Catch-all route for custom 404 Not Found response #

Build a Go web server without external dependencies — just the standard library.

In this short example, you’ll create a basic web server with a custom fallback page to replace Go’s standard 404 not found response. Again, we use http.NewServeMux() from the standard library. By the end, you’ll have a working server with a custom fallback page.

Set up the fallback router #

From the previous tutorial, we learned about the /name/{tag} route pattern to dynamically use path values. Today, we’ll catch all other routes that aren’t specified. The only thing we need to do is create a http.HandleFunc, catch all GET requests on /, and return any response we want e.g. fmt.Fprintf(w, "Custom 404 Not Found").

Example

mux.HandleFunc("GET /", func(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintf(w, "Custom 404 Not Found")
})

Return Custom 404 Not Found for all routes that are not explicitly handled.

It’s important to define the catch-all route at the top. The code may not behave as expected if a more specific HandleFunc is declared before it.

Complete Example #


package main

import (
	"errors"
	"fmt"
	"net/http"
	"os"
)

func main() {
    mux := http.NewServeMux()

    mux.HandleFunc("GET /", func(w http.ResponseWriter, r *http.Request) {
      w.Write([]byte("Custom 404 Not Found"))
    })
    // Add new handler(s) below

    srv := &http.Server{
      Addr:    fmt.Sprintf(":%d", 8080),
      Handler: mux,
    }

    err := srv.ListenAndServe()
    if !errors.Is(err, http.ErrServerClosed) {
      os.Exit(1)
    }
}

Simple web server that catches all unspecified routes.

Try It with curl #

Once your server is running, you can test it using:

curl http://localhost:8080/inject/javascript/here

You should see this response:

Custom 404 Not Found

Contributions