1.1 SASL Overview

SASL is a framework for application protocols, such as SMTP or IMAP, to add authentication support. For example, SASL is used to prove to the server who you are when you access an IMAP server to read your e-mail.

The SASL framework does not specify the technology used to perform the authentication, that is the responsibility for each SASL mechanism. Popular SASL mechanisms include CRAM-MD5 and GSSAPI (for Kerberos V5).

Typically a SASL negotiation works as follows. First the client requests authentication (possibly implicitly by connecting to the server). The server responds with a list of supported mechanisms. The client chose one of the mechanisms. The client and server then exchange data, one round-trip at a time, until authentication either succeeds or fails. After that, the client and server knows more about who is on the other end of the channel.

For example, in SMTP communication happens like this:

250-mail.example.com Hello pc.example.org [192.168.1.42], pleased to meet you
250-AUTH DIGEST-MD5 CRAM-MD5 LOGIN PLAIN
250 HELP
AUTH CRAM-MD5
334 PDk5MDgwNDEzMDUwNTUyMTE1NDQ5LjBAbG9jYWxob3N0Pg==
amFzIDBkZDRkODZkMDVjNjI4ODRkYzc3OTcwODE4ZGI5MGY3
235 2.0.0 OK Authenticated

Here the first three lines are sent by the server and contains the list of supported mechanisms (DIGEST-MD5, CRAM-MD5, etc). The next line is sent by the client to select the CRAM-MD5 mechanism. The server replies with a challenge, which is a message that can be generated by calling GNU SASL functions. The client replies with a response, which also is a message that can be generated by GNU SASL functions. Depending on the mechanism, there can be more than one round trip, so do not assume all authentication exchanges consists of one message from the server and one from the client. The server accepts the authentication. At that point it knows it is talking to a authenticated client, and the application protocol can continue.

Essentially, your application is responsible for implementing the framing protocol (e.g., SMTP or XMPP) according to the particular specifications. Your application uses GNU SASL to generate the authentication messages.

1.2 Implementation

GNU SASL is an implementation of the Simple Authentication and Security Layer framework and a few common SASL mechanisms.

GNU SASL consists of a library (libgsasl), a command line utility (gsasl) to access the library from the shell, and a manual. The library includes support for the framework (with authentication functions and application data privacy and integrity functions) and at least partial support for the ANONYMOUS, CRAM-MD5, DIGEST-MD5, EXTERNAL, GS2-KRB5, GSSAPI, LOGIN, NTLM, PLAIN, SCRAM-SHA-1, SCRAM-SHA-1-PLUS, SCRAM-SHA-256, SCRAM-SHA-256-PLUS, SAML20, OPENID20, and SECURID mechanisms.

The library is easily ported because it does not do network communication by itself, but rather leaves it up to the calling application. The library is flexible with regards to the authorization infrastructure used, as it utilizes a callback into the application to decide whether a user is authorized or not.

GNU SASL is developed for the GNU/Linux system, but runs on over 20 platforms including most major Unix platforms and Windows, and many kind of devices including iPAQ handhelds and S/390 mainframes.

GNU SASL is written in pure ANSI C89 to be portable to embedded and otherwise limited platforms. The entire library, with full support for ANONYMOUS, EXTERNAL, PLAIN, LOGIN and CRAM-MD5, and the front-end that supports client and server mode, and the IMAP and SMTP protocols, fits in under 80kb on an Intel x86 platform, without any modifications to the code. (This figure was accurate as of version 1.1.)

The design of the library and the intended interaction between applications and the library through the official API is illustrated below.

Illustration 1.1: Logical overview showing how applications use authentication mechanisms through an abstract interface.

1.3 Features

GNU SASL might have a couple of advantages over other libraries doing a similar job.

It’s Free Software

Anybody can use, modify, and redistribute it under the terms of the GNU General Public License version 3.0 or later. The library uses the GNU Lesser General Public License version 2.1 or later.

It’s thread-safe

No global variables are used and multiple library handles and session handles may be used in parallel.

It’s internationalized

It handles non-ASCII usernames and passwords and user visible strings used in the library (error messages) can be translated into the users’ language.

It’s portable

It should work on all Unix like operating systems, including Windows. The library itself should be portable to any C89 system, not even POSIX is required.

It’s small

The library has been rewritten with embedded platforms in mind. For example, no API consumes more than around 250 bytes of stack space.

Note that the library does not implement any policy to decide whether a certain user is “authenticated” or “authorized” or not. Rather, it uses a callback into the application to answer these questions.

1.4 Requirements

The GNU SASL library does not have any required external dependencies, but some optional features are enabled if you have a specific external library.

LibNTLM

The NTLM mechanism requires the library LibNTLM, http://www.nongnu.org/libntlm/.

GSS-API

The GSSAPI and GS2-KRB5 mechanisms requires a GSS-API library, see GNU GSS (http://www.gnu.org/software/gss/). MIT Kerberos or Heimdal are also supported.

LibIDN

Processing of non-ASCII usernames and passwords requires the SASLprep implementation in LibIDN (http://www.gnu.org/software/libidn/). This is needed for full conformance with the latest SASL protocol drafts, but is optional in the library for improved portability.

Libgcrypt

The GNU SASL library ships with its own cryptographic implementation, but it can use the one in libgcrypt (http://www.gnupg.org/) instead, if it is available. This is typically useful for desktop machines which have libgcrypt installed.

The command-line interface to GNU SASL requires a POSIX or Windows platform for network connectivity. The command-line tool can make use of GnuTLS (http://www.gnutls.org/) to support the STARTTLS modes of IMAP and SMTP, but GnuTLS is not required.

Note that the library does not need a POSIX platform or network connectivity.

1.5 Supported Platforms

GNU SASL has at some point in time been tested on the following platforms.

1. Debian GNU/Linux

   GCC and GNU Make. This is the main development platform. x86_64-linux-gnu, alphaev67-unknown-linux-gnu, alphaev6-unknown-linux-gnu, arm-unknown-linux-gnu, hppa-unknown-linux-gnu, hppa64-unknown-linux-gnu, i686-pc-linux-gnu, ia64-unknown-linux-gnu, m68k-unknown-linux-gnu, mips-unknown-linux-gnu, mipsel-unknown-linux-gnu, powerpc-unknown-linux-gnu, s390-ibm-linux-gnu, sparc-unknown-linux-gnu, armv4l-unknown-linux-gnu.

2. Tru64 UNIX

   Tru64 UNIX C compiler and Tru64 Make. alphaev67-dec-osf5.1, alphaev68-dec-osf5.1.

3. SuSE Linux 7.1

   GCC 2.96 and GNU Make. alphaev6-unknown-linux-gnu, alphaev67-unknown-linux-gnu.

4. SuSE Linux 7.2a

   GCC 3.0 and GNU Make. ia64-unknown-linux-gnu.

5. RedHat Linux 7.2

   GCC 2.96 and GNU Make. alphaev6-unknown-linux-gnu, alphaev67-unknown-linux-gnu, ia64-unknown-linux-gnu.

6. RedHat Linux 8.0

   GCC 3.2 and GNU Make. i686-pc-linux-gnu.

7. RedHat Advanced Server 2.1

   GCC 2.96 and GNU Make. i686-pc-linux-gnu.

8. Slackware Linux 8.0.01

   GCC 2.95.3 and GNU Make. i686-pc-linux-gnu.

9. Mandrake Linux 9.0

   GCC 3.2 and GNU Make. i686-pc-linux-gnu.

10. IRIX 6.5

    MIPS C compiler, IRIX Make. mips-sgi-irix6.5.

11. AIX 4.3.2

    IBM C for AIX compiler, AIX Make. rs6000-ibm-aix4.3.2.0.

12. Microsoft Windows 2000 (Cygwin)

    GCC 3.2, GNU make. i686-pc-cygwin.

13. HP-UX 11

    HP-UX C compiler and HP Make. ia64-hp-hpux11.22, hppa2.0w-hp-hpux11.11.

14. SUN Solaris 2.8

    Sun WorkShop Compiler C 6.0 and SUN Make. sparc-sun-solaris2.8.

15. SUN Solaris 2.9

    Sun Forte Developer 7 C compiler and GNU Make. sparc-sun-solaris2.9.

16. NetBSD 1.6

    GCC 2.95.3 and GNU Make. alpha-unknown-netbsd1.6, i386-unknown-netbsdelf1.6.

17. OpenBSD 3.1 and 3.2

    GCC 2.95.3 and GNU Make. alpha-unknown-openbsd3.1, i386-unknown-openbsd3.1.

18. FreeBSD 4.7

    GCC 2.95.4 and GNU Make. alpha-unknown-freebsd4.7, i386-unknown-freebsd4.7.

19. Cross compiled to uClinux/uClibc on Motorola Coldfire.

    GCC 3.4 and GNU Make m68k-uclinux-elf.

If you port GNU SASL to a new platform, please report it to the author so this list can be updated.

1.6 Getting help

A mailing list where users may help each other exists, and you can reach it by sending e-mail to help-gsasl@gnu.org. Archives of the mailing list discussions, and an interface to manage subscriptions, is available through the World Wide Web at http://lists.gnu.org/mailman/listinfo/help-gsasl/.

1.7 Commercial Support

Commercial support is available for users of GNU SASL. The kind of support that can be purchased may include:

• Implement new features. Such as a new SASL mechanism.
• Port GNU SASL to new platforms. This could include porting to an embedded platforms that may need memory or size optimization.
• Integrating SASL as a security environment in your existing project.
• System design of components related to SASL.

The following companies have expressed an interest in providing this support:

• Simon Josefsson Datakonsult AB

  Contact through https://josefsson.org/ and email simon@josefsson.org.

If your company provides support related to GNU SASL and would like to be mentioned here, please let us know (see Bug Reports).

