Книга: Linux Network Administrator Guide, Second Edition
Demand Dialing
Demand Dialing
When there is IP traffic to be carried across the link, demand dialing causes your telephone modem to dial and to establish a connection to a remote host. Demand dialing is most useful when you can't leave your telephone line permanently switched to your Internet provider. For example, you might have to pay timed local calls, so it might be cheaper to have the telephone line switched on only when you need it and disconnected when you aren't using the Internet.
Traditional Linux solutions have used the diald command, which worked well but was fairly tricky to configure. Versions 2.3.0 and later of the PPP daemon have built-in support for demand dialing and make it very simple to configure. You must use a modern kernel for this to work, too. Any of the later 2.0 kernels will work just fine.
To configure pppd for demand dialing, all you need to do is add options to your options file or the pppd command line. The following table summarizes the options related to demand dialing:
| Option | Description |
|---|---|
| demand | This option specifies that the PPP link should be placed in demand dial mode. The PPP network device will be created, but the connect command will not be used until a datagram is transmitted by the local host. This option is mandatory for demand dialing to work. |
| active-filter expression | This option allows you to specify which data packets are to be considered active traffic. Any traffic matching the specified rule will restart the demand dial idle timer, ensuring that pppd waits again before closing the link. The filter syntax has been borrowed from the tcpdump command. The default filter matches all datagrams. |
| holdoff n | This option allows you to specify the minimum amount of time, in seconds, to wait before reconnecting this link if it terminates. If the connection fails while pppd believes it is in active use, it will be re-established after this timer has expired. This timer does not apply to reconnections after an idle timeout. |
| idle n | If this option is configured, pppd will disconnect the link whenever this timer expires. Idle times are specified in seconds. Each new active data packet will reset the timer. |
A simple demand dialing configuration would therefore look something like this:
demand
holdoff 60
idle 180
This configuration would enable demand dialing, wait 60 seconds before re-establishing a failed connection, and drop the link if 180 seconds pass without any active data on the link.
Оглавление книги
