Conditional logging in Nginx - Mini Project: Build & Apply

Conditional logging in nginx

📖 Scenario: You are managing a web server using nginx. You want to log only requests that have a status code of 400 or higher to keep your logs focused on errors and warnings.

🎯 Goal: Configure nginx to log only error requests (status code 400 and above) using conditional logging.

📋 What You'll Learn

Create a variable to check if the status code is 400 or higher

Use this variable to conditionally enable logging

Configure the access_log directive to use this conditional logging variable

Verify the configuration by printing the relevant nginx config lines

💡 Why This Matters

🌍 Real World

Web servers often generate large logs. Conditional logging helps focus on errors, saving disk space and making troubleshooting easier.

💼 Career

DevOps engineers and system administrators use conditional logging to optimize server monitoring and improve incident response.

1

Create a variable to detect error status codes

In the nginx configuration, create a variable called $log_errors that is set to 1 if the response status code is 400 or higher, otherwise 0. Use the map directive with $status to do this.

Nginx

# Your code here

Hint

Use the map directive to assign 1 to $log_errors when $status starts with 4 or higher.

2

Add conditional logging using the variable

Add an access_log directive that uses the variable $log_errors to enable logging only when $log_errors is 1. Use the built-in main log format and set the log file path to /var/log/nginx/error_requests.log.

Nginx

map $status $log_errors {
    default 0;
    ~^[4-9] 1;
}

# Your code here

Hint

Use access_log with the if= parameter to conditionally log requests.

3

Add a server block with the conditional logging

Create a server block listening on port 80 that includes the access_log directive with conditional logging using $log_errors. Inside the server block, add a simple location / that returns status 200 with the text OK.

Nginx

map $status $log_errors {
    default 0;
    ~^[4-9] 1;
}

access_log /var/log/nginx/error_requests.log main if=$log_errors;

# Your code here

Hint

Wrap the access_log and location inside a server block listening on port 80.

4

Print the final nginx configuration

Print the entire nginx configuration you created so far to verify the conditional logging setup.

Nginx

map $status $log_errors {
    default 0;
    ~^[4-9] 1;
}

server {
    listen 80;
    access_log /var/log/nginx/error_requests.log main if=$log_errors;

    location / {
        return 200 'OK';
    }
}

# Print the nginx config below
# Your code here

Hint

Use a print statement to output the full nginx configuration text exactly as written.

Practice

1. What is the main purpose of conditional logging in nginx?

easy

A. To log only specific requests based on conditions

B. To disable all logging permanently

C. To log every request without any filter

D. To log errors only, ignoring access logs

Solution

1. Step 1: Understand logging basics in nginx

   Logging records requests to help monitor and debug web traffic.

2. Step 2: Identify the role of conditional logging

   Conditional logging allows filtering which requests get logged based on rules.

3. Final Answer:

   To log only specific requests based on conditions -> Option A

4. Quick Check:

   Conditional logging = selective logging [OK]

Hint: Conditional logging means logging only some requests [OK]

Common Mistakes:

• Thinking conditional logging disables all logs
• Confusing conditional logging with error-only logging
• Assuming it logs every request without filtering

2. Which of the following is the correct syntax to enable conditional logging in nginx using the access_log directive?

easy

A. access_log /var/log/nginx/access.log on_condition=$if;

B. access_log /var/log/nginx/access.log if=$condition;

C. access_log /var/log/nginx/access.log condition=$if;

D. access_log if=$condition /var/log/nginx/access.log;

Solution

1. Step 1: Recall the correct order of parameters in access_log

   The syntax is: access_log <path> [format] [if=condition];

2. Step 2: Identify the correct use of the if= option

   The condition must be specified as if=$variable after the log path.

3. Final Answer:

   access_log /var/log/nginx/access.log if=$condition; -> Option B

4. Quick Check:

   access_log path if=condition [OK]

Hint: Remember: if= comes after log file path in access_log [OK]

Common Mistakes:

• Placing if= before the log file path
• Using wrong parameter names like condition= or on_condition=
• Omitting the $ sign before the variable

3. Given the following nginx configuration snippet, what will be the effect on logging?

map $request_uri $loggable {
    default 1;
    "/health" 0;
}

access_log /var/log/nginx/access.log combined if=$loggable;

medium

A. Only requests to /health will be logged

B. No requests will be logged

C. All requests except to /health will be logged

D. All requests will be logged regardless of URI

Solution

1. Step 1: Understand the map directive

   The map sets $loggable to 0 for "/health" and 1 for all others.

2. Step 2: Analyze the access_log condition

   Logging happens only if $loggable is true (1), so requests to "/health" (0) are skipped.

3. Final Answer:

   All requests except to /health will be logged -> Option C

4. Quick Check:

   map 0 disables logging for /health [OK]

Hint: map 0 disables logging; 1 enables it [OK]

Common Mistakes:

• Assuming /health requests are logged
• Thinking map disables all logging
• Confusing default and specific URI values

4. Identify the error in this nginx configuration for conditional logging:

map $status $loggable {
    200 1;
    default 0;
}

access_log /var/log/nginx/access.log combined if=loggable;

medium

A. access_log path is invalid

B. map directive syntax is incorrect

C. Cannot use $status variable in map

D. Missing $ before loggable in access_log condition

Solution

1. Step 1: Check variable usage in access_log

   Variables must be prefixed with $ in conditions, so if=loggable is wrong.

2. Step 2: Confirm correct syntax

   Correct syntax is if=$loggable to reference the variable properly.

3. Final Answer:

   Missing $ before loggable in access_log condition -> Option D

4. Quick Check:

   Variables need $ prefix in if= [OK]

Hint: Always prefix variables with $ in if= conditions [OK]

Common Mistakes:

• Omitting $ before variable in if= condition
• Miswriting map syntax
• Assuming $status cannot be used in map

5. You want to log all requests except those with user agent containing "Googlebot". Which configuration correctly implements this conditional logging?

hard

A. map $http_user_agent $loggable {
default 1;
~Googlebot 0;
}
access_log /var/log/nginx/access.log combined if=$loggable;

B. map $http_user_agent $loggable {
default 0;
~Googlebot 1;
}
access_log /var/log/nginx/access.log combined if=$loggable;

C. map $http_user_agent $loggable {
default 1;
Googlebot 0;
}
access_log /var/log/nginx/access.log combined if=$loggable;

D. map $http_user_agent $loggable {
default 1;
~Googlebot 1;
}
access_log /var/log/nginx/access.log combined if=$loggable;

Solution

1. Step 1: Use map with regex to detect "Googlebot" in user agent

   The ~ prefix allows regex matching; setting 0 disables logging for matching agents.

2. Step 2: Set default to 1 to log all other requests

   Default 1 means log unless user agent matches Googlebot.

3. Step 3: Use if=$loggable in access_log to apply condition

   This ensures only requests with $loggable=1 are logged.

4. Final Answer:

   map $http_user_agent $loggable { default 1; ~Googlebot 0; } access_log /var/log/nginx/access.log combined if=$loggable; -> Option A

5. Quick Check:

   Regex ~Googlebot disables logging for bots [OK]

Hint: Use ~ for regex in map to match user agents [OK]

Common Mistakes:

• Using exact string without ~ for regex
• Reversing default values causing wrong logging
• Not prefixing variable with $ in if= condition