Clarify IPv4 and IPv6 configuration

Since leaving the quotes out in the config file causes the confusing "did not
find expected key" error, so do not assume that the user is familiar enough
with YAML syntax as to identify the issue.

CONFIGURATION.md

@@ -10,14 +10,19 @@ via the `--config` argument.
 
 Address to listen for connections on in the format of `interface:port`.
 
-### Listen on localhost
-
-`localhost:1965`
-
-### Listen on all interfaces
+### Listen on all addresses (IPv4 and IPv6)
 
 `:1965`
 
+### Listen on a specific address (IPv4 or IPv6)
+
+`192.0.2.1:1965` or `"[2001:db8::1]:1965"` (please note the quotes and brackets
+for IPv6 addresses).
+
+### Listen on localhost (IPv4 only)
+
+`localhost:1965`
+
 ## Types
 
 Content types may be defined by file extension. When a type is not defined for

config.go

@@ -127,13 +127,13 @@ func readconfig(configPath string) error {
 		newLine = "\r\n"
 	}
 
-	split := strings.Split(config.Listen, ":")
-	if len(split) != 2 {
+	listenRe := regexp.MustCompile("(.*):([0-9]+)$")
+	if !listenRe.MatchString(config.Listen) {
 		config.hostname = config.Listen
 		config.Listen += ":1965"
 	} else {
-		config.hostname = split[0]
-		config.port, err = strconv.Atoi(split[1])
+		config.hostname = listenRe.ReplaceAllString(config.Listen, "$1")
+		config.port, err = strconv.Atoi(listenRe.ReplaceAllString(config.Listen, "$2"))
 		if err != nil {
 			log.Fatalf("invalid port specified: %s", err)
 		}