could not connect to server

What this means

The client was unable to establish a TCP connection to the Postgres server at all. Unlike 08006 (which is a drop of an established connection), 08001 means the initial TCP handshake or the Postgres startup protocol never completed.

Why it happens

1. 1Postgres is not running on the target host and port
2. 2Firewall or security group blocks inbound TCP on the Postgres port (default 5432)
3. 3Incorrect hostname or IP address in the connection string
4. 4Postgres is listening on a different port or only on localhost (127.0.0.1) when a remote connection is attempted
5. 5max_connections reached and all connection slots are in use

How to reproduce

A client attempts to connect to a Postgres host that is unreachable.

-- From psql:
-- psql -h 10.0.0.1 -p 5432 -U postgres mydb
-- (when the server is not running or unreachable)

Fix 1: Verify Postgres is running and listening on the correct address

When the server may be down or misconfigured.

-- On the server OS (not SQL):
-- pg_lsclusters (Debian/Ubuntu)
-- systemctl status postgresql

-- Check listening addresses in postgresql.conf:
SHOW listen_addresses;
SHOW port;

-- Verify from pg_hba.conf that the host is allowed.

Fix 2: Check and open firewall rules

When the server is running but network access is blocked.

-- OS-level (not SQL); example for ufw:
-- ufw allow 5432/tcp

-- Cloud security group: add inbound rule for TCP port 5432
-- from the client IP range.

What not to do

Sources

Confidence assessment

See also