CURLOPT_ECH(3) Library Functions Manual CURLOPT_ECH(3)
NAME
CURLOPT_ECH - configuration for Encrypted Client Hello
SYNOPSIS
#include <curl/curl.h>
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_ECH, char *config);
DESCRIPTION
ECH is only compatible with TLSv1.3.
This experimental feature requires a special build of OpenSSL, as ECH is
not yet supported in OpenSSL releases. In contrast ECH is supported by
the latest BoringSSL, wolfSSL and rustls-ffi releases.
There is also a known issue with using wolfSSL which does not support
ECH when the HelloRetryRequest mechanism is used.
Pass a string that specifies configuration details for ECH. In all
cases, if ECH is attempted, it may fail for various reasons. The key-
words supported are:
false Turns off ECH.
grease Instructs client to emit a GREASE ECH extension. (The connection
fails if ECH is attempted but fails.)
true Instructs client to attempt ECH, if possible, but to not fail if
attempting ECH is not possible.
hard Instructs client to attempt ECH and fail if attempting ECH is not
possible.
ecl:<base64-value>
If the string starts with ecl: then the remainder of the string
should be a base64-encoded ECHConfigList that is used for ECH
rather than attempting to download such a value from the DNS.
pn:<name>
If the string starts with pn: then the remainder of the string
should be a DNS/hostname that is used to over-ride the pub-
lic_name field of the ECHConfigList that is used for ECH.
The application does not have to keep the string around after setting
this option.
Using this option multiple times makes the last set string override the
previous ones. Set it to NULL or "false" to disable its use again.
DEFAULT
NULL, meaning ECH is disabled.
PROTOCOLS
This functionality affects all TLS based protocols: HTTPS, FTPS, IMAPS,
POP3S, SMTPS etc.
This option works only with the following TLS backends: OpenSSL, rustls
and wolfSSL
EXAMPLE
int main(void)
{
CURL *curl = curl_easy_init();
const char *config = \
"ecl:AED+DQA87wAgACB/RuzUCsW3uBbSFI7mzD63TUXpI8sGDTnFTbFCDpa+" \
"CAAEAAEAAQANY292ZXIuZGVmby5pZQAA";
if(curl) {
curl_easy_setopt(curl, CURLOPT_ECH, config);
curl_easy_perform(curl);
}
}
AVAILABILITY
Added in curl 8.8.0
RETURN VALUE
curl_easy_setopt(3) returns a CURLcode indicating success or error.
CURLE_OK (0) means everything was OK, non-zero means an error occurred,
see libcurl-errors(3).
SEE ALSO
CURLOPT_DOH_URL(3)
libcurl 2026-07-12 CURLOPT_ECH(3)
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