Database Administration - PostgreSQL: Up and Running - page 21

A common culprit is setting shared_buffers too high. Another suspect is an old
postmaster.pid left over from a failed shutdown. You can safely delete this file, which is
located in the data cluster folder, and try restarting again.
pg_hba.conf
The pg_hba.conf file controls which and how users can connect to PostgreSQL databa‐
ses. Changes to the file require a reload or a server restart to take effect. A typical
pg_hba.conf looks like Example 2-3 .
Example 2-3. Sample pg_hba.conf
# TYPE DATABASE USER ADDRESS METHOD
# IPv4 local connections:
host all all 127.0.0.1/32 ident
# IPv6 local connections:
host all all ::1/128 trust
host all all 192.168.54.0/24 md5
hostssl all all 0.0.0.0/0 md5
# Allow replication connections from localhost, by a user with the
# replication privilege.
#host replication postgres 127.0.0.1/32 trust
#host replication postgres ::1/128 trust
Authentication method. The usual choices are ident , trust , md5 , and pass
word . Version 9.1 introduced the peer authentication method. The ident and
peer options are available only on Linux, Unix, and the Mac, not on Windows.
More esoteric options, such as gss , radius , ldap , and pam , may not always be
installed.
IPv4 syntax for defining network range. The first part—in this case,
192.168.54.0 —is the network address, followed by /24 as the bit mask. In our
pg_hba.conf , we allow anyone in our subnet of 192.168.54.0 to connect as long
as they provide a valid md5 hashed password.
IPv6 syntax for defining network range. This applies only to servers with IPv6
support and may prevent pg_hba.conf from loading if you add this section
without actually having IPv6 networking.
SSL connection rule. In our example, we allow anyone to connect to our server
as long as they connect using SSL and have a valid md5 password.
Definition of a range of IP addresses allowed to replicate with this server. This
is new in version 9.0. These lines are remarked out in this example.
For each connection request, the postgres service checks the pg_hba.conf file from the
top down. As soon as a rule granting access is encountered, processing stops and the
connection is allowed. As soon as a rule rejecting access is encountered, processing stops
and the connection is denied. If the end of the file is reached without any matching