httpserver: Add {labelN} placeholders for parts of hostnames

For example, {label1} would match "sub" in "sub.example.com" or whatever
value is in the wildcard spot of "*.example.com". Useful for rewrite!

caddyhttp/httpserver/replacer.go

@@ -375,6 +375,20 @@ func (r *replacer) getSubstitution(key string) string {
 		}
 		elapsedDuration := time.Since(r.responseRecorder.start)
 		return strconv.FormatInt(convertToMilliseconds(elapsedDuration), 10)
+	default:
+		// {labelN}
+		if strings.HasPrefix(key, "{label") {
+			nStr := key[6 : len(key)-1] // get the integer N in "{labelN}"
+			n, err := strconv.Atoi(nStr)
+			if err != nil || n < 1 {
+				return r.emptyValue
+			}
+			labels := strings.Split(r.request.Host, ".")
+			if n > len(labels) {
+				return r.emptyValue
+			}
+			return labels[n-1]
+		}
 	}
 
 	return r.emptyValue

caddyhttp/httpserver/replacer_test.go

@@ -53,7 +53,7 @@ func TestReplace(t *testing.T) {
 	recordRequest := NewResponseRecorder(w)
 	reader := strings.NewReader(`{"username": "dennis"}`)
 
-	request, err := http.NewRequest("POST", "http://localhost/?foo=bar", reader)
+	request, err := http.NewRequest("POST", "http://localhost.local/?foo=bar", reader)
 	if err != nil {
 		t.Fatalf("Failed to make request: %v", err)
 	}
@@ -87,7 +87,7 @@ func TestReplace(t *testing.T) {
 		expect   string
 	}{
 		{"This hostname is {hostname}", "This hostname is " + hostname},
-		{"This host is {host}.", "This host is localhost."},
+		{"This host is {host}.", "This host is localhost.local."},
 		{"This request method is {method}.", "This request method is POST."},
 		{"The response status is {status}.", "The response status is 200."},
 		{"{when}", "02/Jan/2006:15:04:05 +0000"},
@@ -97,7 +97,7 @@ func TestReplace(t *testing.T) {
 		{"The CustomAdd header is {>CustomAdd}.", "The CustomAdd header is caddy."},
 		{"The Custom response header is {<Custom}.", "The Custom response header is CustomResponseHeader."},
 		{"Bad {>Custom placeholder", "Bad {>Custom placeholder"},
-		{"The request is {request}.", "The request is POST /?foo=bar HTTP/1.1\\r\\nHost: localhost\\r\\n" +
+		{"The request is {request}.", "The request is POST /?foo=bar HTTP/1.1\\r\\nHost: localhost.local\\r\\n" +
 			"Cookie: foo=bar; taste=delicious\\r\\nCustom: foobarbaz\\r\\nCustomadd: caddy\\r\\n" +
 			"Shorterval: 1\\r\\n\\r\\n."},
 		{"The cUsToM header is {>cUsToM}...", "The cUsToM header is foobarbaz..."},
@@ -112,6 +112,7 @@ func TestReplace(t *testing.T) {
 		{"Query string is {query}", "Query string is foo=bar"},
 		{"Query string value for foo is {?foo}", "Query string value for foo is bar"},
 		{"Missing query string argument is {?missing}", "Missing query string argument is "},
+		{"Label1 is {label1} and label2 is {label2} but label 3 is {label3}. {label4}", "Label1 is localhost and label2 is local but label 3 is -. -"},
 	}
 
 	for _, c := range testCases {