Planetlab and scapy: MAC address to reach destination not found

Question

Using the minimal Fedora 14 distribution provided in PlanetLab, I'm forging packets through Scapy and sending them with tcpreplay.

I thought I had installed all the necessary packages to run scapy and tcpreplay, but for some reason scapy can't access the mac address of the machine.

For instance, if I send a packet through Scapy, I always get:

"WARNING: Mac address to reach destination not found. Using broadcast."

Also, if I add an Ethernet layer to an existing IP packet, the resulting destination is always ff:ff:ff:ff:ff:ff.

What am I missing here?

Solution

Ok, so I figured out what was happening:

• On PlanetLab you can't make any ARP requests, even if you are root. You can only read the ARP table that is provided to you
• When Scapy builds an Ethernet layer with no input destination address, it makes an ARP request to fill out that field of the Ethernet header.

=> Solution: I had to lookup the ARP table myself and specify the gateway's MAC address in each packet.