libssh/doc/tutorial.dox

/**
 * @page tutorial The Tutorial
 *
 * @section introduction Introduction
 *
 * libssh is a C library that enables you to write a program that uses the
 * SSH protocol. With it, you can remotely execute programs, transfer
 * files, or use a secure and transparent tunnel for your remote programs.
 * The SSH protocol is encrypted, ensures data integrity, and provides strong
 * means of authenticating both the server of the client. The library hides
 * a lot of technical details from the SSH protocol, but this does not
 * mean that you should not try to know about and understand these details.
 *
 * libssh is a Free Software / Open Source project. The libssh library
 * is distributed under LGPL license. The libssh project has nothing to do with
 * "libssh2", which is a completly different and independant project.
 *
 * libssh supports both client and server sides of the SSH protocol.
 * The following document explains how to set up a client-side connection.
 * If you are going to program a server, you should try first to code some
 * client-side programs in order to understand how libssh works.
 *
 * This tutorial describes libssh version 0.5.0.
 *
 *
 * Table of contents:
 *
 * @subpage session
 *
 * @subpage details
 *
 * @subpage tbd
 *
 *
 * @page session Chapter 1: A typical SSH session
 * @section ssh_session A typical SSH session
 *
 * A SSH session goes through the following steps:
 *
 *  - Before connecting to the server, you can set up if you wish one or other
 *    server public key authentication, i.e. DSA or RSA. You can choose
 *    cryptographic algorithms you trust and compression algorithms if any. You
 *    must of course set up the hostname.
 *
 *  - The connection is established. A secure handshake is made, and resulting from
 *    it, a public key from the server is gained. You MUST verify that the public
 *    key is legitimate, using for instance the MD5 fingerprint or the known hosts
 *    file.
 *
 *  - The client must authenticate: the classical ways are password, or
 *    public keys (from dsa and rsa key-pairs generated by openssh).
 *    If a SSH agent is running, it is possible to use it.
 *
 *  - Now that the user has been authenticated, you must open one or several
 *    channels. Channels are different subways for information into a single ssh
 *    connection. Each channel has a standard stream (stdout) and an error stream
 *    (stderr). You can theoretically open an infinity of channels.
 *
 *  - With the channel you opened, you can do several things:
 *    - Execute a single command.
 *    - Open a shell. You may want to request a pseudo virtual terminal before.
 *    - Invoke the sftp subsystem to transfer files.
 *    - Invoke the scp subsystem to transfer files.
 *    - Invoke your own subsystem. This is outside the scope of this document,
 *      but it is easy to do.
 *
 *  - When everything is finished, just close the channels, and then the connection.
 *
 * All the libssh functions which return an error code also set an error message.
 * Error codes are typically SSH_ERROR for integer values, or NULL for pointers.
 *  
 *
 * @subsection setup Creating the session and setting options
 *
 * The most important object in a SSH connection is the SSH session. In order
 * to allocate a new SSH session, you use ssh_new(). Don't forget to
 * always verify that the allocation successed.
 * @code
 * #include <libssh/libssh.h> 
 * #include <stdlib.h>
 *
 * int main()
 * {
 *   ssh_session my_ssh_session = ssh_new();
 *   if (my_ssh_session == NULL)
 *     exit(-1);
 *   ...
 *   ssh_free(my_ssh_session);
 * }
 * @endcode
 *
 * libssh follows the allocate-it-deallocate-it pattern. Each object that you allocate
 * using xxxxx_new() must be deallocated using xxxxx_free(). In this case, ssh_new()
 * does the allocation and ssh_free() does the contrary.
 *
 * The ssh_options_set() function sets the options of the session. The most important options are:
 *  - SSH_OPTIONS_HOST: the name of the host you want to connect to
 *  - SSH_OPTIONS_PORT: the used port (default is port 22)
 *  - SSH_OPTIONS_USER: the system user under which you want to connect
 *  - SSH_OPTIONS_LOG_VERBOSITY: the quantity of messages that are printed
 *
 * The complete list of options can be found in the documentation of ssh_options_set().
