Add hostname placeholder. Header uses replacer

On matched header rules, replacer is used to replace any placeholders
defined in header rules iex. X-Backend {hostname} where {hostname} will
be replaced by the hostname key present in the replacer

hostname key added to replacer. The value is determined by the output of
`os.Hostname()`

middleware/headers/headers.go

@@ -20,13 +20,14 @@ type Headers struct {
 // ServeHTTP implements the middleware.Handler interface and serves requests,
 // setting headers on the response according to the configured rules.
 func (h Headers) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
+	replacer := middleware.NewReplacer(r, nil, "")
 	for _, rule := range h.Rules {
 		if middleware.Path(r.URL.Path).Matches(rule.Path) {
 			for _, header := range rule.Headers {
 				if strings.HasPrefix(header.Name, "-") {
 					w.Header().Del(strings.TrimLeft(header.Name, "-"))
 				} else {
-					w.Header().Set(header.Name, header.Value)
+					w.Header().Set(header.Name, replacer.Replace(header.Value))
 				}
 			}
 		}

middleware/headers/headers_test.go

@@ -3,12 +3,17 @@ package headers
 import (
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"testing"
 
 	"github.com/mholt/caddy/middleware"
 )
 
 func TestHeaders(t *testing.T) {
+	hostname, err := os.Hostname()
+	if err != nil {
+		t.Fatalf("Could not determine hostname: %v", err)
+	}
 	for i, test := range []struct {
 		from  string
 		name  string
@@ -17,6 +22,7 @@ func TestHeaders(t *testing.T) {
 		{"/a", "Foo", "Bar"},
 		{"/a", "Bar", ""},
 		{"/a", "Baz", ""},
+		{"/a", "ServerName", hostname},
 		{"/b", "Foo", ""},
 		{"/b", "Bar", "Removed in /a"},
 	} {
@@ -27,6 +33,7 @@ func TestHeaders(t *testing.T) {
 			Rules: []Rule{
 				{Path: "/a", Headers: []Header{
 					{Name: "Foo", Value: "Bar"},
+					{Name: "ServerName", Value: "{hostname}"},
 					{Name: "-Bar"},
 				}},
 			},

middleware/replacer.go

@@ -4,6 +4,7 @@ import (
 	"net"
 	"net/http"
 	"net/url"
+	"os"
 	"path"
 	"strconv"
 	"strings"
@@ -52,6 +53,13 @@ func NewReplacer(r *http.Request, rr *ResponseRecorder, emptyValue string) Repla
 				}
 				return "http"
 			}(),
+			"{hostname}": func() string {
+				name, err := os.Hostname()
+				if err != nil {
+					return ""
+				}
+				return name
+			}(),
 			"{host}":          r.Host,
 			"{path}":          r.URL.Path,
 			"{path_escaped}":  url.QueryEscape(r.URL.Path),

middleware/replacer_test.go

@@ -3,6 +3,7 @@ package middleware
 import (
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"strings"
 	"testing"
 )
@@ -53,6 +54,14 @@ func TestReplace(t *testing.T) {
 	request.Header.Set("ShorterVal", "1")
 	repl := NewReplacer(request, recordRequest, "-")
 
+	hostname, err := os.Hostname()
+	if err != nil {
+		t.Fatal("Failed to determine hostname\n")
+	}
+	if expected, actual := "This hostname is "+hostname, repl.Replace("This hostname is {hostname}"); expected != actual {
+		t.Errorf("{hostname} replacement: expected '%s', got '%s'", expected, actual)
+	}
+
 	if expected, actual := "This host is localhost.", repl.Replace("This host is {host}."); expected != actual {
 		t.Errorf("{host} replacement: expected '%s', got '%s'", expected, actual)
 	}