Remote connection reports ORA-12547 after Oracle is installed

I. Error code: ORA-12547
Symptoms:
(1) Oracle listening and Service Startup are normal and no error is reported
(2) Check the log without any exception information
(3) The local commands on the server can connect to oracle and execute query commands.
(4) The customer service machine cannot be connected, error ORA-12547

Solution: Modify three files:
(1) Modify/etc/hosts as follows:

127.0.0.1 localhost. localdomain localhost
: 1 localhost6.localdomain6 localhost6
192.168.1.168 your_hostname.com your_hostname

(2) listener. ora (Note: need to modify to the following content, otherwise the error will be reported: ORA-12514)

SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(GLOBAL_DBNAME = ORCL)
(ORACLE_HOME =/opt/oracle/product/10g)
(SID_NAME = ORCL)
)
(SID_DESC =
(PROGRAM = extproc)
(SID_NAME = PLSExtProc)
(ORACLE_HOME =/opt/oracle/product/10g)
)
)

LISTENER =
(DESCRIPTION_LIST =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = IPC) (KEY = EXTPROC1 ))
(ADDRESS = (PROTOCOL = TCP) (HOST = your_hostname.com) (PORT = 1521 ))
)
)

(3) tnsnames. ora
ORCL =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP) (HOST = your_hostname.com) (PORT = 1521 ))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = ORCL)
)
)

EXTPROC_CONNECTION_DATA =
(DESCRIPTION =
(ADDRESS_LIST =
(ADDRESS = (PROTOCOL = IPC) (KEY = EXTPROC1 ))
)
(CONNECT_DATA =
(SID = PLSExtProc)
(PRESENTATION = RO)
)
)

2. Error code: ORA-01034 ORA-27101

Check the following three items:
(1) Whether the environment variable oracle_sid is consistent with the settings in the listener
(2) check whether the environment oracle_home is consistent with the settings in the listener.
(3) Whether the memory and disk space are sufficient (for example, the disk space in the oracle partition may be insufficient)

Others: if all of the above are normal, restarting oracle still cannot solve the problem,

Note that the following methods are used to start oracle: sqlplus '/as sysdba'

Instead of (1) sqlplus/nolog (2) connect/as sysdba

Do not use shutdown abort when closing oracle; it is best to use shutdown immediate;