caddyhttp: Support placeholders in header matcher values (close #3916)

modules/caddyhttp/matchers.go

@@ -513,7 +513,8 @@ func (m *MatchHeader) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 
 // Match returns true if r matches m.
 func (m MatchHeader) Match(r *http.Request) bool {
-	return matchHeaders(r.Header, http.Header(m), r.Host)
+	repl := r.Context().Value(caddy.ReplacerCtxKey).(*caddy.Replacer)
+	return matchHeaders(r.Header, http.Header(m), r.Host, repl)
 }
 
 // getHeaderFieldVals returns the field values for the given fieldName from input.
@@ -530,7 +531,7 @@ func getHeaderFieldVals(input http.Header, fieldName, host string) []string {
 // matchHeaders returns true if input matches the criteria in against without regex.
 // The host parameter should be obtained from the http.Request.Host field since
 // net/http removes it from the header map.
-func matchHeaders(input, against http.Header, host string) bool {
+func matchHeaders(input, against http.Header, host string, repl *caddy.Replacer) bool {
 	for field, allowedFieldVals := range against {
 		actualFieldVals := getHeaderFieldVals(input, field, host)
 		if allowedFieldVals != nil && len(allowedFieldVals) == 0 && actualFieldVals != nil {
@@ -546,6 +547,9 @@ func matchHeaders(input, against http.Header, host string) bool {
 	fieldVals:
 		for _, actualFieldVal := range actualFieldVals {
 			for _, allowedFieldVal := range allowedFieldVals {
+				if repl != nil {
+					allowedFieldVal = repl.ReplaceAll(allowedFieldVal, "")
+				}
 				switch {
 				case allowedFieldVal == "*":
 					match = true
@@ -985,7 +989,7 @@ func (rm ResponseMatcher) Match(statusCode int, hdr http.Header) bool {
 	if !rm.matchStatusCode(statusCode) {
 		return false
 	}
-	return matchHeaders(hdr, rm.Headers, "")
+	return matchHeaders(hdr, rm.Headers, "", nil)
 }
 
 func (rm ResponseMatcher) matchStatusCode(statusCode int) bool {

modules/caddyhttp/matchers_test.go

@@ -397,6 +397,9 @@ func TestPathREMatcher(t *testing.T) {
 }
 
 func TestHeaderMatcher(t *testing.T) {
+	repl := caddy.NewReplacer()
+	repl.Set("a", "foobar")
+
 	for i, tc := range []struct {
 		match  MatchHeader
 		input  http.Header // make sure these are canonical cased (std lib will do that in a real request)
@@ -490,8 +493,26 @@ func TestHeaderMatcher(t *testing.T) {
 			input:  http.Header{"Must-Not-Exist": []string{"do not match"}},
 			expect: false,
 		},
+		{
+			match:  MatchHeader{"Foo": []string{"{a}"}},
+			input:  http.Header{"Foo": []string{"foobar"}},
+			expect: true,
+		},
+		{
+			match:  MatchHeader{"Foo": []string{"{a}"}},
+			input:  http.Header{"Foo": []string{"asdf"}},
+			expect: false,
+		},
+		{
+			match:  MatchHeader{"Foo": []string{"{a}*"}},
+			input:  http.Header{"Foo": []string{"foobar-baz"}},
+			expect: true,
+		},
 	} {
 		req := &http.Request{Header: tc.input, Host: tc.host}
+		ctx := context.WithValue(req.Context(), caddy.ReplacerCtxKey, repl)
+		req = req.WithContext(ctx)
+
 		actual := tc.match.Match(req)
 		if actual != tc.expect {
 			t.Errorf("Test %d %v: Expected %t, got %t for '%s'", i, tc.match, tc.expect, actual, tc.input)