Эти скрипты относятся к pppd. Например, в ppp-2.4.2, входящего в slackware 10, их нет. Нет проблем их сделать руками.
man pppd
/etc/ppp/ip-up
A program or script which is executed when the link is available for sending and receiving IP packets (that is, IPCP has come up). It is executed with the parameters
interface-name tty-device speed local-IP-address remote-IP-address ipparam
/etc/ppp/ip-down
A program or script which is executed when the link is no longer available for sending and receiving IP packets. This script can be used for undoing the effects of the /etc/ppp/ip-up script. It is invoked in the same manner and with the same parameters as the ip-up script.