Postgres not allowing localhost but works with 127.0.0.1

The Problem

Postgres will potentially use IPv6 when specifying -h localhost which given the above pg_hba.conf specifies ident, a password prompt will be returned.

However when -h 127.0.0.1 is specified, it forces Postgres to use IPv4, which is set to trust in above config and allows access without password.

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The Answer

Thus the answer is to modify the IPv6 host line in pg_hba.conf to use trust:

# IPv6 local connections:
host    all         all         ::1/128               trust

Remembering to restart the Postgres service after making config changes.

In pg_hba.conf, the first match counts. The manual:

The first record with a matching connection type, client address, requested database, and user name is used to perform authentication. There is no "fall-through" or "backup": if one record is chosen and the authentication fails, subsequent records are not considered. If no record matches, access is denied.

Note the reversed order:

host    all         all         127.0.0.1/32          trust
host    all         all         127.0.0.1/32          ident

But:

host    all         all        localhost             ident
host    all         all        localhost             trust

Remember to reload after saving changes to pg_hba.conf. (Restart is not necessary.) The manual:

The pg_hba.conf file is read on start-up and when the main server process receives a SIGHUP signal. If you edit the file on an active system, you will need to signal the postmaster (using pg_ctl reload, calling the SQL function pg_reload_conf(), or using kill -HUP) to make it re-read the file.

If you really "add" the lines like you wrote, there should not be any effect at all. But if you replace the lines, there is.

In the first case, you get trust authentication method, which is an open-door policy. The manual:

PostgreSQL assumes that anyone who can connect to the server is authorized to access the database with whatever database user name they specify (even superuser names)

But in the second case you get the ident authentication method, which has to be set up properly to work.

Plus, as Cas pointed out later, localhost covers both IPv4 and IPv6, while 127.0.0.1/32 only applies to IPv4.

If you are actually using the outdated version 8.4, go to the old manual for 8.4. You are aware that 8.4 has reached EOL in 2014 and is not supported any more? Consider upgrading to a current version.

In Postgres 9.1 or later you would rather use peer than ident.

More:

• Run batch file with psql command without password