今天OJ數據庫服務器掛了，我這邊很奇怪的ssh連不上，后來連上了但是很卡，看了下mysql/error.log沒看出來什么原因。學長那邊進去mysql用了下showprocesslist發現大量的unauthenticated user Connect的信息。於是查了一下原因。
手冊中的解釋是：unauthenticated user refers to a thread that has become associated with a client connection but for which authentication of the client user has not yet been done。意即：有一個線程在處理客戶端的連接，但是該客戶端還沒通過用戶驗證
MySQL處理用戶連接時進行如下過程來驗證用戶的合法性：
When a new client connects to mysqld, mysqld spawns a new thread to handle the request. This thread first checks whether the host name is in the host name cache. If not, the thread attempts to resolve the host name: The thread takes the IP address and resolves it to a host name (using gethostbyaddr()). It then takes that host name and resolves it back to the IP address (using gethostbyname()) and compares to ensure it is the original IP address.
即：先作反向解析IP>Hostname,然后作Hostname>IP的正向解析。如果結果符合，則驗證為合法用戶允許登錄，如果不符合則定義為“unauthenticated user”。
原因可能有：
1、服務器在做DNS反響解析，解決辦法有兩個：
1-）在 hosts 中添加客戶端ip，如
192.168.0.1 yejr
2-） MySQL啟動參數增加一個skip-name-resolve，即不啟用DNS反響解析。即修改mysql配置文件/etc/mysql/my.cnf,在 [mysqld] 行下添加 –skip-name-resolve ,重新啟動mysql服務
2、服務器的線程還處於排隊狀態，因此可以加大 back_log
采用第二個方法成功了。
建議創建用戶權限的時候，不要使用hostname作為限制而是直接用IP；更干脆的話就主機不配置DNS Server