Whoops, sorry, I made some typos and copy&paste errors when posting the commands. Here we go again: # sysctl -w net.ipv4.conf.all.log_martians=1 # sysctl -w net.ipv4.conf.all.rp_filter=0 # tcpdump -w /tmp/dhcp4.pcapng -s 65535 -Uvnfi any 'port 67 or port 68'