In day-to-day cloud operations, some incidents stand out not because of their complexity, but quite the opposite.
They are those cases where, once you discover the root cause, you think:
“this can’t be it”.
This is one of them.
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The scenario
The EC2 instance was running, with a public IP and everything seemingly correct.
- Status: running
- Public IP available
- No visible alerts
Even so, every attempt to access it resulted in a timeout.
No clear error. No direct clue.
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Where to look first
In situations like this, before assuming something more complex, the first step is always to review the Security Group.
More specifically: the inbound rules.
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The problem
When analyzing the Security Group, the situation was straightforward:
No inbound rules configured.
This means the instance was not accepting any external connections.
No SSH, no HTTP, no any other port.
This is expected behavior in AWS:
everything is denied by default unless explicitly allowed.
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Why this happens
This type of situation is more common than it seems.
Some examples:
- Creating an instance without reviewing the Security Group
- Using a default Security Group with no rules
- Changes made during testing that were not reverted
- Switching Security Groups without proper validation
In the middle of daily operations, this kind of detail is easy to overlook.
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How to fix
The fix is simple: add the required rule to the Security Group.
Example for SSH access:
- Type: SSH
- Port: 22
- Source: your IP (recommended) or 0.0.0.0/0 for testing
After the fix
Once the rule was added, access to the instance was restored immediately.
No restart required. No additional changes needed.
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Practical takeaway
Before assuming complex issues, always check the basics.
In many cases, what looks like a serious incident is just a simple configuration that went unnoticed.
And precisely because it looks simple, it happens more often than expected.
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Quick checklist for unreachable EC2
- Security Group
- Network ACL
- Route Table
- Instance status
In most cases, the issue is in the first item.
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Further reading
If you want to dive deeper, the official documentation explains how Security Groups work and how rules are evaluated:
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Note
This content is based on real-world scenarios from day-to-day operations.
AI tools were used only for text review.