 * The only mandatory option is SSH_OPTIONS_HOST. If you don't use SSH_OPTIONS_USER,
 * the local username of your account will be used.
 *
 * Here is a small example of how to use it:
 * @code
 * #include <libssh/libssh.h> 
 * #include <stdlib.h>
 *
 * int main()
 * {
 *   ssh_session my_ssh_session;
 *   int verbosity = SSH_LOG_PROTOCOL;
 *   int port = 22;
 *
 *   my_ssh_session = ssh_new();
 *   if (my_ssh_session == NULL)
 *     exit(-1);
 *
 *   ssh_options_set(my_ssh_session, SSH_OPTIONS_HOST, "localhost");
 *   ssh_options_set(my_ssh_session, SSH_OPTIONS_LOG_VERBOSITY, &verbosity);
 *   ssh_options_set(my_ssh_session, SSH_OPTIONS_PORT, &port);
 *
 *   ...
 *
 *   ssh_free(my_ssh_session);
 * }
 * @endcode
 *
 * Please notice that all parameters are passed to ssh_options_set() as pointers,
 * even if you need to set an integer value.
 *
 * @see ssh_new
 * @see ssh_free
 * @see ssh_options_set
 * @see ssh_options_parse_config
 * @see ssh_options_copy
 * @see ssh_options_getopt
 *
 *
 * @subsection connect Connecting to the server
 *
 * Once all settings have been made, you can connect using ssh_connect(). That
 * function will return SSH_OK if the connection worked, SSH_ERROR otherwise.
 *
 * You can get the error string using ssh_get_error() in order to show the
 * user what went wrong. Then, use ssh_disconnect() when you want to stop
 * the session.
 *
 * Here's an example:
 *
 * @code
 * #include <libssh/libssh.h>
 * #include <stdlib.h>
 * #include <stdio.h> 
 *
 * int main()
 * {
 *   ssh_session my_ssh_session;
 *   int ret;
 *
 *   my_ssh_session = ssh_new();
 *   if (my_ssh_session == NULL)
 *     exit(-1);
 *
 *   ssh_options_set(my_ssh_session, SSH_OPTIONS_HOST, "localhost");
 *
 *   ret = ssh_connect(my_ssh_session);
 *   if (ret != SSH_OK)
 *   {
 *     fprintf(stderr, "Error connecting to localhost: %s\n",
 *             ssh_get_error(my_ssh_session));
 *     exit(-1);
 *   }
 *
 *   ...
 *
 *   ssh_disconnect(my_ssh_session);
 *   ssh_free(my_ssh_session);
 * }
 * @endcode
 *
 *
 * @subsection serverauth Authenticating the server
 *
 * Once you're connected, the following step is mandatory: you must check that the server
 * you just connected to is known and safe to use (remember, SSH is about security and
 * authentication).
 *
 * There are two ways of doing this:
 *  - The first way (recommended) is to use the ssh_is_server_known()
 *    function. This function will look into the known host file
 *    (~/.ssh/known_hosts on UNIX), look for the server hostname's pattern,
 *    and determine whether this host is present or not in the list.
 *  - The second way is to use ssh_get_pubkey_hash() to get a binary version
 *    of the public key hash value. You can then use your own database to check
 *    if this public key is known and secure.
 *
 * You can also use the ssh_get_pubkey_hash() to show the public key hash
 * value to the user, in case he knows what the public key hash value is
 * (some paranoid people write their public key hash values on paper before
 * going abroad, just in case ...).
 *
 * If the remote host is being used to for the first time, you can ask the user whether
 * he/she trusts it. Once he/she concluded that the host is valid and worth being
 * added in the known hosts file, you use ssh_write_knownhost() to register it in
 * the known hosts file, or any other way if you use your own database.