1.8 Downloading and Installing

The package can be downloaded from several places, including:

ftp://ftp.gnu.org/gnu/gsasl/

The latest version is stored in a file, e.g., ‘gsasl-1.10.0.tar.gz’ where the ‘1.10.0’ value is the highest version number in the directory.

The package is then extracted, configured and built like many other packages that use Autoconf. For detailed information on configuring and building it, refer to the INSTALL file that is part of the distribution archive.

Here is an example terminal session that downloads, configures, builds and install the package. You will need a few basic tools, such as ‘sh’, ‘make’ and ‘cc’.

$ wget -q ftp://ftp.gnu.org/gnu/gsasl/gsasl-1.10.0.tar.gz
$ tar xfz gsasl-1.10.0.tar.gz
$ cd gsasl-1.10.0/
$ ./configure
...
$ make
...
$ make install
...

After that gsasl should be properly installed and ready for use.

A few configure options may be relevant, summarized in the table.

--disable-client

--disable-server

If your target system require a minimal implementation, you may wish to disable the client or the server part of the code. This does not remove symbols from the library, so if you attempt to call an application that uses server functions in a library built with --disable-server, the function will return an error code.

--disable-obsolete

Remove backwards compatibility (see Old Functions). Use if you want to limit the size of the library.

--disable-anonymous

--disable-external

--disable-plain

--disable-login

--disable-securid

--disable-ntlm

--disable-cram-md5

--disable-digest-md5

--disable-gssapi

--disable-gs2

--enable-kerberos_v5

--disable-scram-sha1

--disable-scram-sha256

--disable-saml20

--disable-openid20

Disable or enable individual mechanisms (see Mechanisms).

--without-stringprep

Disable internationalized string processing. Note that this will result in a SASL library that is compatible with RFC 2222 but not RFC 4422.

For the complete list, refer to the output from configure --help.

• Installing under Windows
• Kerberos on Windows

$ mkdir ~/kfw
$ cd ~/kfw
$ wget -q http://web.mit.edu/kerberos/dist/kfw/3.2/kfw-3.2.2/kfw-3-2-2-sdk.zip
$ unzip kfw-3-2-2-sdk.zip

perl -pi -e 's,sys\\,sys/,' ~/kfw/kfw-3-2-2-final/inc/krb5/win-mac.h

$ wget -q ftp://alpha.gnu.org/gnu/gsasl/gsasl-1.10.0.tar.gz
$ tar xfz gsasl-1.10.0.tar.gz
$ cd gsasl-1.10.0

$ lt_cv_deplibs_check_method=pass_all ./configure --host=i586-mingw32msvc --build=i686-pc-linux-gnu --with-gssapi-impl=kfw LDFLAGS="-L$HOME/kfw/kfw-3-2-2-final/lib/i386" CPPFLAGS="-I$HOME/kfw/kfw-3-2-2-final/inc/krb5 -DSSIZE_T_DEFINED"

$ make

$ make check

err:module:import_dll Library gssapi32.dll (which is needed by L"Z:\\home\\jas\\src\\gsasl-1.5.2\\lib\\src\\.libs\\libgsasl-7.dll") not found

lib/src/.libs/libgsasl-7.dll
src/.libs/gsasl.exe

gsasl.exe -d interop.josefsson.org

1.9 Bug Reports

If you think you have found a bug in GNU SASL, please investigate it and report it.

• Please make sure that the bug is really in GNU SASL, and preferably also check that it hasn’t already been fixed in the latest version.
• You have to send us a test case that makes it possible for us to reproduce the bug.
• You also have to explain what is wrong; if you get a crash, or if the results printed are not good and in that case, in what way. Make sure that the bug report includes all information you would need to fix this kind of bug for someone else.

Please make an effort to produce a self-contained report, with something definite that can be tested or debugged. Vague queries or piecemeal messages are difficult to act on and don’t help the development effort.

If your bug report is good, we will do our best to help you to get a corrected version of the software; if the bug report is poor, we won’t do anything about it (apart from asking you to send better bug reports).

If you think something in this manual is unclear, or downright incorrect, or if the language needs to be improved, please also send a note.

Send your bug report to:

1.10 Contributing

If you want to submit a patch for inclusion – from solve a typo you discovered, up to adding support for a new feature – you should submit it as a bug report (see Bug Reports). There are some things that you can do to increase the chances for it to be included in the official package.

Unless your patch is very small (say, under 10 lines) we require that you assign the copyright of your work to the Free Software Foundation. This is to protect the freedom of the project. If you have not already signed papers, we will send you the necessary information when you submit your contribution.

For contributions that doesn’t consist of actual programming code, the only guidelines are common sense. Use it.

For code contributions, a number of style guides will help you:

• Coding Style. Follow the GNU Standards document (see (standards)GNU Coding Standards).

  If you normally code using another coding standard, there is no problem, but you should use ‘indent’ to reformat the code (see (indent)GNU Indent) before submitting your work.

• Use the unified diff format ‘diff -u’.
• Return errors. No reason whatsoever should abort the execution of the library. Even memory allocation errors, e.g. when malloc return NULL, should work although result in an error code.
• Design with thread safety in mind. Don’t use global variables. Don’t even write to per-handle global variables unless the documented behaviour of the function you write is to write to the per-handle global variable.
• Avoid using the C math library. It causes problems for embedded implementations, and in most situations it is very easy to avoid using it.
• Document your functions. Use comments before each function headers, that, if properly formatted, are extracted into Texinfo manuals and GTK-DOC web pages.
• Supply a ChangeLog and NEWS entries, where appropriate.

2.1 Header

All interfaces (data types and functions) of the library are defined in the header file gsasl.h. You must include this in all programs using the library, either directly or through some other header file, like this:

#include <gsasl.h>

The name space is gsasl_* for function names, Gsasl* for data types and GSASL_* for other symbols. In addition the same name prefixes with one prepended underscore are reserved for internal use and should never be used by an application.

2.2 Initialization

The library must be initialized before it can be used. The library is initialized by calling gsasl_init (see Global Functions). The resources allocated by the initialization process can be released if the application no longer has a need to call ‘Libgsasl’ functions, this is done by calling gsasl_done. For example:

int
main (int argc, char *argv[])
{
  Gsasl *ctx = NULL;
  int rc;
...
  rc = gsasl_init (&ctx);
  if (rc != GSASL_OK)
    {
      printf ("SASL initialization failure (%d): %s\n",
              rc, gsasl_strerror (rc));
      return 1;
    }
...

In order to make error messages from gsasl_strerror be translated (see GNU Gettext) the application must set the current locale using setlocale before calling gsasl_init. For example:

int
main (int argc, char *argv[])
{
  Gsasl *ctx = NULL;
  int rc;
...
  setlocale (LC_ALL, "");
...
  rc = gsasl_init (&ctx);
  if (rc != GSASL_OK)
    {
      printf (gettext ("SASL initialization failure (%d): %s\n"),
              rc, gsasl_strerror (rc));
      return 1;
    }
...

In order to take advantage of the secure memory features in Libgcrypt¹, you need to initialize secure memory in your application, and for some platforms even make your application setuid root. See the Libgcrypt documentation for more information. Here is example code to initialize secure memory in your code:

#include <gcrypt.h>
...
int
main (int argc, char *argv[])
{
  Gsasl *ctx = NULL;
  int rc;
...
  /* Check version of libgcrypt. */
  if (!gcry_check_version (GCRYPT_VERSION))
    die ("version mismatch\n");

  /* Allocate a pool of 16k secure memory.  This also drops priviliges
     on some systems. */
  gcry_control (GCRYCTL_INIT_SECMEM, 16384, 0);

  /* Tell Libgcrypt that initialization has completed. */
  gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0);
...
  rc = gsasl_init (&ctx);
  if (rc != GSASL_OK)
    {
      printf ("SASL initialization failure (%d): %s\n",
              rc, gsasl_strerror (rc));
      return 1;
    }
...

If you do not do this, keying material will not be allocated in secure memory (which, for most applications, is not the biggest secure problem anyway). Note that the GNU SASL Library has not been audited to make sure it stores passwords or keys in secure memory.

2.3 Version Check

It is often desirable to check that the version of the library used is indeed one which fits all requirements. Even with binary compatibility, new features may have been introduced but, due to problem with the dynamic linker, an old version may actually be used. So you may want to check that the version is okay right after program startup.

gsasl_check_version

Function: const char * gsasl_check_version (const char * req_version) ¶

req_version: version string to compare with, or NULL.

Check GNU SASL Library version.

See GSASL_VERSION for a suitable req_version string.

This function is one of few in the library that can be used without a successful call to gsasl_init() .

Return value: Check that the version of the library is at minimum the one given as a string in req_version and return the actual version string of the library; return NULL if the condition is not met. If NULL is passed to this function no check is done and only the version string is returned.

The normal way to use the function is to put something similar to the following early in your main:

  if (!gsasl_check_version (GSASL_VERSION))
    {
      printf ("gsasl_check_version failed:\n"
              "Header file incompatible with shared library.\n");
      exit(1);
    }

2.4 Building the source

If you want to compile a source file including the gsasl.h header file, you must make sure that the compiler can find it in the directory hierarchy. This is accomplished by adding the path to the directory in which the header file is located to the compilers include file search path (via the -I option).

However, the path to the include file is determined at the time the source is configured. To solve this problem, the library uses the external package pkg-config that knows the path to the include file and other configuration options. The options that need to be added to the compiler invocation at compile time are output by the --cflags option to pkg-config libgsasl. The following example shows how it can be used at the command line:

gcc -c foo.c `pkg-config libgsasl --cflags`

Adding the output of ‘pkg-config libgsasl --cflags’ to the compiler command line will ensure that the compiler can find the gsasl.h header file.

