TLS: ESNI support in curl and libcurl


ESNI means Encrypted Server Name Indication, a TLS 1.3 extension which is currently the subject of an IETF Draft.

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, built using an experimental fork of OpenSSL, which in turn provided an implementation of ESNI, was demonstrated interoperating with a server belonging to the DEfO Project.

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.

DEfOTLS librarysftcd/opensslTag esni-2019-08-30 avoids bleeding edge
curl forkniallor/curlTag esni-2019-08-30 likewise


PR 4011 (Jun 2019) expected in curl release 7.67.0 (Oct 2019)

  • Details below;

  • 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.


  • (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


  • 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) to implement ESNI for OpenSSL, as this is the immediate motivation for the proposed change.


Cloudflare blog: Encrypting SNI: Fixing One of the Core Internet Bugs

Cloudflare blog: Encrypt it or lose it: how encrypted SNI works

IETF Draft: Encrypted Server Name Indication for TLS 1.3