diff --git a/caskethttp/httpserver/replacer.go b/caskethttp/httpserver/replacer.go
index 3c447418..fe66572f 100644
--- a/caskethttp/httpserver/replacer.go
+++ b/caskethttp/httpserver/replacer.go
@@ -296,6 +296,19 @@ func (r *replacer) getSubstitution(key string) string {
 		name := key[2 : len(key)-1]
 		return query.Get(name)
 	}
+	// next check for environment variable
+	if key[1] == '$' {
+		name := key[2 : len(key)-1]
+		// check for a default value
+		if i := strings.Index(name, "="); i != -1 {
+			if value := os.Getenv(name[:i]); value != "" {
+				return value
+			}
+			return name[i+1:]
+		}
+
+		return os.Getenv(name)
+	}
 
 	// search default replacements in the end
 	switch key {
diff --git a/caskethttp/httpserver/replacer_test.go b/caskethttp/httpserver/replacer_test.go
index d3e578a4..c4705237 100644
--- a/caskethttp/httpserver/replacer_test.go
+++ b/caskethttp/httpserver/replacer_test.go
@@ -75,6 +75,9 @@ func TestReplace(t *testing.T) {
 	// add some headers after creating replacer
 	request.Header.Set("CustomAdd", "casket")
 	request.Header.Set("Cookie", "foo=bar; taste=delicious")
+	// add some environment variables
+	os.Setenv("TEST_ENV", "test")
+	os.Setenv("TEST_ENV_EMPTY", "")
 
 	// add some response headers
 	recordRequest.Header().Set("Custom", "CustomResponseHeader")
@@ -108,6 +111,11 @@ func TestReplace(t *testing.T) {
 		{"The Custom header is {>Custom}.", "The Custom header is foobarbaz."},
 		{"The CustomAdd header is {>CustomAdd}.", "The CustomAdd header is casket."},
 		{"The Custom response header is {<Custom}.", "The Custom response header is CustomResponseHeader."},
+		{"The env variable TEST_ENV is {$TEST_ENV}.", "The env variable TEST_ENV is test."},
+		{"The env variable TEST_ENV is {$TEST_ENV=default}, not default.", "The env variable TEST_ENV is test, not default."},
+		{"The env variable TEST_ENV_EMPTY is {$TEST_ENV_EMPTY}.", "The env variable TEST_ENV_EMPTY is ."},
+		{"The env variable TEST_ENV_EMPTY defaults to {$TEST_ENV_EMPTY=default}.", "The env variable TEST_ENV_EMPTY defaults to default."},
+		{"The env variable NO_EXIST defaults to {$NO_EXIST=default}.", "The env variable NO_EXIST defaults to default."},
 		{"Bad {>Custom placeholder", "Bad {>Custom placeholder"},
 		{"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: casket\\r\\n" +
diff --git a/go.sum b/go.sum
index 27649606..483788b6 100644
--- a/go.sum
+++ b/go.sum
@@ -783,6 +783,7 @@ github.com/tmpim/casket-plugins v0.0.4-0.20210411234607-8b023eeb664d h1:vn0yn5M2
 github.com/tmpim/casket-plugins v0.0.4-0.20210411234607-8b023eeb664d/go.mod h1:RYBOo9WtK3LmyjGZVzEm1ptZV9uvPprzvz0V6i9qyuI=
 github.com/tmpim/certmagic v0.12.4 h1:k8MDMZT65fadsCu7fUONdksu5L7NONN6kgtX6dLd18w=
 github.com/tmpim/certmagic v0.12.4/go.mod h1:GrjIQ+gs5GW+jmqRkOnfoBjIiuk12VV4zbxoWBs8lQc=
+github.com/tmpim/dnsproviders v0.4.2/go.mod h1:QNqmtAefbbf/2BYM2hhVHaFglA584r3ayGps32Gu/c8=
 github.com/tmpim/dnsproviders v0.4.3-0.20211231213508-66e13a82678d h1:5IkgvDcbqfqrb635H58scO8qwiCy9Qpq7rp7zEq9VJw=
 github.com/tmpim/dnsproviders v0.4.3-0.20211231213508-66e13a82678d/go.mod h1:QNqmtAefbbf/2BYM2hhVHaFglA584r3ayGps32Gu/c8=
 github.com/transip/gotransip v0.0.0-20190812104329-6d8d9179b66f/go.mod h1:i0f4R4o2HM0m3DZYQWsj6/MEowD57VzoH0v3d7igeFY=