Not able to connect to PostgreSQL

In this document I am going to discuss about some of the most frequent errors faced by users while connecting to PostgreSQL. This is document is to help the beginners who started working on PostgreSQL. PostgreSQL has a process called postmaster which acts as listener process, by default the process listens on 5432.

There are different tools available to connect to PostgreSQL, here I am going to use the psql command line tool but there is no difference in error messages.


[root@localhost ~]# psql
psql: error: FATAL: role “root” does not exist

By default “psql” command line tool use the operating system user as the database user and localhost the hostname if the user name is not passed as argument, here I logged in as the root user and tried to login to PostgreSQL without username which caused the error.


give the username when logging into the database, by default the psql command line take the username as the database name, so no need to pass the database as I am trying to connect to PostgreSQL.

psql -U postgres -h localhost


[root@localhost ~]# psql -U postgres
psql: error: FATAL: Peer authentication failed for user “postgres”


By default PostgreSQL installation using yum repository configure the pg_hba.conf with peer, ident authentication. So change the peer and ident authentication methods to md5 and reload the configuration.

[root@localhost data]# psql -U postgres
Password for user postgres: 
psql (12.5)
Type "help" for help.



[root@localhost data]# psql -h -U postgres -d postgres
psql: error: could not connect to server: Connection refused
Is the server running on host “” and accepting
TCP/IP connections on port 5432?

This is the common error users get when they connect to PostgreSQL. The error represents there is no listener for host and port combination.


Most common issue is listener_address parameter is set to ‘localhost’ or port set to a non default value.

postgres=# show listen_addresses ;
(1 row)

postgres=# show port;
(1 row)

Here the issue is listener_address is localhost, so changed it to ‘*’ where it listens on all ip’s in my host. I have only one ip configured, if your server has multiple ip’s then set the ip address on which the PostgreSQL connections should happen.

Configure the listener_address or change the port and restart the PostgreSQL cluster.

postgres=# alter system set listen_addresses to '*';
postgres=# \q
-bash-4.2$ /usr/pgsql-12/bin/pg_ctl -D /var/lib/pgsql/project/data restart
waiting for server to shut down.... done
server stopped
waiting for server to start....2020-12-20 07:12:20.317 PST [3051] LOG:  starting PostgreSQL 12.5 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39), 64-bit
2020-12-20 07:12:20.318 PST [3051] LOG:  listening on IPv4 address "", port 5432
2020-12-20 07:12:20.318 PST [3051] LOG:  listening on IPv6 address "::", port 5432
2020-12-20 07:12:20.319 PST [3051] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5432"
2020-12-20 07:12:20.320 PST [3051] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2020-12-20 07:12:20.332 PST [3051] LOG:  redirecting log output to logging collector process
2020-12-20 07:12:20.332 PST [3051] HINT:  Future log output will appear in directory "log".
server started


-bash-4.2$ psql -h -U postgres -d postgres
psql: error: FATAL: no pg_hba.conf entry for host “”, user “postgres”, database “postgres”, SSL off

This is another common error which users will face due to missing entry for the user host address (ip address) in pg_hba.conf file. host based authentication file presents in data directory which has a list of ip addresses or host addresses allowed to connect to PostgreSQL.


Make an entry like below for the user host or subnet in pg_hba.conf file. Any ip address with 192.168.X.X will be able to connect to the host.

host	all		all		md5


-bash-4.2$ psql -h -U postgres -d postgres
psql: error: FATAL: sorry, too many clients already

-bash-4.2$ psql -h -U viswamitra -d postgres
psql: error: FATAL: remaining connection slots are reserved for non-replication superuser connections

-bash-4.2$ psql -U viswamitra -d postgres -h
psql: error: FATAL: too many connections for role “viswamitra”

This is also a common error users face after started using the database in application. This errors will come after the connection limit is reached to the maximum configured value.

Connection limit can be set at different levels

  1. cluster – which is applicable for all users and databases
  2. user – which is applicable for the user
  3. database – which is applicable for the given database


Login to the database as a super user, if there are any idle connections from longer time close them and check the application connection pool settings to close the idle connections after some time interval.

postgres=# select pid,datname,usename,application_name,state,now()-state_change as idle_duration from pg_stat_activity where state = 'idle';
 pid  | datname  |  usename   | application_name | state |  idle_duration  
 3656 | alex     | postgres   | psql             | idle  | 00:14:06.647055
 3652 | alex     | postgres   | psql             | idle  | 00:14:11.718486
 3623 | postgres | viswamitra | psql             | idle  | 00:15:47.530912
(3 rows)

postgres=# select pg_terminate_backend(3656);
(1 row)

If there are no idle sessions to cancel and the error is first and second one in the list increase the “max_connections” in postgresql.conf file. This requires a reboot of the PostgreSQL cluster to make it effect.

postgres=# alter system set max_connections to '1000';

/usr/pgsql-12/bin/pg_ctl -D /var/lib/pgsql/project/data restart

For third error (5.3) , the connection limit is configured at the user level, it can be changed using the alter user statement.

postgres=# alter user viswamitra connection limit 100;

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