I would like to capture traffic on Linux virtual interfaces, for debugging purposes. I have been experimenting with
dummy interface types; on all three, I am having trouble getting
tcpdump to show anything.
Here is how I set up the dummy interface:
ip link add dummy10 type dummy ip addr add 126.96.36.199 dev dummy10 ip link set dummy10 up
In one terminal, watch it with
tcpdump -i dummy10
In a second, listen on it with
nc -l 188.8.131.52 2048
In a third, make an HTTP request with
Although in terminal 2 we can see the data from the
curl request, nothing shows up from
A Tun/Tap tutorial clarifies some situations where the kernel may not actually send any packets when one is operating on a local interface:
Looking at the output of tshark, we see...nothing. There is no traffic going through the interface. This is correct: since we're pinging the interface's IP address, the operating system correctly decides that no packet needs to be sent "on the wire", and the kernel itself is replying to these pings. If you think about it, it's exactly what would happen if you pinged another interface's IP address (for example eth0): no packets would be sent out. This might sound obvious, but could be a source of confusion at first (it was for me).
However, it is hard to see how this could apply to TCP data packets.
tcpdump should be bound to the interface a different way?