What should the destination mac address be set to when sending packet outside of your local network?

Question

If I use arp and arping on machines in my local network I get the mac addresses from them. In the same way I can construct and send a ARP request and collect the response to these machines. This is used since I build raw packets completely from scratchy (to allow spoofing of every possible field, including mac addresses if needed). But, when I try arping or arp on external ip's and hosts such as google.com it doesn't get any reply. What should the destination mac address be set to when sending packets to targets outside my local network? I guess the router since that's what passes it on... am I correct? Is there a quick way in ANSI C to collect the mac address of the router in use by the computer? Or at least the IP so I can send a ARP request to it.

Thanx in advance

Answer 1

It should be set to the gateway (assuming ethernet on that link...).

Answer 2

MAC operations are limited to machines directly connected within your subnet. So you should use the router's MAC address for packets intended for hosts outside your subnet.

There are numerous ways to obtain the router's IP address.

1. You can parse the configuration files on your local host if the interface is statically configured.

2. You can see if your compute platform has an API that lets you access the interface configuration information directly. This would work in both static and dhcp cases.

3. You can write socket code to send an ICMP message to an outside address then parse the incoming responses. They will be from the router. The stack will, in this case, find the router for you.