Request size limits (client_max_body_size) in Nginx - Step-by-Step Execution

Process Flow - Request size limits (client_max_body_size)

Client sends HTTP request

↓

Nginx receives request

↓

Check request body size

↓

Size <= limit

↓

Process request

↓

Send response

Nginx checks the size of the incoming request body against the client_max_body_size limit. If the size is within the limit, it processes the request; otherwise, it rejects it with an error.

Execution Sample

Nginx

server {
    client_max_body_size 1m;
    location /upload {
        proxy_pass http://backend;
    }
}

This config sets the max allowed client request body size to 1 megabyte for the /upload path.

Process Table

Step	Request Body Size	Condition (<= 1m?)	Action	Response
1	500kB	Yes	Process request	200 OK
2	1m	Yes	Process request	200 OK
3	1.5m	No	Reject request	413 Payload Too Large

💡 Request with body size exceeding 1m is rejected with 413 error.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
request_body_size	0	500kB	1m	1.5m
allowed_limit	1m	1m	1m	1m
request_accepted	false	true	true	false

Key Moments - 3 Insights

Why does a request with exactly 1m body size get accepted?

Because the condition checks if the size is less than or equal to the limit (<= 1m), so 1m is allowed as shown in execution_table row 2.

What happens if the request body size is larger than client_max_body_size?

Nginx rejects the request immediately with a 413 Payload Too Large error, as shown in execution_table row 3.

Does client_max_body_size affect the response size?

No, it only limits the size of the request body sent by the client, not the response size.

Visual Quiz - 3 Questions

Look at the execution table, what is the response when the request body size is 1.5m?

A 200 OK

B 413 Payload Too Large

C 404 Not Found

D 500 Internal Server Error

💡 Hint

Check the row where Request Body Size is 1.5m in the execution_table.

At which step does the request get accepted with the maximum allowed size?

A Step 1

B Step 3

C Step 2

D None

💡 Hint

Look at the Condition column in execution_table for size exactly equal to 1m.

If client_max_body_size is changed to 2m, what will happen to the request with 1.5m size?

A It will be accepted and processed

B It will be rejected with 413 error

C It will cause a server crash

D It will be redirected

💡 Hint

Refer to variable_tracker and understand the comparison logic with allowed_limit.

Concept Snapshot

Syntax: client_max_body_size <size>;
Limits max size of client request body.
If request body > limit, nginx returns 413 error.
Default is 1m if not set.
Set in http, server, or location block.
Example: client_max_body_size 2m;

Full Transcript

Nginx uses the client_max_body_size directive to limit how large a client's request body can be. When a client sends a request, nginx checks the size of the body. If the size is less than or equal to the configured limit, nginx processes the request normally. If the size is larger, nginx rejects the request and responds with a 413 Payload Too Large error. This prevents very large uploads or requests from overwhelming the server. The directive can be set in different configuration blocks like http, server, or location. For example, setting client_max_body_size 1m means requests with body size up to 1 megabyte are accepted. Requests larger than that are rejected immediately. This check happens before the request is fully processed, saving server resources.