psql: FATAL: Peer authentication failed for user "dev"

when i create a new user, but it cannot login the database.
I do that like this:

postgres@Aspire:/home/XXX$ createuser dev
Shall the new role be a superuser? (y/n) n
Shall the new role be allowed to create databases? (y/n) y
Shall the new role be allowed to create more new roles? (y/n) y

then create a database:

postgres@Aspire:/home/XXX$ createdb -O dev test_development

after that, I try psql -U dev -W test_development to login, but get the error:

psql: FATAL:  Peer authentication failed for user "dev"

I tried to solve the problem but failed.

Try:

psql -U user_name  -h 127.0.0.1 -d db_name

where

• -U is the database user name
• -h is the hostname/IP of the local server, thus avoiding Unix domain sockets
• -d is the database name to connect to

This is then evaluated as a "network" connection by Postgresql rather than a Unix domain socket connection, thus not evaluated as a "local" connect as you might see in pg_hba.conf:

local   all             all                                     peer

Your connection failed because by default psql connects over UNIX sockets using peer authentication, that requires the current UNIX user to have the same user name as psql. So you will have to create the UNIX user dev and then login as dev or use sudo -u dev psql test_development for accessing the database (and psql should not ask for a password).

If you cannot or do not want to create the UNIX user, like if you just want to connect to your database for ad hoc queries, forcing a socket connection using psql --host=localhost --dbname=test_development --username=dev (as pointed out by @meyerson answer) will solve your immediate problem.

But if you intend to force password authentication over Unix sockets instead of the peer method, try changing the following pg_hba.conf* line:

from

# TYPE DATABASE USER ADDRESS METHOD
local  all      all          peer

to

# TYPE DATABASE USER ADDRESS METHOD
local  all      all          md5

• peer means it will trust the identity (authenticity) of UNIX user. So not asking for a password.

• md5 means it will always ask for a password, and validate it after hashing with MD5.

You can, of course, also create more specific rules for a specific database or user, with some users having peer and others requiring passwords.

After changing pg_hba.conf if PostgreSQL is running you'll need to make it re-read the configuration by reloading (pg_ctl reload) or restarting (sudo service postgresql restart).

^{* The file pg_hba.conf will most likely be at /etc/postgresql/9.x/main/pg_hba.conf}

Edited: Remarks from @Chloe, @JavierEH, @Jonas Eicher, @fccoelho, @Joanis, @Uphill_What comments incorporated into answer.

Peer authentication means that postgres asks the operating system for your login name and uses this for authentication. To login as user "dev" using peer authentication on postgres, you must also be the user "dev" on the operating system.

You can find details to the authentication methods in the Postgresql documentation.

Hint: If no authentication method works anymore, disconnect the server from the network and use method "trust" for "localhost" (and double check that your server is not reachable through the network while method "trust" is enabled).

When you specify:

psql -U user

it connects via UNIX Socket, which by default uses peer authentication, unless specified in pg_hba.conf otherwise.

You can specify:

host    database             user             127.0.0.1/32       md5
host    database             user             ::1/128            md5

to get TCP/IP connection on loopback interface (both IPv4 and IPv6) for specified database and user.

After changes you have to restart postgres or reload it's configuration. Restart that should work in modern RHEL/Debian based distros:

service postgresql restart

Reload should work in following way:

pg_ctl reload

but the command may differ depending of PATH configuration - you may have to specify absolute path, which may be different, depending on way the postgres was installed.

Then you can use:

psql -h localhost -U user -d database

to login with that user to specified database over TCP/IP. md5 stands for encrypted password, while you can also specify password for plain text passwords during authorisation. These 2 options shouldn't be of a great matter as long as database server is only locally accessible, with no network access.

Important note: Definition order in pg_hba.conf matters - rules are read from top to bottom, like iptables, so you probably want to add proposed rules above the rule:

host    all             all             127.0.0.1/32            ident

While @flaviodesousa's answer would work, it also makes it mandatory for all users (everyone else) to enter a password.

Sometime it makes sense to keep peer authentication for everyone else, but make an exception for a service user. In that case you would want to add a line to the pg_hba.conf that looks like:

local   all             some_batch_user                         md5

I would recommend that you add this line right below the commented header line:

# TYPE  DATABASE        USER            ADDRESS                 METHOD
local   all             some_batch_user                         md5

You will need to restart PostgreSQL using

sudo service postgresql restart

If you're using 9.3, your pg_hba.conf would most likely be:

/etc/postgresql/9.3/main/pg_hba.conf