I need to simulate network latency to a program running on the local computer, connecting to a local service. Thus far I have tried dummynet (a windows build of ipfw) which I got working after some trial and error. While it generally works, I can't seem to get it to filter localhost traffic. Even after adding a rule from any to any which affects external traffic, this makes no difference for local connections. I would appreciate if anyone knows how to simulate local latency using dummynet or a different tool. The tool should be able to simulate latency generically in IP packets, (TCP and UDP) and not be protocol specific.

2 Answers 2


On windows 7 you should take at Clumsy:

clumsy makes your network condition on Windows significantly worse, but in a managed and interactive manner.

clumsy demo

  • Yes, this is the right tool with visible effect. Unfortunately it does not have effect on 'localhost ping' statistics.
    – TomR
    Jul 12, 2017 at 13:07
  • It tired Clumsy 0.2 with a local application connecting to a local SQL Server using the SQL Server Native Client 11.0 ODBC driver. There was not affect on performance, even when turning on all functions at once. (Nice animated GIF by the way)
    – Ben
    Oct 8, 2020 at 16:20

If you're just testing HTTP traffic the Fiddler Web Debugger is pretty easy to use. You just select the stream, flip to the AutoResponder tab, select "Add Rule", then right click on the rule and select "Set Latency".

  • Sorry, it's not http traffic.
    – nitro2k01
    Oct 17, 2013 at 15:55
  • 1
    @nitro2k01 then you should edit your question and include the information so people don't need to go hunting in comments of answers to get all of the information. Oct 17, 2013 at 16:47
  • 4
    I have edited the question now, but I would think that if people make extra assumptions about the question that are not mentioned in the post, that's their prerogative.
    – nitro2k01
    Oct 17, 2013 at 21:09

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