--- a/pvl/hosts/host.py	Mon Mar 02 18:08:30 2015 +0200
+++ b/pvl/hosts/host.py	Mon Mar 02 18:17:47 2015 +0200
@@ -74,7 +74,7 @@
         Parse the dhcp boot=... option
 
         >>> print parse_dhcp_boot(None)
-        {}
+        None
         >>> print parse_dhcp_boot({'filename': '/foo'})
         {'filename': '/foo'}
         >>> print parse_dhcp_boot({'filename': '/foo', 'next-server': 'bar'})
@@ -90,28 +90,37 @@
             ...
         ValueError: invalid boot=foo
     """
-
+    
+    # normalize to dict
     if not boot:
-        return { }
+        boot = { }
+    elif not isinstance(boot, dict):
+        boot = { None: boot }
+    else:
+        boot = dict(boot)
+    
+    # support either an instanced dict or a plain str or a mixed instanced-with-plain-str
+    boot_str = boot.pop(None, None)
 
-    elif isinstance(boot, dict):
-        if set(boot) <= set(('filename', 'next-server')):
-            return boot
-        else:
-            raise ValueError("invalid boot={boot}".format(boot=boot))
+    if not (set(boot) <= set(('filename', 'next-server', None))):
+        raise ValueError("Invalid boot.*: {instances}".format(instances=' '.join(boot)))
 
-    elif boot.startswith('/'):
-        return {'filename': boot}
+    # any boot= given overrides boot.* fields
+    if not boot_str:
+        pass
+    elif boot_str.startswith('/'):
+        boot['filename'] = boot_str
 
-    elif boot.endswith(':'):
-        return {'next-server': boot[:-1]}
+    elif boot_str.endswith(':'):
+        boot['next-server'] = boot_str[:-1]
 
-    elif ':' in boot :
-        next_server, filename = boot.split(':', 1)
-        return {'next-server': next_server, 'filename': filename}
+    elif ':' in boot_str:
+        boot['next-server'], boot['filename'] = boot_str.split(':', 1)
 
     else :
-        raise ValueError("invalid boot={boot}".format(boot=boot))
+        raise ValueError("invalid boot={boot}".format(boot=boot_str))
+    
+    return boot
 
 def parse_str(value):
     """