This is the online reference for developing with the libssh library. It
documents the libssh C API and the C++ wrapper.
@section main-linking Linking
We created a small howto how to link libssh against your application, read
@section main-tutorial Tutorial
You should start by reading @subpage libssh_tutorial, then reading the documentation of
the interesting functions as you go.
@section main-features Features
The libssh library provides:
- <strong>Key Exchange Methods</strong>: <i>firstname.lastname@example.org, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521</i>, diffie-hellman-group1-sha1, diffie-hellman-group14-sha1
- <strong>Hostkey Types</strong>: <i>ssh-ed25519, ecdsa-sha2-nistp256, ecdsa-sha2-nistp384, ecdsa-sha2-nistp521</i>, ssh-dss, ssh-rsa
- <strong>Ciphers</strong>: <i>aes256-ctr, aes192-ctr, aes128-ctr</i>, aes256-cbc (email@example.com), aes192-cbc, aes128-cbc, 3des-cbc, blowfish-cbc, none
- <strong>Compression Schemes</strong>: zlib, <i>firstname.lastname@example.org</i>, none
- <strong>MAC hashes</strong>: hmac-sha1, hmac-sha2-256, hmac-sha2-384, hmac-sha2-512, hmac-md5, none
- <strong>Authentication</strong>: none, password, public-key, hostbased, keyboard-interactive, <i>gssapi-with-mic</i>
- <strong>Channels</strong>: shell, exec (incl. SCP wrapper), direct-tcpip, subsystem, <i>email@example.com</i>
- <strong>Global Requests</strong>: tcpip-forward, forwarded-tcpip
- <strong>Channel Requests</strong>: x11, pty, <i>exit-status, signal, exit-signal, firstname.lastname@example.org, email@example.com</i>
- <strong>Subsystems</strong>: sftp(version 3), <i>OpenSSH Extensions</i>
- <strong>SFTP</strong>: <i>firstname.lastname@example.org, email@example.com</i>
- <strong>Thread-safe</strong>: Just don't share sessions
- <strong>Non-blocking</strong>: it can be used both blocking and non-blocking
- <strong>Your sockets</strong>: the app hands over the socket, or uses libssh sockets
- <b>OpenSSL</b> or <b>gcrypt</b>: builds with either
@section main-additional-features Additional Features
- Client <b>and</b> server support
- SSHv2 and SSHv1 protocol support
- Supports <a href="http://test.libssh.org/" target="_blank">Linux, UNIX, BSD, Solaris, OS/2 and Windows</a>
- Automated test cases with nightly <a href="http://test.libssh.org/" target="_blank">tests</a>
- Event model based on poll(2), or a poll(2)-emulation.
@section main-copyright Copyright Policy
libssh is a project with distributed copyright ownership, which means we prefer
the copyright on parts of libssh to be held by individuals rather than
corporations if possible. There are historical legal reasons for this, but one
of the best ways to explain it is that it’s much easier to work with
individuals who have ownership than corporate legal departments if we ever need
to make reasonable compromises with people using and working with libssh.
We track the ownership of every part of libssh via git, our source code control
system, so we know the provenance of every piece of code that is committed to
So if possible, if you’re doing libssh changes on behalf of a company who
normally owns all the work you do please get them to assign personal copyright
ownership of your changes to you as an individual, that makes things very easy
for us to work with and avoids bringing corporate legal departments into the
If you can’t do this we can still accept patches from you owned by your
employer under a standard employment contract with corporate copyright
ownership. It just requires a simple set-up process first.
We use a process very similar to the way things are done in the Linux Kernel
community, so it should be very easy to get a sign off from your corporate
legal department. The only changes we’ve made are to accommodate the license we
use, which is LGPLv2 (or later) whereas the Linux kernel uses GPLv2.
The process is called signing.
How to sign your work
Once you have permission to contribute to libssh from your employer, simply
email a copy of the following text from your corporate email address to:
libssh Developer's Certificate of Origin. Version 1.0
By making a contribution to this project, I certify that:
(a) The contribution was created in whole or in part by me and I
have the right to submit it under the appropriate
version of the GNU General Public License; or
(b) The contribution is based upon previous work that, to the best of
my knowledge, is covered under an appropriate open source license
and I have the right under that license to submit that work with
modifications, whether created in whole or in part by me, under
the GNU General Public License, in the appropriate version; or
(c) The contribution was provided directly to me by some other
person who certified (a) or (b) and I have not modified it.
(d) I understand and agree that this project and the contribution are
public and that a record of the contribution (including all
metadata and personal information I submit with it, including my
sign-off) is maintained indefinitely and may be redistributed
consistent with the libssh Team's policies and the requirements of
the GNU GPL where they are relevant.
(e) I am granting this work to this project under the terms of the
GNU Lesser General Public License as published by the
Free Software Foundation; either version 2.1 of
the License, or (at the option of the project) any later version.
