diff --git a/src/morevna.py b/src/morevna.py
--- a/src/morevna.py
+++ b/src/morevna.py
@@ -7,27 +7,29 @@ from client import Client
 from server import Server
 
 
-def _checkDatafile(datafile):
-	if not os.path.isfile(datafile):
-		print("invalid file specified:",args.datafile,file=sys.stderr)
+def _checkFile(f):
+	if not os.path.isfile(f):
+		print("invalid file specified:",f,file=sys.stderr)
 		sys.exit(1)
 
 
 def buildTree(args):
-	_checkDatafile(args.datafile)
+	_checkFile(args.datafile)
 
 	tree=HashTree.fromFile(args.datafile)
 	tree.save(args.treefile)
 
 def update(args):
-	_checkDatafile(args.datafile)
+	_checkFile(args.datafile)
 
 	c=Client(args.datafile)
 	blocksToTransfer=c.negotiate()
 	c.sendData(blocksToTransfer)
 
 def serve(args):
-	_checkDatafile(args.datafile)
+	_checkFile(args.datafile)
+	if args.tree:
+		_checkFile(args.tree)
 
 	# debug copy default file
 	import shutil
@@ -35,7 +37,7 @@ def serve(args):
 	filename=origFilename+"_"
 	shutil.copyfile(origFilename,filename)
 
-	s=Server(filename)
+	s=Server(filename,args.tree)
 	s.serve()
 
 
diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -24,9 +24,14 @@ class Connection:
 
 
 class Server:
-	def __init__(self,filename):
+	def __init__(self,filename,treeFile=""):
 		self.filename=filename
-		self.tree=HashTree.fromFile(filename)
+
+		if treeFile:
+			self.tree=HashTree.load(treeFile)
+		else:
+			self.tree=HashTree.fromFile(filename)
+
 		self.BLOCK_SIZE=self.tree.BLOCK_SIZE
 
 		self.ss = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
@@ -34,6 +39,7 @@ class Server:
 		self.ss.listen(1)
 
 		self._lastWrite=-1
+		self.dataFile=None
 
 	def serve(self):
 		while self._serveOne():
@@ -55,6 +61,8 @@ class Server:
 
 			elif jsonData["command"]=="end":
 				log.info("closing session...")
+				if self.dataFile:
+					self.dataFile.close()
 				return False
 
 			else:
@@ -75,11 +83,12 @@ class Server:
 	def _receiveData(self,jsonData,binData):
 		log.info("received data block #{0}: {1}...{2}".format(jsonData["index"],binData[:5],binData[-5:]))
 
+		if not self.dataFile:
+			self.dataFile=open(self.filename,mode="rb+")
 		i=jsonData["index"]
-		with open(self.filename,mode="rb+") as dataFile:
-			if self._lastWrite+1!=i:
-				dataFile.seek(i*self.BLOCK_SIZE)
-			dataFile.write(binData)
+		if self._lastWrite+1!=i:
+			self.dataFile.seek(i*self.BLOCK_SIZE)
+		self.dataFile.write(binData)
 		self._lastWrite=i
 
 		# never update the hash tree