How to fix Unable to establish connection in PostgreSQL

Check if the PostgreSQL daemon is active on the target machine.

On Linux/Mac with systemd:

sudo systemctl status postgresql

On macOS with Homebrew:

brew services list

On Windows, check Services.msc for "postgresql-x64" service.

If the service is not running, start it:

sudo systemctl start postgresql
# or
brew services start postgresql

Verify that PostgreSQL is listening on port 5432 (or your configured port).

# Check listening ports on Linux/Mac
sudo lsof -i :5432
# or
sudo netstat -ltnp | grep postgres

If PostgreSQL is not listening, it may not be running or listen_addresses is misconfigured. Verify your postgresql.conf:

sudo -u postgres grep -E "^listen_addresses|^port" /etc/postgresql/*/main/postgresql.conf

Edit postgresql.conf to ensure the server accepts connections from your client IP.

sudo -u postgres vi /etc/postgresql/*/main/postgresql.conf

For local connections only (default):

listen_addresses = 'localhost'

For accepting connections from any IP (use with caution):

listen_addresses = '*'

For specific network:

listen_addresses = '192.168.1.5, localhost'

After changing this setting, restart PostgreSQL:

sudo systemctl restart postgresql

The pg_hba.conf file controls which hosts can connect and how they authenticate.

sudo -u postgres cat /var/lib/postgresql/*/main/pg_hba.conf

Look for a line matching your connection attempt. For local TCP connections, ensure you have:

host    all             all             127.0.0.1/32            md5
host    all             all             ::1/128                 md5

For remote connections from specific network:

host    all             all             192.168.1.0/24          md5

After modifying pg_hba.conf, reload the configuration:

sudo -u postgres psql -c "SELECT pg_reload_conf();"

Use psql to diagnose connection issues with verbose output.

psql -h localhost -U postgres -d postgres -v ON_ERROR_STOP=on

Add verbose flags for more debugging:

psql -h 192.168.1.10 -p 5432 -U myuser -d mydb -v

Check if port is reachable using netstat or telnet:

telnet 192.168.1.10 5432
# or on macOS
nc -zv 192.168.1.10 5432

If telnet/nc connects, PostgreSQL is listening. If it times out, check firewall rules.

Firewall blocking is a common cause of connection failures on remote connections.

# Check UFW (Ubuntu)
sudo ufw status
sudo ufw allow 5432/tcp

# Check firewalld (CentOS/RHEL)
sudo firewall-cmd --list-ports
sudo firewall-cmd --permanent --add-service=postgresql
sudo firewall-cmd --reload

# Check iptables directly
sudo iptables -L -n | grep 5432

On Windows, ensure Windows Firewall has a rule allowing inbound TCP on port 5432.

If PostgreSQL is running but rejecting connections, you may have hit the max_connections limit.

# Connect as superuser to check
sudo -u postgres psql -c "SHOW max_connections;"
sudo -u postgres psql -c "SELECT count(*) FROM pg_stat_activity;"

Increase the limit in postgresql.conf:

max_connections = 200

Restart PostgreSQL after changing:

sudo systemctl restart postgresql

Alternatively, implement connection pooling with PgBouncer to manage connections more efficiently.