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140 lines
4.7 KiB
Markdown
140 lines
4.7 KiB
Markdown
# TLS: ESNI support in curl and libcurl
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## Summary
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**ESNI** means **Encrypted Server Name Indication**, a TLS 1.3
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extension which is currently the subject of an
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[IETF Draft][tlsesni].
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This file is intended to show the latest current state of ESNI support
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in **curl** and **libcurl**.
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At end of August 2019, an [experimental fork of curl][niallorcurl],
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built using an [experimental fork of OpenSSL][sftcdopenssl], which in
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turn provided an implementation of ESNI, was demonstrated
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interoperating with a server belonging to the [DEfO
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Project][defoproj].
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Further sections here describe
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- resources needed for building and demonstrating **curl** support
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for ESNI,
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- progress to date,
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- TODO items, and
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- additional details of specific stages of the progress.
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## Resources needed
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To build and demonstrate ESNI support in **curl** and/or **libcurl**,
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you will need
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- a TLS library, supported by **libcurl**, which implements ESNI;
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- an edition of **curl** and/or **libcurl** which supports the ESNI
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implementation of the chosen TLS library;
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- an environment for building and running **curl**, and at least
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building **OpenSSL**;
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- a server, supporting ESNI, against which to run a demonstration
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and perhaps a specific target URL;
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- some instructions.
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The following set of resources is currently known to be available.
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| Set | Component | Location | Remarks |
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|:-----|:-------------|:------------------------------|:-------------------------------------------|
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| DEfO | TLS library | [sftcd/openssl][sftcdopenssl] | Tag *esni-2019-08-30* avoids bleeding edge |
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| | curl fork | [niallor/curl][niallorcurl] | Tag *esni-2019-08-30* likewise |
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| | instructions | [ESNI-README][niallorreadme] | |
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## Progress
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### PR 4011 (Jun 2019) expected in curl release 7.67.0 (Oct 2019)
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- Details [below](#pr4011);
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- New **curl** feature: `CURL_VERSION_ESNI`;
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- New configuration option: `--enable-esni`;
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- Build-time check for availability of resources needed for ESNI
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support;
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- Pre-processor symbol `USE_ESNI` for conditional compilation of
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ESNI support code, subject to configuration option and
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availability of needed resources.
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## TODO
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- (next PR) Add libcurl options to set ESNI parameters.
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- (next PR) Add curl tool command line options to set ESNI parameters.
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- (WIP) Extend DoH functions so that published ESNI parameters can be
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retrieved from DNS instead of being required as options.
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- (WIP) Work with OpenSSL community to finalize ESNI API.
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- Track OpenSSL ESNI API in libcurl
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- Identify and implement any changes needed for CMake.
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- Optimize build-time checking of available resources.
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- Encourage ESNI support work on other TLS/SSL backends.
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## Additional detail
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### PR 4011
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**TLS: Provide ESNI support framework for curl and libcurl**
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The proposed change provides a framework to facilitate work to
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implement ESNI support in curl and libcurl. It is not intended
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either to provide ESNI functionality or to favour any particular
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TLS-providing backend. Specifically, the change reserves a
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feature bit for ESNI support (symbol `CURL_VERSION_ESNI`),
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implements setting and reporting of this bit, includes dummy
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book-keeping for the symbol, adds a build-time configuration
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option (`--enable-esni`), provides an extensible check for
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resources available to provide ESNI support, and defines a
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compiler pre-processor symbol (`USE_ESNI`) accordingly.
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Proposed-by: @niallor (Niall O'Reilly)\
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Encouraged-by: @sftcd (Stephen Farrell)\
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See-also: [this message](https://curl.haxx.se/mail/lib-2019-05/0108.html)
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Limitations:
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- Book-keeping (symbols-in-versions) needs real release number, not 'DUMMY'.
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- Framework is incomplete, as it covers autoconf, but not CMake.
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- Check for available resources, although extensible, refers only to
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specific work in progress ([described
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here](https://github.com/sftcd/openssl/tree/master/esnistuff)) to
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implement ESNI for OpenSSL, as this is the immediate motivation
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for the proposed change.
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## References
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Cloudflare blog: [Encrypting SNI: Fixing One of the Core Internet Bugs][corebug]
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Cloudflare blog: [Encrypt it or lose it: how encrypted SNI works][esniworks]
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IETF Draft: [Encrypted Server Name Indication for TLS 1.3][tlsesni]
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---
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[tlsesni]: https://datatracker.ietf.org/doc/draft-ietf-tls-esni/
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[esniworks]: https://blog.cloudflare.com/encrypted-sni/
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[corebug]: https://blog.cloudflare.com/esni/
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[defoproj]: https://defo.ie/
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[sftcdopenssl]: https://github.com/sftcd/openssl/
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[niallorcurl]: https://github.com/niallor/curl/
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[niallorreadme]: https://github.com/niallor/curl/blob/master/ESNI-README.md
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