I am running Debian Linux on an embedded system. It has an ethernet port and it also uses 3G USB modem. Wvdial and pppd are used and I have pppd configured to make the interface (ppp0) the default route when it comes up.
In the field these systems only use the modem but I like to login over ethernet (SSH) to monitor things while testing on the bench. In particular I'm trying to test the situation where the modem loses its carrier. In this case PPPD eventually fails and the ppp0 interface is removed. My LAN then becomes the default route allowing my software to behave normally. But I don't want my software to know about that LAN.
Is the thing to do to create a static route only for things destined for my LAN such that when the ppp0 interface disappears it will fail when trying to access our servers? Other suggestions?