 *
 * The following example is part of the examples suite available in the
 * examples/ directory:
 *
 * @code
 * int verify_knownhost(ssh_session session)
 * {
 *   int state, hlen;
 *   unsigned char *hash = NULL;
 *   char *hexa;
 *   char buf[10];
 *
 *   state = ssh_is_server_known(session);
 *
 *   hlen = ssh_get_pubkey_hash(session, &hash);
 *   if (hlen < 0)
 *     return -1;
 *
 *   switch (state)
 *   {
 *     case SSH_SERVER_KNOWN_OK:
 *       break; /* ok */
 *
 *     case SSH_SERVER_KNOWN_CHANGED:
 *       fprintf(stderr, "Host key for server changed: it is now:\n");
 *       ssh_print_hexa("Public key hash", hash, hlen);
 *       fprintf(stderr, "For security reasons, connection will be stopped\n");
 *       free(hash);
 *       return -1;
 *
 *     case SSH_SERVER_FOUND_OTHER:
 *       fprintf(stderr, "The host key for this server was not found but an other"
 *         "type of key exists.\n");
 *       fprintf(stderr, "An attacker might change the default server key to"
 *         "confuse your client into thinking the key does not exist\n"
 *       free(hash);
 *       return -1;
 *
 *     case SSH_SERVER_FILE_NOT_FOUND:
 *       fprintf(stderr, "Could not find known host file.\n");
 *       fprintf(stderr, "If you accept the host key here, the file will be"
 *        "automatically created.\n");
 *       /* fallback to SSH_SERVER_NOT_KNOWN behavior */
 *
 *     case SSH_SERVER_NOT_KNOWN:
 *       hexa = ssh_get_hexa(hash, hlen);
 *       fprintf(stderr,"The server is unknown. Do you trust the host key?\n");
 *       fprintf(stderr, "Public key hash: %s\n", hexa);
 *       free(hexa);
 *       if (fgets(buf, sizeof(buf), stdin) == NULL)
 *       {
 *         free(hash);
 *         return -1;
 *       }
 *       if (strncasecmp(buf, "yes", 3) != 0)
 *       {
 *         free(hash);
 *         return -1;
 *       }
 *       if (ssh_write_knownhost(session) < 0)
 *       {
 *         fprintf(stderr, "Error %s\n", strerror(errno));
 *         free(hash);
 *         return -1;
 *       }
 *       break;
 *
 *     case SSH_SERVER_ERROR:
 *       fprintf(stderr, "Error %s", ssh_get_error(session));
 *       free(hash);
 *       return -1;
 *   }
 *
 *   free(hash);
 *   return 0;
 * }
 * @endcode
 *
 * @see ssh_connect
 * @see ssh_disconnect
 * @see ssh_get_error
 * @see ssh_get_error_code
 * @see ssh_get_pubkey_hash
 * @see ssh_is_server_known
 * @see ssh_write_knownhost
 *
 *
 * @subsection auth Authenticating yourself
 *
 * The authentication process is the way a service provider can identify a
 * user and verify his/her identity. The authorization process is about enabling
 * the authenticated user the access to ressources. In SSH, the two concepts
 * are linked. After authentication, the server can grant the user access to
 * several ressources such as port forwarding, shell, sftp subsystem, and so on.
 *
 * libssh supports several methods of authentication:
 *  - "none" method. This method allows to get the available authentications
 *    methods. It also gives the server a chance to authenticate the user with
 *    just his/her login. Some very old hardware uses this feature to fallback
 *    the user on a "telnet over SSH" style of login.
 *  - password method. A password is sent to the server, which accepts it or not.
 *  - keyboard-interactive method. The server sends several challenges to the
 *    user, who must answer correctly. This makes possible the authentication
 *    via a codebook for instance ("give code at 23:R on page 3").
 *  - public key method. The host knows the public key of the user, and the
 *    user must prove he knows the associated private key. This can be done
 *    manually, or delegated to the SSH agent as we'll see later.
