You are running a LAPP application on Ubuntu Linux with PostgreSQL. A user has complained that she cannot log into your Postgres database via a LAPP application. You are not sure if the attempts are even registering of if the user is entering an incorrect username. How do you look at a log to see a history of unsuccessful attempts at logging into the Postgresql database?
Examine this file: /var/log/postgresql/postgresql-x.y-main.log
(Substitute x.y with the major and minor versions of PostgreSQL you are running e.g., 9.5.)
It logs failed Postgres authentication attempts -- even if they come from a PHP web page (a web-based UI). If a role, like an OS user but for postgres, tries to log in from a web application with the wrong password, it will be logged in to /var/log/postgresql/postgresql-9.5-main.log (if you are using PostgreSQL 9.5). Please note that this log does not show successful database login attempts. It is not a complete history -- it only records failed attempts.
To check operating-system-level failed logins, check these files (the top two pertain to SSH both successful and unsuccessful while btmp pertains to other failed remote login attempts):