500 OOPS: vsftpd: cannot locate user specified in ‘ftp_username’:ftp

500
OOPS: vsftpd: cannot locate user specified in 'ftp_username':ftp

的錯誤訊息。我在google上搜了一把後來發現有一封mail非常有用：
http://lists.debian.org/debian-user/.../msg02648.html 

其中有一段話提到：

Yep, I did. There is no occurrence of the string "tunable" anywhere in the vsftpd.conf man page ...

However, I've been delving into the source code, and now understand this - the author uses the name of a config file directive *prefixed by* "tunable_" inside the source code as a variable name. So what I needed to do was define "ftp_username" in vsftpd.conf.
For some reason the Debian package leaves a little to be desired in this area, and neither asks you how you want this directive set, nor creates an account for use with vsftpd anonymous connections. The default for this directive is the name "ftp", but there
has never been an "ftp" account on my Woody system, nor was one created by the vsftpd installation.

作者提到收到一個500 Oops但是其中提到的tunable_ftp_username哪裡也找不到。後來他查看源碼，發現源碼中是使用一個以"tunable_"為首碼的變數，實際上相應在vsftpd.conf中應該是ftp_username這個directive。所以他在vsftpd.conf中加入了ftp_username=xxx（使用者）這一行。但是他也抱怨Debian的發行版沒有在任何地方標識這樣的問題。而且這個ftp_username的預設使用者應該是ftp，但是woody系統預設安裝後並沒有這個ftp使用者存在，而且vsftpd安裝後也沒有添加這個使用者。

註：我是用ftp_username=nobody的，然後/etc/init.d/vsftpd restart後成功登陸。