 *
 * All these methods can be combined. You can for instance force the user to
 * authenticate with at least two of the authentication methods. In that case,
 * one speaks of "Partial authentication". A partial authentication is a
 * response from authentication functions stating that your credential was
 * accepted, but yet another one is required to get in.
 *
 * The example below shows an authentication with password:
 *
 * @code
 * #include <libssh/libssh.h>
 * #include <stdlib.h>
 * #include <stdio.h>
 *
 * int main()
 * {
 *   ssh_session my_ssh_session;
 *   int ret;
 *   char *password;
 *
 *   // Open session and set options
 *   my_ssh_session = ssh_new();
 *   if (my_ssh_session == NULL)
 *     exit(-1);
 *   ssh_options_set(my_ssh_session, SSH_OPTIONS_HOST, "localhost");
 *
 *   // Connect to server
 *   ret = ssh_connect(my_ssh_session);
 *   if (ret != SSH_OK)
 *   {
 *     fprintf(stderr, "Error connecting to localhost: %s\n",
 *             ssh_get_error(my_ssh_session));
 *     ssh_free(my_ssh_session);
 *     exit(-1);
 *   }
 *
 *   // Verify the server's identity
 *   // For the source code of verify_knowhost(), check previous example
 *   if (verify_knownhost(my_ssh_session) < 0)
 *   {
 *     ssh_disconnect(my_ssh_session);
 *     ssh_free(my_ssh_session);
 *     exit(-1);
 *   }
 *
 *   // Authenticate ourselves
 *   password = getpass("Password: ");
 *   ret = ssh_userauth_password(my_ssh_session, NULL, password);
 *   if (ret != SSH_AUTH_SUCCESS)
 *   {
 *     fprintf(stderr, "Error authenticating with password: %s\n",
 *             ssh_get_error(my_ssh_session));
 *     ssh_disconnect(my_ssh_session);
 *     ssh_free(my_ssh_session);
 *     exit(-1);
 *   }
 *
 *   ...
 *
 *   ssh_disconnect(my_ssh_session);
 *   ssh_free(my_ssh_session);
 * }
 * @endcode
 *
 * @see @ref authentication_details
 *
 *
 * @subsection using_ssh Doing something
 *
 * At this point, the authenticity of both server and client is established.
 * Time has come to take advantage of the many possibilities offered by the SSH
 * protocol: execute remote commands, open remote shells, transfer files,
 * forward ports, etc.
 *
 * The example below puts the final touch to our step-by-step discovery
 * of a SSH connection:
 *
 * @code
 * /* *** To be replaced with something simpler *** */
 * #include <sys/stat.h>
 *
 * int scp_helloworld(ssh_session session)
 * {
 *   ssh_scp scp_session;
 *   int ret;
 *   const char *helloworld = "Hello, world!\n";
 *   int length = strlen(helloworld);
 *
 *   // Open a scp session
 *   scp_session = ssh_scp_new
 *     (session, SSH_SCP_WRITE | SSH_SCP_RECURSIVE, ".");
 *   if (scp_session == NULL)
 *   {
 *     fprintf(stderr, "Error allocating scp session.\n");
 *     return SSH_ERROR;
 *   }
 *
 *   // Initialize the scp session
 *   ret = ssh_scp_init(scp_session);
 *   if (ret != SSH_OK)
 *   {
 *     fprintf(stderr, "Error initializing scp session.\n");
 *     ssh_scp_free(scp_session);
 *     return ret;
 *   }
 *
 *   // Open the remote file
 *   ret = ssh_scp_push_file
 *     (scp_session, "helloworld.txt", length, S_IRUSR |  S_IWUSR);
 *   if (ret != SSH_OK)
 *   {
 *     fprintf(stderr, "Can't open remote file.\n");
 *     ssh_scp_close(scp_session);
 *     ssh_scp_free(scp_session);
 *     return ret;
 *   }
 *
 *   // Write the data into the remote file
 *   ret = ssh_scp_write(scp_session, helloworld, length);
 *   if (ret != SSH_OK)
 *   {
 *     fprintf(stderr, "Can't write to remote file.\n");
 *     ssh_scp_close(scp_session);
 *     ssh_scp_free(scp_session);
 *     return ret;
 *   }
 *
 *   ssh_scp_close(scp_session);
 *   ssh_scp_free(scp_session);
 *   return ret;
 * }
 * @endcode
 *
 * @see @ref opening_shell
 * @see @ref remote_commands
 * @see @ref sftp_subsystem
 * @see @ref scp_subsystem
 *
 *
 * @page details Chapter 2: A deeper insight on authentication
