HSTS header in Nginx - Step-by-Step Execution

Process Flow - HSTS header

Client makes HTTPS request

↓

Server responds with HSTS header

↓

Client stores HSTS policy

↓

Future requests forced to HTTPS

↓

No HTTP fallback allowed

↓

Secure connection enforced

The server sends the HSTS header to tell browsers to always use HTTPS for future requests, improving security by preventing HTTP fallback.

Execution Sample

Nginx

server {
    listen 443 ssl;
    add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
}

This nginx config adds the HSTS header to HTTPS responses, telling browsers to enforce HTTPS for 1 year including subdomains.

Process Table

Step	Action	Header Sent	Client Behavior	Result
1	Client sends HTTPS request	No	No HSTS policy yet	Server processes request
2	Server responds with HSTS header	Strict-Transport-Security: max-age=31536000; includeSubDomains	Client stores HSTS policy	Client remembers to use HTTPS
3	Client sends HTTP request next time	No	Client upgrades to HTTPS automatically	Connection is secure
4	Client sends HTTPS request again	No new header needed	Uses HTTPS directly	Secure connection maintained
5	Client tries HTTP after max-age expires	No	HSTS policy expired, may allow HTTP	Potential insecure connection

💡 HSTS header instructs client to enforce HTTPS until max-age expires

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
HSTS Policy Stored	False	True	True	True	False (expired)
Connection Protocol	HTTPS	HTTPS	Upgraded to HTTPS	HTTPS	HTTP possible

Key Moments - 2 Insights

Why does the client upgrade HTTP requests to HTTPS after receiving the HSTS header?

Because the client stores the HSTS policy at Step 2 (see execution_table row 2), it automatically upgrades any HTTP requests to HTTPS to follow the security rule.

What happens if the max-age time expires?

After max-age expires (Step 5), the client no longer enforces HTTPS strictly and may allow HTTP connections again, as shown by the HSTS Policy Stored variable becoming False.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the client store the HSTS policy?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Check the 'Client Behavior' column in execution_table row 2

According to variable_tracker, what is the value of 'Connection Protocol' after Step 3?

A HTTP

B HTTPS

C Upgraded to HTTPS

D No connection

💡 Hint

Look at the 'Connection Protocol' row under 'After Step 3' in variable_tracker

If the server did not send the HSTS header at Step 2, what would happen to the client behavior?

A Client would store HSTS policy anyway

B Client would not enforce HTTPS automatically

C Client would upgrade HTTP requests to HTTPS

D Client would block all HTTP requests

💡 Hint

Refer to execution_table row 2 where header is sent and client stores policy

Concept Snapshot

HSTS header in nginx:
add_header Strict-Transport-Security "max-age=SECONDS; includeSubDomains" always;
Sends policy to browsers to enforce HTTPS
Browsers upgrade HTTP to HTTPS automatically
max-age defines how long policy lasts
Improves security by preventing downgrade attacks

Full Transcript

HSTS header is a security feature where the server tells browsers to always use HTTPS for future requests. In nginx, you add it with add_header Strict-Transport-Security and set max-age for how long the browser should remember this. When a client first connects over HTTPS, the server sends this header. The client stores the policy and upgrades any HTTP requests to HTTPS automatically. This prevents insecure connections and downgrade attacks. After the max-age expires, the client may allow HTTP again. This trace shows each step from request to client enforcing HTTPS.