Bug in Cloudflare Warp

Could someone in the Cloudflare community who is able please make sure that the maintainers of Cloudflare Warp receive the following bug report? I reported this via Warp’s in-app “report a bug” function a couple weeks ago, but never received a reply from a human.

I am testing Cloudflare Warp version (20211004.9) on macOS.

When Cloudflare Warp is running and in the “connected” state, it incorrectly handles other programs trying to bind to a server socket.

This bug can be reproduced with a very simple (and commonly-needed) command:

python3 -m http.server

When Cloudflare Warp is enabled, this command hangs forever, never starting a local HTTP server.

After disabling or quitting Cloudflare Warp, it works.

I spent some time debugging this before I narrowed it down to Cloudflare Warp.

Eventually I figured out that it’s possible to work around this by explicitly passing an address via the “-b” flag, e.g.:

python3 -m http.server -b localhost


python3 -m http.server -b

Stepping through the path that the code in the Python standard library’s http/server.py is taking right before the hang occurs, this looks related to dual stack, which I think explains why passing an explicit -b argument works around the bug:

Please let me know if any of the Cloudlfare Warp maintainers receive this bug report and are interested in fixing the bug.

Hi, an internal ticket has been filed for this bug and assigned to an engineer.


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