A similar problem occurs when linking the program with the library. Again, the compiler has to find the library files. For this to work, the path to the library files has to be added to the library search path (via the -L option). For this, the option --libs to pkg-config libgsasl can be used. For convenience, this option also outputs all other options that are required to link the program with the library (for instance, the ‘-lidn’ option). The example shows how to link foo.o with the library to a program foo.

gcc -o foo foo.o `pkg-config libgsasl --libs`

Of course you can also combine both examples to a single command by specifying both options to pkg-config:

gcc -o foo foo.c `pkg-config libgsasl --cflags --libs`

2.5 Autoconf tests

If you work on a project that uses Autoconf (see (autoconf)GNU Autoconf) to help find installed libraries, the suggestions in the previous section are not the entire story. There are a few methods to detect and incorporate the GNU SASL Library into your Autoconf based package. The preferred approach, is to use Libtool in your project, and use the normal Autoconf header file and library tests.

• Autoconf test via ‘pkg-config’
• Standalone Autoconf test using Libtool

AC_ARG_ENABLE(gsasl,
  AC_HELP_STRING([--disable-gsasl], [don't use GNU SASL]),
  gsasl=$enableval)
if test "$gsasl" != "no" ; then
  PKG_CHECK_MODULES(GSASL, libgsasl >= 1.10.0,
    [gsasl=yes],
    [gsasl=no])
  if test "$gsasl" != "yes" ; then
    gsasl=no
    AC_MSG_WARN([Cannot find GNU SASL, disabling])
  else
    gsasl=yes
    AC_DEFINE(USE_GSASL, 1, [Define to 1 if you want GNU SASL.])
  fi
fi
AC_MSG_CHECKING([if GNU SASL should be used])
AC_MSG_RESULT($gsasl)

AC_CHECK_HEADER(gsasl.h,
  AC_CHECK_LIB(gsasl, gsasl_check_version,
    [gsasl=yes AC_SUBST(GSASL_LIBS, -lgsasl)],
    gsasl=no),
  gsasl=no)
AC_ARG_ENABLE(gsasl,
  AC_HELP_STRING([--disable-gsasl], [don't use GNU SASL]),
  gsasl=$enableval)
if test "$gsasl" != "no" ; then
  AC_DEFINE(USE_SASL, 1, [Define to 1 if you want GNU SASL.])
else
  AC_MSG_WARN([Cannot find GNU SASL, diabling])
fi
AC_MSG_CHECKING([if GNU SASL should be used])
AC_MSG_RESULT($gsasl)

3.1 Choosing a mechanism

Our earlier code was hard coded to use a specific mechanism. This is rarely a good idea. Instead, it is recommended to select the best mechanism available from the list of mechanisms supported by the server. Note that without TLS or similar, the list may have been maliciously altered, by an attacker. This means that you should abort if you cannot find any mechanism that exceeds your minimum security level. There is a function gsasl_client_suggest_mechanism (see Global Functions) that will try to pick the “best” available mechanism from a list of mechanisms. Our simple interactive example client (see Example 3) includes the following function to decide which mechanism to use. Note that the code doesn’t blindly use what is returned from gsasl_client_suggest_mechanism, rather it lets some logic (in this case the user, through an interactive query) decide which mechanism is acceptable.

const char *client_mechanism (Gsasl *ctx)
{
  static char mech[GSASL_MAX_MECHANISM_SIZE + 1] = "";
  char mechlist[BUFSIZ] = "";
  const char *suggestion;

  printf ("Enter list of server supported mechanisms, separate by SPC:\n");
  fgets (mechlist, sizeof (mechlist) - 1, stdin);

  suggestion = gsasl_client_suggest_mechanism (ctx, mechlist);
  if (suggestion)
    printf ("Library suggests use of `%s'.\n", suggestion);

  printf ("Enter mechanism to use:\n");
  fgets (mech, sizeof (mech) - 1, stdin);
  mech[strlen (mech) - 1] = '\0';

  return mech;
}

When running this example code, it might look like in the following output.

Enter list server supported mechanisms, separate by SPC:
CRAM-MD5 DIGEST-MD5 GSSAPI FOO BAR
Library suggests use of `GSSAPI'.
Enter mechanism to use:
CRAM-MD5
Input base64 encoded data from server:
Zm5vcmQ=
Output:
amFzIDkyY2U1NWE5MTM2ZTY4NzEyMTUyZTFjYmFmNjVkZjgx

If server accepted us, we're done.

3.2 Using a callback

Our earlier code specified the username and password before the authentication loop, as in:

gsasl_property_set (ctx, GSASL_AUTHID, "jas");
gsasl_property_set (ctx, GSASL_PASSWORD, "secret");

This may work for simple mechanisms, that need only a username and a password. But some mechanism requires more information, such as an authorization identity, a special PIN or passcode, a realm, a hostname, a service name, or an anonymous identifier. Querying the user for all that information, without knowing exactly which of it is really needed will result in a poor user interface. The user should not have to input private information, if it isn’t required.

The approach is a bad idea for another reason. What if the server aborts the authentication process? Then your application has already queried the user for a username and password. It would be better if you only asked the user for this information, annoying to input, when it is known to be needed.

A better approach to this problem is to use a callback. Then the mechanism may query your application whenever it needs some information, like the username and password. It will only do this at the precise step in the authentication when the information is actually needed. Further, if the user aborts, e.g., a password prompt, the mechanism is directly informed of this (because it invoked the callback), and could recover somehow.

Our final example (see Example 4) specifies a callback function, inside main as below.

/* Set the callback handler for the library. */
gsasl_callback_set (ctx, callback);

The function itself is implemented as follows.

int callback (Gsasl * ctx, Gsasl_session * sctx, Gsasl_property prop)
{
  char buf[BUFSIZ] = "";
  int rc = GSASL_NO_CALLBACK;

  /* Get user info from user. */

  printf ("Callback invoked, for property %d.\n", prop);

  switch (prop)
    {
    case GSASL_PASSCODE:
      printf ("Enter passcode:\n");
      fgets (buf, sizeof (buf) - 1, stdin);
      buf[strlen (buf) - 1] = '\0';

      gsasl_property_set (sctx, GSASL_PASSCODE, buf);
      rc = GSASL_OK;
      break;

    case GSASL_AUTHID:
      printf ("Enter username:\n");
      fgets (buf, sizeof (buf) - 1, stdin);
      buf[strlen (buf) - 1] = '\0';

      gsasl_property_set (sctx, GSASL_AUTHID, buf);
      rc = GSASL_OK;
      break;

    default:
      printf ("Unknown property!  Don't worry.\n");
      break;
    }

  return rc;
}

Again, it is bad style to use a fixed size buffer. Mmm’kay.

Which properties you should handle is up to you. If you don’t know how to respond to a certain property, simply return GSASL_NO_CALLBACK. The basic properties to support are authentication identity (GSASL_AUTHID), authorization identity (GSASL_AUTHZID), and password (GSASL_PASSWORD). See Properties, for the list of all properties, and what your callback should (ideally) do for them, and which properties each mechanism require in order to work.

5.1 The EXTERNAL mechanism

The EXTERNAL mechanism is used to authenticate a user to a server based on out-of-band authentication. EXTERNAL is typically used over TLS authenticated channels. Note that in the server, you need to make sure that TLS actually authenticated the client successfully and that the negotiated ciphersuite and other parameters are acceptable. It is generally not sufficient that TLS is used, since TLS supports anonymous and other variants that generally provide less assurance than you normally want.

In the client, this mechanism is always enabled, and it will send the GSASL_AUTHZID property as the authorization name to the server, if the property is set. If the property is not set, an empty authorization name is sent. You need not implement a callback.

In the server, this mechanism will request the GSASL_VALIDATE_EXTERNAL callback property to decide whether the client is authenticated and authorized to log in. Your callback can retrieve the GSASL_AUTHZID property to inspect the requested authorization name from the client.

The EXTERNAL mechanism was initially specified in the core SASL framework RFC 2222 and later revised in RFC 4422.

5.2 The ANONYMOUS mechanism

The ANONYMOUS mechanism is used to “authenticate” clients to anonymous services; or rather, just indicate that the client wishes to use the service anonymously. The client sends a token, usually her email address, which serve the purpose of some trace information suitable for logging. The token cannot be empty.

In the client, this mechanism is always enabled, and will send the GSASL_ANONYMOUS_TOKEN property as the trace information to the server.

In the server, this mechanism will invoke the GSASL_VALIDATE_ANONYMOUS callback to decide whether the client should be permitted to log in. Your callback can retrieve the GSASL_ANONYMOUS_TOKEN property to, for example, record it in a log file. The token is normally not used to decide whether the client should be permitted to log in or not.

The ANONYMOUS mechanism was initially specified in RFC 2245 and later revised in RFC 4505.

5.3 The PLAIN mechanism

The PLAIN mechanism uses username and password to authenticate users. Two user names are relevant. The first, the authentication identity, indicates the credential holder, i.e., whom the provided password belongs to. The second, the authorization identity, is typically empty to indicate that the user requests to log on to the server as herself (i.e., the authentication identity). If the authorization identity is not empty, the server should decide whether the authenticated user may log on as the authorization identity. This is typically used for super-user accounts like ‘admin’ to take on the role of a regular user.

In the client, this mechanism is always enabled, and require the GSASL_AUTHID and GSASL_PASSWORD properties. If set, GSASL_AUTHZID will also be used.

In the server, the mechanism is always enabled. Two approaches to authenticate and authorize the client are provided.

In the first approach, the server side of the mechanism will request the GSASL_VALIDATE_SIMPLE callback property to decide whether the client should be accepted or not. The callback may inspect the GSASL_AUTHID, GSASL_AUTHZID, and GSASL_PASSWORD properties. These property values will be normalized.

