diff --git a/modules/caddyhttp/headers/headers.go b/modules/caddyhttp/headers/headers.go
index 71e575dd..813b9fec 100644
--- a/modules/caddyhttp/headers/headers.go
+++ b/modules/caddyhttp/headers/headers.go
@@ -79,15 +79,8 @@ func (h Handler) Validate() error {
 func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhttp.Handler) error {
 	repl := r.Context().Value(caddy.ReplacerCtxKey).(caddy.Replacer)
 
-	h.Request.applyTo(r.Header, repl)
-
-	// request header's Host is handled specially by the
-	// Go standard library, so if that header was changed,
-	// change it in the Host field since the Header won't
-	// be used
-	if intendedHost := r.Header.Get("Host"); intendedHost != "" {
-		r.Host = intendedHost
-		r.Header.Del("Host")
+	if h.Request != nil {
+		h.Request.ApplyToRequest(r)
 	}
 
 	if h.Response != nil {
@@ -99,7 +92,7 @@ func (h Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyhtt
 				headerOps:             h.Response.HeaderOps,
 			}
 		} else {
-			h.Response.applyTo(w.Header(), repl)
+			h.Response.ApplyTo(w.Header(), repl)
 		}
 	}
 
@@ -160,11 +153,9 @@ type RespHeaderOps struct {
 	Deferred bool                       `json:"deferred,omitempty"`
 }
 
-func (ops *HeaderOps) applyTo(hdr http.Header, repl caddy.Replacer) {
-	if ops == nil {
-		return
-	}
-
+// ApplyTo applies ops to hdr using repl.
+func (ops HeaderOps) ApplyTo(hdr http.Header, repl caddy.Replacer) {
+	// add
 	for fieldName, vals := range ops.Add {
 		fieldName = repl.ReplaceAll(fieldName, "")
 		for _, v := range vals {
@@ -172,22 +163,28 @@ func (ops *HeaderOps) applyTo(hdr http.Header, repl caddy.Replacer) {
 		}
 	}
 
+	// set
 	for fieldName, vals := range ops.Set {
 		fieldName = repl.ReplaceAll(fieldName, "")
+		var newVals []string
 		for i := range vals {
-			vals[i] = repl.ReplaceAll(vals[i], "")
+			// append to new slice so we don't overwrite
+			// the original values in ops.Set
+			newVals = append(newVals, repl.ReplaceAll(vals[i], ""))
 		}
-		hdr.Set(fieldName, strings.Join(vals, ","))
+		hdr.Set(fieldName, strings.Join(newVals, ","))
 	}
 
+	// delete
 	for _, fieldName := range ops.Delete {
 		hdr.Del(repl.ReplaceAll(fieldName, ""))
 	}
 
+	// replace
 	for fieldName, replacements := range ops.Replace {
 		fieldName = repl.ReplaceAll(fieldName, "")
 
-		// perform replacements across all fields
+		// all fields...
 		if fieldName == "*" {
 			for _, r := range replacements {
 				search := repl.ReplaceAll(r.Search, "")
@@ -205,7 +202,7 @@ func (ops *HeaderOps) applyTo(hdr http.Header, repl caddy.Replacer) {
 			continue
 		}
 
-		// perform replacements only with the named field
+		// ...or only with the named field
 		for _, r := range replacements {
 			search := repl.ReplaceAll(r.Search, "")
 			replace := repl.ReplaceAll(r.Replace, "")
@@ -220,6 +217,42 @@ func (ops *HeaderOps) applyTo(hdr http.Header, repl caddy.Replacer) {
 	}
 }
 
+// ApplyToRequest applies ops to r, specially handling the Host
+// header which the standard library does not include with the
+// header map with all the others. This method mutates r.Host.
+func (ops HeaderOps) ApplyToRequest(r *http.Request) {
+	repl := r.Context().Value(caddy.ReplacerCtxKey).(caddy.Replacer)
+
+	// capture the current Host header so we can
+	// reset to it when we're done
+	origHost, hadHost := r.Header["Host"]
+
+	// append r.Host; this way, we know that our value
+	// was last in the list, and if an Add operation
+	// appended something else after it, that's probably
+	// fine because it's weird to have multiple Host
+	// headers anyway and presumably the one they added
+	// is the one they wanted
+	r.Header["Host"] = append(r.Header["Host"], r.Host)
+
+	// apply header operations
+	ops.ApplyTo(r.Header, repl)
+
+	// retrieve the last Host value (likely the one we appended)
+	if len(r.Header["Host"]) > 0 {
+		r.Host = r.Header["Host"][len(r.Header["Host"])-1]
+	} else {
+		r.Host = ""
+	}
+
+	// reset the Host header slice
+	if hadHost {
+		r.Header["Host"] = origHost
+	} else {
+		delete(r.Header, "Host")
+	}
+}
+
 // responseWriterWrapper defers response header
 // operations until WriteHeader is called.
 type responseWriterWrapper struct {
@@ -236,7 +269,9 @@ func (rww *responseWriterWrapper) WriteHeader(status int) {
 	}
 	rww.wroteHeader = true
 	if rww.require == nil || rww.require.Match(status, rww.ResponseWriterWrapper.Header()) {
-		rww.headerOps.applyTo(rww.ResponseWriterWrapper.Header(), rww.replacer)
+		if rww.headerOps != nil {
+			rww.headerOps.ApplyTo(rww.ResponseWriterWrapper.Header(), rww.replacer)
+		}
 	}
 	rww.ResponseWriterWrapper.WriteHeader(status)
 }