# Mac/iOS sync issue: unrecognized sync protocol

**URL:** <https://discuss.zetetic.net/t/mac-ios-sync-issue-unrecognized-sync-protocol/4795>\
**Category:** Codebook FAQ\
**Created:** [January 23, 2021, 12:33am UTC](https://discuss.zetetic.net/t/mac-ios-sync-issue-unrecognized-sync-protocol/4795 "2021-01-23T00:33:11Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![mmoore](https://sea2.discourse-cdn.com/flex016/user_avatar/discuss.zetetic.net/mmoore/32/858_2.png) [@mmoore](https://discuss.zetetic.net/u/mmoore)\
**Post date:** [January 25, 2021, 3:51pm UTC](https://discuss.zetetic.net/t/mac-ios-sync-issue-unrecognized-sync-protocol/4795/2 "2021-01-25T15:51:13Z")

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Hey @siriusly

Thanks for using Codebook and reaching out. The sync protocol changed in [Codebook for macOS 4.1.0](https://discuss.zetetic.net/t/codebook-4-1-0-released-new-sync-operation-options/4396) and we discontinued support for version of macOS older than 10.12 in [Codebook for macOS 4.3.0](https://discuss.zetetic.net/t/codebook-for-macos-4-3-0-released/4690)

I’ve just messaged you directly with instructions for downloading an old version of Codebook for macOS (4.2) that supports both your version of macOS and the new sync protocol.

If anyone else is running into a similar issue, please reach out to our support at [support@zetetic.net](mailto:support@zetetic.net?subject=Codebook%20macOS%20LTS)

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_[View the full topic](https://discuss.zetetic.net/t/mac-ios-sync-issue-unrecognized-sync-protocol/4795)._