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When I listen on a port on localhost, Windows will popup a dialogue in which the user must add my program to the list of firewall exceptions. This is annoying, and requires administrator-rights, which the user may not have.

Why does Windows do this for loopback connections (127.0.0.1) and is there some trick to prevent this?

Maestro
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2 Answers2

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The answer was to specify:

IPEndPoint localEndPoint = new IPEndPoint(IPAddress.Loopback, Port);

instead of

IPEndPoint localEndPoint = new IPEndPoint(IPAddress.Any, Port);

for the listening socket. At first sight this seems to prevent any firewall warnings and doesn't require any rules to be added to the firewall. But I have to do some more extensive testing to be sure this works on all Windows configurations.

Peter Mortensen
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Maestro
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    Well this shows that your question was based on wrong assumptions.You didn't listen on localhost, you listened on `Any` = `0.0.0.0` = all interfaces. – BatteryBackupUnit Feb 12 '16 at 10:43
  • @BatteryBackupUnit You are right, but I was not aware of that because 'Any' is the default – Maestro Feb 13 '16 at 11:21
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It does this to prevent people from doing bad things. If a program is accessing something via localhost, it might do things at higher privileges than it might be able to do if it does it via non-localhost.

Example:

  • A localhost administrative port
  • Applications that only listen to localhost to prevent remote access, file indexing services
  • etc.

There is no way to avoid the popup. Otherwise, what would be the use of it? You can, if your program has administrative privileges, add a firewall exception rule, thus preventing this popup.

Peter Mortensen
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Noam Rathaus
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    If I click 'Deny' when that dialogue appears, my program still accepts local incoming connections, so that popup is completely useless for preventing any of the risks you just mentioned. – Maestro Jan 20 '14 at 09:55