Raspberry Pi Select Correct Network Interface eth0 ppp0

Question

I have a Raspberry Pi that has two network interfaces. 1, eth0 which is the Pi's ethernet port connected to my LAN (gets internet via Comcast, I connect to the Pi via SSH on my MBP). 2, ppp0 which is a 4G USB dongle (connected to AT&T).

I would like to keep the ability to connect to my Pi via LAN but route requests to/from the internet via ppp0. This project is a Pi camera hooked up to gstreamer, I have it working on the local LAN now I want it to work via the IP address assigned to the 4G USB dongle.

How would I change my routing tables to "force a specific network".

Here are my interface configurations and routing table configurations.

eth0      Link encap:Ethernet  HWaddr b8:27:eb:bb:ef:7d  
      inet addr:192.168.1.185  Bcast:192.168.1.255  Mask:255.255.255.0
      UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
      RX packets:1556 errors:0 dropped:0 overruns:0 frame:0
      TX packets:926 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:1000 
      RX bytes:110795 (108.1 KiB)  TX bytes:133594 (130.4 KiB)

ppp0      Link encap:Point-to-Point Protocol  
      inet addr:10.39.55.100  P-t-P:10.64.64.64  Mask:255.255.255.255
      UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
      RX packets:39 errors:0 dropped:0 overruns:0 frame:0
      TX packets:42 errors:0 dropped:0 overruns:0 carrier:0
      collisions:0 txqueuelen:3 
      RX bytes:2526 (2.4 KiB)  TX bytes:2741 (2.6 KiB)


pi@rpi ~ $ netstat -rn
Kernel IP routing table
Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface
0.0.0.0         192.168.1.1     0.0.0.0         UG        0 0          0 eth0
192.168.1.0     0.0.0.0         255.255.255.0   U         0 0          0 eth0

Answer 1

You must change the default gateway.

To delete the old default gateway:

route del default

To add the new gateway:

route add default gw 10.64.64.64 dev ppp0