When configuring a PostgreSQL server, one of the first things that needs to be done is define remote access. By default, PostgreSQL does not accept remote connections. The important thing here is that PostgreSQL does not even reject the connection because it simply does not listen on the port. If we try to connect, the error message will actually come from the operating system because PostgreSQL does not care at all.

Assuming that there is a database server using the default configuration on 192.168.0.123, the following will happen:

iMac:~ hs$ telnet 192.168.0.123 5432 
Trying 192.168.0.123... 
telnet: connect to address 192.168.0.123: Connection refused 
telnet: Unable to connect to remote host

Telnet tries to create a connection on port 5432 and is instantly rejected by the remote box. From the outside, it looks as if PostgreSQL is not running at all.

The key to success can be found in the postgresql.conf file:

# - 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)

The listen_addresses setting will tell PostgreSQL which addresses to listen on. Technically speaking, those addresses are bind addresses. What does that actually mean? Suppose we have four network cards in our machine. We can listen on, say, three of those IP addresses. PostgreSQL takes requests to those three cards into account and does not listen on the fourth one. The port is simply closed.

If we put an * in, PostgreSQL will listen to every IP assigned to your machine.

However, there are more settings related to connection management that are highly important to understand. They are as follows:

#port = 5432 
              # (change requires restart) 
max_connections = 100 
              # (change requires restart) 
# Note: Increasing max_connections costs ~400 bytes of 
# shared memory per 
# connection slot, plus lock space 
# (see max_locks_per_transaction). 
#superuser_reserved_connections = 3 
              # (change requires restart) 
#unix_socket_directories = '/tmp' 
              # comma-separated list of directories 
              # (change requires restart) 
#unix_socket_group = '' 
              # (change requires restart) 
#unix_socket_permissions = 0777 
              # begin with 0 to use octal notation 
              # (change requires restart) 

First of all, PostgreSQL listens to a single TCP port, the default value of which is is 5432. Keep in mind that PostgreSQL will listen on a single port only. Whenever a request comes in, the postmaster will fork and create a new process to handle the connection. By default, up to 100 normal connections are allowed. On top of that, three additional connections are reserved for superusers. This means that we can either have 97 connections plus three superusers or 100 superuser connections.