If the first approach fails (because there is no callback or your callback returns ‘GSASL_NO_CALLBACK’ to signal that it does not implement GSASL_VALIDATE_SIMPLE) the mechanism will continue to query the application for a password, via the GSASL_PASSWORD property. Your callback may use the GSASL_AUTHID and GSASL_AUTHZID properties to select the proper password. The password is then normalized and compared to the client credential.

Which approach to use? If your database stores hashed passwords, you have no option, but must use the first approach. If passwords in your user database are stored in prepared (SASLprep) form, the first approach will be faster. If you do not have prepared passwords available, you can use the second approach to make sure the password is prepared properly before comparison.

The PLAIN mechanism was initially specified in RFC 2595 and later revised in RFC 4616.

5.4 The LOGIN mechanism

The LOGIN mechanism is a non-standard mechanism, and is similar to the PLAIN mechanism except that LOGIN lacks the support for authorization identities. Always use PLAIN instead of LOGIN in new applications.

The callback behaviour is the same as for PLAIN, except that GSASL_AUTHZID is neither used nor required, and that the server does not normalize the password using SASLprep.

See Use of SASLprep in LOGIN, for a proposed clarification of the interpretation of a hypothetical LOGIN specification.

5.5 The CRAM-MD5 mechanism

CRAM-MD5 is a widely used challenge-response mechanism that transfers hashed passwords instead of clear text passwords. It is official deprecated, initially in favor of first DIGEST-MD5 but today SCRAM-SHA-1. For insecure channels (e.g., when TLS is not used), it is has better properties than PLAIN since the unhashed password is not leaked. The CRAM-MD5 mechanism does not support authorization identities; that make the relationship between CRAM-MD5 and DIGEST-MD5/SCRAM-SHA-* similar to the relationship between LOGIN and PLAIN.

The disadvantage with hashed passwords is that the server cannot use normal authentication infrastructures such as PAM, because the server must have access to the unhashed password in order to validate every authentication attempt.

In the client, this mechanism is always enabled, and it requires the GSASL_AUTHID and GSASL_PASSWORD properties.

In the server, the mechanism will require the GSASL_PASSWORD callback property, which may use the GSASL_AUTHID property to determine which users’ password should be used. The GSASL_AUTHID will be in normalized form. The server will then normalize the password, and compare the client response with the computed correct response, and accept the user accordingly.

See Use of SASLprep in CRAM-MD5, for a clarification on the interpretation of the CRAM-MD5 specification that this implementation rely on.

The CRAM-MD5 mechanism was initially specified in RFC 2095 but quickly revised in RFC 2195. Note that both were published before the core SASL framework, which explains its lack of authorization identity.

5.6 The DIGEST-MD5 mechanism

The DIGEST-MD5 mechanism is based on repeated hashing using MD5. After the MD5 break may be argued to be weaker than HMAC-MD5 that CRAM-MD5 builds on, but DIGEST-MD5 supports other features. For example, authorization identities and data integrity and privacy protection are supported. Like CRAM-MD5, only a hashed password is transferred. Consequently, DIGEST-MD5 needs access to the correct password to verify the client response – however the server can store the password in hashed form, another improvement compared to CRAM-MD5 . Alas, this makes it impossible to use, e.g., PAM on the server side.

In the client, this mechanism is always enabled, and it requires the GSASL_AUTHID, GSASL_PASSWORD, GSASL_SERVICE, and GSASL_HOSTNAME properties. If set, GSASL_AUTHZID and GSASL_REALM will also be used.

In the server, the mechanism will first request the GSASL_DIGEST_MD5_HASHED_PASSWORD callback property to get the user’s hashed password. If the callback doesn’t supply a hashed password (i.e., it returns ‘GSASL_NO_CALLBACK’), the GSASL_PASSWORD callback property will be requested. Both callbacks may use the GSASL_AUTHID, GSASL_AUTHZID and GSASL_REALM properties to determine which users’ password should be used. The server will then compare the client response with a computed correct response, and accept the user accordingly.

The server uses the GSASL_QOPS callback to get the set of quality of protection values to use. By default, it advertises support for authentication (qop-auth) only. You can use the callback, for example, to make the server advertise support for authentication with integrity layers.

The client uses the GSASL_QOP callback to get the quality of protection value to request. The client must choose one of the QOP values offered by the server (which may be inspected through the GSASL_QOPS property). If the client does not return a value, qop-auth is used by default.

The security layers of DIGEST-MD5 are rarely used in practice due to interoperability and security reasons. You are recommended to use TLS instead.

The DIGEST-MD5 mechanism is specified in RFC 2831. RFC 6331 labels DIGEST-MD5 as historic and it contains a good exposition of the disadvantages with DIGEST-MD5.

5.7 The SCRAM mechanisms

SCRAM is a family of mechanisms and we support SCRAM-SHA-1 and SCRAM-SHA-256. They differ only in the use of underlying hash function, SHA-1 and SHA-256 respectively. Channel bindings are supported through the SCRAM-SHA-1-PLUS and SCRAM-SHA-256-PLUS mechanisms, and will bind the authentication to a particular TLS channel. SCRAM provides mutual authentication, i.e., after a succesful authentication the client will know that the server knows the password, and the server will know that the client knows the password. Further, this can be achieved without storing the password in clear text on either side.

The SCRAM family is designed to provide the same capabilities that CRAM-MD5 and DIGEST-MD5 provides but with modern cryptographic techniques such as HMAC hashing and PKCS#5 PBKDF2 key derivation. SCRAM supports authorization identities. Like CRAM-MD5 and DIGEST-MD5, only a hashed password is transferred. Consequently, SCRAM servers needs access to the correct password (or the derived ServerKey/StoredKey) to verify the client response.

In the client, the non-PLUS mechanism is always enabled, and it requires the GSASL_AUTHID property, and either GSASL_PASSWORD or GSASL_SCRAM_SALTED_PASSWORD. When the GSASL_CB_TLS_UNIQUE property is available, the SCRAM-SHA-1-PLUS mechanism is also available and it will negotiate channel bindings when the server also supports it. If set, GSASL_AUTHZID will be used by the client. To be able to return the proper GSASL_SCRAM_SALTED_PASSWORD value, the callback needs to check the GSASL_SCRAM_ITER and GSASL_SCRAM_SALT values which are available when the GSASL_SCRAM_SALTED_PASSWORD property is queried for. The client/server may retrieve the calculated GSASL_SCRAM_SALTED_PASSWORD value by retrieving it after the final authentication step. The GSASL_SCRAM_SALTED_PASSWORD value can also be derived by using the gsasl_scram_secrets_from_password function (see Utilities), or through the --mkpasswd parameter for the gsasl utility (see Invoking gsasl).

In the server, the GSASL_AUTHID property (and, when provided by the client, the GSASL_AUTHZID property) will be set in order for the callback to retrieve the user credentials. The server mechanism will request the GSASL_SERVERKEY and GSASL_STOREDKEY properties first, and will use them to complete authentication. Using ServerKey/StoredKey on the server make it possible for the server to avoid storing the clear-text password. If ServerKey/StoredKey is not available, the GSASL_PASSWORD property is requested, which will be used to derive the ServetKey/StoredKey secrets. The mechanism uses the credentials to authenticate the user. The application may set the GSASL_SCRAM_ITER and GSASL_SCRAM_SALT properties which allow the server to tell the clients what values to use for deriving a key from a password. When the application do not supply them, the SCRAM server will default to using a fresh random salt and an iteration count of 4096. After the final authentication step, the application may retrieve the GSASL_SCRAM_ITER, GSASL_SCRAM_SALT, GSASL_SCRAM_SALTED_PASSWORD, GSASL_SERVERKEY, and GSASL_STOREDKEY properties for potential storage in a database to avoid the need to store the cleartext password. Note that servers should normally not store GSASL_SCRAM_SALTED_PASSWORD as stealing that allow someone to impersonate the client, which is not (directly) possible otherwise. When the GSASL_CB_TLS_UNIQUE property is set, the SCRAM-*-PLUS mechanism is supported and is used to negotiate channel bindings.

The GSASL_CB_TLS_UNIQUE property signal that this side of the authentication supports channel bindings. Setting the property will enable the SCRAM-SHA-1-PLUS and SCRAM-SHA-256-PLUS mechanisms. For clients, this also instructs the SCRAM-SHA-1 and SCRAM-SHA-256 mechanism to tell servers that the client believes the server does not support channel bindings if it is used (remember that clients should otherwise have chosen the SCRAM-SHA-1-PLUS mechanism instead of the SCRAM-SHA-1 mechanism). For servers, it means the SCRAM-SHA-1/SCRAM-SHA-256 mechanism will refuse to authenticate against a client that signals that it believes the server does not support channel bindings.

The SCRAM-SHA-*-PLUS mechanisms will never complete authentication successfully if channel bindings are not confirmed.

To offer the intended security, the SCRAM-SHA-*-PLUS mechanisms MUST be used over a TLS channel that has had the session hash extension (RFC 7627) negotiated, or session resumption MUST NOT have been used. The library cannot enforce this, so it is up to the application to only provide the GSASL_CB_TLS_UNIQUE property when this condition holds.

The SCRAM-SHA-1 mechanism is specified in RFC 5802 and SCRAM-SHA-256 is specified in RFC 7677. How to store SCRAM credentials in LDAP on the server side is described in RFC 5803.

5.8 The NTLM mechanism

The NTLM is a non-standard mechanism. Do not use it in new applications, and do not expect it to be secure. Currently only the client side is supported.

In the client, this mechanism is always enabled, and it requires the GSASL_AUTHID and GSASL_PASSWORD properties. It will set the ‘domain’ field in the NTLM request to the value of GSASL_REALM. Some servers reportedly need non-empty but arbitrary values in that field.

5.9 The SECURID mechanism

The SECURID mechanism uses authentication and authorization identity together with a passcode from a hardware token to authenticate users.

