Avoid erasing default gateway when ifup another interface—WiFi & 3G—routing table

http://stackoverflow.com/questions/34873991/avoid-erasing-default-gateway-when-ifup-another-interface

I'm working on an custom embedded board with two interfaces: WiFi & 3G.

I can only work with 1 routing table so I have defined prioritiesWhen 3G is ON and WiFi is connected to a Router, Gateway has to be the 3G interface in order to make the embedded externally accesible. So when the embedded try to connect to 3G he changes the default GW to this interface by itself (i think udhpc does this).

My problem comes when I'm connected by WiFi to a Router with Internte connection but I try to start a new 3G connection with wrong parameters (bad APN). In this case I can only set a timeout (40-60s) waiting for the IP, after this time I return an error code (APN or signal problem) and continue just with the WiFi connection.The problem is that "someone" removes default gw (wifi) when is trying to connect to 3G and I lose Internet connection for 40-60s...

I need to keep the default gateway on the WiFi interface and just replace it when the connection suceed (udhcp job after obtaining ip).

First I thought that was udhcpc who was removing that gateway but as I can see, it is removed sometimes before and other after the udhcpc leasing to background.