diff --git a/gns3server/modules/base_vm.py b/gns3server/modules/base_vm.py
index b5030c26..a3684b69 100644
--- a/gns3server/modules/base_vm.py
+++ b/gns3server/modules/base_vm.py
@@ -403,11 +403,10 @@ class BaseVM:
         """
 
         path = self._manager.config.get_section_config("Server").get("ubridge_path", "ubridge")
-        if path == "ubridge":
-            path = shutil.which("ubridge")
+        path = shutil.which(path)
 
         if path is None or len(path) == 0:
-            raise VMError("uBridge is not installed")
+            raise VMError("uBridge is not installed or uBridge path is invalid")
         return path
 
     @asyncio.coroutine
diff --git a/gns3server/modules/dynamips/__init__.py b/gns3server/modules/dynamips/__init__.py
index e5df157a..c781952e 100644
--- a/gns3server/modules/dynamips/__init__.py
+++ b/gns3server/modules/dynamips/__init__.py
@@ -336,16 +336,16 @@ class Dynamips(BaseManager):
     def find_dynamips(self):
 
         # look for Dynamips
-        dynamips_path = self.config.get_section_config("Dynamips").get("dynamips_path")
-        if not dynamips_path:
-            dynamips_path = shutil.which("dynamips")
+        dynamips_path = self.config.get_section_config("Dynamips").get("dynamips_path", "dynamips")
+        if not os.path.isabs(dynamips_path):
+            dynamips_path = shutil.which(dynamips_path)
 
         if not dynamips_path:
             raise DynamipsError("Could not find Dynamips")
         if not os.path.isfile(dynamips_path):
             raise DynamipsError("Dynamips {} is not accessible".format(dynamips_path))
         if not os.access(dynamips_path, os.X_OK):
-            raise DynamipsError("Dynamips is not executable")
+            raise DynamipsError("Dynamips {} is not executable".format(dynamips_path))
 
         self._dynamips_path = dynamips_path
         return dynamips_path
diff --git a/gns3server/modules/iou/iou_vm.py b/gns3server/modules/iou/iou_vm.py
index b6dd548f..4105b55c 100644
--- a/gns3server/modules/iou/iou_vm.py
+++ b/gns3server/modules/iou/iou_vm.py
@@ -230,9 +230,11 @@ class IOUVM(BaseVM):
         :returns: path to IOUYAP
         """
 
-        path = self._manager.config.get_section_config("IOU").get("iouyap_path", "iouyap")
-        if path == "iouyap":
-            path = shutil.which("iouyap")
+        search_path = self._manager.config.get_section_config("IOU").get("iouyap_path", "iouyap")
+        path = shutil.which(search_path)
+        # shutil.which return None if the path doesn't exists
+        if not path:
+            return search_path
         return path
 
     @property
diff --git a/gns3server/modules/virtualbox/__init__.py b/gns3server/modules/virtualbox/__init__.py
index e5299a66..45085b07 100644
--- a/gns3server/modules/virtualbox/__init__.py
+++ b/gns3server/modules/virtualbox/__init__.py
@@ -66,7 +66,10 @@ class VirtualBox(BaseManager):
             elif sys.platform.startswith("darwin"):
                 vboxmanage_path = "/Applications/VirtualBox.app/Contents/MacOS/VBoxManage"
             else:
-                vboxmanage_path = shutil.which("vboxmanage")
+                vboxmanage_path = "vboxmanage"
+
+        if not os.path.abspath(vboxmanage_path):
+            vboxmanage_path = shutil.which(vboxmanage_path)
 
         if not vboxmanage_path:
             raise VirtualBoxError("Could not find VBoxManage")
diff --git a/gns3server/modules/vmware/__init__.py b/gns3server/modules/vmware/__init__.py
index 802911d4..a9cd6108 100644
--- a/gns3server/modules/vmware/__init__.py
+++ b/gns3server/modules/vmware/__init__.py
@@ -105,7 +105,10 @@ class VMware(BaseManager):
             elif sys.platform.startswith("darwin"):
                 vmrun_path = "/Applications/VMware Fusion.app/Contents/Library/vmrun"
             else:
-                vmrun_path = shutil.which("vmrun")
+                vmrun_path = "vmrun"
+
+        if not os.path.abspath(vmrun_path):
+            vmrun_path = shutil.which(vmrun_path)
 
         if not vmrun_path:
             raise VMwareError("Could not find VMware vmrun, please make sure it is installed")
diff --git a/gns3server/modules/vpcs/vpcs_vm.py b/gns3server/modules/vpcs/vpcs_vm.py
index 16984918..e1b11bea 100644
--- a/gns3server/modules/vpcs/vpcs_vm.py
+++ b/gns3server/modules/vpcs/vpcs_vm.py
@@ -139,9 +139,11 @@ class VPCSVM(BaseVM):
         :returns: path to VPCS
         """
 
-        path = self._manager.config.get_section_config("VPCS").get("vpcs_path", "vpcs")
-        if path == "vpcs":
-            path = shutil.which("vpcs")
+        search_path = self._manager.config.get_section_config("VPCS").get("vpcs_path", "vpcs")
+        path = shutil.which(search_path)
+        # shutil.which return None if the path doesn't exists
+        if not path:
+            return search_path
         return path
 
     @BaseVM.name.setter