domO -> 209.67.209.186
domU -> 216.32.93.124
another machine -> 209.67.213.238
dom0 can ping on domU
but another machine can't ping on domU
22:51:22.658974 IP 209.67.209.186 > 216.32.93.124: ICMP echo request, id 9760, seq 5, length 64
22:51:22.739018 IP 216.32.93.124 > 209.67.209.186: ICMP echo reply, id 9760, seq 5, length 64
22:51:22.990500 IP 209.67.213.238 > 216.32.93.124: ICMP echo request, id 31028, seq 82, length 64
22:51:24.060996 IP 209.67.213.238 > 216.32.93.124: ICMP echo request, id 31028, seq 83, length 64