在Linux平台中，对hostname的修改，是否对ORACLE数据库实例或监听进程有影响呢？如果有影响，又要如何解决问题呢？另外/etc/hosts下相关内容的修改，是否也会影响实例或监听呢？这里涉及的场景非常多，当然关系也非常复杂，我们下面通过几个例子来测试验证一下。
如下所示，服务器/etc/hosts 与/etc/sysconfig/network的原始配置信息如下
[root@test ~]# more /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
#::1 localhost6.localdomain6 localhost6
127.0.0.1 localhost.localdomain localhost
192.168.27.134 test test
[root@test ~]#
[root@test ~]# more /etc/sysconfig/network
NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=test
GATEWAY=192.168.27.1
[root@test ~]#
1： 首先假设有个需求，需要修改hostname，使之变成test.edution.com（加上域名部分）, 那么此时是否有问题呢？我们先修改/etc/sysconfig/network下的HOSTNAME，然后重启服务器
[root@test ~]# more /etc/sysconfig/network
NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=test.eduction.com
GATEWAY=192.168.27.1
[root@test ~]# vi /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
#::1 localhost6.localdomain6 localhost6
127.0.0.1 localhost.localdomain localhost
192.168.27.134 test test
然后我们重启数据库实例后，并没有任何问题，但是重启监听的时候遇到下面错误：
[oracle@test ~]$ sqlplus / as sysdba
SQL*Plus: Release 10.2.0.5.0 - Production on Sat Jun 18 16:42:21 2016
Copyright (c) 1982, 2010, Oracle. All Rights Reserved.
Connected to an idle instance.
SQL> startup
ORACLE instance started.
Total System Global Area 1509949440 bytes
Fixed Size 2096472 bytes
Variable Size 1392509608 bytes
Database Buffers 67108864 bytes
Redo Buffers 48234496 bytes
Database mounted.
Database opened.
SQL> exit
Disconnected from Oracle Database 10g Release 10.2.0.5.0 - 64bit Production
[oracle@test ~]$ lsnrctl start
LSNRCTL for Linux: Version 10.2.0.5.0 - Production on 18-JUN-2016 16:42:47
Copyright (c) 1991, 2010, Oracle. All rights reserved.
Starting /u01/app/oracle/product/10.2.0/db_1/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 10.2.0.5.0 - Production
Log messages written to /u01/app/oracle/product/10.2.0/db_1/network/log/listener.log
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=test.eduction.com)(PORT=1521)))
Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
TNS-12535: TNS:operation timed out
TNS-12560: TNS:protocol adapter error
TNS-00505: Operation timed out
Linux Error: 110: Connection timed out
[oracle@test ~]$
出现这个问题时，必须修改/etc/hosts下主机名的部分，使之与/etc/sysconfig/network下的HOSTNAME一致，上面错误就能解决。如下红色部分所示：
[root@test ~]# more /etc/sysconfig/network
NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=test.eduction.com
GATEWAY=192.168.27.1
[root@test ~]# more /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
#::1 localhost6.localdomain6 localhost6
127.0.0.1 localhost.localdomain localhost
192.168.27.134 test.eduction.com test
[root@test ~]#
由于这里测试，我修改了域名，如果服务器真实域名部分跟/etc/resolv.conf一致，那么数据库实例启动过程中，监控告警日志，就会发现告警日志里面会出现大量ORA-07445 & ORA-00108错误
2：修改/etc/sysconfig/network下的hostname并使之生效，如下所示
[oracle@kerry ~]$ more /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
#::1 localhost6.localdomain6 localhost6
127.0.0.1 localhost.localdomain localhost
192.168.27.134 test.eduction.com test
[oracle@kerry ~]$ more /etc/sysconfig/network
NETWORKING=yes
NETWORKING_IPV6=no
HOSTNAME=kerry.eduction.com
GATEWAY=192.168.27.1
数据库实例启动并没有任何问题，但是监听启动出现上面一样的错误。 如果域名使用真实的域名，则会遇到另外一种情况，告警日志里面也会出现下面错误
Errors in file /u01/app/oracle/admin/SCM2/bdump/scm2_ora_4494.trc:
ORA-07445: exception encountered: core dump [kslgetl()+120] [SIGSEGV] [Address not mapped to object] [0x000000210] [] []
ORA-00108: failed to set up dispatcher to accept connection asynchronously
关于这个，可以参考官方文档ORA-07445: [kslgetl()+80] Followed by ORA-108: failed to set up dispatcher to accept connection asynchronously (文档 ID 1298804.1)
3： 如果屏蔽/etc/hosts下的localhost部分，如下所示，此时有可能会影响监听
[root@kerry ~]# more /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
#::1 localhost6.localdomain6 localhost6
#127.0.0.1 localhost.localdomain localhost
192.168.27.134 kerry.eduction.com kerry
[root@kerry ~]#
[oracle@kerry ~]$ lsnrctl start
LSNRCTL for Linux: Version 10.2.0.5.0 - Production on 18-JUN-2016 17:45:37
Copyright (c) 1991, 2010, Oracle. All rights reserved.
