ORA-01034: ORACLE not available on remote connection - oracle

Question

When I try to connect to Oracle database from the remote host (I use oracle instant client), I get following errors:
ERROR:
ORA-01034: ORACLE not available
ORA-27101: shared memory realm does not exist
Linux-x86_64 Error: 2: No such file or directory
Additional information: 3640
Additional information: 1108778635
Process ID: 0
Session ID: 0 Serial number: 0
However, ORACLE_HOME on the host machine is set correctly, as well as ORACLE_SID, and i can easily connect to the database from its host machine:
SQL*Plus: Release 12.1.0.2.0 Production on Wed Jul 10 09:20:09 2019
Copyright (c) 1982, 2014, Oracle. All rights reserved.
Enter user-name: system
Enter password:
Last Successful login time: Wed Jul 10 2019 09:13:57 +03:00
Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options
What can cause this sort of behavior?
UPD:
Status of the listener on the remote DB:
Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=192.168.71.2)(PORT=1521)))
STATUS of the LISTENER
------------------------
Alias LISTENER
Version TNSLSNR for Linux: Version 12.1.0.2.0 - Production
Start Date 05-JUL-2019 08:56:16
Uptime 6 days 23 hr. 29 min. 47 sec
Trace Level off
Security ON: Local OS Authentication
SNMP OFF
Listener Parameter File /u01/app/oracle/product/12.1.0/dbhome_1/network/admin/listener.ora
Listener Log File /u01/app/oracle/product/12.1.0/dbhome_1/log/diag/tnslsnr/epyc-gbu2/listener/alert/log.xml
Listening Endpoints Summary...
(DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=192.168.71.2)(PORT=1521)))
(DESCRIPTION=(ADDRESS=(PROTOCOL=ipc)(KEY=EXTPROC1521)))
Services Summary...
Service "AGRO.GBUBDD.RU" has 1 instance(s).
Instance "AGRO", status UNKNOWN, has 1 handler(s) for this service...
UPD2:
The command I use to connect to remote DB:
sqlplus /nolog
connect system/oracle#192.168.71.2:1521/agro.gbubdd.ru

Answer 1

This could happen due to 2 reasons.
1) Oracle database is not started correctly.
2) There is no space left and database is crashed / stopped working.
To correct, login to remote db and check if database is up or not and there is enough space for database to perform operations.
To check if database is open or not, on remote machine where db is installed, you can login with sql plus and run query to check status of database.

Answer 2

Having ORACLE_SID set in your environment is only relevant for local connections. The error is telling you that there is no database running with that SID on the machine you are running SQL*Plus on. It doesn't know about the remote DB.
To connect remotely you need to tell SQL*Plus how to reach the remote DB, which is usually either via a TNS entry or using 'easy connect' syntax.
You haven't included any details of the remote DB, but you would need to do something like:
sqlplus username#//database_host:listener_port/remote_service_name
Or start with sqlplus /nolog and then use the connect command, as shown in the documentation.
The port is usually 1521 but might be different, and the service name might be the same as the SID. If you run lsnrctl services or lsnrctl status on the DB server you should be able to see the valid service names, as well as the address and port the listener is using.