Ι have done the following:
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Create a Boto configuration file somewhere with the following contents:
[Boto]
proxy = example-host
proxy_port = port number
After that, point the NO_AUTH_BOTO_CONFIG environment variable to the file you created:
export NO_AUTH_BOTO_CONFIG=/path/to/proxy_webrtc.boto
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As I found from a third-party source. The "notice" disappeared but I still see some "Timeout" logs so I don't think that the proxy settings are used at all.