10.2.4 Configuring a PPP interface
If you access the Internet through a modem connected to a dial-up telephone
line then the connection is negotiated using the Point-to-Point Protocol (PPP).
Such connections are accessed as network interface ppp0,
ppp1, and so on.
A PPP interface is managed by the PPP daemon pppd which comes in
the ppp package. Thus, for the user, configuring a PPP interface
means configuring pppd.
10.2.4.1 Configuring pppd manually
For a network link to be established, a communication port (usually a serial
port) needs to be opened, commands have to be sent to a communication device
(usually a modem), a telephone number may have to be dialed, identity has to be
authenticated to a foreign PPP daemon, a PPP interface has to be created by the
kernel and then routing tables have to be modified so that traffic can be sent
over the link. pppd can do all of this and consequently has a
very long list of operating options. These options are described in
pppd(8).
On a Debian system, global options are set up in /etc/ppp/options.
User-specific options are set up in ~/.ppprc. Options that must
depend on the communication port used are stored in
/etc/ppp/options.portname. For example, suppose you
have two modems—a built-in Lucent LT modem accessed through /dev/LT-modem
and an external modem accessed through /dev/ttyS0. Create the following two
options files.
# cat > /etc/ppp/options.LT-modem <<EOF
115200
init "/usr/sbin/chat -f /etc/chatscripts/setup-LT-modem"
EOF
# cat > /etc/ppp/options.ttyS0 <<EOF
115200
init "/usr/sbin/chat -f /etc/chatscripts/setup-ttyS0"
EOF
These refer to the following chat scripts. First,
/etc/chatscripts/setup-LT-modem.
ABORT ERROR
'' ATZ
OK 'ATW2X2 S7=70 S11=55'
OK AT
Second, /etc/chatscripts/setup-ttyS0.
ABORT ERROR
'' ATZ
OK 'ATL1M1Q0V1W2X4&C1&D2 S6=4 S7=70 S11=55 S95=63 S109=1 +FCLASS=0'
OK AT
The contents of these files must depend on your hardware, of course.
Options can also be given to pppd as arguments.
In Debian pppd is usually started using the pon
command. When pon is used its first argument names an options
file in /etc/ppp/peers/ which is also read by pppd.
[
54] This is where you set up
options that are specific to a particular peer—for example, a particular
Internet Service Provider (ISP).
Suppose for example you commute between Amsterdam and Den Haag. In each city
you have access to two ISP services—Planet and KPN. First create a basic
options file for each ISP.
# cat > /etc/ppp/peers/KPN <<EOF
remotename KPN
noauth
user kpn
noipdefault
ipparam KPN
EOF
# cat > /etc/ppp/peers/Planet <<EOF
remotename Planet
auth
user [email protected]
noipdefault
mru 1000
mtu 1000
ipparam Planet
EOF
These files set options that differ between the two ISPs. Options common to
both ISPs can be placed in /etc/ppp/options or in one of the
interface-specific options files as appropriate.
Now create options files for each ISP in each city. In our example the only
difference between connecting to an ISP in one location versus connecting in
another is the chatscript that is required. (The chatscript is different
because the local access telephone number is different.)
# cat > /etc/ppp/peers/KPN-Amsterdam <<EOF
connect "/usr/sbin/chat -v -f /etc/chatscripts/KPN-Amsterdam"
file /etc/ppp/peers/KPN
EOF
# cat > /etc/ppp/peers/KPN-DenHaag <<EOF
connect "/usr/sbin/chat -v -f /etc/chatscripts/KPN-DenHaag"
file /etc/ppp/peers/KPN
EOF
# cat > /etc/ppp/peers/Planet-Amsterdam <<EOF
connect "/usr/sbin/chat -v -f /etc/chatscripts/Planet-Amsterdam"
file /etc/ppp/peers/Planet
EOF
# cat > /etc/ppp/peers/Planet-DenHaag <<EOF
connect "/usr/sbin/chat -v -f /etc/chatscripts/Planet-DenHaag"
file /etc/ppp/peers/Planet
EOF
The file directives each include one of the options files shown
earlier. The connect directive specifies the command that
pppd uses to make the connection. Normally one uses the
chat program for this, adapting the chatscript to the ISP. Here
are the chatscripts for Den Haag; the chatscripts for Amsterdam might be
similar except for the telephone number or they might be different if the ISP
offers service through another company there.
# cat > /etc/chatscripts/KPN-DenHaag <<EOF
ABORT BUSY
ABORT 'NO CARRIER'
ABORT VOICE
ABORT 'NO DIALTONE'
ABORT 'NO DIAL TONE'
ABORT 'NO ANSWER'
ABORT ERROR
OK-AT-OK ATDT 0676012321
CONNECT \d\c
EOF
# cat > /etc/chatscripts/Planet-DenHaag <<EOF
ABORT BUSY
ABORT 'NO CARRIER'
ABORT VOICE
ABORT 'NO DIALTONE'
ABORT 'NO DIAL TONE'
ABORT 'NO ANSWER'
ABORT ERROR
OK-AT-OK ATDT 0676002505
CONNECT \d\c
EOF
To be able to connect to these ISPs you need client names and passwords that
pppd can supply to the peer on demand. This information is stored
either in /etc/ppp/pap-secrets (if the PAP protocol is used) or in
/etc/ppp/chap-secrets (if the CHAP protocol is used). Although
CHAP is more secure, PAP is still more widely used. Because these files
contain secrets, group and world should not have permission to read or write
them. The format of these files is explained in pppd(8). A
"secret" (third field) is looked up in the file by finding the client
name (first field) and/or the server name (second field). When connecting to
an ISP one generally doesn't know the server name, so one supplies a client
name instead; this was done on the user lines in
peers/KPN and peers/Planet above.
# client name server name secret
kpn * kpn
[email protected] * myfavoritepet
See /usr/share/doc/ppp/README.Debian.gz for more information.
10.2.4.2 Configuring pppd using pppconfig
A quick way to configure pppd is to use the pppconfig
program which comes in the package of the same name. pppconfig
sets up files like those above after asking the user questions through a menu
interface.
10.2.4.3 Configuring a PPP interface using wvdial
A different approach to using pppd is to run it from
wvdial which comes in the wvdial package. Instead of
pppd running chat to dial in and negotiate the
connection, wvdial does the dialing and initial negotiating and
then starts pppd to do the rest. Given only phone number,
username, and password wvdial succeeds in making the connection in
most cases.
