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PgBouncer Pro Tip: Use auth_user

Moderator

Introduction

 

Anyone running a database in a production environment with over a hundred users should seriously consider employing a connection pooler to keep resource usage under control. PgBouncer is one such tool, and it’s great because it’s lightweight and yet has a handful of nifty features for DBAs that have very specific needs.

One of these nifty features that I want to share about is the auth_user and auth_query combo that serves as an alternative to the default authentication process that uses userlist.txt “What’s wrong with userlist.txt” you may ask. For starters, it makes user/role administration a little tricky. Every time you add a new user to PG, you need to add it to userlist.txt in PgBouncer. And every time you change a password, you have to change it in userlist.txt as well. Multiply that by the 30+ servers you’re managing, and you’ve got a sysadmin’s nightmare on your hands. With auth_user and auth_query, you can centralize the password management and take one item off your checklist.

 

What’s auth_user?

 

In the [databases] section of your pgbouncer.ini, you would typically specify a user= and password= with which PgBouncer will connect to the Postgres database with. If left blank, the user/password are declared at the connection string (i.e., psql -U <username> <database>). When this happens, PgBouncer will perform a lookup of the provided username/password against userlist.txt to verify that the credentials are correct, and then the username/password are sent to Postgres for an actual database login.

When auth_user is provided, PgBouncer will still read in credentials from the connection string, but instead of comparing against userlist.txt, it logs in to Postgres with the specified auth_user(preferably a non-superuser) and runs auth_query to pull the corresponding md5 password hash for the desired user. The validation is performed at this point, and if correct, the specified user is allowed to log in.

 

An Example

 

Assuming Postgres is installed and running, you can get the auth_user and auth_query combo running with the following steps:

  1. Create a Postgres user to use as auth_user
  2. Create the user/password lookup function in Postgres
  3. Configure pgbouncer.ini

Create a Postgres user to use as auth_user

 

In your terminal, run psql -c "CREATE ROLE myauthuser WITH PASSWORD 'abc123'" to create myauthuser. Note that myauthuser should be an unprivileged user, wiht no GRANTs to read/write any tables. myauthuser is used strictly for assisting with PgBouncer authentication.

For the purposes of this example, we’ll also have a database user called mydbuser, which can be created with:

CREATE ROLE mydbuser WITH PASSWORD 'mysecretpassword'
GRANT SELECT ON emp TO mydbuser;

 

Create the user/password lookup function in Postgres

 

 

In your psql prompt, create a function that will be used by myauthuser to perform the user/password lookup:

CREATE OR REPLACE FUNCTION user_search(uname TEXT) RETURNS TABLE (usename name, passwd text) as
$$
  SELECT usename, passwd FROM pg_shadow WHERE usename=$1;
$$
LANGUAGE sql SECURITY DEFINER;

As mentioned in the documentation, the SECURITY DEFINER clause enables the non-privileged myauthuser to view the contents of pg_shadow, which would otherwise be limited to only admin users.

 

Configure pgbouncer.ini

 

Configure your [databases] section with an alias, like:

[databases]
foodb = host=db1 dbname=edb auth_user=myauthuser

Then, configure auth_query in the [pgbouncer] section with:

auth_query = SELECT usename, passwd FROM user_search($1)

 

Let ‘er rip!

 

Spin up PgBouncer and try logging in:

PGPASSWORD=thewrongpassword psql -h 127.0.0.1 -p 6432 -U mydbuser -Atc 'SELECT '\''success'\''' foodb
psql: ERROR:  Auth failed
PGPASSWORD=mysecretpassword psql -h 127.0.0.1 -p 6432 -U mydbuser -Atc 'SELECT '\''success'\''' foodb
success

As you can see, providing the wrong password for mydbuser led to a pg_shadow lookup failure, and the user was prevented from logging in. The subsequent psql call used the correct password, and successfully logged in.

 

Some Caveats

 

I’ve seen a few customers try to implement this, and one of the common mistakes I’ve seen is the failure to set pg_hba.conf properly in Postgres. Bear in mind that once the provided credentials are validated, PgBouncer will attempt to log in with the specified user. Therefore, if your auth_user is myauthuser and you’ve got a pg_hba.conf with host all myauthuser 127.0.0.1/32 md5, but you want to ultimately login with mydbuser, you won’t be able to do so because there’s no pg_hba.conf entry for mydbuser, and you’ll probably see something like this:

server login failed: FATAL no pg_hba.conf entry for host "127.0.0.1", user "mydbuser", database "edb", SSL off

Also, make sure auth_type is not set to trust in pgbouncer.ini – instead, you should set trust in pg_hba.conf for auth_user and clamp it down to only the IP(s) that will be running PgBouncer. Set auth_type to md5 so that your login attempt will be challenged with a password request.

Enjoy!