Add check for referrer in req.Referer

- typo as per standard

README.md

@@ -35,6 +35,13 @@ URL_PREFIX=
 # served using HTTP.
 TLS_CERT=
 TLS_KEY=
+# List of accepted HTTP referrers. Return 403 if HTTP header `Referer` does not
+# match prefixes provided in the list.
+# Examples:
+#   'REFERRERS=http://localhost,https://...,https://another.name'
+#   To accept missing referrer header, add a blank entry (start comma):
+#   'REFERRERS=,http://localhost,https://another.name'
+REFERRERS=
 ```
 
 ### YAML Configuration File
@@ -53,6 +60,9 @@ folder: /web
 url-prefix: ""
 tls-cert: ""
 tls-key: ""
+referrers:
+  - http://localhost
+  - https://my.site
 ```
 
 ## Deployment

cli/help/help.go

@@ -63,6 +63,10 @@ ENVIRONMENT VARIABLES
         The prefix to use in the URL path. If supplied, then the prefix must
         start with a forward-slash and NOT end with a forward-slash. If not
         supplied then no prefix is used.
+    REFERRERS
+        A comma-separated list of valid HTTP Referer prefixes. If incoming header
+        value is not in this list, a 403 HTTP error is returned.
+        Examples: 'http://localhost,https://some.site,http://other.site:8080'
 
 CONFIGURATION FILE
     Configuration can also managed used a YAML configuration file. To select the
@@ -81,6 +85,8 @@ CONFIGURATION FILE
     tls-cert: ""
     tls-key: ""
     url-prefix: ""
+    referrers:
+      - http://localhost
     ----------------------------------------------------------------------------
 
 USAGE
@@ -99,7 +105,7 @@ USAGE
         export PORT=80
         static-file-server
             Retrieve with: wget http://my.machine/sub/my.file
-        
+
         export FOLDER=/var/www
         static-file-server -c config.yml
             Result: Runs with values from config.yml, but with the folder being

cli/server/server.go

@@ -34,11 +34,16 @@ func Run() error {
 // configuration.
 func handlerSelector() (handler http.HandlerFunc) {
 	var serveFileHandler handle.FileServerFunc
+
 	serveFileHandler = http.ServeFile
 	if config.Get.Debug {
 		serveFileHandler = handle.WithLogging(serveFileHandler)
 	}
 
+	if config.Get.Referrers != nil {
+		serveFileHandler = handle.WithReferrers(serveFileHandler, config.Get.Referrers)
+	}
+
 	// Choose and set the appropriate, optimized static file serving function.
 	if 0 == len(config.Get.URLPrefix) {
 		handler = handle.Basic(serveFileHandler, config.Get.Folder)

config/config.go

@@ -22,6 +22,7 @@ var (
 		TLSCert     string `yaml:"tls-cert"`
 		TLSKey      string `yaml:"tls-key"`
 		URLPrefix   string `yaml:"url-prefix"`
+		Referrers   []string `yaml:"referrers"`
 	}
 )
 
@@ -34,6 +35,7 @@ const (
 	tlsCertKey     = "TLS_CERT"
 	tlsKeyKey      = "TLS_KEY"
 	urlPrefixKey   = "URL_PREFIX"
+	referrersKey   = "REFERRERS"
 )
 
 const (
@@ -61,6 +63,7 @@ func setDefaults() {
 	Get.TLSCert = defaultTLSCert
 	Get.TLSKey = defaultTLSKey
 	Get.URLPrefix = defaultURLPrefix
+	Get.Referrers = nil
 }
 
 // Load the configuration file.
@@ -108,6 +111,7 @@ func overrideWithEnvVars() {
 	Get.TLSCert = envAsStr(tlsCertKey, Get.TLSCert)
 	Get.TLSKey = envAsStr(tlsKeyKey, Get.TLSKey)
 	Get.URLPrefix = envAsStr(urlPrefixKey, Get.URLPrefix)
+	Get.Referrers = strAsArray(envAsStr(referrersKey, ""))
 }
 
 // validate the configuration.
@@ -211,3 +215,11 @@ func strAsBool(value string) (result bool, err error) {
 	}
 	return
 }
+
+func strAsArray(value string) (result []string) {
+	if (value == "") {
+		return nil
+	}
+	result = strings.Split(value, ",");
+	return
+}

handle/handle.go

@@ -26,12 +26,54 @@ type ListenerFunc func(string, http.HandlerFunc) error
 // requesting client.
 type FileServerFunc func(http.ResponseWriter, *http.Request, string)
 
+func validReferrer(s []string, e string) bool {
+	if (s == nil) {
+		// log.Printf("No referrers specified, all fine.")
+		return true
+	}
+
+	// log.Printf("Checking referrers " + strings.Join(s, ",")  + " against " + e)
+
+	for _, a := range s {
+		// Handle blank HTTP Referer header, if configured
+		if (a == "") {
+			if (e == "") {
+				// log.Printf("No referrer in request. Allowing.");
+				return true;
+			}
+			// Continue loop (all strings start with "")
+			continue;
+		}
+
+		// Compare header with allowed prefixes
+		if strings.HasPrefix(e, a) {
+			// log.Printf(strings.Join([]string{ "Referrer match", e, a }, " "));
+			return true
+		}
+	}
+	return false
+}
+
+func WithReferrers(serveFile FileServerFunc, referrers []string) FileServerFunc {
+	return func(w http.ResponseWriter, r *http.Request, name string) {
+		if (validReferrer(referrers, r.Referer())) {
+			// log.Printf("Serving file.")
+			serveFile(w, r, name)
+		} else {
+				// log.Printf(strings.Join([]string{"Invalid referrer", r.Referer(), "Not in", strings.Join(referrers, ",")}, " "))
+				http.Error(w, strings.Join([]string{ "Invalid source", r.Referer() }, " "), 403)
+				return
+		}
+	}
+}
+
 // WithLogging returns a function that logs information about the request prior
 // to serving the requested file.
 func WithLogging(serveFile FileServerFunc) FileServerFunc {
 	return func(w http.ResponseWriter, r *http.Request, name string) {
 		log.Printf(
-			"REQ: %s %s %s%s -> %s\n",
+			"REQ from %s: %s %s %s%s -> %s\n",
+			r.Referer(),
 			r.Method,
 			r.Proto,
 			r.Host,