diff --git a/weed/server/filer_server_handlers_read.go b/weed/server/filer_server_handlers_read.go
index 56aee18be..8037b1d94 100644
--- a/weed/server/filer_server_handlers_read.go
+++ b/weed/server/filer_server_handlers_read.go
@@ -21,6 +21,64 @@ import (
 	"github.com/chrislusf/seaweedfs/weed/util"
 )
 
+
+// Validates the preconditions. Returns true if GET/HEAD operation should not proceed.
+// Preconditions supported are:
+//  If-Modified-Since
+//  If-Unmodified-Since
+//  If-Match
+//  If-None-Match
+func checkPreconditions(w http.ResponseWriter, r *http.Request, entry *filer.Entry) bool {
+
+	etag := filer.ETagEntry(entry)
+	/// When more than one conditional request header field is present in a
+	/// request, the order in which the fields are evaluated becomes
+	/// important.  In practice, the fields defined in this document are
+	/// consistently implemented in a single, logical order, since "lost
+	/// update" preconditions have more strict requirements than cache
+	/// validation, a validated cache is more efficient than a partial
+	/// response, and entity tags are presumed to be more accurate than date
+	/// validators. https://tools.ietf.org/html/rfc7232#section-5
+	if entry.Attr.Mtime.IsZero() {
+		return false
+	}
+	w.Header().Set("Last-Modified", entry.Attr.Mtime.UTC().Format(http.TimeFormat))
+
+	ifMatchETagHeader := r.Header.Get("If-Match")
+	ifUnmodifiedSinceHeader := r.Header.Get("If-Unmodified-Since")
+	if ifMatchETagHeader != "" {
+		if util.CanonicalizeETag(etag) != util.CanonicalizeETag(ifMatchETagHeader) {
+			w.WriteHeader(http.StatusPreconditionFailed)
+			return true
+		}
+	} else if ifUnmodifiedSinceHeader != "" {
+		if t, parseError := time.Parse(http.TimeFormat, ifUnmodifiedSinceHeader); parseError == nil {
+			if t.Before(entry.Attr.Mtime) {
+				w.WriteHeader(http.StatusPreconditionFailed)
+				return true
+			}
+		}
+	}
+
+	ifNoneMatchETagHeader := r.Header.Get("If-None-Match")
+	ifModifiedSinceHeader := r.Header.Get("If-Modified-Since")
+	if ifNoneMatchETagHeader != "" {
+		if util.CanonicalizeETag(etag) == util.CanonicalizeETag(ifNoneMatchETagHeader) {
+			w.WriteHeader(http.StatusNotModified)
+			return true
+		}
+	} else if ifModifiedSinceHeader != "" {
+		if t, parseError := time.Parse(http.TimeFormat, ifModifiedSinceHeader); parseError == nil {
+			if t.After(entry.Attr.Mtime) {
+				w.WriteHeader(http.StatusNotModified)
+				return true
+			}
+		}
+	}
+
+	return false
+}
+
 func (fs *FilerServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request) {
 
 	path := r.URL.Path
@@ -61,10 +119,8 @@ func (fs *FilerServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request)
 		return
 	}
 
-	// set etag
 	etag := filer.ETagEntry(entry)
-	if ifm := r.Header.Get("If-Match"); ifm != "" && (ifm != "\""+etag+"\"" && ifm != etag) {
-		w.WriteHeader(http.StatusPreconditionFailed)
+	if checkPreconditions(w, r, entry) {
 		return
 	}
 
@@ -81,19 +137,6 @@ func (fs *FilerServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request)
 		w.Header().Set("Content-Type", mimeType)
 	}
 
-	// if modified since
-	if !entry.Attr.Mtime.IsZero() {
-		w.Header().Set("Last-Modified", entry.Attr.Mtime.UTC().Format(http.TimeFormat))
-		if r.Header.Get("If-Modified-Since") != "" {
-			if t, parseError := time.Parse(http.TimeFormat, r.Header.Get("If-Modified-Since")); parseError == nil {
-				if !t.Before(entry.Attr.Mtime) {
-					w.WriteHeader(http.StatusNotModified)
-					return
-				}
-			}
-		}
-	}
-
 	// print out the header from extended properties
 	for k, v := range entry.Extended {
 		if !strings.HasPrefix(k, "xattr-") {
@@ -123,10 +166,6 @@ func (fs *FilerServer) GetOrHeadHandler(w http.ResponseWriter, r *http.Request)
 		w.Header().Set(xhttp.AmzTagCount, strconv.Itoa(tagCount))
 	}
 
-	if inm := r.Header.Get("If-None-Match"); inm == "\""+etag+"\"" {
-		w.WriteHeader(http.StatusNotModified)
-		return
-	}
 	setEtag(w, etag)
 
 	filename := entry.Name()
diff --git a/weed/util/parse.go b/weed/util/parse.go
index 0955db682..502f3a80f 100644
--- a/weed/util/parse.go
+++ b/weed/util/parse.go
@@ -61,3 +61,8 @@ func ParseHostPort(hostPort string) (filerServer string, filerPort int64, err er
 
 	return
 }
+
+func CanonicalizeETag(etag string) string {
+	canonicalETag := strings.TrimPrefix(etag, "\"")
+	return strings.TrimSuffix(canonicalETag, "\"")
+}