Demand-dialled modem configuration on Debian (Sarge)

Debian GNU/Linux has excellent configuration dialogues for almost every aspect of the newly installed operating system. One fly in the ointment is that -- just as when I installed Red Hat 6.1 six years ago -- they do not account for people who might want their sysem to automatically dial up to the internet on demand. (For the sake of Google, I mention the terms auto-dial, autodial, and intermittent connection.)

Nowadays if one is configuring a PPP connection, chances are excellent that one's ISP is the other peer, in which case demand-dial is surely the obvious choice (unless there is some subtle problem with it I am unaware of). As it is it seems the only way is, after running pppconfig, to enable demand-dialling, one must read the pppd documentation and then edit /etc/ppp/peers/provider by hand to add demand and idle keywords.

One great advantage of demand-dialling is that it simplifies the rest of the configuration. For example, I use my modem to connect to Demon Internet to collect mail and for that I need to set my PPP connection to (a) not be defaultroute, and (b) to route packets for Demon's SMTP servers to the modem (rather than via my broadband gateway). This is because Demon uses SMTP to send me my mail, triggered by my modem dial-up, and restricted to the modem connection for security reasons.

Now without demand dialling, this is tricky: you cannot use Debian's standard repository for network info, /etc/network/interfaces, because the pppd command cannot instantly enable the interface, and so the on commands in the interfaces file will fail; as a result, I was instructed to create scripts in /etc/ppp/if-up.d, which is a nontrivial proposition. I wasted several evenings over this (and eliminating red herrings like wvdial). But with demand dialling, I can create routes via /etc/network/interfaces and it all just works. Yay!

Now all I need to do is add a configuration file for hot-plug events (my modem is on a PC Card) and add a cron job to ping once a day to cause the demand dialler to dial Demon.