Almost Internet with SLiRP and PPP

With SLiRP installed on your remote host and pppd installed on your Linux machine, it's time to try to get them to work together. It's a good idea to dial into your Internet service provider (the remote machine) using your favorite communications package (such as kermit, minicom, or Seyon) for the first few times in order to test the login and connection process. There are several steps to take:

Figure 3. Initial Routing Table

Figure 4. Final Routing Table

Once you have pppd and SLiRP talking to each other, you can create a more sophisticated ppp-on script to automatically dial the remote host, log you in, start slirp, and then start pppd on your Linux machine. I have created such a script which uses the “chat” utility that came with the pppd package; it is shown in Figure 5.

Figure 5. Sophisticated ppp-on Script With chat Options

This shell script is designed to look in a certain directory (such as ~/.ppp or /usr/local/ppp/script) for “chat scripts” to use in dialing and logging in. When started with an argument, e.g., ppp-on my-isp-name, this version of ppp-on will use a chat script called ~/.ppp/my-isp-name.chat for dialing and logging in. Such a feature allows a single user to have multiple dialing scripts for multiple Internet Service Providers, and allows multiple users to keep their passwords private.

When started with the -c option, this ppp-on script can also execute a custom script after starting the PPP connection. For example, ppp-on -c would perform a script called ~/.ppp/default.ppp after starting pppd, and ppp-on -c my-isp-name would dial and automatically log me in using the chat script as described above, then perform ~/.ppp/my-isp-name.ppp after starting pppd. This can be a useful substitute for the /etc/ppp/ip-up script in a multi-user environment. Additionally, this ppp-on script allows a verbose option -v to log the execution of the chat script to the system logs for debugging.

A chat script consists of a sequence of “expect-send” strings separated by whitespace. chat expects to receive on its standard input the first non-keyword string in the script; when it does, it sends the next non-keyword string followed by a carriage return to the standard output, and so on. To make a customized chat script, you can use the session log you made above while logging into your remote host. A sample session log and the resulting chat script are shown here; you may wish to consult chat's manual page as well.

First, the sample session log:

at
OK
atz
OK
at&b0s0=0
OK
atdtMY-PHONE-NUMBER
CARRIER 28800
PROTOCOL: LAP-M
CONNECT 115200
Welcome to My Internet Service Provider
suchandsuch login: MY-LOGIN-ID
Password:
--SYSTEM NEWS--
        blah blah blah blah
        blah blah blah blah blah blah.
TERM = unknown
TERM = (vt100) vt102
MY-SHELL-PROMPT% slirp

And this is the chat script that matches the session log:

ABORT BUSY
ABORT "NO CARRIER"
ABORT "NO DIALTONE"
TIMEOUT 2   OK-at-OK atz
TIMEOUT 5   OK at&b0s0=0
            OK atdtMY-PHONE-NUMBER
TIMEOUT 60  CONNECT ""
TIMEOUT 10  ogin:--ogin: MY-LOGIN-ID
            ssword: \qMY-PASSWORD
            (vt100) vt102
            MY-SHELL-PROMPT% slirp

The script uses several features of chat which make it easier to perform automated logins:

If we can make it easier to log into a remote host using SLiRP, pppd, and chat, we ought to be able to make it easier to log out as well. The ppp-off script (see Figure 2) does some of this, but not quite enough. pppd does have a disconnect option, but since you may wish to use pppd to connect to multiple Internet service providers, one or more of which may not use SLiRP, there's a more flexible solution: instead of trying to use pppd to hang up the phone (which is not its job), we'll use chat to exit SLiRP and hang up the phone after pppd has disconnected the PPP link.

Figure 6. The remote-slirp-off script

Figure 6 contains a shell script called remote-slirp-off, which does just that. This shell script shows chat scripts included as arguments to chat instead of read from a script file. Note the use of double-backslashes for chat escape sequences—if they weren't doubled, the shell would remove them, and the characters which follow would simply appear to be part of the send string. The \d escape delays for one second, while the \p escape delays for a tenth of a second. \c indicates not to send a carriage return after the send string in which it appears. The script also provides a verbose option -v to log the chats to the system logs for debugging. Like chat, remote-slirp-off expects the modem to be on both standard input and standard output.

Armed with the “glue” of the foregoing scripts, including your custom-made chat script for dialing into your Internet account, you can start a PPP connection between pppd and SLiRP using the simple command:

ppp-on name-of-chat-script

and you can stop the connection using the simple command sequence:

ppp-off
remote-slirp-ff </dev/modem >/dev/modem

If you simply must have simple, one-word commands to perform these actions, you can easily put them into an alias, shell function, or yet another shell script.