Olaf Kirch - Linux Network Administrator Guide, Second Edition

$ chat -v '' ATZ OK ATDT318714 CONNECT '' ogin: ppp word: GaGariN

By definition, the first string must be an expect string, but as the modem won't say anything before we have kicked it, we make chat skip the first expect by specifying an empty string. We then send ATZ , the reset command for Hayes-compatible modems, and wait for its response ( OK ). The next string sends the dial command along with the phone number to chat, and expects the CONNECT message in response. This is followed by an empty string again because we don't want to send anything now, but rather wait for the login prompt. The remainder of the chat script works exactly as described previously. This description probably looks a bit confusing, but we'll see in a moment that there is a way to make chat scripts a lot easier to understand.

The -v option makes chat log all activities to the syslog daemon local2 facility. [50] If you edit syslog.conf to redirect these log messages to a file, make sure this file isn't world readable, as chat also logs the entire chat script by default - including passwords.

Specifying the chat script on the command line bears a certain risk because users can view a process's command line with the ps command. You can avoid this risk by putting the chat script in a file like dial-c3po . You make chat read the script from the file instead of the command line by giving it the -f option, followed by the filename. This action has the added benefit of making our chat expect sequences easier to understand. To convert our example, our dial-c3po file would look like:

'' ATZ

OK ATDT318714

CONNECT ''

ogin: ppp

word: GaGariN

When we use a chat script file in this way, the string we expect to receive is on the left and the response we will send is on the right. They are much easier to read and understand when presented this way.

The complete pppd incantation would now look like this:

# pppd connect "chat -f dial-c3po" /dev/ttyS3 38400 -detach \

crtscts modem defaultroute

Besides the connect option that specifies the dialup script, we have added two more options to the command line: -detach , which tells pppd not to detach from the console and become a background process, and the modem keyword, which makes it perform modem-specific actions on the serial device, like disconnecting the line before and after the call. If you don't use this keyword, pppd will not monitor the port's DCD line and will therefore not detect whether the remote end hangs up unexpectedly.

The examples we have shown are rather simple; chat allows for much more complex scripts. For instance, it can specify strings on which to abort the chat with an error. Typical abort strings are messages like BUSY or NO CARRIER that your modem usually generates when the called number is busy or doesn't answer. To make chat recognize these messages immediately rather than timing out, you can specify them at the beginning of the script using the ABORT keyword:

$ chat -v ABORT BUSY ABORT 'NO CARRIER' '' ATZ OK…

Similarly, you can change the timeout value for parts of the chat scripts by inserting TIMEOUT options.

Sometimes you also need to have conditional execution for parts of the chat script: when you don't receive the remote end's login prompt, you might want to send a BREAK or a carriage return. You can achieve this by appending a subscript to an expect string. The subscript consists of a sequence of send and expect strings, just like the overall script itself, which are separated by hyphens. The subscript is executed whenever the expected string it is appended to is not received in time. In the example above, we would modify the chat script as follows:

ogin:-BREAK-ogin: ppp ssword: GaGariN

When chat doesn't see the remote system send the login prompt, the subscript is executed by first sending a BREAK, and then waiting for the login prompt again. If the prompt now appears, the script continues as usual; otherwise, it will terminate with an error.

IPCP is used to negotiate a number of IP parameters at link configuration time. Usually, each peer sends an IPCP Configuration Request packet, indicating which values it wants to change from the defaults and the new value. Upon receipt, the remote end inspects each option in turn and either acknowledges or rejects it.

pppd gives you a lot of control over which IPCP options it will try to negotiate. You can tune it through various command-line options that we will discuss in this section.

All IP interfaces require IP addresses assigned to them; a PPP device always has an IP address. The PPP suite of protocols provides a mechanism that allows the automatic assignment of IP addresses to PPP interfaces. It is possible for the PPP program at one end of a point-to-point link to assign an IP address for the remote end to use, or each may use its own.

Some PPP servers that handle a lot of client sites assign addresses dynamically; addresses are assigned to systems only when calling in and are reclaimed after they have logged off again. This allows the number of IP addresses required to be limited to the number of dialup lines. While limitation is convenient for managers of the PPP dialup server, it is often less convenient for users who are dialing in. We discussed the way that hostnames are mapped to IP addresses by use of a database in Chapter 6. In order for people to connect to your host, they must know your IP address or the hostname associated with it. If you are a user of a PPP service that assigns you an IP address dynamically, this knowledge is difficult without providing some means of allowing the DNS database to be updated after you are assigned an IP address. Such systems do exist, but we won't cover them in detail here; instead, we will look at the more preferable approach, which involves you being able to use the same IP address each time you establish your network connection. [51] More information on two dynamic host assignment mechanisms can be found at http://www.dynip.com/ and http://www.justlinux.com/dynamic_dns.html.

In the previous example, we had pppd dial up c3po and establish an IP link. No provisions were taken to choose a particular IP address on either end of the link. Instead, we let pppd take its default action. It attempts to resolve the local hostname, vlager in our example, to an IP address, which it uses for the local end, while letting the remote machine, c3po , provide its own. PPP supports several alternatives to this arrangement.

To ask for particular addresses, you generally provide pppd with the following option:

local_addr : remote_addr

local_addr and remote_addr may be specified either in dotted quad notation or as hostnames. [52] Using hostnames in this option has consequences for CHAP authentication. Please refer to the "Authentication with PPP" section later in this chapter. This option makes pppd attempt to use the first address supplied as its own IP address, and the second as the peer's. If the peer rejects either of the addresses during IPCP negotiation, no IP link will be established. [53] The ipcp-accept-local and ipcp-accept-remote options instruct your pppd to accept the local and remote IP addresses being offered by the remote PPP, even if you've supplied some in your configuration. If these options are not configured, your pppd will reject any attempt to negotiate the IP addresses used.