问题描述：
SUSE LINUX，root用户通过ftp登陆到服务器失败： pam_listfile(vsftpd:auth): Refused user root for service vsftpd
db2b(192.168.187.139)机器：
db2b:~ #
ftp 192.168.187.138
Wrapper for lftp to simulate compatibility with lukemftp
Name (root):
root
Password: <--确认输入了正确的密码
lftp root@192.168.187.138:~>
ls
---- Connecting to 192.168.187.138 (192.168.187.138) port 21
<--- 220 (vsFTPd 3.0.2)
ls: Login failed: 530
Login incorrect.
db2a(192.168.187.138)机器上查看状态,ftp服务确实是active的：
db2a:~ #
service vsftpd status
鈼m vsftpd.service - Vsftpd ftp daemon
Loaded: loaded (/usr/lib/systemd/system/vsftpd.service; disabled; vendor preset: disabled)
Active: active (running) since Thu 2018-02-01 17:00:01 CST; 16h ago
Main PID: 17368 (vsftpd)
Tasks: 1 (limit: 512)
CGroup: /system.slice/vsftpd.service
鈹斺攢17368 /usr/sbin/vsftpd /etc/vsftpd.conf
Feb 01 17:00:01 db2a systemd[1]: Started Vsftpd ftp daemon.
Feb 01 17:06:39 db2a vsftpd[18011]: CONNECT: Client "::ffff:192.168.187.139"
Feb 01 17:06:39 db2a vsftpd[18010]:
pam_listfile(vsftpd:auth): Refused user root for service vsftpd
Feb 01 17:06:41 db2a vsftpd[18010]: [root] FAIL LOGIN: Client "::ffff:192.168.187.139"
Feb 01 17:06:50 db2a vsftpd[18016]: CONNECT: Client "::ffff:192.168.187.139"
Feb 01 17:06:50 db2a vsftpd[18015]: pam_listfile(vsftpd:auth): Refused user root for service vsftpd
Feb 01 17:06:53 db2a vsftpd[18015]: [root] FAIL LOGIN: Client "::ffff:192.168.187.139"
Feb 01 17:07:17 db2a vsftpd[18053]: CONNECT: Client "::ffff:192.168.187.139"
Feb 01 17:07:17 db2a vsftpd[18052]: pam_listfile(vsftpd:auth): Refused user root for service vsftpd
Feb 01 17:07:18 db2a vsftpd[18052]: [root] FAIL LOGIN: Client "::ffff:192.168.187.139"
==============
原因：
决定一个用户是否可以通过ftp登录有两个配置文件：ftpusers和user_list. 首先，在ftpusers里的用户都不能通过ftp登录；其次user_list里的用户能否登陆是由参数userlist_deny决定的：如果userlist_deny=YES，则user_list里的用户不能登陆，如果userlist_deny=NO，则user_list里的用户可以通过ftp登陆。
可以看到我的环境里，root用户是在ftpusers列表里的：所以会报错
db2a:~ #
cat /etc/ftpusers
#
# ftpusers
This file describes the names of the users that may
#
_*NOT*_ log into the system via the FTP server.
#
This usually includes "root", "uucp", "news" and the
#
like, because those users have too much power to be
#
allowed to do "just" FTP...
#
adabas
amanda
..
root
...
db2a:~ #
ls /etc/*ftp*
/etc/ftpusers /etc/lftp.conf /etc/vsftpd.conf
解决方法：
将root从/etc/ftpusers文件里删除或者加上注释,并重启ftp服务: service vsftpd restart