Ravi Sir's Cyber Security Cards › Revision Cards
Subdomains
Subdomains A subdomain is a name under your domain, like blog.example.com . Add a DNS record with Name blog only, not blog.example.com. Then add a server block for it, test and reload. Forgotten subdomains (dev., test., old.) often run old software with no one watching. If a subdomain points (CNAME)
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What this card says
What it is
A subdomain is a name under your domain, like blog.example.com.
How
Add a DNS record with Name blog only, not blog.example.com.
Then add a server block for it, test and reload.
Why it matters for security
Forgotten subdomains (dev., test., old.) often run old software with no one watching.
If a subdomain points (CNAME) to a cloud resource you deleted, someone else can claim it. That is subdomain takeover.
| Type | Name | Value |
|---|---|---|
| A | blog | 203.0.113.10 |
| A | api | 203.0.113.20 |
| CNAME | shop | my-alb-123.ap-south-1.elb.amazonaws.com |
| A | 203.0.113.10 |
Learn more in the lesson: 12.7 Subdomains