I have a Linux box (Debian squeeze) with several NICs. The ones of interest are: eth3 - my main link to the network (dhcp on 10.20.30.0/24) eth0 - the first connection to my test network (static: 192.168.1.2) eth4 - the second connection to my test network (static: 192.168.1.1) My routing table looks like this: $ sudo route Kernel IP routing table Destination Gateway Genmask Flags Metric Ref Use Iface 10.20.30.0 * 255.255.255.0 U 0 0 0 eth3 default 10.20.30.254 0.0.0.0 UG 0 0 0 eth3 I have the 2 test net ports connected to each other with a crossover cable and an instance of wireshark running on each port. Every once in a while, I'll see a packet like the following show up. Who could be doing this, and how do I convince them to stop? I do have Samba running on the machine (for a cifs mount) but don't see why it would be sending packets out to unrouted ports. I had a Windows VM running in VMWare Client and thought that might be causing it, but it still happens without it. What I want is totally silent interfaces so I can run some tests with Scapy over them.