Docker (legacy)
What is Port 4243?
Port 4243 was the original default port for the Docker daemon API before it was changed to ports 2375 (unencrypted) and 2376 (TLS). Some older Docker documentation, tutorials, and configurations still reference port 4243. Early versions of Docker exposed the API on this port for remote management. The change to 2375/2376 was made to better distinguish between encrypted and unencrypted Docker API access. If you encounter port 4243 in use, it's likely from a legacy Docker configuration or a custom application. Some Docker management tools and container orchestration systems may still use this port in certain configurations.
Common Uses
- Legacy Docker daemon API access
- Older Docker tooling and configurations
- Docker management tool integrations
- Custom container management interfaces
Technical Details
Copy-paste commands to check port 4243 (Docker (legacy)) from your terminal
Test connectivity
nc -zv example.com 4243
curl -v --max-time 5 http://example.com:4243/
telnet example.com 4243
nmap -p 4243 -sV example.com
(echo > /dev/tcp/example.com/4243) 2>/dev/null && echo "open" || echo "closed"
Open port in firewall
sudo ufw allow 4243/tcp
sudo iptables -A INPUT -p tcp --dport 4243 -j ACCEPT
sudo firewall-cmd --permanent --add-port=4243/tcp && sudo firewall-cmd --reload
Security Considerations
The same security concerns as port 2375 apply — unencrypted Docker API access provides root-equivalent access. Migrate to ports 2375/2376 with TLS. If this port is open, verify whether it's an outdated Docker configuration that should be updated or a different service.
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