ping_command
char *ping_command(int family);
snprintf(cmd, sizeof(cmd), "%s ::1 -A -c 10000 -q > /dev/null", ping_command(AF_INET6));
SYS(fail, "ip netns exec " NS_SRC " %s " PING_ARGS " %s > /dev/null", ping_command(family), addr);
SYS(fail, "%s %s %s > /dev/null", ping_command(family), PING_ARGS, addr);