In the client, this mechanism is always enabled, and it requires the GSASL_AUTHID and GSASL_PASSCODE properties. If set, GSASL_AUTHZID will also be used. If the server requests it, the GSASL_PIN property is also required, and its callback may inspect the GSASL_SUGGESTED_PIN property to discover a server-provided PIN to use.

In the server, this mechanism will invoke the GSASL_VALIDATE_SECURID callback. The callback may inspect the GSASL_AUTHID, GSASL_AUTHZID, and GSASL_PASSCODE properties. The callback can return GSASL_SECURID_SERVER_NEED_ADDITIONAL_PASSCODE to ask for another additional passcode from the client. The callback can return GSASL_SECURID_SERVER_NEED_NEW_PIN to ask for a new PIN code from the client, in which case it may also set the GSASL_SUGGESTED_PIN property to indicate a recommended new PIN. If the callbacks is invoked again, after having returned GSASL_SECURID_SERVER_NEED_NEW_PIN, it may also inspect the GSASL_PIN property, in addition to the other properties, to find out the client selected PIN code.

The SECURID mechanism is specified in RFC 2808.

5.10 The GSSAPI mechanism

The GSSAPI mechanism allows you to authenticate using Kerberos V5. The mechanism was originally designed to allow for any GSS-API mechanism to be used, but problems with the protocol made it unpractical and it is today restricted for use with Kerberos V5. See the GS2 mechanism (see The GS2-KRB5 mechanism) for a general solution. However, GSSAPI continues to be widely used in Kerberos V5 environments.

In the client, the mechanism is enabled only if the user has acquired credentials (i.e., a ticket granting ticket), and it requires the GSASL_AUTHZID, GSASL_SERVICE, and GSASL_HOSTNAME properties. (For historical reasons, if the GSASL_AUTHZID property is not specified, this mechanism checks for the GSASL_AUTHZID property and if present will use that as the authorization identity – this behaviour will be removed after the year 2012 so you should update your code to use only GSASL_AUTHZID.)

In the server, the mechanism requires the GSASL_SERVICE and GSASL_HOSTNAME properties, and it will invoke the GSASL_VALIDATE_GSSAPI callback property in order to validate the user. The callback may inspect the GSASL_AUTHZID and GSASL_GSSAPI_DISPLAY_NAME properties to decide whether to authorize the user. Note that authentication is performed by the GSS-API library and that GSASL_AUTHID is not used by the server mechanism, its role is played by GSASL_GSSAPI_DISPLAY_NAME.

This implementation does not support security layers. You are recommended to use TLS instead.

The GSSAPI mechanism was specified as part of the initial core SASL framework, in RFC 2222, but later revised in RFC 4752 to only apply to Kerberos V5.

5.11 The GS2-KRB5 mechanism

GS2 is a protocol bridge between GSS-API and SASL, and allows every GSS-API mechanism that supports mutual authentication and channel bindings to be used as a SASL mechanism. Currently GS2-KRB5 is supported, for Kerberos V5 authentication, however our GS2 implementation is flexible enough to easily support other GSS-API mechanism if any gains popularity.

In the client, the mechanism is enabled only if the user has acquired credentials (i.e., a ticket granting ticket), and it requires the GSASL_AUTHZID, GSASL_SERVICE, and GSASL_HOSTNAME properties.

In the server, the mechanism requires the GSASL_SERVICE and GSASL_HOSTNAME properties, and it will invoke the GSASL_VALIDATE_GSSAPI callback property in order to validate the user. The callback may inspect the GSASL_AUTHZID and GSASL_GSSAPI_DISPLAY_NAME properties to decide whether to authorize the user. Note that authentication is performed by the GSS-API library and that GSASL_AUTHID is not used by the server mechanism, its role is played by GSASL_GSSAPI_DISPLAY_NAME.

The GS2 framework supports a variant of each mechanism, called the PLUS variant, which can also bind the authentication to a secure channel through channel bindings. Currently this is not supported by GNU SASL.

The GS2 mechanism family was specified in RFC 5801.

5.12 The SAML20 mechanism

The SAML20 mechanism makes it possible to use SAML in SASL, in a way that offloads the authentication exchange to an external web browser.

The mechanism makes use of the following properties: GSASL_AUTHZID, GSASL_SAML20_IDP_IDENTIFIER, GSASL_SAML20_REDIRECT_URL, GSASL_SAML20_AUTHENTICATE_IN_BROWSER and GSASL_VALIDATE_SAML20.

In client mode, the mechanism will retrieve the GSASL_AUTHZID and GSASL_SAML20_IDP_IDENTIFIER properties and form a request to the server. The server will respond with a redirect URL stored in the GSASL_SAML20_REDIRECT_URL property, which the client can retrieve from the GSASL_SAML20_AUTHENTICATE_IN_BROWSER callback. The intention is that the client launches a browser to the given URL, and then proceeds with authentication. The server responds whether authentication was successful or not.

In server mode, the mechanism will invoke the GSASL_SAML20_REDIRECT_URL callback and the application can inspect the GSASL_AUTHZID and GSASL_SAML20_IDP_IDENTIFIER properties when forming the redirect URL. The URL is passed to the client which will hopefully complete authentication in the browser. The server callback GSASL_VALIDATE_SAML20 should check whether the authentication attempt was successful.

Note that SAML itself is not implemented by the GNU SASL library. On the client side, no SAML knowledge is needed, it is only required on the server side. The client only needs to be able to start a web browser to access the redirect URL. The server side is expected to call a SAML library to generate the AuthRequest and to implement an AssertionConsumerService (ACS) to validate the AuthResponse. There is a complete proof-of-concept example of a SMTP server with SAML 2.0 support distributed with GNU SASL in the examples/saml20/ sub-directory. The example uses the Lasso SAML implementation (http://lasso.entrouvert.org/) and require a web server for the ACS side. The example may be used as inspiration for your own server implementation. The gsasl command line client supports SAML20 as a client.

The SAML20 mechanism is specified in RFC 6595.

5.13 The OPENID20 mechanism

The OPENID20 mechanism makes it possible to use OpenID in SASL, in a way that offloads the authentication exchange to an external web browser.

The mechanism makes use of the following properties: GSASL_AUTHID (for the OpenID User-Supplied Identifier), GSASL_AUTHZID, GSASL_OPENID20_REDIRECT_URL, GSASL_OPENID20_OUTCOME_DATA, GSASL_OPENID20_AUTHENTICATE_IN_BROWSER, and GSASL_VALIDATE_OPENID20.

In the client, the mechanism is enabled by default. The GSASL_AUTHID property is required and should contain the User-Supplied OpenID Identifier (for example http://josefsson.org). If set, GSASL_AUTHZID will be used by the client. The client will be invoked with the GSASL_OPENID20_AUTHENTICATE_IN_BROWSER callback to perform the OpenID authentication in a web browser. The callback can retrieve the GSASL_OPENID20_REDIRECT_URL property to find out the URL to redirect the user to. After authentication, the client can retrieve the GSASL_OPENID20_OUTCOME_DATA property with the OpenID Simple Registry (SREG) attributes sent by the server (if any).

In the server, the mechanism is enabled by default. The server will request the GSASL_OPENID20_REDIRECT_URL property, and your callback may inspect the GSASL_AUTHID to find the OpenID User-Supplied Identifier. The server callback should perform OpenID discovery and return the URL to redirect the user to. After this, the user would access the URL and proceed with authentication in the browser. The server is invoked with the GSASL_VALIDATE_OPENID20 callback to perform the actual validation of the authentication. Usually the callback will perform some IPC communication with an OpenID consumer running in a web server. The callback should return GSASL_OK on successful authentication and GSASL_AUTHENTICATION_ERROR on authentication errors, or any other error code. If the server received some OpenID Simple Registry (SREG) attributes from the OpenID Identity Provider, it may use the GSASL_OPENID20_OUTCOME_DATA property to send these to the client.

Note that OpenID itself is not implemented by the GNU SASL library. On the client side, no OpenID knowledge is required, it is only required on the server side. The client only needs to be able to start a web browser to access the redirect URL. The server side is expected to use an OpenID library to generate the redirect URL and to implement the Service Provider (SP) to validate the response from the IdP. There is a complete proof-of-concept example with a SMTP server with OpenID 2.0 support distributed with GNU SASL in the examples/openid20/ sub-directory. It uses the JanRain PHP5 OpenID implementation and require a web server to implement the OpenID SP. The example may be used as inspiration for your own server implementation. The gsasl command line client supports OPENID20 as a client.

The OPENID20 mechanism is specified in RFC 6616.

5.14 The KERBEROS_V5 mechanism

The KERBEROS_V5 is an experimental mechanism, the protocol specification is available on the GNU SASL homepage. It can operate in three modes, non-infrastructure mode, infrastructure mode and proxied infrastructure mode. Currently only non-infrastructure mode is supported.

In the non-infrastructure mode, it works as a superset of most features provided by PLAIN, CRAM-MD5, DIGEST-MD5 and GSSAPI while at the same time building on what is believed to be proven technology (the RFC 1510 network security system). In the non-infrastructure mode, the client must specify (via callbacks) the name of the user, and optionally the server name and realm. The server must be able to retrieve passwords given the name of the user.

In the infrastructure mode (proxied or otherwise), it allows clients and servers to authenticate via SASL in an RFC 1510 environment, using a trusted third party, a “Key Distribution Central”. In the normal mode, clients acquire tickets out of band and then invokes a one roundtrip AP-REQ and AP-REP exchange. In the proxied mode, which can be used by clients without IP addresses or without connectivity to the KDC (e.g., when the KDC is IPv4 and the client is IPV6-only), the client uses the server to proxy ticket requests and finishes with the AP-REQ/AP-REP exchange. In infrastructure mode (proxied or otherwise), neither the client nor server need to implement any callbacks (this will likely change later, to allow a server to authorize users, similar to the GSSAPI callback).

