diff --git a/src/client.py b/src/client.py
--- a/src/client.py
+++ b/src/client.py
@@ -166,8 +166,10 @@ class Client(NetNode):
 					self._new_leaves[i+self._tree.leaf_start] = hash_block(block)
 
 					t = datetime.now().timestamp()
-					if t-last_flushed >= 60 and self._tree_file:
-						self._update_tree()
+					if t-last_flushed >= 60:
+						if self._tree_file:
+							self._update_tree()
+						self._refresh_lock()
 						last_flushed = t
 
 				stats.log_transferred_block()
diff --git a/src/netnode.py b/src/netnode.py
--- a/src/netnode.py
+++ b/src/netnode.py
@@ -1,4 +1,5 @@
 import os
+from datetime import datetime
 import socket
 import logging as log
 
@@ -62,7 +63,15 @@ class NetNode:
 			f = open(lock_file, "x")
 			f.close()
 		except FileExistsError:
-			raise LockedException()
+			stat = os.stat(lock_file)
+			dt = datetime.now().timestamp()-stat.st_mtime
+			if dt<5*60:
+				raise LockedException()
+			log.warning("Found an old lock file ({0}s), ignoring it.".format(round(dt)))
+			self._refresh_lock()
+
+	def _refresh_lock(self):
+		os.utime(lock_file)
 
 	def _unlock(self):
 		os.remove(lock_file)
diff --git a/src/server.py b/src/server.py
--- a/src/server.py
+++ b/src/server.py
@@ -5,7 +5,7 @@ import multiprocessing
 import logging as log
 
 from hashtree import hash_block
-from netnode import BaseConnection,NetNode
+from netnode import BaseConnection, NetNode
 import config as conf
 import status
 from datafile import DataFile
@@ -150,8 +150,10 @@ class Server(NetNode):
 				self._new_leaves[k+self._tree.leaf_start] = hash_block(block)
 
 		t = datetime.now().timestamp()
-		if t-self._last_flushed>=60 and self._tree_file:
-			self._update_tree()
+		if t-self._last_flushed>=60:
+			if self._tree_file:
+				self._update_tree()
+			self._refresh_lock()
 			self._last_flushed = t
 
 		return ({"command": "ack", "index": indices},)