detect.go

package main

import (
	"fmt"
	"io"
	"net/http"
	"net/url"
	"regexp"
	"strings"
	"time"
)

var (
	nginxVersionRegex = regexp.MustCompile(`(?i)nginx/([0-9]+\.[0-9]+\.[0-9]+)`)
	nginxBodySigRegex = regexp.MustCompile(`(?i)<center>\s*nginx(?:/([0-9]+\.[0-9]+\.[0-9]+))?\s*</center>`)
)

const unknownVersion = "Unknown (version hidden)"

func normalizeURL(target string) (*url.URL, error) {
	if !strings.HasPrefix(target, "http://") && !strings.HasPrefix(target, "https://") {
		target = "http://" + target
	}
	return url.Parse(target)
}

func detectFromServerHeader(result *NginxResult, header, method string) {
	if !strings.Contains(strings.ToLower(header), "nginx") {
		return
	}
	result.IsNginx = true
	if result.DetectionMethod == "" {
		result.DetectionMethod = method
	}

	if matches := nginxVersionRegex.FindStringSubmatch(header); len(matches) > 1 {
		if result.NginxVersion == "" || result.NginxVersion == unknownVersion {
			result.NginxVersion = matches[1]
		}
	} else if result.NginxVersion == "" {
		result.NginxVersion = unknownVersion
	}
}

func probe404(client *http.Client, target *url.URL, result *NginxResult) {
	probeURL := *target
	probeURL.Path = fmt.Sprintf("/_goscouter_non_existent_404_probe_%d", time.Now().UnixNano())

	resp, err := doGet(client, &probeURL)
	if err != nil {
		return
	}
	defer resp.Body.Close()

	probeServer := resp.Header.Get("Server")
	if probeServer != "" && result.ServerHeader == "" {
		result.ServerHeader = probeServer
	}
	detectFromServerHeader(result, probeServer, "404 Probe Server Header")

	body, err := io.ReadAll(io.LimitReader(resp.Body, 4096))
	if err != nil || len(body) == 0 {
		return
	}

	matches := nginxBodySigRegex.FindStringSubmatch(string(body))
	if len(matches) == 0 {
		return
	}
	result.IsNginx = true
	if result.DetectionMethod == "" {
		result.DetectionMethod = "404 Error Page HTML Signature"
	}
	if len(matches) > 1 && matches[1] != "" {
		result.NginxVersion = matches[1]
	}
}