SQLAlchemy attempts PostgreSQL connection over IPv6 before Unix socket or IPv4

Question

How do I ensure that my SQLAlchemy connection to PostgreSQL is over a Unix socket or even IPv4? It will only connect via IPv6.

I configured create_engine as @localhost but note the host IPv6 host "::1" failure at the bottom. I purposely removed the IPv6 trust in pg_hba.conf (see below) to show the error. If I uncomment that line this will connect successfully over IPv6 and skip Unix and IPv4.

>>> from sqlalchemy import *
db = create_engine('postgresql://vaderade@localhost:5432/mydb', echo=True)
>>> metadata = MetaData(db)
>>> users = Table('users', metadata,
... Column('user_id', Integer, primary_key=True),
... Column('name', String(40)),
... Column('age', Integer),
... Column('password', String),
... )
>>> users.create()
Traceback (most recent call last):
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 1044, in _do_get
    return self._pool.get(wait, self._timeout)
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/util/queue.py", line 145, in get
    raise Empty
sqlalchemy.util.queue.Empty

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/engine/base.py", line 2074, in _wrap_pool_connect
    return fn()
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 376, in connect
    return _ConnectionFairy._checkout(self)
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 713, in _checkout
    fairy = _ConnectionRecord.checkout(pool)
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 480, in checkout
    rec = pool._do_get()
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 1060, in _do_get
    self._dec_overflow()
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/util/langhelpers.py", line 60, in __exit__
    compat.reraise(exc_type, exc_value, exc_tb)
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/util/compat.py", line 186, in reraise
    raise value
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 1057, in _do_get
    return self._create_connection()
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 323, in _create_connection
    return _ConnectionRecord(self)
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 449, in __init__
    self.connection = self.__connect()
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/pool.py", line 607, in __connect
    connection = self.__pool._invoke_creator(self)
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/engine/strategies.py", line 97, in connect
    return dialect.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.5/site-packages/sqlalchemy/engine/default.py", line 385, in connect
    return self.dbapi.connect(*cargs, **cparams)
  File "/usr/local/lib/python3.5/site-packages/psycopg2/__init__.py", line 164, in connect
    conn = _connect(dsn, connection_factory=connection_factory, async=async)
psycopg2.OperationalError: FATAL:  no pg_hba.conf entry for host "::1", user "vaderade", database "mydb", SSL off

Relevant info from /var/lib/pgsql/9.5/data/pg_hba.conf

# TYPE  DATABASE        USER            ADDRESS                 METHOD

# "local" is for Unix domain socket connections only
local   all             all                                     trust
# IPv4 local connections:
host    all             all             127.0.0.1/32            trust
# IPv6 local connections:
#host    all             all             ::1/128                 trust

Relevant info from /var/lib/pgsql/9.5/data/postgresql.conf

#------------------------------------------------------------------------------
# CONNECTIONS AND AUTHENTICATION
#------------------------------------------------------------------------------

# - Connection Settings -

listen_addresses = 'localhost'          # what IP address(es) to listen on;
                                        # comma-separated list of addresses;
                                        # defaults to 'localhost'; use '*' for all
                                        # (change requires restart)
port = 5432                             # (change requires restart)

Specifics:

Red Hat 7.2
PostgreSQL 9.5.4
Python 3.5.2
SQLAlchemy==1.0.14
psycopg2==2.6.2

MatheusOl MatheusOl · Accepted Answer · 2016-09-07T15:19:37

When you use localhost it will use a TCP connection trough whatever is configured in your system to respond localhost as. On Linux it is generally mapped on /etc/hosts, being commonly IPv4 127.0.0.1 or IPv6 ::1 (as it is probably your case).

Now, if you want to connect locally you have the options:

postgresql://vaderade@localhost:5432/mydb: connect through TCP localhost (if IPv4 or IPv6 depends on your system)
postgresql://[email protected]:5432/mydb: connect through TCP IPv4 localhost
postgresql://vaderade@[::1]:5432/mydb: connect through TCP IPv6 localhost
postgresql://vaderade@:5432/mydb: connect through default unix domain socket location (that is the answer you are looking for)
postgresql://vaderade@:5432/mydb?host=/path/to/socket: connect through unix domain socket at non-default location of /path/to/socket

SQLAlchemy attempts PostgreSQL connection over IPv6 before Unix socket or IPv4

1 Answers