It is always a bit of a surprise that many people do not know peer authentication in PostgreSQL. You might ask why that is important as initdb creates a default pg_hba.conf which does not allow any connections from outside the PostgreSQL server. While that is true there is another important point to consider.

Let’s assume you executed initdb without any options like this:

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postgres@centos8pg:/home/postgres/ [pgdev] mkdir /var/tmp/test
postgres@centos8pg:/home/postgres/ [pgdev] initdb -D /var/tmp/test
The files belonging to this database system will be owned by user "postgres".
This user must also own the server process.
 
The database cluster will be initialized with locale "en_US.UTF-8".
The default database encoding has accordingly been set to "UTF8".
The default text search configuration will be set to "english".
 
Data page checksums are disabled.
 
fixing permissions on existing directory /var/tmp/test ... ok
creating subdirectories ... ok
selecting dynamic shared memory implementation ... posix
selecting default max_connections ... 100
selecting default shared_buffers ... 128MB
selecting default time zone ... Europe/Zurich
creating configuration files ... ok
running bootstrap script ... ok
performing post-bootstrap initialization ... ok
syncing data to disk ... ok
 
initdb: warning: enabling "trust" authentication for local connections
You can change this by editing pg_hba.conf or using the option -A, or
--auth-local and --auth-host, the next time you run initdb.
 
Success. You can now start the database server using:
 
pg_ctl -D /var/tmp/test -l logfile start

Did you ever notice the warning at the end of the output?

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initdb: warning: enabling "trust" authentication for local connections
You can change this by editing pg_hba.conf or using the option -A, or
--auth-local and --auth-host, the next time you run initdb.

You might think that this is not important as only the DBAs will have access to the operating system user postgres (or whatever user you used when you executed initdb). Although this might be true in your case, the server eventually might have other local users. Before creating a new user lets start the instance:

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postgres@centos8pg:/home/postgres/ [pgdev] export PGPORT=9999
postgres@centos8pg:/home/postgres/ [pgdev] pg_ctl -D /var/tmp/test/ start -l /dev/null
waiting for server to start.... done
server started

You really need to be aware of is this:

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postgres@centos8pg:/home/postgres/ [pgdev] sudo useradd test
postgres@centos8pg:/home/postgres/ [pgdev] sudo su - test
[test@centos8pg ~]$ /u01/app/postgres/product/DEV/db_1/bin/psql -p 9999 -U postgres postgres
psql (13devel)
Type "help" for help.
 
postgres=#

… and you are in as the superuser! So any local user can connect as the superuser by default. What you might want to do is this:

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postgres@centos8pg:/home/postgres/ [pgdev] sudo chmod o-rwx /u01/app/postgres/product
postgres@centos8pg:/home/postgres/ [pgdev] sudo su - test
Last login: Tue Oct 22 21:19:58 CEST 2019 on pts/0
[test@centos8pg ~]$ /u01/app/postgres/product/DEV/db_1/bin/psql -p 9999 -U postgres postgres
-bash: /u01/app/postgres/product/DEV/db_1/bin/psql: Permission denied

This prevents all other users on the system from executing the psql binary. If you can guarantee that nobody installs psql in another way on the system that might be sufficient. As soon as psql is available somewhere on the system you’re lost again:

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postgres@centos8pg:/home/postgres/ [pgdev] sudo dnf provides psql
Last metadata expiration check: 0:14:53 ago on Tue 22 Oct 2019 09:09:23 PM CEST.
postgresql-10.6-1.module_el8.0.0+15+f57f353b.x86_64 : PostgreSQL client programs
Repo        : AppStream
Matched from:
Filename    : /usr/bin/psql
 
postgres@centos8pg:/home/postgres/ [pgdev] sudo dnf install -y postgresql-10.6-1.module_el8.0.0+15+f57f353b.x86_64
[test@centos8pg ~]$ /usr/bin/psql -p 9999 -U postgres -h /tmp postgres
psql (10.6, server 13devel)
WARNING: psql major version 10, server major version 13.
Some psql features might not work.
Type "help" for help.
 
postgres=#

Not really an option. This is where peer authentication becomes very handy.

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postgres@centos8pg:/home/postgres/ [pgdev] sed -i 's/local   all             all                                     trust/local   all             all                                     peer/g' /var/tmp/test/pg_hba.conf

Once you switched from trust to peer for local connections only the operating system user that created the instance will be able to connect locally without providing a password:

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postgres@centos8pg:/home/postgres/ [pgdev] pg_ctl -D /var/tmp/test/ reload
server signaled
postgres@centos8pg:/home/postgres/ [pgdev] psql postgres
psql (13devel)
Type "help" for help.
 
[local]:9999 postgres@postgres=#

Other local users will not be able to connect anymore:

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postgres@centos8pg:/home/postgres/ [pgdev] sudo su - testLast login: Tue Oct 22 21:25:36 CEST 2019 on pts/0
[test@centos8pg ~]$ /usr/bin/psql -p 9999 -U postgres -h /tmp postgres
psql: FATAL:  Peer authentication failed for user "postgres"
[test@centos8pg ~]$

So, please, consider enabling peer authentication or at least go for md5 for local connections as well.

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Daniel Westermann