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License: see COPYING

Source code: https://github.com/libssh2/libssh2

Web site source code: https://github.com/libssh2/www

Installation instructions are in docs/INSTALL

To build libssh2 you will need CMake v2.8 or later [1] and one of the following cryptography libraries:

  • OpenSSL
  • Libgcrypt
  • WinCNG
  • mbedTLS

Getting started

If you are happy with the default options, make a new build directory, change to it, configure the build environment and build the project:

mkdir bin cd bin cmake .. cmake --build .

libssh2 will be built as a static library and will use any cryptography library available. The library binary will be put in bin/src, with the examples in bin/example and the tests in bin/tests.

Customising the build

Of course, you might want to customise the build options. You can pass the options to CMake on the command line:

cmake -D= ..

The following options are available:

  • BUILD_SHARED_LIBS=OFF

    Determines whether libssh2 is built as a static library or as a shared library (.dll/.so). Can be ON or OFF.

  • CRYPTO_BACKEND=

    Chooses a specific cryptography library to use for cryptographic operations. Can be OpenSSL (https://www.openssl.org), Libgcrypt (https://www.gnupg.org/), WinCNG (Windows Vista+), mbedTLS (https://tls.mbed.org/) or blank to use any library available.

    CMake will attempt to locate the libraries automatically. See [2] for more information.

  • ENABLE_ZLIB_COMPRESSION=OFF

    Will use zlib (http://www.zlib.org) for payload compression. Can be ON or OFF.

  • ENABLE_CRYPT_NONE=OFF

    The SSH2 Transport allows for unencrypted data transmission using the "none" cipher. Because this is such a huge security hole, it is typically disabled on SSH2 implementations and is disabled in libssh2 by default as well.

    Enabling this option will allow for "none" as a negotiable method, however it still requires that the method be advertized by the remote end and that no more-preferable methods are available.

  • ENABLE_MAC_NONE=OFF

    The SSH2 Transport also allows implementations to forego a message authentication code. While this is less of a security risk than using a "none" cipher, it is still not recommended as disabling MAC hashes removes a layer of security.

    Enabling this option will allow for "none" as a negotiable method, however it still requires that the method be advertized by the remote end and that no more-preferable methods are available.

  • ENABLE_GEX_NEW=ON

    The diffie-hellman-group-exchange-sha1 (dh-gex) key exchange method originally defined an exchange negotiation using packet type 30 to request a generation pair based on a single target value. Later refinement of dh-gex provided for range and target values. By default libssh2 will use the newer range method.

    If you experience trouble connecting to an old SSH server using dh-gex, try this option to fallback on the older more reliable method.

  • ENABLE_DEBUG_LOGGING=ON in Debug, =OFF in Release

    Will enable the libssh2_trace() function for showing debug traces.

  • CLEAR_MEMORY=ON

    Securely zero memory before freeing it (if the backend supports this).

Build tools

The previous examples used CMake to start the build using:

cmake --build .

Alternatively, once CMake has configured your project, you can just use your own build tool, e.g GNU make, Visual Studio, etc., from that point onwards.

Tests

To test the build, run the appropriate test target for your build system. For example:

cmake --build . --target test or cmake --build . --target RUN_TESTS

How do I use libssh2 in my project if my project doesn't use CMake?

If you are not using CMake for your own project, install libssh2

cmake cmake --build . cmake --build . --target install or cmake --build . --target INSTALL

and then specify the install location to your project in the normal way for your build environment. If you don't like the default install location, add -DCMAKE_INSTALL_PREFIX=<chosen prefix> when initially configuring the project.

How can I use libssh2 in my project if it also uses CMake?

If your own project also uses CMake, you don't need to worry about setting it up with libssh2's location. Just add just the following lines and CMake will find libssh2 on your system, set up the necessary paths and link the library with your binary.

find_package(Libssh2 REQUIRED CONFIG)
target_link_libraries(my_project_target Libssh2::libssh2)

Of course, you still have to make libssh2 available on your system first. You can install it in the traditional way shown above, but you don't have to. Instead you can just build it, which will export its location to the user package registry [3] where find_package will find it.

You can even combine the two steps using a so-called 'superbuild' project [4] that downloads, builds and exports libssh2, and then builds your project:

include(ExternalProject)

ExternalProject_Add(
    Libssh2
    URL <libssh2 download location>
    URL_HASH SHA1=<libssh2 archive SHA1>
    INSTALL_COMMAND "")

ExternalProject_Add(
    MyProject DEPENDS Libssh2
    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/src
    INSTALL_COMMAND "")

[1] https://www.cmake.org/cmake/resources/software.html [2] https://www.cmake.org/cmake/help/v3.0/manual/cmake-packages.7.html [3] https://www.cmake.org/cmake/help/v3.0/manual/cmake-packages.7.html#package-registry [4] https://blog.kitware.com/wp-content/uploads/2016/01/kitware_quarterly1009.pdf