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<h1>SASL Programmer's Guide</h1>
<h3>NOTE: This is a work in progress. Any contributions would be
<i>very</i> appreciated</h3>
<ul><h2>Contents</h2>
<li><a href="#intro">Introduction</a>
<ul>
<li><a href="#about_this_guide">About this guide</a>
<li><a href="#what_is_sasl">What is SASL?</a>
</ul>
<li><a href="#background">Background</a>
<ul>
<li><a href="#world_before">How did the world work before SASL?</a>
<li><a href="#sasl_rescue">SASL to the rescue!</a>
</ul>
<li><a href="#briefly">Briefly</a>
<ul>
<li><a href="#what_good_for">What is the Cyrus SASL library good for?</a>
<li><a href="#what_do">What does the Cyrus SASL library do?</a>
<li><a href="#what_doesnt">What doesn't the Cyrus SASL library do?</a>
</ul>
<li><a href="#client_only">Client-only Section</a>
<ul>
<li><a href="#client_typical">A typical interaction from the client's perspective</a>
<li><a href="#client_code">How does this look in code?</a>
<li><a href="#sasl_client_init">sasl_client_init</a>
<li><a href="#sasl_client_new">sasl_client_new</a>
<li><a href="#sasl_client_start">sasl_client_start</a>
<li><a href="#sasl_client_step">sasl_client_step</a>
</ul>
<li><a href="#server_section">Server-only Section</a>
<ul>
<li><a href="#server_typical">A typical interaction from the server's perspective</a>
<li><a href="#server_code">How does this look in code?</a>
<li><a href="#sasl_server_init">sasl_server_init</a>
<li><a href="#sasl_server_new">sasl_server_new</a>
<li><a href="#sasl_server_start">sasl_server_start</a>
<li><a href="#sasl_server_step">sasl_server_step</a>
<li><a href="#sasl_listmech">sasl_listmech</a>
<li><a href="#sasl_checkpass">sasl_checkpass</a>
</ul>
<li><a href="#common_section">Common Section</a>
<ul>
<li><a href="#callbacks_interactions">Callbacks and Interaction</a>
<li><a href="#sasl_encode">sasl_encode</a>
<li><a href="#sasl_decode">sasl_decode</a>
</ul>
<li><a href="#example_section">Example applications that come with the Cyrus SASL library</a>
<ul>
<li><a href="#sample_client"><tt>sample-client</tt> and <tt>sample-server</tt></a>
<li><a href="#cyrus_imapd">Cyrus imapd v1.6.0 or later</a>
<li><a href="#imtest"><tt>imtest</tt>, from cyrus imapd 1.6.0 or later</a>
</ul>
<li><a href="#random_things">Miscelaneous Information</a>
<ul>
<li><a href="#empty_exchanges">Empty exchanges</a>
</ul>
<li><a href="#not_implemented">What's not implemented</a>
<ul>
<li><a href="#credentials">Credentials</a>
<li><a href="#secrets">Secrets</a>
<li><a href="#idle">Idle</a>
</ul>
</ul>
<a name="intro"><h2>Introduction</h2></a>
<a name="about_this_guide"><h3>About this Guide</h3></a>
<p>This guide gives a tutorial on the use of the Cyrus SASL library
for a client or server application. It complies with versions up
to and including 1.5.15. The following pages should only be
considered a guide, not the final word on programming with the
Cyrus SASL library. Consult the sasl.h file in the distribution in
the case of ambiguities.
<a name="what_is_sasl"><h3>What is SASL?</h3></a>
<p>SASL stands for Simple Authentication Security Layer and is
explained in <a href="http://ftp.ietf.org/rfc/rfc2222.txt">RFC
2222</a>. That document is very difficult to understand however
and it should be unnecessary to consult it.
<a name="background"><h2>Background</h2></a>
<a name="world_before"><h3>How did the world work before SASL?</h3></a>
<p>Before SASL, when a new protocol was written which required
authentication (users proving who they are to an entity), the
protocol had to allow explicitly for each individual
authentication mechanism. There had to be a distinct way to say "I
want to log in with Kerberos V4". There had to be another distinct
way to say "I want to log in with CRAM-MD5". There had to be yet a
different way to say "I want to log in anonymously," and so
on. This was non-ideal for both the protocol and application
writers.
<p>Additionally, many programmers were not very familiar with
security, so the protocol did support many mechanisms, or worse,
they were supported incorrectly. Moreover, when a new
authentication method was invented the protocol needed to be
modified to support that mechanism.
<p>This system also was not ideal for application writer. She had
to have a special case for each mechanism she wished her
application to support. Also, the mechanisms were difficult to
implement. Even with a good library, an understanding of how the
mechanism worked was still necessary. Finally if an application
used more than one protocol (for example a mail client might use
IMAP, POP, and SMTP) then "Kerberos V4 for IMAP", "Kerberos V4 for
POP", "Kerberos V4 for SMTP", "CRAM MD5 for IMAP", "CRAM-MD5 for
POP", etc... would need to be written. This could quickly create a
huge number of different mechanism-protocol pairs to implement.
<a name="sasl_rescue"><h3>SASL to the rescue!</h3></a>
<p>SASL hopefully solves all these problems. In practice it makes
many of them easier to deal with.
