localhost vs 127.0.0.1 vs 0.0.0.0: Network Binding Explained

Why localhost, 127.0.0.1, 0.0.0.0, and * behave differently for MySQL clients and services, and how to find and fix a local port conflict between Docker and SSH.

14 Sept 2025 · 2 min read

database #mysql #networking #docker #ssh

Audio summary

Running a Dockerized database alongside an SSH tunnel to a remote one, both on the same local port, produces confusing failures: the wrong service answers, or one client works while another times out. That comes down to what address each service actually binds to and which one your client resolves to.

Localhost vs. 127.0.0.1

Term	Type	IP Address	MySQL Client Behavior
`localhost`	Hostname	Usually `127.0.0.1`	Often tries a Unix socket connection (not TCP/IP), which can fail for Docker or SSH tunnels.
`127.0.0.1`	IPv4 Loopback	`127.0.0.1`	Always forces a TCP/IP connection (recommended for local testing).

Step 2: Understanding `0.0.0.0` and `*` Bindings

These addresses specify which network interfaces a service should listen on.

Term	Meaning	Interfaces Covered
`0.0.0.0`	IPv4 Wildcard	Listens on all available IPv4 interfaces, including `127.0.0.1` and your external network IP.
*``**	Shorthand Wildcard	Acts the same as `0.0.0.0` or `::` (IPv6 wildcard) in most contexts: listens on all interfaces.

Step 3: The Port Conflict Scenario & Identifying the Full Command

A port conflict is often avoided because two services listen on different interfaces, such as one on the universal 0.0.0.0:3306 and one on the specific 127.0.0.1:3306.

To confirm exactly which process is which, you must inspect the full command.

Identifying the Full Process Command

The lsof tool shows the PID, but the ps tool shows the full command associated with that PID.

Run lsof to find the PIDs listening on the port (e.g., 3306):
```
sudo lsof -i :3306 -t
```
Run ps to see the full command for those PIDs:
```
ps -fp $(sudo lsof -i :3306 -t)
```

Command Snippet in `ps` Output	Service Confirmed
`docker-proxy -host-ip 0.0.0.0 -container-port 3306 ...`	Docker Container
`ssh -L 3307:localhost:3306 user@...`	SSH Tunnel
`/usr/sbin/mysqld --daemonize --pid-file=...`	Local/Host MySQL Server

Step 4: Connecting to the Correct MySQL Instance

Use the -h (host) and -P (port) flags on the mysql client to connect explicitly to the correct service.

To Docker (using the global binding): Connect using your machine’s actual network IP address (e.g., 192.168.1.10) to ensure you hit the Docker proxy.
```
mysql -h 192.168.1.10 -P 3306 -u docker_user -p
```
To SSH Tunnel (using the loopback binding): Connect using the loopback IP address.
```
mysql -h 127.0.0.1 -P 3306 -u remote_user -p
```

Step 5: Avoid the Conflict Entirely

To maintain a stable development environment, always configure a unique host port for each service.

Service	Host Port	SSH Command / Docker Config	MySQL Connection Command
Docker DB	`3306`	`-p 3306:3306`	`mysql -h 127.0.0.1 -P 3306 -u user -p`
SSH Tunnel	`3307`	`ssh -L 3307:remote_db:3306 user@host`	`mysql -h 127.0.0.1 -P 3307 -u user -p`