XXX: update when implementation has matured

12.1 Error values

Errors are returned as int values.

The value of the symbol GSASL_OK is guaranteed to always be 0, and all other error codes are guaranteed to be non-0, so you may use that information to build boolean expressions involving return codes. Otherwise, an application should not depend on the particular value for error codes, and are encouraged to use the constants even for GSASL_OK to improve readability. Possible values are:

GSASL_OK

Libgsasl success

GSASL_NEEDS_MORE

SASL mechanism needs more data

GSASL_UNKNOWN_MECHANISM

Unknown SASL mechanism

GSASL_MECHANISM_CALLED_TOO_MANY_TIMES

SASL mechanism called too many times

GSASL_TOO_SMALL_BUFFER

SASL function needs larger buffer (internal error)

GSASL_FOPEN_ERROR

Could not open file in SASL library

GSASL_FCLOSE_ERROR

Could not close file in SASL library

GSASL_MALLOC_ERROR

Memory allocation error in SASL library

GSASL_BASE64_ERROR

Base 64 coding error in SASL library

GSASL_CRYPTO_ERROR

Low-level crypto error in SASL library

GSASL_NEED_CLIENT_ANONYMOUS_CALLBACK

SASL mechanism needs gsasl_client_callback_anonymous() callback (application error)

GSASL_NEED_CLIENT_PASSWORD_CALLBACK

SASL mechanism needs gsasl_client_callback_password() callback (application error)

GSASL_NEED_CLIENT_PASSCODE_CALLBACK

SASL mechanism needs gsasl_client_callback_passcode() callback (application error)

GSASL_NEED_CLIENT_PIN_CALLBACK

SASL mechanism needs gsasl_client_callback_pin() callback (application error)

GSASL_NEED_CLIENT_AUTHORIZATION_ID_CALLBACK

SASL mechanism needs gsasl_client_callback_authorization_id() callback (application error)

GSASL_NEED_CLIENT_AUTHENTICATION_ID_CALLBACK

SASL mechanism needs gsasl_client_callback_authentication_id() callback (application error)

GSASL_NEED_CLIENT_SERVICE_CALLBACK

SASL mechanism needs gsasl_client_callback_service() callback (application error)

GSASL_NEED_SERVER_VALIDATE_CALLBACK

SASL mechanism needs gsasl_server_callback_validate() callback (application error)

GSASL_NEED_SERVER_CRAM_MD5_CALLBACK

SASL mechanism needs gsasl_server_callback_cram_md5() callback (application error)

GSASL_NEED_SERVER_DIGEST_MD5_CALLBACK

SASL mechanism needs gsasl_server_callback_digest_md5() callback (application error)

GSASL_NEED_SERVER_EXTERNAL_CALLBACK

SASL mechanism needs gsasl_server_callback_external() callback (application error)

GSASL_NEED_SERVER_ANONYMOUS_CALLBACK

SASL mechanism needs gsasl_server_callback_anonymous() callback (application error)

GSASL_NEED_SERVER_REALM_CALLBACK

SASL mechanism needs gsasl_server_callback_realm() callback (application error)

GSASL_NEED_SERVER_SECURID_CALLBACK

SASL mechanism needs gsasl_server_callback_securid() callback (application error)

GSASL_NEED_SERVER_SERVICE_CALLBACK

SASL mechanism needs gsasl_server_callback_service() callback (application error)

GSASL_NEED_SERVER_GSSAPI_CALLBACK

SASL mechanism needs gsasl_server_callback_gssapi() callback (application error)

GSASL_NEED_SERVER_RETRIEVE_CALLBACK

SASL mechanism needs gsasl_server_callback_retrieve() callback (application error)

GSASL_UNICODE_NORMALIZATION_ERROR

Failed to perform Unicode Normalization on string.

GSASL_SASLPREP_ERROR

Could not prepare internationalized (non-ASCII) string.

GSASL_MECHANISM_PARSE_ERROR

SASL mechanism could not parse input

GSASL_AUTHENTICATION_ERROR

Error authenticating user

GSASL_CANNOT_GET_CTX

Cannot get internal library handle (library error)

GSASL_INTEGRITY_ERROR

Integrity error in application payload

GSASL_NO_MORE_REALMS

No more realms available (non-fatal)

GSASL_NO_CLIENT_CODE

Client-side functionality not available in library (application error)

GSASL_NO_SERVER_CODE

Server-side functionality not available in library (application error)

GSASL_GSSAPI_RELEASE_BUFFER_ERROR

GSSAPI library could not deallocate memory in gss_release_buffer() in SASL library. This is a serious internal error.

GSASL_GSSAPI_IMPORT_NAME_ERROR

GSSAPI library could not understand a peer name in gss_import_name() in SASL library. This is most likely due to incorrect service and/or hostnames.

GSASL_GSSAPI_INIT_SEC_CONTEXT_ERROR

GSSAPI error in client while negotiating security context in gss_init_sec_context() in SASL library. This is most likely due insufficient credentials or malicious interactions.

GSASL_GSSAPI_ACCEPT_SEC_CONTEXT_ERROR

GSSAPI error in server while negotiating security context in gss_accept_sec_context() in SASL library. This is most likely due insufficient credentials or malicious interactions.

GSASL_GSSAPI_UNWRAP_ERROR

GSSAPI error while decrypting or decoding data in gss_unwrap() in SASL library. This is most likely due to data corruption.

GSASL_GSSAPI_WRAP_ERROR

GSSAPI error while encrypting or encoding data in gss_wrap() in SASL library.

GSASL_GSSAPI_ACQUIRE_CRED_ERROR

GSSAPI error acquiring credentials in gss_acquire_cred() in SASL library. This is most likely due to not having the proper Kerberos key available in /etc/krb5.keytab on the server.

GSASL_GSSAPI_DISPLAY_NAME_ERROR

GSSAPI error creating a display name denoting the client in gss_display_name() in SASL library. This is probably because the client supplied bad data.

GSASL_GSSAPI_UNSUPPORTED_PROTECTION_ERROR

Other entity requested integrity or confidentiality protection in GSSAPI mechanism but this is currently not implemented.

GSASL_KERBEROS_V5_INIT_ERROR

Kerberos V5 initialization failure.

GSASL_KERBEROS_V5_INTERNAL_ERROR

Kerberos V5 internal error.

GSASL_SECURID_SERVER_NEED_ADDITIONAL_PASSCODE

SecurID needs additional passcode.

GSASL_SECURID_SERVER_NEED_NEW_PIN

SecurID needs new pin.

GSASL_INVALID_HANDLE

The provided library handle was invalid (application error)

GSASL_NO_CALLBACK

No callback specified by caller (application error).

GSASL_NO_ANONYMOUS_TOKEN

Authentication failed because the anonymous token was not provided.

GSASL_NO_AUTHID

Authentication failed because the authentication identity was not provided.

GSASL_NO_AUTHZID

Authentication failed because the authorization identity was not provided.

GSASL_NO_PASSWORD

Authentication failed because the password was not provided.

GSASL_NO_PASSCODE

Authentication failed because the passcode was not provided.

GSASL_NO_PIN

Authentication failed because the pin code was not provided.

GSASL_NO_SERVICE

Authentication failed because the service name was not provided.

GSASL_NO_HOSTNAME

Authentication failed because the host name was not provided.

GSASL_GSSAPI_ENCAPSULATE_TOKEN_ERROR

GSSAPI error encapsulating token.

GSASL_GSSAPI_DECAPSULATE_TOKEN_ERROR

GSSAPI error decapsulating token.

GSASL_GSSAPI_INQUIRE_MECH_FOR_SASLNAME_ERROR

GSSAPI error getting OID for SASL mechanism name.

GSASL_GSSAPI_TEST_OID_SET_MEMBER_ERROR

GSSAPI error testing for OID in OID set.

GSASL_GSSAPI_RELEASE_OID_SET_ERROR

GSSAPI error releasing OID set.

GSASL_NO_CB_TLS_UNIQUE

Authentication failed because a tls-unique CB was not provided.

GSASL_NO_SAML20_IDP_IDENTIFIER

Callback failed to provide SAML20 IdP identifier.

GSASL_NO_SAML20_REDIRECT_URL

Callback failed to provide SAML20 redirect URL.

GSASL_NO_OPENID20_REDIRECT_URL

Callback failed to provide OPENID20 redirect URL.

12.2 Error strings

gsasl_strerror

Function: const char * gsasl_strerror (int err) ¶

err: libgsasl error code

Convert return code to human readable string explanation of the reason for the particular error code.

This string can be used to output a diagnostic message to the user.

This function is one of few in the library that can be used without a successful call to gsasl_init() .

Return value: Returns a pointer to a statically allocated string containing an explanation of the error code err .

gsasl_strerror_name

Function: const char * gsasl_strerror_name (int err) ¶

err: libgsasl error code

Convert return code to human readable string representing the error code symbol itself. For example, gsasl_strerror_name(GSASL_OK ) returns the string "GSASL_OK".

This string can be used to output a diagnostic message to the user.

This function is one of few in the library that can be used without a successful call to gsasl_init() .

Return value: Returns a pointer to a statically allocated string containing a string version of the error code err , or NULL if the error code is not known.

