First Steps - PostgreSQL 9 Administration - page 22

F Check for explicit rejections: If you receive the following error message:
pg_hba.conf rejects connection for host …
then your connection attempt has been explicitly rejected by the database
administrator for that server. You will not be able to connect from the current
client system using those credentials. There is little point attempting to contact
the administrator, as you are violating an explicit security policy in what you are
attempting to do.
F Check for implicit rejections: If the error message you receive is:
no pg_hba.conf entry for …
then there is no explicit rule that matches your credentials. This is likely an oversight
on the part of the administrator, and is common in very complex networks. Please
contact the administrator, and request a ruling on whether your connection should be
allowed (hopefully) or explicitly rejected in the future.
F Check whether the connection works with psql: If you're trying to connect to PostgreSQL
from anything other than the psql command-line utility, switch to that now. If you can
make psql connect successfully, yet cannot make your main connection work correctly,
then the problem may be in the local interface you are using.
F Check whether the server is up: If a server is shut down, then you cannot connect. The
typical problem here is simply mixing up to which server you are connecting. You need
to specify the hostname and port, so it's possible you are mixing up those details.
F Check whether the server is up and accepting new connections: A server that is
shutting down will not accept new connections, apart from superusers. Also, a
standby server may not have the hot_standby parameter enabled, preventing you
from connecting.
F Check whether the server is listening correctly.
F Check the port on which the server is actually listening: Confirm that the incoming
request is arriving on interface listed in the listen_addresses parameter, or
whether it is set to * for remote connections, or localhost for local connections.
F Check whether the database name and username exist: It's possible the database or
user no longer exists.
F Check the connection request: Check whether the connection request was
successful, yet was somehow dropped after connection. You can confirm this by
looking at the server log when the following parameters are enabled:
log_connections = on
log_disconnections = on
F Check for other disconnection reasons: If you are connecting to a standby server, it is
possible that you have been disconnected because of hot standby conflicts. See the
section on Replication and Upgrades .