tldr: starting now, you need to select which TLS to use when you run curl’s configure script.
How it started
In June 1998, three months after the first release of curl, we added support for HTTPS. We decided that we would use an external library for this purpose – for providing SSL support – and therefore the first dependency was added. The build would optionally use SSLeay. If you wanted HTTPS support enabled, we would use that external library.
SSLeay ended development at the end of that same year, and OpenSSL rose as a new project and library from its ashes. Of course, even later the term “SSL” would also be replaced by “TLS” but the entire world has kept using them interchangeably.
The initial configure script we wrote and provided back then (it appeared for the first time in November 1998) would look for OpenSSL and use it if found present.
In the spring of 2005, we merged support for an alternative TLS library, GnuTLS, and now you would have to tell the configure script to not use OpenSSL but instead use GnuTLS if you wanted that in your build. That was the humble beginning of the explosion of TLS libraries supported by curl.
As time went on we added support for more and more TLS libraries, giving the users the choice to select exactly which particular one they wanted their curl build to use. At the time of this writing, we support 14 different TLS libraries.
OpenSSL was still default
The original logic from when we added GnuTLS back in 2005 was however still kept so whatever library you wanted to use, you would have to tell configure to not use OpenSSL and instead use your preferred library.
Also, as the default configure script would try to find and use OpenSSL it would result in some surprises to users who maybe didn’t want TLS in their build or even when something was just not correctly setup and configure unexpectedly didn’t find OpenSSL and the build then went on and was made completely without TLS support! Sometimes even without getting noticed for a long time.
Not doing it anymore
Starting now, curl’s configure will not select TLS backend by default.
It will not decide for you which one you use, as there are many decisions involved when selecting TLS backend and there are many users who prefer something else than OpenSSL. We will no longer give any special treatment to that library at build time. We will not impose our bias onto others anymore.
Not selecting any TLS backend at all will just make configure exit quickly with a help message prompting you to make a decision, as shown below. Notice that going completely without a TLS library is still fine but similarly also requires an active decision (
Effect on configure users
With this change, every configure invoke needs to clearly state which TLS library or even libraries (in plural since curl supports building with support for more than one library) to use.
The biggest change is of course for everyone who invokes configure and wants to build with OpenSSL since they now need to add an option and say that explicitly. For virtually everyone else life can just go on like before.
Everyone who builds curl automatically from source code might need to update their build scripts.
The first release shipping with this change will be curl 7.77.0.