Skip to content
Gin Web Framework

Limit upload size

Use http.MaxBytesReader to strictly limit the maximum size of uploaded files. When the limit is exceeded, the reader returns an error and you can respond with a 413 Request Entity Too Large status.

This is important for preventing denial-of-service attacks where clients send excessively large files to exhaust server memory or disk space.

How it works

  1. Define limit — A constant MaxUploadSize (1 MB) sets the hard cap for uploads.
  2. Enforce limit — http.MaxBytesReader wraps c.Request.Body. If the client sends more bytes than allowed, the reader stops and returns an error.
  3. Parse and check — c.Request.ParseMultipartForm triggers the read. The code checks for *http.MaxBytesError to return a 413 status with a clear message.
package main


import (
  "fmt"
  "net/http"


  "github.com/gin-gonic/gin"
)


const (
  MaxUploadSize = 1 << 20 // 1 MB
)


func uploadHandler(c *gin.Context) {
  // Wrap the body reader so only MaxUploadSize bytes are allowed
  c.Request.Body = http.MaxBytesReader(c.Writer, c.Request.Body, MaxUploadSize)


  // Parse multipart form
  if err := c.Request.ParseMultipartForm(MaxUploadSize); err != nil {
    if _, ok := err.(*http.MaxBytesError); ok {
      c.JSON(http.StatusRequestEntityTooLarge, gin.H{
        "error": fmt.Sprintf("file too large (max: %d bytes)", MaxUploadSize),
      })
      return
    }
    c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
    return
  }


  file, _, err := c.Request.FormFile("file")
  if err != nil {
    c.JSON(http.StatusBadRequest, gin.H{"error": "file form required"})
    return
  }
  defer file.Close()


  c.JSON(http.StatusOK, gin.H{
    "message": "upload successful",
  })
}


func main() {
  r := gin.Default()
  r.POST("/upload", uploadHandler)
  r.Run(":8080")
}

Test it

Terminal window
# Upload a small file (under 1 MB) -- succeeds
curl -X POST http://localhost:8080/upload \
  -F "file=@/path/to/small-file.txt"
# Output: {"message":"upload successful"}


# Upload a large file (over 1 MB) -- rejected
curl -X POST http://localhost:8080/upload \
  -F "file=@/path/to/large-file.zip"
# Output: {"error":"file too large (max: 1048576 bytes)"}

See also