# PostgreSQL: could not connect to server / connection refused — Is the server running on host and accepting TCP/IP connections?

**Code:** `could not connect` · **Category:** PostgreSQL · **Seen on:** PostgreSQL, Linux, Docker

The client reached no PostgreSQL listener at that host/port — the server is down, listening elsewhere, or not accepting remote connections.

## Meaning

By default PostgreSQL listens only on localhost (`listen_addresses = 'localhost'`) and port 5432. Remote clients also need a matching `pg_hba.conf` line and an open firewall port. In Docker, `localhost` means the app container itself.

## Common causes

- PostgreSQL service stopped or crashed (disk full, bad config after edit)
- `listen_addresses` limited to localhost while connecting remotely
- Different port (5433 when multiple versions are installed)
- Firewall / cloud security group blocking 5432
- Docker: using localhost instead of the service name
- Socket path mismatch: “could not connect to server: No such file or directory … /var/run/postgresql/.s.PGSQL.5432”

## Quick fix

1. `sudo systemctl status postgresql` and read the log if it’s down
2. Check `SHOW listen_addresses;` and `SHOW port;`
3. Set `listen_addresses = '*'` + a pg_hba.conf host line for remote access, then restart
4. In Docker Compose, connect to `db:5432`

## Detailed fix by platform

### Linux

1. Check service, port and listeners:

```bash
sudo systemctl status postgresql
sudo ss -ltnp | grep postgres
pg_lsclusters                      # Debian/Ubuntu: shows version, port, status
sudo tail -n 50 /var/log/postgresql/postgresql-*-main.log
```


### Docker

1. `DATABASE_URL=postgres://app:secret@db:5432/app` — use the compose service name; add a healthcheck so the app waits for the DB.

## How to diagnose

1. **Service** — Running?
2. **Listener** — Which address/port does it listen on?
3. **Network** — Firewall / container network?
4. **Auth** — Next error after connecting: pg_hba.conf?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/postgresql/could-not-connect-to-server)