今日發現數據庫的連接非常慢，甚至是無法連接，想要我檢查看看
#su - oracle
$ lsnrctl status
LSNRCTL for Linux: Version 11.2.0.1.0 - Production on 17-MAY-2018 12:29:11
Copyright (c) 1991, 2009, Oracle. All rights reserved.
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=oracletest)(PORT=1521))) ## 到這裏，
hang住了，一直沒有響應從其他主機做了個tnsping測試：
oracle@IBlvs02:/u01/app/oracle/11.2/db_1/network/admin> tnsping oracletest
TNS Ping Utility for Linux: Version 11.2.0.1.0 - Production on 17-MAY-2018 12:54:20
Copyright (c) 1991, 2009, Oracle. All rights reserved.
Used parameter files:
Used TNSNAMES adapter to resolve the alias
Attempting to contact (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = 192.168.121.134)(PORT = 1521))) (CONNECT_DATA = (SERVICE_NAME = wdadb))) ## 這裏也是hang住, 一直沒有響應最後檢查DNS配置時，發現有：
# cat /etc/resolv.conf
nameserver 211.141.90.68
系統配置有DNS，這是內網主機，根本用不到DNS，所以把這個DNS去掉後恢復正常。其實可以考慮開啓listener的trace進行查看的。
或修改sshd配置文件UseDNS參數爲no，不解析dns
#vi /etc/ssh/sshd_config
UseDNS no這樣即使保留了/etc/resolv.conf系統配置DNS也沒有關係。