 * @section authentication_details A deeper insight on authentication
 *
 *
 * @subsection pubkeys Authenticating using public keys
 *
 * The public key authentication is the only method that does not compromise
 * your key if the remote host has been compromised (the server can't do
 * anything more than getting your public key). This is not the
 * case of a password authentication (the server can get your plaintext
 * password). On the other hand, if the client machine is compromised and
 * the private key has no passphrase, then the attacker has gained automatic
 * access to your server. It is not the purpose of this document to do
 * a detailed review of the advantages and drawbacks of each authentication
 * method, so refer to the abundant documentation on this topic on the
 * Internet to fully understand the advantages and risks linked to each method.
 *
 * libssh is fully compatible with the openssh public and private keys. You
 * can either use the automatic public key authentication method provided by
 * libssh, or roll your own using the public key functions.
 *
 * The process of authenticating by public key to a server is the following:
 *  - you scan a list of files that contain public keys. each key is sent to
 *    the SSH server, until the server acknowledges a key (a key it knows can be
 *    used to authenticate the user).
 *  - then, you retrieve the private key for this key and send a message
 *    proving that you know that private key.
 *
 * The function ssh_userauth_autopubkey() does this using the available keys in
 * "~/.ssh/".  The return values are the following:
 *  - SSH_AUTH_ERROR: some serious error happened during authentication
 *  - SSH_AUTH_DENIED: no key matched
 *  - SSH_AUTH_SUCCESS: you are now authenticated
 *  - SSH_AUTH_PARTIAL: some key matched but you still have to provide an other
 *                       mean of authentication (like a password).
 *
 * The ssh_userauth_autopubkey function also tries to authenticate using the
 * SSH agent, if you have one running, or the "none" method otherwise.
 *
 * If you wish to authenticate with public key by your own, follow these steps:
 *  - Retrieve the public key in a ssh_string using publickey_from_file().
 *  - Offer the public key to the SSH server using ssh_userauth_offer_pubkey().
 *    If the return value is SSH_AUTH_SUCCESS, the SSH server accepts to
 *    authenticate using the public key and you can go to the next step.
 *  - Retrieve the private key, using the privatekey_from_file() function. If
 *    a passphrase is needed, either the passphrase specified as argument or
 *    a callback (see callbacks section) will be used.
 *  - Authenticate using ssh_userauth_pubkey() with your public key string
 *    and private key.
 *  - Do not forget cleaning up memory using string_free() and privatekey_free().
 *
 * Here is a minimalistic example of public key authentication:
 *
 * @code
 * int authenticate_pubkey(ssh_session session)
 * {
 *   int rc;
 *
 *   rc = ssh_userauth_autopubkey(session, NULL);
 *
 *   if (rc == SSH_AUTH_ERROR)
 *   {
 *      fprintf(stderr, "Authentication failed: %s\n",
 *        ssh_get_error(session));
 *      return SSH_AUTH_ERROR;
 *   }
 *
 *   return rc;
 * }
 * @endcode
 *
 * @see ssh_userauth_autopubkey
 * @see ssh_userauth_offer_pubkey
 * @see ssh_userauth_pubkey
 * @see publickey_from_file
 * @see publickey_from_privatekey
 * @see string_free
 * @see privatekey_from_file
 * @see privatekey_free
 *
 *
 * @subsection password Authenticating using a password
 *
 * The function ssh_userauth_password() serves the purpose of authenticating
 * using a password. It will return SSH_AUTH_SUCCESS if the password worked,
 * or one of other constants otherwise. It's your work to ask the password
 * and to deallocate it in a secure manner.
