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