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Error: An unknown option was passed in to libcurl when using CURLOPT_SSL_ENABLE_ALPN #4668

shailcompsit opened this issue Dec 3, 2019 · 4 comments


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What I did,

As per this man page for CURLOPT_SSL_ENABLE_ALPN, the option is enabled by default, and we can see the ALPN extension set in Client Hello using Wireshark. However, I want to disable that option, so that ALPN extension is not present in Client Hello (TLS Handshake). I tried below code,

CURLcode res = CURLE_OK;
res = curl_easy_setopt(curl_, CURLOPT_SSL_ENABLE_ALPN, 0L);
if(res == CURLE_OK){
cout<<"CURLOPT_SSL_ENABLE_ALPN to 0L res = "<<res;
cout<<"Error: res = "<<res<<"\t"<<curl_easy_strerror(res);


Error: res = 48 An unknown option was passed in to libcurl

I expected the following


curl/libcurl version

libcurl Version: 7.67.0
openssl version: OpenSSL 1.1.0l

libcurl configuration options

./configure --prefix= " << full path to installation directory >> " --disable-manual --enable-ipv6 --disable-ntlm-wb --without-krb4 --without-gssapi --with-ssl=" << full path to openssl folder containing library and include files >> " --without-gnutls --without-polarssl --without-cyassl --without-nss --without-axtls --without-libmetalink --without-libssh2 --disable-ldap --without-libidn LDFLAGS="-Wl,-rpath -Wl, << full path to openssl library folder >> "

operating system


Even though this option throws error in MacOSX also, ALPN extension is not present in TLS Handshake on MacOSX
Is this option deprecated? Or am I missing something here?

@jay jay added the HTTP/2 label Dec 4, 2019
jay added a commit to jay/curl that referenced this issue Dec 4, 2019
- Stop treating lack of HTTP2 as an unknown option error result for

Prior to this change it was impossible to disable ALPN / NPN if libcurl
was built without HTTP2. Setting either option would result in
CURLE_UNKNOWN_OPTION and the respective internal option would not be
set. That was incorrect since ALPN and NPN are used independent of

Reported-by: Shailesh Kapse

Fixes curl#4668
Closes #xxxx
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jay commented Dec 4, 2019

Those options erroneously require HTTP/2 support:


Lines 2615 to 2622 in 2e9b725

#ifdef USE_NGHTTP2
data->set.ssl_enable_npn = (0 != va_arg(param, long)) ? TRUE : FALSE;
data->set.ssl_enable_alpn = (0 != va_arg(param, long)) ? TRUE : FALSE;

If that is your issue then please try the fix in #4672. If that doesn't work then give us this output:

    curl_version_info_data *info = curl_version_info(CURLVERSION_NOW);
    printf("SSPI is %s\n", ((info->features & CURL_VERSION_SSPI) ? "on" : "off"));
    printf("HTTP2 is %s\n", ((info->features & CURL_VERSION_HTTP2) ? "on" : "off"));
    printf("%s\n", curl_version());

@jay jay added the libcurl API label Dec 4, 2019
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@jay Replicated the changes committed by you. This solves the issue.
I can see the output as expected: "CURLOPT_SSL_ENABLE_ALPN to 0L res = 0"
Also, monitored the TLS Handshake, and ALPN extension is absent in Client Hello now.
Thanks a lot!
When can we expect the next release of libcurl ?

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bagder commented Dec 4, 2019

When can we expect the next release of libcurl ?

See the pending release-notes page: January 8th 2020 is the scheduled release date for the coming 7.68.0 release.

You can however always download the current state from git or as a daily snapshot to try out the coming changes even before they ship!

@jay jay closed this as completed in 0edf758 Dec 6, 2019
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jay commented Dec 6, 2019


@lock lock bot locked as resolved and limited conversation to collaborators Mar 10, 2020
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3 participants