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Date: February 11, 2007
Author: Daniel Stenberg <>
This document is written to describe the situation as it is right now.
libcurl 7.16.1 is currently the latest version available. Things may of
course change in the future.
This document reflects my view and understanding of these things. Please tell
me where and how you think I'm wrong, and I'll try to correct my mistakes.
The Free Software Foundation has deemed the Original BSD license[1] to be
"incompatible"[2] with GPL[3]. I'd rather say it is the other way around, but
the point is the same: if you distribute a binary version of a GPL program,
it MUST NOT be linked with any Original BSD-licensed parts or libraries.
Doing so will violate the GPL license. For a long time, very many GPL
licensed programs have avoided this license mess by adding an exception[8] to
their license. And many others have just closed their eyes for this problem.
libcurl is MIT-style[4] licensed - how on earth did this dilemma fall onto
our plates?
libcurl is only a little library. libcurl can be built to use OpenSSL for its
SSL/TLS capabilities. OpenSSL is basically Original BSD licensed[5].
If libcurl built to use OpenSSL is used by a GPL-licensed application and you
decide to distribute a binary version of it (Linux distros - for example -
tend to), you have a clash. GPL vs Original BSD.
This dilemma is not libcurl-specific nor is it specific to any particular
Linux distro. (This article mentions and refers to Debian several times, but
only because Debian seems to be the only Linux distro to have faced this
issue yet since no other distro is shipping libcurl built with two SSL
Part of the Operating System
This would not be a problem if the used lib would be considered part of the
underlying operating system, as then the GPL license has an exception
clause[6] that allows applications to use such libs without having to be
allowed to distribute it or its sources. Possibly some distros will claim
that OpenSSL is part of their operating system.
Debian does however not take this stance and has officially(?) claimed that
OpenSSL is not a required part of the Debian operating system
Some people claim that this paragraph cannot be exploited this way by a Linux
distro, but I am not a lawyer and that is a discussion left outside of this
Since May 2005 libcurl can get built to use GnuTLS instead of OpenSSL. GnuTLS
is an LGPL[7] licensed library that offers a matching set of features as
OpenSSL does. Now, you can build and distribute an TLS/SSL capable libcurl
without including any Original BSD licensed code.
I believe Debian is the first (only?) distro that provides libcurl/GnutTLS
libcurl can get also get built to use yassl for the TLS/SSL layer. yassl is a
GPL[3] licensed library.
GnuTLS vs OpenSSL vs yassl
While these three libraries offer similar features, they are not equal.
libcurl does not (yet) offer a standardized stable ABI if you decide to
switch from using libcurl-openssl to libcurl-gnutls or vice versa. The GnuTLS
and yassl support is very recent in libcurl and it has not been tested nor
used very extensively, while the OpenSSL equivalent code has been used and
thus matured since 1999.
- LGPL licensened
- supports SRP
- lacks SSLv2 support
- lacks MD2 support (used by at least some CA certs)
- lacks the crypto functions libcurl uses for NTLM
- Original BSD licensened
- lacks SRP
- supports SSLv2
- older and more widely used
- provides crypto functions libcurl uses for NTLM
- libcurl can do non-blocking connects with it in 7.15.4 and later
- GPL licensed
- much untested and unproven in the real work by (lib)curl users so we don't
know a lot about restrictions or benefits from using this
The Better License, Original BSD, GPL or LGPL?
It isn't obvious or without debate to any objective interested party that
either of these licenses are the "better" or even the "preferred" one in a
generic situation.
Instead, I think we should accept the fact that the SSL/TLS libraries and
their different licenses will fit different applications and their authors
differently depending on the applications' licenses and their general usage
pattern (considering how GPL and LGPL libraries for example can be burdensome
for embedded systems usage).
In Debian land, there seems to be a common opinion that LGPL is "maximally
compatible" with apps while Original BSD is not. Like this:
More SSL Libraries
In libcurl, there's no stopping us here. There are more Open Source/Free
SSL/TLS libraries out there and we would very much like to support them as
well, to offer application authors an even wider scope of choice.
Application Angle of this Problem
libcurl is built to use one SSL/TLS library. It uses a single fixed name (by
default) on the built/created lib file, and applications are built/linked to
use that single lib. Replacing one libcurl instance with another one that
uses the other SSL/TLS library might break one or more applications (due to
ABI differences and/or different feature set). You want your application to
use the libcurl it was built for.
Project cURL Angle of this Problem
We distribute libcurl and everyone may build libcurl with either library at
their choice. This problem is not directly a problem of ours. It merely
affects users - GPL application authors only - of our lib as it comes
included and delivered on some distros.
libcurl has different ABI when built with different SSL/TLS libraries due to
these reasons:
1. No one has worked on fixing this. The mutex/lock callbacks should be set
with a generic libcurl function that should use the proper underlying
2. The CURLOPT_SSL_CTX_FUNCTION option is not possible to "emulate" on GnuTLS
but simply requires OpenSSL.
3. There might be some other subtle differences just because nobody has yet
tried to make a fixed ABI like this.
Distro Angle of this Problem
To my knowledge there is only one distro that ships libcurl built with either
OpenSSL or GnuTLS.
Debian Linux is now (since mid September 2005) providing two different
libcurl packages, one for libcurl built with OpenSSL and one built with
GnuTLS. They use different .so names and can this both be installed in a
single system simultaneously. This has been said to be a transitional system
not desired to keep in the long run.
[1] =
[2] =
[3] =
[4] =
[5] =
[6] = end of section 3
[7] =
[8] =
Feedback/Updates provided by
Eric Cooper