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[https://www.postgresql.org/ PostgreSQL] also known as Postgres, is a free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance.
 
This article extends the documentation in the [https://nixos.org/manual/nixos/stable/#module-postgresql NixOS manual].


=== Getting started ===
=== Getting started ===


To try out Postgresql add the following minimal example to your [https://nixos.wiki/wiki/NixOS_modules NixOS module]:
To try out Postgresql add the following minimal example to your [[NixOS modules | NixOS module]]:


<syntaxhighlight lang="nix">
<syntaxhighlight lang="nix">
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* [https://search.nixos.org/options?query=services.postgresql Available NixOS Postgresql service options]
* [https://search.nixos.org/options?query=services.postgresql Available NixOS Postgresql service options]
It's also possible to setup PostgreSQL with [[Nix Darwin]] similar to how you would on NixOS, see the [https://daiderd.com/nix-darwin/manual/index.html#opt-services.postgresql.enable options].


=== Verify  setup ===
=== Verify  setup ===
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</syntaxhighlight>
</syntaxhighlight>


We have to switch to a system user like "postgres" with <code>sudo -u postgres</code, because by default <code>psql</code> logs you into the DB user of the same name as the current Linux/system user. By default, NixOS creates a system and DB user names "postgres".
We have to switch to a system user like "postgres" with <code>sudo -u postgres</code>, because by default <code>psql</code> logs you into the DB user of the same name as the current Linux/system user. By default, NixOS creates a system and DB user names "postgres".
So the line <code>postgres=# </code> shows that we are now logged-in as DB user "postgres".
So the line <code>postgres=# </code> shows that we are now logged-in as DB user "postgres".


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=== Set the Postgresql versions ===
=== Set the Postgresql versions ===


By default, NixOS uses whatever Postgres version that comes with the  [https://search.nixos.org/packages?query=postgresql "pkgs.postgresql" package]. To avoid sudden breaking changes you can fix the Postgres version by using a more specific Nix package:
By default, NixOS uses whatever Postgres version shipped as default for your [https://search.nixos.org/options?show=system.stateVersion system.stateVersion]. To use a different or more recent version, you can set it manually:


<syntaxhighlight lang="nix">
<syntaxhighlight lang="nix">
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};
};
</syntaxhighlight>
</syntaxhighlight>
Note that changing the package version does not trigger any automatic migrations of your existing databases: if you update Postgres you should be ready to migrate any existing databases manually.


