diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -1,8 +1,9 @@
-import socket
+import hashlib
+import socket
+import logging as log
+
 from hashtree import HashTree
 from networkers import NetworkReader,NetworkWriter
-import logging as log
-
 import config as conf
 
 
@@ -26,12 +27,14 @@ class Connection:
 class Server:
 	def __init__(self,filename,treeFile=""):
 		self.filename=filename
+		self._treeFile=treeFile
 
 		if treeFile:
 			self.tree=HashTree.load(treeFile)
 		else:
 			self.tree=HashTree.fromFile(filename)
 
+		self._newLeaves=dict()
 		self.BLOCK_SIZE=self.tree.BLOCK_SIZE
 
 		self.ss = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
@@ -65,9 +68,7 @@ class Server:
 			outcoming.writeMsg(*self._receiveData(jsonData,binData))
 
 		elif jsonData["command"]=="end":
-			log.info("closing session...")
-			if self.dataFile:
-				self.dataFile.close()
+			self._finalize()
 			return False
 
 		else:
@@ -95,6 +96,18 @@ class Server:
 			self.dataFile.seek(i*self.BLOCK_SIZE)
 		self.dataFile.write(binData)
 		self._lastWrite=i
+		if self._treeFile:
+			self._newLeaves[i+self.tree.leafStart]=hashlib.sha256(binData).digest()[HashTree.HASH_LEN:]
 
 		return ({"command": "ack", "index": i},)
-		# never update the hash tree
+
+	def _finalize(self):
+		log.info("closing session...")
+		if self.dataFile:
+			self.dataFile.close()
+		if self._treeFile:
+			log.info("updating hash tree...")
+			for (k,v) in self._newLeaves:
+				self.tree.updateLeaf(k,v)
+			self.tree.save(self._treeFile)
+		log.info("done")