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Python implementation of the Happy Eyeballs Algorithm described in RFC 6555.

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Python implementation of the Happy Eyeballs Algorithm described in RFC 6555. Provided with a single file and dead-simple API to allow easy vendoring and integration into other projects.


When a server’s IPv4 path and protocol are working, but the server’s IPv6 path and protocol are not working, a dual-stack client application experiences significant connection delay compared to an IPv4-only client. This is undesirable because it causes the dual- stack client to have a worse user experience. This document specifies requirements for algorithms that reduce this user-visible delay and provides an algorithm.

~ Abstract from RFC 6555


$ python -m pip install rfc6555


The main API for the rfc6555 module is via rfc6555.create_connection() which functions identically to socket.create_connection() with the same arguments. This function will automatically fall back on a socket.create_connection() call if RFC 6555 is not supported (for instance on platforms not capable of IPv6) or if RFC 6555 is disabled via setting rfc6555.RFC6555_ENABLED equal to False.

IMPORTANT: Caching is NOT thread-safe by default. If you require thread-safe caching one should create their own implementation of rfc6555._RFC6555CacheManager object that is thread-safe and assign an instance to rfc6555.cache.

import rfc6555
sock = rfc6555.create_connection(('', 80), timeout=10, source_address=('::1', 0))

# This will disable the Happy Eyeballs algorithm for future
# calls to create_connection()
rfc6555.RFC6555_ENABLED = False

# Use this to set a different duration for cache entries.
rfc6555.cache.validity_duration = 10  # 10 second validity time.

# Use this to disable caching.
rfc6555.cache.enabled = False


This module supports Python 2.6 or newer and supports all major platforms. Additionally if you have selectors2>=2.0.0 installed this module will also support Jython in addition to CPython.


The rfc6555 package is released under the Apache-2.0 license.

See full license text in LICENSE file for more information.

              Copyright 2017 Seth Michael Larson

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
See the License for the specific language governing permissions and
limitations under the License.



  • Initial release of rfc6555.

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