Today I found I can't ping the localhost
ping 127.0.0.1 PING 127.0.0.1 (127.0.0.1): 56 data bytes Request timeout for icmp_seq 0 Request timeout for icmp_seq 1 Request timeout for icmp_seq 2 ^C --- 127.0.0.1 ping statistics --- 4 packets transmitted, 0 packets received, 100.0% packet loss
And After some Google, it's related with the StealthMode, so you can turn it off
sudo /usr/libexec/ApplicationFirewall/socketfilterfw --setstealthmode off