Unreachable Amazon EC2 Instance Issues
-
Cannot Connect via SSH (Timeout)
Description: When attempting to SSH into an EC2 instance, the connection times out.
Cause:
- The security group does not allow inbound SSH traffic.
- The instance does not have a public IP or Elastic IP.
- The network ACL blocks SSH traffic.
Solution:
- Verify that port 22 is open in the security group:
- Ensure the instance has a public or Elastic IP:
- Check the network ACL settings to allow inbound SSH:
aws ec2 describe-security-groups --group-ids sg-xxxxxxxxxxxxxxaws ec2 describe-instances --instance-ids i-xxxxxxxxxxxxxxxxx --query 'Reservations[*].Instances[*].PublicIpAddress'aws ec2 describe-network-acls --filters Name=association.subnet-id,Values=subnet-xxxxxxxx -
Cannot Connect via RDP (Windows Instances)
Description: RDP connection to a Windows instance fails.
Cause:
- RDP (port 3389) is blocked in the security group.
- The instance is in a private subnet without a VPN or bastion host.
- The Windows Firewall is blocking RDP connections.
Solution:
- Ensure port 3389 is open in the security group:
- Check if the instance is in a private subnet and use a VPN or bastion host if necessary.
- Modify the Windows Firewall settings to allow RDP access.
aws ec2 authorize-security-group-ingress --group-id sg-xxxxxxxxxxxxxx --protocol tcp --port 3389 --cidr 0.0.0.0/0 -
Instance Does Not Respond to Ping
Description: ICMP ping requests to the instance time out.
Cause:
- ICMP is blocked in the security group or network ACL.
- The instance is in a private subnet without NAT Gateway.
- The instance firewall blocks incoming ICMP traffic.
Solution:
- Allow ICMP in the security group:
- Modify the network ACL to allow inbound ICMP traffic.
- Ensure the instance is in a public subnet if external access is needed.
aws ec2 authorize-security-group-ingress --group-id sg-xxxxxxxxxxxxxx --protocol icmp --port -1 --cidr 0.0.0.0/0 -
Instance Becomes Unreachable After a Reboot
Description: The EC2 instance was accessible before rebooting but is now unreachable.
Cause:
- The instance lost its public IP if it was not assigned an Elastic IP.
- The network configuration inside the instance changed.
- A startup script or firewall rule is blocking traffic.
Solution:
- Check if the instance has a new public IP:
- Assign an Elastic IP to prevent IP changes:
- Verify network settings inside the instance using EC2 Serial Console or Systems Manager Session Manager.
aws ec2 describe-instances --instance-ids i-xxxxxxxxxxxxxxxxx --query 'Reservations[*].Instances[*].PublicIpAddress'aws ec2 associate-address --instance-id i-xxxxxxxxxxxxxxxxx --allocation-id eipalloc-xxxxxxxx -
DNS Resolution Issues
Description: The instance is running but cannot resolve domain names.
Cause:
- The instance is using incorrect DNS settings.
- The VPC does not have the correct DHCP options set.
- The outbound traffic is blocked by network ACLs.
Solution:
- Ensure the instance uses Amazon-provided DNS settings.
- Check the VPC DHCP options:
- Use a different DNS server such as 8.8.8.8 (Google DNS).
aws ec2 describe-dhcp-options --dhcp-options-ids dopt-xxxxxxxx
Favourites
Java
HTML and CSS 
Python
Javascript
TroubleshootingMonitor your IT infrastructure effortlessly with Site24x7 and get comprehensive insights and ensure smooth operations with 24/7 monitoring.
Sign up now
Programming Cheatsheets: Quick Reference for Productivity
Welcome to our comprehensive collection of programming language cheatsheets! Whether you're a seasoned developer or a beginner, these quick reference guides provide essential tips and key information for all major languages. They focus on core concepts, commands, and functions—designed to enhance your efficiency and productivity.
ManageEngine Site24x7, a leading IT monitoring and observability platform, is committed to equipping developers and IT professionals with the tools and insights needed to excel in their fields.
Monitor your IT infrastructure effortlessly with Site24x7 and get comprehensive insights and ensure smooth operations with 24/7 monitoring.
Sign up now!
Blogs
Learn
Tools