Conditional logging in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is conditional logging in nginx?

Conditional logging in nginx means logging requests only when certain conditions are met, like logging errors but not successful requests.

beginner

Which nginx directive is used to control conditional logging?

The access_log directive with a condition parameter controls conditional logging.

intermediate

How do you disable logging for requests with status code 200 in nginx?

Use access_log /path/to/log combined if=$loggable; where $loggable is 0 for status 200, for example: map $status $loggable { 200 0; default 1; } and then access_log /path/to/log combined if=$loggable;

intermediate

What is the purpose of the map directive in conditional logging?

The map directive creates a variable based on conditions, which can be used to decide if logging should happen.

intermediate

Can you log only error responses (status 400 and above) in nginx?

Yes, by using a map to set a variable to 1 for status codes 400 and above, then using access_log with if to log only when that variable is 1.

Which directive controls logging in nginx?

A access_log

B error_log

C log_format

D server_log

The access_log directive controls logging of client requests.

How do you disable logging for certain requests in nginx?

A Remove the log file

B Use <code>access_log off;</code> globally

C Use <code>error_log off;</code>

D Use <code>access_log /path/to/log combined if=$condition;</code>

You can disable logging conditionally using access_log /path/to/log combined if=$condition;.

What does the map directive do in conditional logging?

A Sets log format

B Defines log file location

C Creates a variable based on conditions

D Starts the nginx server

map creates variables that help decide logging conditions.

To log only error responses (status 400+), what condition should be set?

A $status < 400

B $status >= 400

C $status = 200

D $status = 500

You set a condition to log when status is 400 or higher.

Which of these is a valid way to conditionally log in nginx?

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

B error_log /var/log/nginx/error.log if=$loggable;

C log_format conditional;

D server_log conditional;

access_log supports conditional logging with the if parameter.

Explain how to set up conditional logging in nginx to log only requests with status codes 400 and above.

Think about how to create a variable that marks error responses and use it in access_log.

Describe the role of the access_log directive and how it can be customized for conditional logging.

Focus on how access_log controls logging and how conditions affect it.

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