This appendix is a reference for dip, pppd,
and chat. These tools are used to
create dial-up IP connections for the Point-to-Point Protocol (PPP). dip and
chat are both scripting languages.
Creating a script that initializes the modem, dials the remote server,
logs in, and configures the remote server is the biggest task in
configuring a PPP connection. Chapter 6
provides examples and tutorial information about all three of the programs
covered here. This appendix provides a reference to the programs.
dip is a scripting tool
designed specifically for creating SLIP and PPP
connections.[154]
The syntax of the dip command
is:
dip [options] [scriptfile]
The dip command is invoked with
an option set, a script file specified, or both. When
scriptfile is specified, dip executes the commands contained in the
script file to create a point-to-point connection. Examples of scripts
and dip are shown in Chapter 6. The
options
valid with script files are:
-vRuns dip in verbose mode.
In this mode, dip echoes each
line of the script file as it is executed and displays enhanced
status messages.
-m
mtuSets the maximum transmission unit (MTU) to the number of
bytes specified by mtu. The default MTU
is 296 bytes.
-p
protoSelects the serial line protocol. Possible values for
proto are: SLIP, CSLIP, PPP, or
TERM.
The other dip command-line
options are:
-kKills the last dip
process you started. You can only kill a process you own, unless
of course you’re root.
-l
deviceSpecifies that the process to be killed is the one that has
locked the specified device. This
option is valid only when used with the -k option.
-i [
username ]Runs dip as a login shell
to provide a PPP server. The diplogin command is equivalent to
dip -i. These two forms of the
command are used interchangeably, but diplogin is the most common form.
diplogin is placed in the login
shell field of the /etc/passwd file entry for
each PPP client. From there it is run by login. The username from the
/etc/passwd file is used to retrieve
additional configuration information from
/etc/diphosts. If the optional
username is specified with the diplogin command, that username is used
to retrieve the information from the
/etc/diphosts file. Chapter 6 provides a tutorial and
examples of creating a PPP server.
-aPrompts for the username and password. The -a option is valid only when used with
the -i option. The diplogini command is equivalent to
dip -i -a. diplogini is used as a login shell in
the /etc/passwd file where it is run by
login.
-tRuns dip in test mode,
which allows you to input individual script language commands
directly from the keyboard. The -t option is frequently used in
combination with -v so that the
result of each command can be better observed. As shown in Chapter 6, this option is used to
debug a dip script.
diplogin and diplogini are used only on servers and are not
used with a script file. The script file is used on the PPP clients when
dip is configured to dial into a
remote server. The script file contains the instructions used to do
this.
The script file is made up of comments, labels, variables, and commands. Any line that begins with a sharp sign (#) is a comment. A label is a line that contains only a string ending in a colon. Labels are used to divide the script into separate procedures. For example, the section of the script that dials the remote host might begin with the label:
Dial-in:
A variable stores a value. A variable name is a string that
begins with a dollar sign ($). You might, for example, create a
variable to hold a loop counter and give it the name $loopcntr. It is possible to create your own
variables, but this is rarely done. The variables that are used in
most scripts are the special variables defined by dip. Table A-1 lists the special
variables and the value that each holds.
Table A-1. dip special variables
Variable | Value stored |
|---|---|
$errlvl | The return code of the last command |
$locip | The IP address of the local host |
$local | The fully qualified domain name of the local host |
$rmtip | The IP address of the remote host |
$remote | The fully qualified domain name of the remote host |
$mtu | The maximum transmission unit in bytes |
$modem | The modem type; currently this must be HAYES |
$port | The name of the serial device, e.g., cua0 |
$speed | The transmission speed of the port |
The final component of the script file is the command list.
There are many script commands. Because this appendix is a reference,
we cover them all. However, most scripts are built using only a few of
these commands. See the sample scripts in Chapter 6 and at the end of this section
for realistic dip scripts. The
complete list of script commands is:
beep [
n ]Tells the system to beep the user. Repeat
n times.
bootpTells the system to use the BOOTP protocol to obtain the
local and remote IP addresses. This command applies only to
SLIP. PPP has its own protocol for assigning addresses; SLIP
does not. Usually SLIP addresses are statically set inside the
script. However, some SLIP servers have evolved techniques for
dynamic address assignment. The most common method is for the
server to display the address as clear text immediately after
the connection is made. Use the get $locip remote command to retrieve the address from
this type of SLIP server. Other SLIP servers require you to send
them a command before they will display the address. Put the
required server command in the script and follow it with the
get command. Finally, a few
SLIP servers use BOOTP to distribute addresses. Use the bootp command in your script to enable
BOOTP when it is required by your SLIP server.
breakSends a BREAK. Some remote servers may require a BREAK as an attention character.
chatkey
keyword codeMaps a modem response keyword
to a numeric code. The predefined
mappings are:
config [interface|routing] [pre|up|down|post]
arguments...Modifies interface characteristics (interface) or the routing table
(routing) either before
(pre) the link comes up, when
it is up, when it goes
down, or after (post) the link is shut down. For
example:
config up routing add canary gw ibis
adds a route to canary using
ibis as the gateway when the link is up.