We will maintain a copy of that email as a record that you have the rights to
contribute code to libssh under the required licenses whilst working for the
company where the email came from.
Then when sending in a patch via the normal mechanisms described above, add a
line that states:
Signed-off-by: Random J Developer <firstname.lastname@example.org>
using your real name and the email address you sent the original email you used
to send the libssh Developer’s Certificate of Origin to us (sorry, no
pseudonyms or anonymous contributions.)
That’s it! Such code can then quite happily contain changes that have copyright
messages such as:
(c) Example Corporation.
and can be merged into the libssh codebase in the same way as patches from any
other individual. You don’t need to send in a copy of the libssh Developer’s
Certificate of Origin for each patch, or inside each patch. Just the sign-off
message is all that is required once we’ve received the initial email.
Have fun and happy libssh hacking!
The libssh Team
@section main-rfc Internet standard
@subsection main-rfc-secsh Secure Shell (SSH)
The following RFC documents described SSH-2 protcol as an Internet standard.
- <a href="http://tools.ietf.org/html/rfc4250" target="_blank">RFC 4250</a>,
The Secure Shell (SSH) Protocol Assigned Numbers
- <a href="http://tools.ietf.org/html/rfc4251" target="_blank">RFC 4251</a>,
The Secure Shell (SSH) Protocol Architecture
- <a href="http://tools.ietf.org/html/rfc4252" target="_blank">RFC 4252</a>,
The Secure Shell (SSH) Authentication Protocol
- <a href="http://tools.ietf.org/html/rfc4253" target="_blank">RFC 4253</a>,
The Secure Shell (SSH) Transport Layer Protocol
- <a href="http://tools.ietf.org/html/rfc4254" target="_blank">RFC 4254</a>,
The Secure Shell (SSH) Connection Protocol
- <a href="http://tools.ietf.org/html/rfc4255" target="_blank">RFC 4255</a>,
Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
- <a href="http://tools.ietf.org/html/rfc4256" target="_blank">RFC 4256</a>,
Generic Message Exchange Authentication for the Secure Shell Protocol (SSH)
- <a href="http://tools.ietf.org/html/rfc4335" target="_blank">RFC 4335</a>,
The Secure Shell (SSH) Session Channel Break Extension
- <a href="http://tools.ietf.org/html/rfc4344" target="_blank">RFC 4344</a>,
The Secure Shell (SSH) Transport Layer Encryption Modes
- <a href="http://tools.ietf.org/html/rfc4345" target="_blank">RFC 4345</a>,
Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer Protocol
It was later modified and expanded by the following RFCs.
- <a href="http://tools.ietf.org/html/rfc4419" target="_blank">RFC 4419</a>,
Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
- <a href="http://tools.ietf.org/html/rfc4432" target="_blank">RFC 4432</a>,
RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol
- <a href="http://tools.ietf.org/html/rfc4462" target="_blank">RFC 4462</a>,
Generic Security Service Application Program Interface (GSS-API)
Authentication and Key Exchange for the Secure Shell (SSH) Protocol
- <a href="http://tools.ietf.org/html/rfc4716" target="_blank">RFC 4716</a>,
The Secure Shell (SSH) Public Key File Format
- <a href="http://tools.ietf.org/html/rfc5647" target="_blank">RFC 5647</a>,
AES Galois Counter Mode for the Secure Shell Transport Layer Protocol
- <a href="http://tools.ietf.org/html/rfc5656" target="_blank">RFC 5656</a>,
Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer
Interesting cryptography documents:
- <a href="http://www.cryptsoft.com/pkcs11doc/" target="_blank">PKCS #11</a>, PKCS #11 reference documents, describing interface with smartcards.
@subsection main-rfc-sftp Secure Shell File Transfer Protocol (SFTP)
The protocol is not an Internet standard but it is still widely implemented.
OpenSSH and most other implementation implement Version 3 of the protocol. We
do the same in libssh.
- <a href="http://tools.ietf.org/html/draft-ietf-secsh-filexfer-02" target="_blank">
SSH File Transfer Protocol
@subsection main-rfc-extensions Secure Shell Extensions
The libssh project has an extension to support Curve25519 which is also supported by
the OpenSSH project.
- <a href="http://email@example.com" target="_blank">firstname.lastname@example.org</a>,
Curve25519-SHA256 for ECDH KEX
The OpenSSH project has defined some extensions to the protocol. We support some of
them like the statvfs calls in SFTP or the ssh-agent.
- <a href="http://api.libssh.org/rfc/PROTOCOL" target="_blank">
OpenSSH's deviations and extensions</a>
- <a href="http://api.libssh.org/rfc/PROTOCOL.agent" target="_blank">
- <a href="http://api.libssh.org/rfc/PROTOCOL.certkeys" target="_blank">
OpenSSH's pubkey certificate authentication</a>