 *
 * If your server complains that the password is wrong, but you can still
 * authenticate using openssh's client (issuing password), it's probably
 * because openssh only accept keyboard-interactive. Switch to
 * keyboard-interactive authentication, or try to configure plain text passwords
 * on the SSH server.
 *
 * Here is a small example of password authentication:
 *
 * @code
 * int authenticate_password(ssh_session session)
 * {
 *   char *password;
 *   int rc;
 *
 *   password = getpass("Enter your password: ");
 *   rc = ssh_userauth_password(session, NULL, password);
 *   if (rc == SSH_AUTH_ERROR)
 *   {
 *      fprintf(stderr, "Authentication failed: %s\n",
 *        ssh_get_error(session));
 *      return SSH_AUTH_ERROR;
 *   }
 *
 *   return rc;
 * }
 * @endcode
 *
 * @see ssh_userauth_password
 *
 *
 * @subsection keyb_int The keyboard-interactive authentication method
 *
 * The keyboard-interactive method is, as its name tells, interactive. The
 * server will issue one or more challenges that the user has to answer,
 * until the server takes an authentication decision.
 *
 * ssh_userauth_kbdint() is the the main keyboard-interactive function. 
 * It will return SSH_AUTH_SUCCESS,SSH_AUTH_DENIED, SSH_AUTH_PARTIAL,
 * SSH_AUTH_ERROR, or SSH_AUTH_INFO, depending on the result of the request.
 *
 * The keyboard-interactive authentication method of SSH2 is a feature that
 * permits the server to ask a certain number of questions in an interactive
 * manner to the client, until it decides to accept or deny the login.
 *
 * To begin, you call ssh_userauth_kbdint() (just set user and submethods to
 * NULL) and store the answer.
 *
 * If the answer is SSH_AUTH_INFO, it means that the server has sent a few
 * questions that you should ask the user. You can retrieve these questions
 * with the following functions: ssh_userauth_kbdint_getnprompts(),
 * ssh_userauth_kbdint_getname(), ssh_userauth_kbdint_getinstruction(), and
 * ssh_userauth_kbdint_getprompt().
 *
 * Set the answer for each question in the challenge using
 * ssh_userauth_kbdint_setanswer().
 *
 * Then, call again ssh_userauth_kbdint() and start the process again until
 * these functions returns something else than SSH_AUTH_INFO.
 *
 * Here are a few remarks:
 *  - Even the first call can return SSH_AUTH_DENIED or SSH_AUTH_SUCCESS.
 *  - The server can send an empty question set (this is the default behavior
 *    on my system) after you have sent the answers to the first questions.
 *    You must still parse the answer, it might contain some
 *    message from the server saying hello or such things. Just call
 *    ssh_userauth_kbdint() until needed.
 *  - The meaning of "name", "prompt", "instruction" may be a little
 *    confusing. An explanation is given in the RFC section that follows.
 *
 * Here is a little note about how to use the information from
 * keyboard-interactive authentication, coming from the RFC itself (rfc4256):
 *
 * @verbatim
 *
 *  3.3 User Interface Upon receiving a request message, the client SHOULD
 *  prompt the user as follows: A command line interface (CLI) client SHOULD
 *  print the name and instruction (if non-empty), adding newlines. Then for
 *  each prompt in turn, the client SHOULD display the prompt and read the
 *  user input.