Since: 0.2.29

13.1 Example 1

/* client.c --- Example SASL client.
 * Copyright (C) 2004-2021 Simon Josefsson
 *
 * This file is part of GNU SASL.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include <config.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <gsasl.h>

static void
client_authenticate (Gsasl_session * session)
{
  char buf[BUFSIZ] = "";
  char *p;
  int rc;

  /* This loop mimics a protocol where the client send data first. */

  do
    {
      /* Generate client output. */
      rc = gsasl_step64 (session, buf, &p);

      if (rc == GSASL_NEEDS_MORE || rc == GSASL_OK)
        {
          /* If sucessful, print it. */
          printf ("Output:\n%s\n", p);
          gsasl_free (p);
        }

      if (rc == GSASL_NEEDS_MORE)
        {
          /* If the client need more data from server, get it here. */
          printf ("Input base64 encoded data from server:\n");
          p = fgets (buf, sizeof (buf) - 1, stdin);
	  if (p == NULL)
	    {
	      perror ("fgets");
	      return;
	    }
          if (buf[strlen (buf) - 1] == '\n')
            buf[strlen (buf) - 1] = '\0';
        }
    }
  while (rc == GSASL_NEEDS_MORE);

  printf ("\n");

  if (rc != GSASL_OK)
    {
      printf ("Authentication error (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* The client is done.  Here you would typically check if the server
     let the client in.  If not, you could try again. */

  printf ("If server accepted us, we're done.\n");
}

static void
client (Gsasl * ctx)
{
  Gsasl_session *session;
  const char *mech = "PLAIN";
  int rc;

  /* Create new authentication session. */
  if ((rc = gsasl_client_start (ctx, mech, &session)) != GSASL_OK)
    {
      printf ("Cannot initialize client (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* Set username and password in session handle.  This info will be
     lost when this session is deallocated below.  */
  gsasl_property_set (session, GSASL_AUTHID, "jas");
  gsasl_property_set (session, GSASL_PASSWORD, "secret");

  /* Do it. */
  client_authenticate (session);

  /* Cleanup. */
  gsasl_finish (session);
}

int
main (void)
{
  Gsasl *ctx = NULL;
  int rc;

  /* Initialize library. */
  if ((rc = gsasl_init (&ctx)) != GSASL_OK)
    {
      printf ("Cannot initialize libgsasl (%d): %s", rc, gsasl_strerror (rc));
      return 1;
    }

  /* Do it. */
  client (ctx);

  /* Cleanup. */
  gsasl_done (ctx);

  return 0;
}

13.2 Example 2

/* client-serverfirst.c --- Example SASL client, where server send data first.
 * Copyright (C) 2004-2021 Simon Josefsson
 *
 * This file is part of GNU SASL.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include <config.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <gsasl.h>

static void
client_authenticate (Gsasl_session * session)
{
  char buf[BUFSIZ] = "";
  char *p;
  int rc;

  /* This loop mimics a protocol where the server send data first. */

  do
    {
      printf ("Input base64 encoded data from server:\n");
      p = fgets (buf, sizeof (buf) - 1, stdin);
      if (p == NULL)
	{
	  perror ("fgets");
	  return;
	}
      if (buf[strlen (buf) - 1] == '\n')
        buf[strlen (buf) - 1] = '\0';

      rc = gsasl_step64 (session, buf, &p);

      if (rc == GSASL_NEEDS_MORE || rc == GSASL_OK)
        {
          printf ("Output:\n%s\n", p);
          gsasl_free (p);
        }
    }
  while (rc == GSASL_NEEDS_MORE);

  printf ("\n");

  if (rc != GSASL_OK)
    {
      printf ("Authentication error (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* The client is done.  Here you would typically check if the server
     let the client in.  If not, you could try again. */

  printf ("If server accepted us, we're done.\n");
}

static void
client (Gsasl * ctx)
{
  Gsasl_session *session;
  const char *mech = "CRAM-MD5";
  int rc;

  /* Create new authentication session. */
  if ((rc = gsasl_client_start (ctx, mech, &session)) != GSASL_OK)
    {
      printf ("Cannot initialize client (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* Set username and password in session handle.  This info will be
     lost when this session is deallocated below.  */
  gsasl_property_set (session, GSASL_AUTHID, "jas");
  gsasl_property_set (session, GSASL_PASSWORD, "secret");

  /* Do it. */
  client_authenticate (session);

  /* Cleanup. */
  gsasl_finish (session);
}

int
main (void)
{
  Gsasl *ctx = NULL;
  int rc;

  /* Initialize library. */
  if ((rc = gsasl_init (&ctx)) != GSASL_OK)
    {
      printf ("Cannot initialize libgsasl (%d): %s", rc, gsasl_strerror (rc));
      return 1;
    }

  /* Do it. */
  client (ctx);

  /* Cleanup. */
  gsasl_done (ctx);

  return 0;
}

13.3 Example 3

/* client-mech.c --- Example SASL client, with a choice of mechanism to use.
 * Copyright (C) 2004-2021 Simon Josefsson
 *
 * This file is part of GNU SASL.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include <config.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <gsasl.h>

static void
client_authenticate (Gsasl_session * session)
{
  char buf[BUFSIZ] = "";
  char *p;
  int rc;

  /* This loop mimics a protocol where the server send data first. */

  do
    {
      printf ("Input base64 encoded data from server:\n");
      p = fgets (buf, sizeof (buf) - 1, stdin);
      if (p == NULL)
	{
	  perror ("fgets");
	  return;
	}
      if (buf[strlen (buf) - 1] == '\n')
        buf[strlen (buf) - 1] = '\0';

      rc = gsasl_step64 (session, buf, &p);

      if (rc == GSASL_NEEDS_MORE || rc == GSASL_OK)
        {
          printf ("Output:\n%s\n", p);
          gsasl_free (p);
        }
    }
  while (rc == GSASL_NEEDS_MORE);

  printf ("\n");

  if (rc != GSASL_OK)
    {
      printf ("Authentication error (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* The client is done.  Here you would typically check if the server
     let the client in.  If not, you could try again. */

  printf ("If server accepted us, we're done.\n");
}

static const char *
client_mechanism (Gsasl * ctx)
{
  static char mech[GSASL_MAX_MECHANISM_SIZE + 1] = "";
  char mechlist[BUFSIZ] = "";
  const char *suggestion;
  char *p;

  printf ("Enter list of server supported mechanisms, separate by SPC:\n");
  p = fgets (mechlist, sizeof (mechlist) - 1, stdin);
  if (p == NULL)
    {
      perror ("fgets");
      return NULL;
    }

  suggestion = gsasl_client_suggest_mechanism (ctx, mechlist);
  if (suggestion)
    printf ("Library suggests use of `%s'.\n", suggestion);

  printf ("Enter mechanism to use:\n");
  p = fgets (mech, sizeof (mech) - 1, stdin);
  if (p == NULL)
    {
      perror ("fgets");
      return NULL;
    }

  mech[strlen (mech) - 1] = '\0';

  return mech;
}

static void
client (Gsasl * ctx)
{
  Gsasl_session *session;
  const char *mech;
  int rc;

  /* Find out which mechanism to use. */
  mech = client_mechanism (ctx);

  /* Create new authentication session. */
  if ((rc = gsasl_client_start (ctx, mech, &session)) != GSASL_OK)
    {
      printf ("Cannot initialize client (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* Set username and password in session handle.  This info will be
     lost when this session is deallocated below.  */
  gsasl_property_set (session, GSASL_AUTHID, "jas");
  gsasl_property_set (session, GSASL_PASSWORD, "secret");

  /* Do it. */
  client_authenticate (session);

  /* Cleanup. */
  gsasl_finish (session);
}

int
main (void)
{
  Gsasl *ctx = NULL;
  int rc;

  /* Initialize library. */
  if ((rc = gsasl_init (&ctx)) != GSASL_OK)
    {
      printf ("Cannot initialize libgsasl (%d): %s", rc, gsasl_strerror (rc));
      return 1;
    }

  /* Do it. */
  client (ctx);

  /* Cleanup. */
  gsasl_done (ctx);

  return 0;
}

13.4 Example 4

/* client-callback.c --- Example SASL client, with callback for user info.
 * Copyright (C) 2004-2021 Simon Josefsson
 *
 * This file is part of GNU SASL.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

#include <config.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <gsasl.h>

static void
client_authenticate (Gsasl_session * session)
{
  char buf[BUFSIZ] = "";
  char *p;
  int rc;

  /* This loop mimics a protocol where the server send data first. */

  do
    {
      printf ("Input base64 encoded data from server:\n");
      p = fgets (buf, sizeof (buf) - 1, stdin);
      if (p == NULL)
	{
	  perror ("fgets");
	  return;
	}
      if (buf[strlen (buf) - 1] == '\n')
        buf[strlen (buf) - 1] = '\0';

      rc = gsasl_step64 (session, buf, &p);

      if (rc == GSASL_NEEDS_MORE || rc == GSASL_OK)
        {
          printf ("Output:\n%s\n", p);
          gsasl_free (p);
        }
    }
  while (rc == GSASL_NEEDS_MORE);

  printf ("\n");

  if (rc != GSASL_OK)
    {
      printf ("Authentication error (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* The client is done.  Here you would typically check if the server
     let the client in.  If not, you could try again. */

  printf ("If server accepted us, we're done.\n");
}

static void
client (Gsasl * ctx)
{
  Gsasl_session *session;
  const char *mech = "SECURID";
  int rc;

  /* Create new authentication session. */
  if ((rc = gsasl_client_start (ctx, mech, &session)) != GSASL_OK)
    {
      printf ("Cannot initialize client (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    }

  /* Do it. */
  client_authenticate (session);

  /* Cleanup. */
  gsasl_finish (session);
}

static int
callback (Gsasl * ctx, Gsasl_session * sctx, Gsasl_property prop)
{
  char buf[BUFSIZ] = "";
  int rc = GSASL_NO_CALLBACK;
  char *p;

  /* Get user info from user. */

  printf ("Callback invoked, for property %u.\n", prop);

  switch (prop)
    {
    case GSASL_PASSCODE:
      printf ("Enter passcode:\n");
      p = fgets (buf, sizeof (buf) - 1, stdin);
      if (p == NULL)
	{
	  perror ("fgets");
	  break;
	}
      buf[strlen (buf) - 1] = '\0';

      gsasl_property_set (sctx, GSASL_PASSCODE, buf);
      rc = GSASL_OK;
      break;

    case GSASL_AUTHID:
      printf ("Enter username:\n");
      p = fgets (buf, sizeof (buf) - 1, stdin);
      if (p == NULL)
	{
	  perror ("fgets");
	  break;
	}
      buf[strlen (buf) - 1] = '\0';

      gsasl_property_set (sctx, GSASL_AUTHID, buf);
      rc = GSASL_OK;
      break;

    default:
      printf ("Unknown property!  Don't worry.\n");
      break;
    }

  return rc;
}

int
main (void)
{
  Gsasl *ctx = NULL;
  int rc;

  /* Initialize library. */
  if ((rc = gsasl_init (&ctx)) != GSASL_OK)
    {
      printf ("Cannot initialize libgsasl (%d): %s", rc, gsasl_strerror (rc));
      return 1;
    }

  /* Set the callback handler for the library. */
  gsasl_callback_set (ctx, callback);

  /* Do it. */
  client (ctx);

  /* Cleanup. */
  gsasl_done (ctx);

  return 0;
}

13.5 Example 5

/* smtp-server.c --- Example SMTP server with SASL authentication
 * Copyright (C) 2012-2021 Simon Josefsson
 *
 * This file is part of GNU SASL.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 */

/* This is a minimal SMTP server with GNU SASL authentication support.

