I have a C++ program which runs on many Linux operating systems. One of the things it does for initial setup is to try to determine the default network interface name, which is often, but not always, eth0. It accomplishes this by giving the "route" command and looking for the line which contains the word "default." I now face the prospect of it being run on routers (running Linux) which have several NICs. Apparently, on the router on which it was tried, the route command showed neither the word default nor the address 0.0.0.0, making it impossible for my program to determine the default interface name.

Is there a reasonably graceful way to find out which of the interface names listed is connected to the LAN by giving a command or calling a library function? I hasten to add that I am not very familiar with networking.

Any such technique must be something that a program can do, it cannot require any special setup on the machine in question, and it cannot be specific to some 3rd party software which is not widespread.

Thanks.

If there isn't 0.0.0.0 or default entry in routing table, that means no default route in the system.
Can you upload the routing table?

Sure, how do I obtain it? I know how to do the route command, etc., but I do not know how to just find the routing table itself.

Just using route command to dump routing table.

I've modified a few things so as to protect confidential information, but this is basically it:

route

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
109.33.116.105 * 255.255.255.105 U 0 0 0 hc.eth0
192.168.0.0 * 255.255.255.0 U 0 0 0 eth1
192.0.2.0 * 255.255.255.0 U 0 0 0 dummy0
192.0.2.0 * 255.255.255.0 U 0 0 0 utun

As you can see, neither "deafult" nor a flag of UG occurs.

All route entry is interface or host route. I think that the system doesn't config default route. To confirm it, you add default route manually and then execute route command.

The question was how to identify the default interface name from a program.

If there isn't default route in system, that means no default interface.

Fair enough, but it's still true that one of the interfaces faces the LAN, and I would like a way for my program to be able tell which one. Short of having the program ping them all and parsing the result to see which ping worked, and assuming that ping will even be active, is there any way to do that?

maybe?

"route -n" here isn't much help, as it lists all my available Ifaces.
arp at least shows which one is being used, no?

I guess I should stop this. This question arose out of a problem that ocurred when my program was installed on some routers. In the end, having no other answer and needing a solution that would work everywhere, I created a new parameter in a config file to allow a user to write in an interface name for the program to use if it failed to discover a default in any other way. This worked and I no longer have access to that environment. I was keeping this discussion going out of a desire to understand the subject, but I have no further capacity to test suggestions. Would the use of "arp" work everywhere under all (reasonable) conditions, though?