--- a/pkg/config/config.go	Thu Dec 20 14:18:30 2018 +0100
+++ b/pkg/config/config.go	Thu Dec 20 14:39:23 2018 +0100
@@ -106,11 +106,12 @@
 func TmpDir() string { return viper.GetString("tmp-dir") }
 
 var (
-	proxyKeyOnce sync.Once
-	proxyKey     []byte
-
+	proxyKeyOnce    sync.Once
+	proxyKey        []byte
 	proxyPrefixOnce sync.Once
 	proxyPrefix     string
+	externalURLOnce sync.Once
+	externalURL     string
 )
 
 // ProxyKey is a crytographic key to sign the URLs generated by the proxy.
@@ -142,13 +143,29 @@
 func ProxyPrefix() string {
 	fetchPrefix := func() {
 		if proxyPrefix == "" {
-			proxyPrefix = fmt.Sprintf("http://%s:%d", WebHost(), WebPort())
+			if proxyPrefix = viper.GetString("proxy-prefix"); proxyPrefix == "" {
+				proxyPrefix = fmt.Sprintf("http://%s:%d", WebHost(), WebPort())
+			}
 		}
 	}
 	proxyPrefixOnce.Do(fetchPrefix)
 	return proxyPrefix
 }
 
+// ExternalURL is the URL to find this server from the outside.
+// It defauls to http://${WebHost}:${WebPort}".
+func ExternalURL() string {
+	fetchExternal := func() {
+		if externalURL == "" {
+			if externalURL = viper.GetString("external-url"); externalURL == "" {
+				externalURL = fmt.Sprintf("http://%s:%d", WebHost(), WebPort())
+			}
+		}
+	}
+	externalURLOnce.Do(fetchExternal)
+	return externalURL
+}
+
 // RootCmd is cobra command to be bound th the cobra/viper infrastructure.
 var RootCmd = &cobra.Command{
 	Use:   "gemma",
@@ -219,10 +236,16 @@
 	str("geoserver-password", "geoserver", "GeoServer password")
 	bl("geoserver-clean", false, "Clean GeoServer setup")
 
-	str("proxy-key", "", `signing key for proxy URLs. Defaults to random key.`)
-	str("proxy-prefix", "", `URL prefix of proxy. Defaults to "http://${web-host}:${web-port}"`)
+	str("proxy-key", "", "signing key for proxy URLs.\n"+
+		"Defaults to random key.")
+	str("proxy-prefix", "", "URL prefix of proxy.\n"+
+		"Defaults to 'http://${web-host}:${web-port}'")
 
-	str("tmp-dir", "", "Temp directory of gemma server. Defaults to system temp directory.")
+	str("external-url", "", "URL to find the server from the outside.\n"+
+		"Defaults to 'http://${web-host}:${web-port}'")
+
+	str("tmp-dir", "", "Temp directory of gemma server.\n"+
+		"Defaults to system temp directory.")
 }
 
 var (