   The only valid password is "sesam".  This server will complete
   authentications using LOGIN, PLAIN, DIGEST-MD5, CRAM-MD5,
   SCRAM-SHA-1 and SCRAM-SHA-256.

   It accepts an optional command line parameter specifying the
   service name (i.e., a numerical port number or /etc/services name).
   By default it listens on port "2000".
*/

#include <config.h>
#include <string.h>
#include <stdlib.h>
#include <stdarg.h>
#include <netdb.h>
#include <signal.h>
#include <errno.h>

#include <gsasl.h>

static int
callback (Gsasl * ctx, Gsasl_session * sctx, Gsasl_property prop)
{
  int rc = GSASL_NO_CALLBACK;

  switch (prop)
    {
    case GSASL_PASSWORD:
      gsasl_property_set (sctx, prop, "sesam");
      rc = GSASL_OK;
      break;

    default:
      /* You may want to log (at debug verbosity level) that an
         unknown property was requested here, possibly after filtering
         known rejected property requests. */
      break;
    }

  return rc;
}

static ssize_t
gettrimline (char **line, size_t * n, FILE * fh)
{
  ssize_t s = getline (line, n, fh);

  if (s >= 2)
    {
      if ((*line)[strlen (*line) - 1] == '\n')
	(*line)[strlen (*line) - 1] = '\0';
      if ((*line)[strlen (*line) - 1] == '\r')
	(*line)[strlen (*line) - 1] = '\0';

      printf ("C: %s\n", *line);
    }

  return s;
}

#define print(fh, ...)							\
  printf ("S: "), printf (__VA_ARGS__), fprintf (fh, __VA_ARGS__)

static void
server_auth (FILE * fh, Gsasl_session * session, char *initial_challenge)
{
  char *line = initial_challenge != NULL ? strdup (initial_challenge) : NULL;
  size_t n = 0;
  char *p;
  int rc;

  /* The ordering and the type of checks in the following loop has to
     be adapted for each protocol depending on its SASL properties.
     SMTP is normally a "server-first" SASL protocol, but if
     INITIAL_CHALLENGE is supplied by the client it turns into a
     client-first SASL protocol.  This implementation do not support
     piggy-backing of the terminating server response.  See RFC 2554
     and RFC 4422 for terminology.  That profile results in the
     following loop structure.  Ask on the help-gsasl list if you are
     uncertain.  */
  do
    {
      rc = gsasl_step64 (session, line, &p);
      if (rc == GSASL_NEEDS_MORE || (rc == GSASL_OK && p && *p))
	{
	  print (fh, "334 %s\n", p);
	  gsasl_free (p);

	  if (gettrimline (&line, &n, fh) < 0)
	    {
	      print (fh, "221 localhost getline failure\n");
	      goto done;
	    }
	}
    }
  while (rc == GSASL_NEEDS_MORE);

  if (rc != GSASL_OK)
    {
      print (fh, "535 gsasl_step64 (%d): %s\n", rc, gsasl_strerror (rc));
      goto done;
    }

  {
    const char *authid = gsasl_property_fast (session, GSASL_AUTHID);
    const char *authzid = gsasl_property_fast (session, GSASL_AUTHZID);
    print (fh, "235 OK [authid: %s authzid: %s]\n",
	   authid ? authid : "N/A", authzid ? authzid : "N/A");
  }

done:
  free (line);
}

static void
smtp (FILE * fh, Gsasl * ctx)
{
  char *line = NULL;
  size_t n = 0;
  int rc;

  print (fh, "220 localhost ESMTP GNU SASL smtp-server\n");

  while (gettrimline (&line, &n, fh) >= 0)
    {
      if (strncmp (line, "EHLO ", 5) == 0 || strncmp (line, "ehlo ", 5) == 0)
	{
	  char *mechlist;

	  rc = gsasl_server_mechlist (ctx, &mechlist);
	  if (rc != GSASL_OK)
	    {
	      print (fh, "221 localhost gsasl_server_mechlist (%d): %s\n",
		     rc, gsasl_strerror (rc));
	      continue;
	    }

	  print (fh, "250-localhost\n");
	  print (fh, "250 AUTH %s\n", mechlist);

	  gsasl_free (mechlist);
	}
      else if (strncmp (line, "AUTH ", 5) == 0
	       || strncmp (line, "auth ", 5) == 0)
	{
	  Gsasl_session *session = NULL;
	  char *p = strchr (line + 5, ' ');

	  if (p)
	    *p++ = '\0';

	  if ((rc = gsasl_server_start (ctx, line + 5, &session)) != GSASL_OK)
	    {
	      print (fh, "221 localhost gsasl_server_start (%d): %s: %s\n",
		     rc, gsasl_strerror (rc), line + 5);
	      continue;
	    }

	  server_auth (fh, session, p);

	  gsasl_finish (session);
	}
      else if (strncmp (line, "MAIL", 4) == 0)
	print (fh, "250 localhost OK\n");
      else if (strncmp (line, "RCPT", 4) == 0)
	print (fh, "250 localhost OK\n");
      else if (strncmp (line, "DATA", 4) == 0)
	{
	  print (fh, "354 OK\n");
	  while (gettrimline (&line, &n, fh) >= 0
		 && strncmp (line, ".", 2) != 0)
	    ;
	  print (fh, "250 OK\n");
	}
      else if (strncmp (line, "QUIT", 4) == 0
	       || strncmp (line, "quit", 4) == 0)
	{
	  print (fh, "221 localhost QUIT\n");
	  break;
	}
      else
	print (fh, "500 unrecognized command\n");
    }

  if (errno)
    print (fh, "221 localhost getline failure: %s\n", strerror (errno));

  free (line);
}

int
main (int argc, char *argv[])
{
  const char *service = argc > 1 ? argv[1] : "2000";
  volatile int run = 1;
  struct addrinfo hints, *addrs;
  int sockfd;
  int rc;
  int yes = 1;
  Gsasl *ctx;

  setvbuf (stdout, NULL, _IONBF, 0);

  rc = gsasl_init (&ctx);
  if (rc < 0)
    {
      printf ("gsasl_init (%d): %s\n", rc, gsasl_strerror (rc));
      exit (EXIT_FAILURE);
    }

  printf ("%s [gsasl header %s library %s]\n",
	  argv[0], GSASL_VERSION, gsasl_check_version (NULL));

  gsasl_callback_set (ctx, callback);

  memset (&hints, 0, sizeof (hints));
  hints.ai_flags = AI_PASSIVE | AI_ADDRCONFIG;
  hints.ai_socktype = SOCK_STREAM;

  rc = getaddrinfo (NULL, service, &hints, &addrs);
  if (rc < 0)
    {
      printf ("getaddrinfo: %s\n", gai_strerror (rc));
      exit (EXIT_FAILURE);
    }

  sockfd = socket (addrs->ai_family, addrs->ai_socktype, addrs->ai_protocol);
  if (sockfd < 0)
    {
      perror ("socket");
      exit (EXIT_FAILURE);
    }

  if (setsockopt (sockfd, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof (yes)) < 0)
    {
      perror ("setsockopt");
      exit (EXIT_FAILURE);
    }

  rc = bind (sockfd, addrs->ai_addr, addrs->ai_addrlen);
  if (rc < 0)
    {
      perror ("bind");
      exit (EXIT_FAILURE);
    }

  freeaddrinfo (addrs);

  rc = listen (sockfd, SOMAXCONN);
  if (rc < 0)
    {
      perror ("listen");
      exit (EXIT_FAILURE);
    }

  signal (SIGPIPE, SIG_IGN);

  while (run)
    {
      struct sockaddr from;
      socklen_t fromlen = sizeof (from);
      char host[NI_MAXHOST];
      int fd;
      FILE *fh;

      fd = accept (sockfd, &from, &fromlen);
      if (fd < 0)
	{
	  perror ("accept");
	  continue;
	}

      rc = getnameinfo (&from, fromlen, host, sizeof (host),
			NULL, 0, NI_NUMERICHOST);
      if (rc == 0)
	printf ("connection from %s\n", host);
      else
	printf ("getnameinfo: %s\n", gai_strerror (rc));

      fh = fdopen (fd, "w+");
      if (!fh)
	{
	  perror ("fdopen");
	  close (fd);
	  continue;
	}

      smtp (fh, ctx);

      fclose (fh);
    }

  close (sockfd);
  gsasl_done (ctx);

  return 0;
}