Cause 1: Network Connectivity Issues
First check basic network connectivity: ping the server IP address to confirm it's online. If ping fails, there may be a network outage or the server is down. Contact your network administrator or cloud provider support.
Cause 2: SSH Service Not Running
Check SSH service status on the server: systemctl status sshd (Linux). If not running, start it: sudo systemctl start sshd && sudo systemctl enable sshd. For Windows Server, check if OpenSSH service is running.
Cause 3: Firewall Blocking SSH Port
Check if firewall rules allow SSH port (default 22). Ubuntu/Debian: sudo ufw allow 22/tcp; CentOS/RHEL: sudo firewall-cmd --permanent --add-service=ssh && sudo firewall-cmd --reload. Also check security group rules on cloud platforms.
Cause 4: Wrong IP Address or Hostname
Carefully verify the server IP address or domain name you entered. Common mistakes include: typos, using internal IP instead of public IP, DNS resolution errors. Use ping command to verify correct IP resolution.
Cause 5: SSH Port is Not Default 22
If your server uses a non-standard SSH port (e.g., 2222, 8022), you must specify the port number when connecting. Enter the correct port in the web SSH tool's port field, or use command line: ssh -p port_number user@hostname.
Cause 6: Cloud Server Security Group Configuration
AWS EC2, Alibaba Cloud ECS, Tencent Cloud CVM and other cloud platforms use security groups to control access. Log into the cloud console and ensure inbound rules allow TCP port 22 (or custom SSH port) from your IP address.
Cause 7: High Server Load
If server CPU or memory usage is near 100%, SSH responses may timeout. Access the server via cloud console VNC or out-of-band management, check resource usage with top or htop commands. Restart services or upgrade configuration if necessary.
Cause 8: SSH Configuration File Limits
Check /etc/ssh/sshd_config settings: LoginGraceTime (login timeout), MaxStartups (max concurrent connections). If values are too small, they may cause connection timeouts. Restart SSH service after modification: sudo systemctl restart sshd.
Cause 9: DNS Resolution Failure
If connecting using a domain name instead of IP address, DNS resolution failure will cause timeout. Test with: nslookup your-domain.com or dig your-domain.com. If resolution fails, check DNS configuration or connect directly using IP address.
Cause 10: ISP or Network Provider Restrictions
Some network providers may restrict outbound connections on SSH port 22. Try switching networks (e.g., to mobile hotspot) to test. Or change SSH port to 443 (HTTPS port), which is usually not restricted: modify Port in sshd_config to 443.
Best Practices to Prevent SSH Connection Timeouts:
- Always use SSH key authentication instead of passwords for better security and connection stability
- Configure KeepAlive parameters to prevent idle connection disconnection: ClientAliveInterval 60
- Regularly check server firewall and security group rules
- Monitor server resource usage to avoid overload