curl uses http/1.1 alpn code when told to use http/1.0

I did this

I started a local https server on port 8000.
I sent a request using curl --http1.0 -vvv --insecure https://localhost:8000
Curl offers http/1.1 via ALPN
Curl sends a http/1.0 request

I expected the following

Curl should offer http/1.0 via ALPN instead of http/1.1 as http/1.0 was specified via the command line and the send request was also http/1.0

curl/libcurl version

curl 8.17.0 (x86_64-pc-linux-gnu) libcurl/8.17.0 OpenSSL/3.6.0 zlib/1.3.1 brotli/1.2.0 zstd/1.5.7 libidn2/2.3.8 libpsl/0.21.5 libssh2/1.11.1 nghttp2/1.67.1 ngtcp2/1.18.0 nghttp3/1.13.1 mit-krb5/1.22.1
Release-Date: 2025-11-05
Protocols: dict file ftp ftps gopher gophers http https imap imaps ipfs ipns mqtt pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS brotli GSS-API HSTS HTTP2 HTTP3 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM PSL SPNEGO SSL threadsafe TLS-SRP UnixSockets zstd

operating system

Linux nixie-tube 6.18.5-zen1 #1-NixOS ZEN SMP PREEMPT_DYNAMIC Tue Jan 1 00:00:00 UTC 1980 x86_64 GNU/Linux

Curl should offer http/1.0 via ALPN instead of http/1.1 as http/1.0 was specified via the command line and the send request was also http/1.0

Ref: #12259
Ref: https://www.iana.org/assignments/tls-extensiontype-values/tls-extensiontype-values.xhtml#alpn-protocol-ids

curl uses ALPN http/1.1 for HTTP/1.0 requests for compatibility reasons.

curl/lib/vtls/vtls.c

Lines 150 to 170 in 2eebc58

    
           static const struct alpn_spec *alpn_get_spec(http_majors wanted, 
        
                                                        http_majors preferred, 
        
                                                        bool use_alpn) 
        
           { 
        
             if(!use_alpn) 
        
               return NULL; 
        
           #ifdef USE_HTTP2 
        
             if(wanted & CURL_HTTP_V2x) { 
        
               if(wanted & CURL_HTTP_V1x) 
        
                 return (preferred == CURL_HTTP_V1x) ? 
        
                   &ALPN_SPEC_H11_H2 : &ALPN_SPEC_H2_H11; 
        
               return &ALPN_SPEC_H2; 
        
             } 
        
           #else 
        
             (void)wanted; 
        
             (void)preferred; 
        
           #endif 
        
             /* Use the ALPN protocol "http/1.1" for HTTP/1.x. 
        
                Avoid "http/1.0" because some servers do not support it. */ 
        
             return &ALPN_SPEC_H11; 
        
           }

@programmerlexi

@programmerlexi is this actually causing a problem for you?

This doesn't cause much of an issue, but i feel it's incorrect to exclusively offer HTTP/1.1 for a HTTP/1.0 connection.
I feel like curl should at least offer HTTP/1.0 as the preferred protocol when --http1.0 is used.

This doesn't cause much of an issue, but i feel it's incorrect to exclusively offer HTTP/1.1 for a HTTP/1.0 connection. I feel like curl should at least offer HTTP/1.0 as the preferred protocol when --http1.0 is used.

Do you know of a server that implements this ALPN (apart from the fact that ALPN http/1.0 is not a registered identifier)? Apache does not. And what should a server do then? Reject an incoming HTTP/1.1 request on such a connection? I do not see anyone implementing this.

Do you know of a server that implements this ALPN (apart from the fact that ALPN http/1.0 is not a registered identifier)?

I am currently implementing a server that does.
http/1.0 it is a registered identifier in the registry.

And what should a server do then? Reject an incoming HTTP/1.1 request on such a connection?

Yes.

Ah, ok. They added it since the last time I looked. Hmm...not sure I see the benefit of such...but you are of course free to implement such a server.

vtls: use ALPN http/1.1 & http/1.0 for HTTP/1.0 requests

I think for HTTP/1.0 requests we should keep sending ALPN http/1.1 for compatibility reasons, however we could also send ALPN http/1.0 for servers that accept only that. In other words send both, see #20533.

vtls: use ALPN http/1.0 & http/1.1 for HTTP/1.0 requests

Thanks. For HTTP/1.0 requests curl will now send ALPNs http/1.0 and http/1.1 and the server can pick which one it recognizes.

Navigation Menu

Repository navigation

curl uses http/1.1 alpn code when told to use http/1.0 #20487

I did this

I expected the following

curl/libcurl version

operating system

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

	static const struct alpn_spec *alpn_get_spec(http_majors wanted,
	http_majors preferred,
	bool use_alpn)
	{
	if(!use_alpn)
	return NULL;
	#ifdef USE_HTTP2
	if(wanted & CURL_HTTP_V2x) {
	if(wanted & CURL_HTTP_V1x)
	return (preferred == CURL_HTTP_V1x) ?
	&ALPN_SPEC_H11_H2 : &ALPN_SPEC_H2_H11;
	return &ALPN_SPEC_H2;
	}
	#else
	(void)wanted;
	(void)preferred;
	#endif
	/* Use the ALPN protocol "http/1.1" for HTTP/1.x.
	Avoid "http/1.0" because some servers do not support it. */
	return &ALPN_SPEC_H11;
	}

curl uses http/1.1 alpn code when told to use http/1.0 #20487

Description

I did this

I expected the following

curl/libcurl version

operating system

Activity

jay commented on Jan 31, 2026

bagder commented on Feb 3, 2026

programmerlexi commented on Feb 4, 2026

icing commented on Feb 5, 2026

programmerlexi commented on Feb 5, 2026

icing commented on Feb 5, 2026

jay commented on Feb 5, 2026

jay commented on Feb 9, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions