How cu and uucp work

The cu(1bnu) and uucp(1bnu) commands request connections through the connection server, cs(1Mbnu). When it receives the request, cs performs the tasks necessary to make the connection, such as dialing a phone number through a modem, or connecting cu or uucp to a direct line.

After the connection is made (that is, when the connected message is received), uucp only has the capability to send login requests and passwords. This is known as the login sequence.

On a dialup or direct-connect request, the first thing cs does is select a line:

• If you specify a line using the command line (as in cu -l line), then cs attempts to use that line.

• If you specify a telephone number (without a line), then cs uses the line speed you specify to search /etc/uucp/Devices for an appropriate device. If you don't specify a line speed, cs selects the speed Any (see Devices(4bnu)).

• If you specify a system name (and not a telephone number or a line), then cs first looks in the /etc/uucp/Systems file (see Systems(4bnu)), and then the /etc/uucp/Devices file for an appropriate device.

Many groups can be defined in the Devices file. The purpose for having groups is to limit the number of entries that are searched through in the Devices file for a particular call, and to identify the type of device entry.

You can use group or type names on the command line (as in cu -t type) to search only entries of type type. To establish dialup connections, two of these groups are used: the ACU group and the Direct group.

Once a line is selected, the connection server cs sends characters out on the selected line and waits for a response.

The cs process looks in the /etc/uucp/Dialers file (see Dialers(4bnu)) for an entry that matches the type (or group) of the system being called.

The Dialers entries are arranged by modem type (or direct-connect type) names. There is a hayes entry, for example, and a direct_modem entry (which is used by the modem utility). The ``Connects to:'' field in the Dialup Setup: Device Properties window lists these Dialers file types.

Each entry in the Dialers file starts with a type (which equates to the device type) and a chat script for that particular device type. An entry may say, for example, to use ATDTnumber when dialing the telephone, and to then wait for the signal CONNECT before returning a connected message to the user.

Summary

To summarize, there are two or three files you go through when you make a dialup request.

If you directly specify a line, or just specify a phone number, the connection server gets an entry from the Devices file. On each device entry there is a type or group name that points to an entry in the Dialers file for the chat script.

If you give cs a system name, it first searches the Systems file for the connection type (modem or direct-connect) and then goes to the Devices file to search through the appropriate group(s) of entries for an available line.

Once a line is opened, cs consults the Dialers file for the chat script for the selected device. In this way you get a line and a dialer type for all calls you make.

If you use uucp to transfer files, the connection sequence involves one more step. That is, after the connection server has worked its way through the Dialers entry chat script, it uses the login sequence in the Systems(4bnu) file to attempt to log in to the remote system.