Conditional redirects with if in Nginx - Step-by-Step Execution

Process Flow - Conditional redirects with if

Request Received

↓

Evaluate if condition

↓

Redirect

↓

End Request

Nginx checks the if condition on each request. If true, it redirects; if false, it serves the request normally.

Execution Sample

Nginx

server {
  listen 80;
  if ($host = 'old.example.com') {
    return 301 https://new.example.com$request_uri;
  }
}

Redirect requests from old.example.com to new.example.com preserving the path.

Process Table

Step	Request Host	Condition ($host = 'old.example.com')	Action	Result
1	old.example.com	True	return 301 redirect	Redirect to https://new.example.com$request_uri
2	new.example.com	False	serve normally	Serve requested content
3	other.example.com	False	serve normally	Serve requested content

💡 Requests with host 'old.example.com' redirect; others serve normally.

Status Tracker

Variable	Start	Request 1	Request 2	Request 3
$host	undefined	old.example.com	new.example.com	other.example.com
Condition Result	undefined	True	False	False

Key Moments - 2 Insights

Why does the redirect only happen for 'old.example.com' and not others?

Because the if condition checks if $host equals 'old.example.com'. Only when this is true (see execution_table row 1) does nginx perform the redirect.

What happens if the condition is false?

Nginx skips the redirect and serves the request normally, as shown in execution_table rows 2 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the action taken when the request host is 'new.example.com'?

A Serve normally

B Redirect to new.example.com

C Return 404 error

D Redirect to old.example.com

💡 Hint

Check execution_table row 2 under 'Action' column.

At which step does the condition become true?

A Step 2

B Step 1

C Step 3

D None

💡 Hint

Look at the 'Condition' column in execution_table.

If the condition was changed to check for 'new.example.com', what would happen at step 1?

A Redirect occurs

B Error occurs

C Serve normally

D Request is blocked

💡 Hint

Refer to variable_tracker and execution_table logic for condition evaluation.

Concept Snapshot

Nginx conditional redirect syntax:
if (condition) {
  return 301 https://newsite$request_uri;
}
Redirects only if condition is true.
Otherwise, serves request normally.
Use for simple host or path based redirects.

Full Transcript

This visual execution shows how nginx uses the if directive to conditionally redirect requests. When a request arrives, nginx checks if the host matches 'old.example.com'. If yes, it sends a 301 redirect to 'https://new.example.com' preserving the path. If not, it serves the request normally. The execution table traces three requests with different hosts, showing when the redirect happens and when it does not. The variable tracker shows the $host value and condition result for each request. Key moments clarify why only matching hosts redirect and what happens otherwise. The quiz tests understanding of condition evaluation and redirect behavior.

Practice

1. What does the if directive do in an nginx configuration for conditional redirects?

easy

A. It logs all user requests to a file.

B. It checks a condition and performs an action like redirect if true.

C. It restarts nginx automatically.

D. It permanently disables the server.

Solution

1. Step 1: Understand the role of if in nginx

   The if directive evaluates a condition inside the server or location block.

2. Step 2: Recognize its use for redirects

   If the condition is true, nginx can perform actions like return to redirect users.

3. Final Answer:

   It checks a condition and performs an action like redirect if true. -> Option B

4. Quick Check:

   if checks condition for redirect [OK]

Hint: Remember: if tests, return redirects [OK]

Common Mistakes:

• Thinking if restarts nginx
• Confusing if with logging
• Assuming if disables server

2. Which of the following is the correct syntax to redirect all requests from /old to /new using if in nginx?

easy

A. if ($request_uri == "/old") return 301 /new;

B. if $request_uri == "/old" { return 301 /new; }

C. if ($request_uri = "/old") { return 301 /new; }

D. if $request_uri = "/old" return 301 /new;

Solution

1. Step 1: Check correct if syntax in nginx

   nginx requires parentheses around the condition and uses a single equals sign for string comparison.

2. Step 2: Confirm the block and return statement

   The condition must be inside parentheses, followed by braces with the return directive inside.

3. Final Answer:

   if ($request_uri = "/old") { return 301 /new; } -> Option C

4. Quick Check:

   Parentheses and braces with single = [OK]

Hint: Use parentheses and single = inside if [OK]

Common Mistakes:

• Using double == instead of single =
• Omitting parentheses around condition
• Missing braces around return

3. Given this nginx snippet:

server {
  listen 80;
  server_name example.com;
  location / {
    if ($http_user_agent ~* "Mobile") {
      return 302 /mobile;
    }
  }
}

What happens when a user with user agent "Mozilla Mobile" visits http://example.com/?

medium

A. They are redirected to /mobile with a 302 status.

B. They see the normal homepage without redirect.

C. They get a 404 error.

D. The server crashes due to syntax error.

Solution

1. Step 1: Understand the if condition with regex

   The condition checks if the user agent contains "Mobile" ignoring case.

2. Step 2: Apply the redirect action

   Since "Mozilla Mobile" matches, nginx returns a 302 redirect to /mobile.

3. Final Answer:

   They are redirected to /mobile with a 302 status. -> Option A

4. Quick Check:

   User agent matches regex, triggers redirect [OK]

Hint: Regex ~* means case-insensitive match [OK]

Common Mistakes:

• Ignoring case sensitivity in regex
• Assuming no redirect happens
• Thinking syntax error occurs

4. Identify the error in this nginx config snippet for redirecting /old to /new:

if $request_uri = "/old" {
  return 301 /new;
}

medium

A. Missing parentheses around the condition in if.

B. Using double equals instead of single equals.

C. Missing semicolon after if statement.

D. Incorrect return status code.

Solution

1. Step 1: Check if syntax requirements

   nginx requires the condition inside parentheses after if.

2. Step 2: Verify the given snippet

   The snippet lacks parentheses around $request_uri = "/old", causing syntax error.

3. Final Answer:

   Missing parentheses around the condition in if. -> Option A

4. Quick Check:

   if needs parentheses [OK]

Hint: Always wrap if conditions in parentheses [OK]

Common Mistakes:

• Forgetting parentheses in if
• Confusing single and double equals
• Thinking semicolon needed after if

5. You want to redirect users accessing /old only if they come from the domain example.com. Which nginx config snippet correctly does this using if?

hard

A. if ($request_uri = "/old" && $http_referer ~* "example.com") { return 301 /new; }

B. if ($request_uri = "/old") { return 301 /new if ($http_referer ~* "example.com"); }

C. if ($request_uri = "/old" || $http_referer ~* "example.com") { return 301 /new; }

D. if ($request_uri = "/old") { if ($http_referer ~* "example.com") { return 301 /new; } }

Solution

1. Step 1: Understand nginx if limitations

   nginx does not support combining conditions with && inside a single if.

2. Step 2: Use nested if for multiple conditions

   Nested if blocks allow checking one condition inside another.

3. Step 3: Analyze options

   if ($request_uri = "/old") { if ($http_referer ~* "example.com") { return 301 /new; } } uses nested if correctly; others use unsupported syntax or wrong logic.

4. Final Answer:

   if ($request_uri = "/old") { if ($http_referer ~* "example.com") { return 301 /new; } } -> Option D

5. Quick Check:

   Nested if for multiple conditions [OK]

Hint: Use nested if for multiple conditions in nginx [OK]

Common Mistakes:

• Trying to use && inside one if
• Using || instead of && for AND logic
• Placing return incorrectly