Starting /u01/app/oracle/product/10.2.0/db_1/bin/tnslsnr: please wait...
TNSLSNR for Linux: Version 10.2.0.5.0 - Production
Log messages written to /u01/app/oracle/product/10.2.0/db_1/network/log/listener.log
Listening on: (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=kerry.eduction.com)(PORT=1521)))
Connecting to (ADDRESS=(PROTOCOL=tcp)(HOST=)(PORT=1521))
TNS-12547: TNS:lost contact
TNS-12560: TNS:protocol adapter error
TNS-00517: Lost contact
Linux Error: 104: Connection reset by peer
这是因为我没有在$ORACLE_HOME/network/admin下配置listener.ora,所以在注释或删除了/etc/hosts下localhost部分后就会出现这个错误，因为在没有listener.ora下的情况下，都会使用默认值（如下官方文档描述），监听进程会使用本机配置127.0.0.1注册监听服务,所以会出现上面错误信息，官方文档关于这方面的描述如下所示：
Oracle Net Listener Configuration Overview
Note:
Oracle Database 10g and later databases require a version 10 or later listener. Earlier versions of the listener are not supported for use with Oracle Database 10g and later databases. However, you can use a version 10 listener with previous versions of Oracle Database.
A listener is configured with one or more listening protocol addresses, information about supported services, and parameters that control its runtime behavior. The listener configuration is stored in a configuration file named listener.ora.
Because all of the configuration parameters have default values, it is possible to start and use a listener with no configuration. This default listener has a name of LISTENER, supports no services on startup, and listens on the following TCP/IP protocol address:
(ADDRESS=(PROTOCOL=tcp)(HOST=host_name)(PORT=1521))
Supported services, that is, the services to which the listener forwards client requests, can be configured in the listener.ora file or this information can be dynamically registered with the listener. This dynamic registration feature is called service registration. The registration is performed by the PMON process—an instance background process—of each database instance that has the necessary configuration in the database initialization parameter file. Dynamic service registration does not require any configuration in the listener.ora file.
解决方案两种：
1：在$ORACLE_HOME/network/admin/下配置listener.ora文件。则屏蔽或删除/etc/hosts下127.0.0.1后，监听不会有任何问题。
2：在配置文件/etc/hosts下增加localhost（红色部分所示）也能解决这个问题。
[root@kerry ~]# more /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
#::1 localhost6.localdomain6 localhost6
#127.0.0.1 localhost.localdomain localhost
192.168.27.134 kerry.eduction.com kerry localhost
[root@kerry ~]#
具体可以参考官方文档Starting TNS Listener or LSNRCTL Start Yields TNS-12541, Linux Error: 111: Connection Refused (文档 ID 343295.1)
另外，我们这里也忽略了lisnter.ora里面的配置，如果该配置文件使用的是hostname而不是IP，那么也会遇到一些问题。