* [https://search.nixos.org/packages?query=postgresql_ Available Nixpkgs Postgresql versions]
* [https://search.nixos.org/packages?query=postgresql_ Available Nixpkgs Postgresql versions]
=== Security ===
Letting every system and DB user have access to all data is dangerous. Postgres supports several layers of protection.
One is to '''prefer "local" connections using Unix sockets''', that aren't accessible from the internet, whenever Postgres and your client app run on the same server.
==== Harden authentication  ====
We can '''limit what system user can connect'''.
Postgres supports [https://www.postgresql.org/docs/current/auth-username-maps.html "user name maps"], which limit which system users can log in as which DB user:
<syntaxhighlight lang="nix">
services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
  identMap = ''
    # ArbitraryMapName systemUser DBUser
      superuser_map      root      postgres
      superuser_map      postgres  postgres
      # Let other names login as themselves
      superuser_map      /^(.*)$  \1
  '';
};
</syntaxhighlight>
This map can have an arbitrary name and defines which system user can login as which DB user. Every other user and combination is rejected.
For example, with the above mapping if we are logged-in as system user "root" but want enter the DB as DB user "postgres" we would be allowed:
<syntaxhighlight lang="nix">
root$ psql -U postgres
# ok
</syntaxhighlight>
==== Limit Access  ====
Once logged-in we can '''limit what DB users can access'''. With the <code>authentication</code> we can limit what
DB user can access which databases. A good default setting is as follows:
<syntaxhighlight lang="nix">
services.postgresql = {
  enable = true;
  ensureDatabases = [ "mydatabase" ];
  authentication = pkgs.lib.mkOverride 10 ''
    #type database  DBuser  auth-method optional_ident_map
    local sameuser  all    peer        map=superuser_map
  '';
};
</syntaxhighlight>
With "sameuser"  Postgres will allow DB user access only to databases of the same name. E.g. DB user "mydatabase" will get access to database "mydatabase" and nothing else. The part <code>map=superuser_map</code> is optional.
One exception is the DB user "postgres", which by default is a superuser/admin with access to everything.
== TLS ==
To turn TLS on in recent versions of postgres it's pretty easy. Their [https://www.postgresql.org/docs/current/ssl-tcp.html docs] are pretty good.
Create a simple cert just to make it work. If you are doing this in production, you need to provide your own server.crt and server.key in the main PGDATA dir (~postgres).
In a shell:
<syntaxhighlight lang="nix">
cd ~postgres
sudo -u postgres openssl req -new -x509 -days 365 -nodes -text -out server.crt  -keyout server.key -subj "/CN=dbhost.yourdomain.com"
chmod og-rwx server.key
</syntaxhighlight>
Then in your nix configuration:
<syntaxhighlight lang="nix">
  services.postgresql = {
    enable = true;
    package = pkgs.postgresql_16;
    enableTCPIP = true;
    ensureDatabases = [ "tootieapp" ];
    settings = {
        ssl = true;
    };
    authentication = pkgs.lib.mkOverride 10 ''
      #type database  DBuser  auth-method
      local all      all    trust
      host  sameuser    all    127.0.0.1/32 scram-sha-256
      host  sameuser    all    ::1/128 scram-sha-256
    '';
  };
</syntaxhighlight>
the `sameuser` mentioned in the authentication section requires the database name be the same as the username, which you may not want, you can change that to `all` to allow an authenticated user the ability to connect to any database.
`scram-sha-256` is the require a password option, but you can authenticate a variety of different ways, see the official docs for other options as part of pg_hba.conf.
user creation and permissions are best described in the PG manual under `CREATE ROLE` and `GRANT`
for example:
<syntaxhighlight lang="sql">
CREATE USER tootieapp WITH PASSWORD 'BIGLONGRANDOMSTRINGHERE';
GRANT ALL PRIVILEGES ON DATABASE tootieapp TO tootieapp;
</syntaxhighlight>


== Debugging with <code>psql</code> ==
== Debugging with <code>psql</code> ==
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</pre>
</pre>


== FAQ ==
== Troubleshooting ==


=== Connection rejected with "Role does not exist" ===
=== Connection rejected with "Role does not exist" ===
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You are trying to login as a system user ("root" in this example) that has no DB user of the same name. Try <code>psql -U postgres</code> or <code>sudo -u postgres psql</code> to log in as a different DB user.
You are trying to login as a system user ("root" in this example) that has no DB user of the same name. Try <code>psql -U postgres</code> or <code>sudo -u postgres psql</code> to log in as a different DB user.


=== Connection rejected with "Peer authentication failed" ===
<syntaxhighlight lang="nix">
root$ psql -U postgres
psql: error: connection to server on socket "/run/postgresql/.s.PGSQL.5432" failed: FATAL:  Peer authentication failed for user "postgress"
</syntaxhighlight>
You are trying to login as a DB user ("postgres" in this example) for which your current system user ("root" in this example) has no permission to switch to. Check your "user name map" in the <code>identMap</code> section.
=== WARNING: database "XXX" has a collation version mismatch ===
The complete error which appears in the system log might look similar to this
<syntaxhighlight>
WARNING:  database "outline" has a collation version mismatch
DETAIL:  The database was created using collation version 2.35, but the operating system provides version 2.38.
HINT:  Rebuild all objects in this database that use the default collation and run ALTER DATABASE outline REFRESH COLLATION VERSION, or build PostgreSQL with the right library version.
</syntaxhighlight>
To fix it, run following commands in the psql console. Replace the database name <code>outline</code> with the name of the database which you want to migrate
<syntaxhighlight>
sudo -u postgres psql
postgres=# \c outline;
outline=# REINDEX DATABASE outline;
outline=# ALTER DATABASE outline REFRESH COLLATION VERSION;
</syntaxhighlight>
== See also ==
== See also ==


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[[Category:Applications]]
[[Category:Applications]]
[[Category:Database]]
[[Category:NixOS Manual]]
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