 *
 *  A graphical user interface (GUI) client has many choices on how to prompt
 *  the user. One possibility is to use the name field (possibly prefixed
 *  with the application's name) as the title of a dialog window in which
 *  the prompt(s) are presented. In that dialog window, the instruction field
 *  would be a text message, and the prompts would be labels for text entry
 *  fields. All fields SHOULD be presented to the user, for example an
 *  implementation SHOULD NOT discard the name field because its windows lack
 *  titles; it SHOULD instead find another way to display this information. If
 *  prompts are presented in a dialog window, then the client SHOULD NOT
 *  present each prompt in a separate window.
 *
 *  All clients MUST properly handle an instruction field with embedded
 *  newlines. They SHOULD also be able to display at least 30 characters for
 *  the name and prompts. If the server presents names or prompts longer than 30
 *  characters, the client MAY truncate these fields to the length it can
 *  display. If the client does truncate any fields, there MUST be an obvious
 *  indication that such truncation has occured.
 *
 *  The instruction field SHOULD NOT be truncated. Clients SHOULD use control
 *  character filtering as discussed in [SSH-ARCH] to avoid attacks by
 *  including terminal control characters in the fields to be displayed.
 *
 *  For each prompt, the corresponding echo field indicates whether or not
 *  the user input should be echoed as characters are typed. Clients SHOULD
 *  correctly echo/mask user input for each prompt independently of other
 *  prompts in the request message. If a client does not honor the echo field
 *  for whatever reason, then the client MUST err on the side of
 *  masking input. A GUI client might like to have a checkbox toggling
 *  echo/mask. Clients SHOULD NOT add any additional characters to the prompt
 *  such as ": " (colon-space); the server is responsible for supplying all
 *  text to be displayed to the user. Clients MUST also accept empty responses
 *  from the user and pass them on as empty strings.
 * @endverbatim
 *
 * The following example shows how to perform keyboard-interactive authentication:
 *
 * @code
 * int authenticate_kbdint(ssh_session session)
 * {
 *   int rc;
 *
 *   rc = ssh_userauth_kbdint(session, NULL, NULL);
 *   while (rc == SSH_AUTH_INFO)
 *   {
 *     const char *name, *instruction;
 *     int nprompts, iprompt;
 *
 *     name = ssh_userauth_kbdint_getname(session);
 *     instruction = ssh_userauth_kbdint_getinstruction(session);
 *     nprompts = ssh_userauth_kbdint_getnprompts(session);
 *
 *     if (strlen(name) > 0)
 *       printf("%s\n", name);
 *     if (strlen(instruction) > 0)
 *       printf("%s\n", instruction);
 *     for (iprompt = 0; iprompt < nprompts; iprompt++)
 *     {
 *       const char *prompt;
 *       char echo;
 *
 *       prompt = ssh_userauth_kbdint_getprompt(session, iprompt, &echo);
 *       if (echo)
 *       {
 *         char buffer[128], *ptr;
 *
 *         printf("%s", prompt);
 *         if (fgets(buffer, sizeof(buffer), stdin) == NULL)
 *           return SSH_AUTH_ERROR;
 *         buffer[sizeof(buffer) - 1] = '\0';
 *         if ((ptr = strchr(buffer, '\n')) != NULL)
 *           *ptr = '\0';
 *         if (ssh_userauth_kbdint_setanswer(session, iprompt, buffer) < 0)
 *           return SSH_AUTH_ERROR;
 *         memset(buffer, 0, strlen(buffer));
 *       }
 *       else
 *       {
 *         char *ptr;
 *
 *         ptr = getpass(prompt);
 *         if (ssh_userauth_kbdint_setanswer(session, iprompt, ptr) < 0)
 *           return SSH_AUTH_ERROR;
 *       }
 *     }
 *     rc = ssh_userauth_kbdint(session, NULL, NULL);
 *   }
 *   return rc;
 * }
 * @endcode
 *
 * @see ssh_userauth_kbdint()
 * @see ssh_userauth_kbdint_getnprompts
 * @see ssh_userauth_kbdint_getname 
 * @see ssh_userauth_kbdint_getinstruction
 * @see ssh_userauth_kbdint_getprompt
 * @see ssh_userauth_kbdint_setanswer()
 *
 *
 * @subsection none Authenticating with "none" method
 *
 * The primary purpose of the "none" method is to get authenticated **without**
 * any credential. Don't do that, use one of the other authentication methods,
 * unless you really want to grant anonymous access.
