more verbose printing of messages. Using this option requires configuration of the system log daemon (syslogd). For more info, read the man pages of 'pppd', 'syslogd' and 'syslog.conf'.
  
  
• When we want to disconnect, we need to kill the 'pppd' process. An easy way to do that:
  
  killall pppd