curl speaks HTTP/2 with proxy

In September 2013 we merged the first code into curl that made it capable of using HTTP/2: HTTP version 2.

This version of HTTP changed a lot of previous presumptions when it comes to transfers, which introduced quite a few challenges to HTTP stack authors all of the world. One of them being that with version 2 there can be more than one transfer using the same connection where as up to that point we had always just had one transfer per connection.

In May 2015 the spec was published.


Now almost eight years since the RFC was published, HTTP/2 is the version seen most frequently in browser responses if we ask the Firefox telemetry data. 44.4% of the responses are HTTP/2.


This year, the curl project has been sponsored by the Sovereign Tech Fund, and one of the projects this funding has covered is what I am here to talk about:

Speaking HTTP/2 with a proxy. More specifically with what is commonly referred to as a “forward proxy.”

Many organizations and companies have setups like the one illustrated in this image below. The user on the left is inside the organization network A and the website they want to reach is on the outside on network B.

HTTP/2 to the proxy

When this is an HTTPS proxy, meaning that the communication to and with the proxy is itself protected with TLS, curl and libcurl are now capable of negotiating HTTP/2 with it.

It might not seem like a big deal to most people, and maybe it is not, but the introduction of this feature comes after some rather heavy lifting and internal refactors over the recent months that have enabled the rearrangement of networking components for this purpose.


To enable this feature in your libcurl-using application, you first need to make sure you use libcurl 8.1.0 when it ships in mid May and then you need to set the proxy type to CURLPROXY_HTTPS2.

In plain C code it could look like this:


This allows HTTP/2 but will proceed with plain old HTTP/1 if it can’t negotiate the higher protocol version using ALPN.

The old proxy type called just CURLPROXY_HTTPS remains for asking libcurl to stick to HTTP/1 when talking to the proxy. We decided to introduce a new option for this simply because we anticipate that there will be proxies out there that will not work correctly so we cannot throw this feature at users without them asking for it.

command line tool

Using the command line tool, you use a HTTPS proxy exactly like before and then you add this flag to tell the tool that it may try HTTP/2 with the proxy: --proxy-http2.

This also happens to be curl’s 251st command line option.

Shipping and credits

This implementation has been done by Stefan Eissing.

These features have already landed in the master branch and will be part of the pending curl 8.1.0 release, scheduled for release on May 17, 2023.