Add root access check to cns server.

media/tools/constrained_network_server/traffic_control.py

Index: media/tools/constrained_network_server/traffic_control.py
diff --git a/media/tools/constrained_network_server/traffic_control.py b/media/tools/constrained_network_server/traffic_control.py
index c27a255365b766cdd68ade24204916fc9d02285d..1b775a0d03ac1af414afbb00bb34a07a1bca68f3 100755
--- a/media/tools/constrained_network_server/traffic_control.py
+++ b/media/tools/constrained_network_server/traffic_control.py
@@ -9,6 +9,7 @@ Traffic to the constrained port is forwarded to a specified server port.
 """
 
 import logging
+import os
 import re
 import subprocess
 
@@ -37,6 +38,28 @@ class TrafficControlError(BaseException):
     self.error = error
 
 
+def CheckRequirements():
+  """Checks if requirements are available to run traffic control commands.

[DaleCurtis, 2012/01/09 19:08:02]

s/requirements/permissions ?

[shadi, 2012/01/09 23:47:44]

Done.

+
+  Raises:
+    TrafficControlError: If requirements to run traffic control commands are not
+    available.
+  """
+  if os.geteuid() == 0:

[DaleCurtis, 2012/01/09 19:08:02]

Not quite correct. You want os.geteuid() == 0 _OR_ sudo -n <blah> passes.

[shadi, 2012/01/09 23:47:44]

Shouldn't the sudo -n <blah> pass if os.geteuid() passes? If it doesn't, don't
we want sudo -n to pass either way? 
On 2012/01/09 19:08:02, DaleCurtis wrote:

+    try:
+      command = ['sudo', '-n', 'iptables', '-L']

[DaleCurtis, 2012/01/09 19:08:02]

For short commands like these you might consider inlining them instead of
declaring a variable first. Up to you.

[shadi, 2012/01/09 23:47:44]

Done.

+      _Exec(command)
+      # tc commands only ask for root access when modifying the network state.
+      command = ['sudo', '-n', 'tc', 'qdisc', 'add']

[DaleCurtis, 2012/01/09 19:08:02]

For both of these it's enough to just run "--help" (or "-help" for tc). All we
need to know is that sudo will allow these commands to execute.

[shadi, 2012/01/09 23:47:44]

I thought we want to try to run a command that *does* require root access. I
didn't know sudo -n checks for root password before trying to run a command :) 
On 2012/01/09 19:08:02, DaleCurtis wrote:

+      _Exec(command)
+    except TrafficControlError as e:
+      if 'No such device' in e.error:
+        return
+      pass
+  raise TrafficControlError('Cannot run traffic control commands. Please check '
+                            'that you have root access.')
+
+
 def CreateConstrainedPort(config):
   """Creates a new constrained port.