 *
 * If the account has no password, and if the server is configured to let you
 * pass, ssh_userauth_none() might answer SSH_AUTH_SUCCESS.
 *
 * The following example shows how to perform "none" authentication:
 *
 * @code
 * int authenticate_kbdint(ssh_session session)
 * {
 *   int rc;
 *
 *   rc = ssh_userauth_none(session, NULL, NULL);
 *   return rc;
 * }
 * @endcode
 *
 * @subsection auth_list Getting the list of supported authentications
 *
 * You are not meant to choose a given authentication method, you can
 * let the server tell you which methods are available. Once you know them,
 * you try them one after the other.
 *
 * The following example shows how to get the list of available authentication
 * methods with ssh_userauth_list() and how to use the result:
 *
 * @code
 * /* Marche pas sans ssh_userauth_none(), du moins en 0.4.2 */
 * int test_several_auth_methods(ssh_session session)
 * {
 *   int method, rc;
 *
 *   method = ssh_userauth_list(session, NULL);
 *
 *   if (method & SSH_AUTH_METHOD_NONE)
 *   { // For the source code of function authenticate_none(),
 *     // refer to the corresponding example
 *     rc = authenticate_none(session);
 *     if (rc == SSH_AUTH_SUCCESS) return rc;
 *   }
 *   if (method & SSH_AUTH_METHOD_PUBLICKEY)
 *   { // For the source code of function authenticate_pubkey(),
 *     // refer to the corresponding example
 *     rc = authenticate_pubkey(session);
 *     if (rc == SSH_AUTH_SUCCESS) return rc;
 *   }
 *   if (method & SSH_AUTH_METHOD_INTERACTIVE)
 *   { // For the source code of function authenticate_kbdint(),
 *     // refer to the corresponding example
 *     rc = authenticate_kbdint(session);
 *     if (rc == SSH_AUTH_SUCCESS) return rc;
 *   }
 *   if (method & SSH_AUTH_METHOD_PASSWORD)
 *   { // For the source code of function authenticate_password(),
 *     // refer to the corresponding example
 *     rc = authenticate_password(session);
 *     if (rc == SSH_AUTH_SUCCESS) return rc;
 *   }
 *   return SSH_AUTH_ERROR;
 * }
 * @endcode
 *
 *
 * @subsection banner
 *
 * The SSH server might send a banner, which you can retrieve with
 * ssh_get_issue_banner(), then display to the user.
 *
 * The following example shows how to retrieve and dispose the issue banner:
 *
 * @code
 * /* Marche pas sans ssh_userauth_none(), du moins en 0.4.2 */
 * int display_banner(ssh_session session)
 * {
 *   int rc;
 *   char *banner;
 *
 *   rc = ssh_userauth_none(session, NULL);
 *   if (rc == SSH_AUTH_ERROR)
 *     return rc;
 *
 *   banner = ssh_get_issue_banner(session);
 *   if (banner)
 *   {
 *     printf("%s\n", banner);
 *     free(banner);
 *   }
 *
 *   return rc;
 * }
 * @endcode
 *
 *
 * @page tbd Chapter 3: To be done
 * @section opening_shell Opening a shell
 *
 * *** To be written ***
 *
 * @section remote_commands Passing remote commands
 *
 * *** To be written ***
 *
 * @section sftp_subsystem The SFTP subsystem
 *
 * *** To be written ***
 *
 * @section scp_subsystem The SCP subsystem
 *
 * *** To be written ***
 *
 * @section threads Working with threads
 *
 * *** To be written ***
 *
 * @section forwarding_connections Forwarding connections
 *
 * *** To be written ***
 *
 */