Allowing users to modify the routing table or interface
characteristics is very dangerous. The config command is disabled in the DIP
code and requires recompilation to be enabled.
databits 7|8Sets the number of data bits to 7 or 8. 8 bits is recommended for PPP and SLIP links.
dec
$variable [ value
]Decrements $variable by
value. The default
value is 1.
defaultSets the PPP connection as the default route.
dial
phonenumber [ timeout
]Dials the phonenumber. If the
remote modem does not answer within
timeout seconds, the connection
aborts. $errlvl is set to a
numeric value based on the keyword returned by the local modem.
Set chatkey for the keyword
to numeric mappings.
echo on|offEnables or disables the display of modem commands.
exit [
n ]Exits the script, optionally returning the number
n as the exit status. Clears the
input buffer.
get
$variable [ask | remote [ timeout ]]
valueSets $variable to
value, unless ask or remote is specified. When ask is specified, the user is prompted
for the value. When remote is
specified, the value is read from the remote machine, optionally
waiting timeout seconds for the
remote system to respond.
goto
labelJumps to the section of the script identified by
label.
helpLists the dip script
commands.
if
expr goto
labelA conditional statement that jumps to the section of the
script identified by label if the
expression evaluates to true. The
expression must compare a variable to a constant using one of
these operators: == (equal), != (not equal), < (less than),
> (greater than), <= (less than or equal to), >=
(greater than or equal to).
inc
$variable [ value
]Increments $variable by
value. The default
value is 1.
init
commandSets the command string used to initialize the modem. The
default is ATE0 Q0 V1
X1.
mode SLIP|CSLIP|PPP|TERMSelects the serial protocol. The default is SLIP, so this should be set to PPP.
modem
typeSets the modem type. Ignore this command. The only legal value is HAYES, and that is the default.
netmask
maskSets the address mask.
parity E|O|NSets the parity to even (E), odd (O), or no (N). No parity (N) is recommended for SLIP and PPP
links.
passwordPrompts the user for the password.
proxyarpInstalls a proxy ARP entry for the remote system in the local host’s ARP table.
print
$variableDisplays the contents of
$variable.
psend
commandExecutes command through the
default shell passing the output to the serial device. The
command runs using the user’s real UID.
port
deviceIdentifies the serial device, such as cua0, that attaches the modem.
quitExits the script with a nonzero exit status, aborting the connection.
resetResets the modem.
send
stringPasses string to the serial
device.
shell
commandExecutes command through the
default shell. The command runs using the user’s real
UID.
skey [
timeout ]Waits for an S/Key challenge from the remote terminal
server, prompts the user for the secret key, and generates and
sends the response. Waits timeout
seconds for the challenge. If the timer expires, $errlvl is set to 1; otherwise, it is
set to 0. S/Key must be compiled into dip.
sleep
timeDelays time seconds.
speed
bits-per-secondSets the port speed. The default is 38400.
stopbits 1|2Sets the number of stop bits to 1 or 2. Enables terminal mode. In terminal mode, keyboard input is passed directly to the serial device.
timeout
timeSets the time in seconds that
the line is allowed to remain inactive. When this timer expires,
the link is closed.
wait
text [ timeout
]Waits timeout seconds for the
text string to arrive from the remote
system. If timeout is not specified,
the script will wait forever.
In the next section we put some of these commands to work in a realistic script.
This script is based on the PPP sample from Chapter 6. Labels and error detection have been added to create a more robust script.
# Select configuration settings setup: # Ask PPP to provide the addresses get $local 0.0.0.0 # Select the port port cua1 # Set the port speed speed 57600 # Create a loop counter get $loopcntr 0 # Dial the remote server dialin: # Reset the modem and clear the input buffer reset flush # Dial the PPP server and check the modem response dial *70,301-555-1234 # If BUSY, dial again if $errlvl == 3 goto redial # If some other error, abort if $errlvl != 1 goto dial-error # Otherwise rest loop counter get $loopcntr 0 # Give the server 2 seconds to get ready sleep 2 # Login to the remote server login: # Send a carriage-return to wake up the server send \r # Wait for the Username> prompt and send the username wait name> 20 if $errlvl != 0 goto try-again send kristin\r # Wait for the Password> prompt and send the password wait word> 10 if $errlvl != 0 goto server-failure password # Wait for the PPP server's command-line prompt wait > 20 if $errlvl != 0 goto server-failure # Send the command required by the PPP server send ppp enabled\r # Success! We're on-line connected: # Set the interface to PPP mode mode PPP # Exit the script exit # Error processing routines # Try dialing 3 times. Wait 5 seconds between attempts redial: inc $loopcntr if $loopcntr > 3 goto busy-failure sleep 5 goto dialin # Try a second carriage return try-again: inc $loopcntr if $loopcntr > 1 goto server-failure goto login dial-error: print Dial up of $remote failed. quit server-failure: print $remote failed to respond. quit busy-failure: print $remote is busy. Try again later. quit
This script provides a realistic example of the commands used
in most scripts. However, you may encounter a particularly tough
scripting problem. If you do, the abundance of scripting commands
available with dip should be able
to handle it. If dip can’t do the
job, try expect. See
Exploring Expect by Don Libes (O’Reilly &
Associates) for a full description of the expect scripting language.
[154] Serial Line IP (SLIP) predates PPP. Today most serial connections are PPP, which is what this appendix emphasizes.