求助:vsftpd 虚拟用户无法更改文件权限?

上传下载删除等都没有问题 就是在filezilla中对文件权限进行更改的时候就提示

命令: SITE CHMOD 755 rand.txt
响应: 550 SITE CHMOD command failed.

vsftpd配置如下:
-------------------------
# cat /etc/vsftpd/vsftpd.conf
guest_enable=YES
guest_username=ftp
user_config_dir=/etc/vsftpd/vuser_conf
virtual_use_local_privs=YES
pam_service_name=/etc/pam.d/vsftpd
#
pasv_enable=YES
pasv_min_port=40000
pasv_max_port=40080
pasv_promiscuous=YES
#
chroot_list_file=/etc/vsftpd/vuser_password
chmod_enable=YES

-------------------------------
# cat /etc/vsftpd/vuser_password
aaa
123456
bbb
123456
ccc
123456


---------------------------------
# cat /etc/pam.d/vsftpd
auth required pam_userdb.so db=/etc/vsftpd/vuser_password
account required pam_userdb.so db=/etc/vsftpd/vuser_password
-------------------------------------
# cat /etc/vsftpd/vuser_conf/aaa

local_root=/var/www/html/aaa
write_enable=YES
anon_umask=022
anon_world_readable_only=NO
anon_upload_enable=YES
anon_mkdir_write_enable=YES
anon_other_write_enable=YES

----------------------------------
# ll /var/www/html
total 48
drwxrwxrwx 13 root ftp 4096 Aug 20 10:21 aaa

请熟悉linux的朋友帮忙看下配置哪里出现了错误