<p>Protocol designers simply have to support SASL (in particular
RFC 2222). Consequently, any mechanism that supports SASL (just
about anything you would want to use does now) is supported by the
protocol. If a new authentication mechanism is invented the
protocol automatically supports it without any modifications.
<p>Application writers, instead of having to support every mechanism
for every protocol, only need to support SASL for every
protocol. Application writers do not need to understand the
authentication mechanisms at all: the SASL library handles all
that. Also with the Cyrus SASL library if a new mechanism is
invented you do not have rewrite your application at all. You may
not even have to restart your application if it is a long running
process. This is because the Cyrus SASL library loads each mechanism
from a shared library. Simply copying a shared library into a
directory will magically make your application support a new
mechanism.
<a name="briefly"><h2>Briefly</h2></a>
<a name="what_good_for"><h3>What is the Cyrus SASL library good for?</h3></a>
<p>The Cyrus SASL library is good for applications that wish to
use protocols that support SASL authentication. An non-exhaustive
list of these are: IMAP, SMTP, ACAP, and LDAP. Also if you are
making a proprietary system and wish to support authentication it
is a good way of supporting many different authentication types.
<a name="what_do"><h3>What does the Cyrus SASL library do?</h3></a>
<p>From a client point of view, the Cyrus SASL library, given a list of
mechanisms the server supports it will decide the best mechanism
to use and tell you what to send to the server at each step of the
authentication. From a server perspective, it handles
authentication requests from clients.
<a name="what_doesnt"><h3>What doesn't the Cyrus SASL library do?</h3></a>
<p>The Cyrus SASL library is neither network nor protocol aware. It
is up to the application to send the data over the wire as well as
to send the data in the protocol specific manner. With IMAP this
means putting it in the form: <tt>+ [base64'ed data]\r\n</tt>. LDAP
just sends data in binary via bind requests. The Cyrus SASL library
has utility base64 encode and decode routines to help with this.
<a name="client_section"><h2>Client-only Section</h2></a>
<a name="client_typical"><h3>A typical interaction from the client's perspective</h3></a>
<ol>
<li>A client makes a few calls (explained later) to initialize SASL.
<li>Every time the client application makes a new connection it should make a new context that is kept for the life of the connection.
<li>Ask the server for the list of supported mechanisms
<li>Feed this list to the library
<li>Start the authentication with the mechanism the library chose
<li>The server will return some bytes
<li>Give these to the library
<li>The library returns some bytes to the application
<li>Application sends these bytes over the network
<li>repeat the last 4 steps until the server tells you that the authentication is completed
</ol>
<a name="client_code"><h3>How does this look in code</h3></a>
Initialize the library. (done once).
<tt><pre>
int result;
/* attempt to start sasl
* See the section on Callbacks and Interactions for an
* explanation of the variable callbacks
*/
result=sasl_client_init(callbacks);
/* check to see if that worked */
if (result!=SASL_OK) [failure]
</pre></tt>
For every network connection make a new SASL connection:
<tt><pre>
sasl_conn_t *conn; /* The SASL context kept for the life of the connection */
/* client new connection */
result=sasl_client_new("imap", /* The service we are using */
serverFQDN, /* The fully qualified domain name
of the server we're connecting to */
NULL,
0,
&conn); /* allocated on success */
/* check to see if that worked */
if (result!=SASL_OK) [failure]
</pre></tt>
Next get the list of SASL mechanisms the server supports. This
is usually done throught a capability command. Format the list
as a single string seperated by spaces.
Feed this string into SASL to begin the authentication process.
<tt><pre>
sasl_interact_t *client_interact=NULL;
char *out;
unsigned outlen;
do {
result=sasl_client_start(conn, /* the same context from above */
mechlist, /* the list of mechanisms from the server */
NULL,
&client_interact, /* filled in if an interaction is needed */
&out, /* filled in on success */
&outlen, /* filled in on success */
&mechusing);
if (result==SASL_INTERACT)
{
[deal with the interactions. See interactions section below]
}
} while (result==SASL_INTERACT); /* the mechanism may ask us to fill in things
many times. result is SASL_CONTINUE on success */
if (result!=SASL_CONTINUE) [failure]
</pre></tt>
If this is sucessful send the protocol specific command to
start the authentication process. This may or may not allow
for initial data to be sent (see the documentation of the
protocol to see).
<pre>
For IMAP this might look like:
{tag} "AUTHENTICATE" {mechusing}\r\n
A01 AUTHENTICATE KERBEROS_V4\r\n
SMTP looks like:
"AUTH" {mechusing}[ {out base64 encoded}]
AUTH DIGEST-MD5 GHGJJGDDFDKHGHJG=
</pre>
<br>
<a name="client_authentication_step">Check Results</a>
<br>
Next, read what the server sent back. It can be one of three things:
<ol>
<li>Authentication failure. Authentication process is
halted. This might look like <tt>A01 NO Authentication
failure</tt> in IMAP or <tt>501 Failed</tt> in SMTP. Either
retry the authentication or abort.
<li>Authentication success. We're now sucessfully
authenticated. This might look like <tt>A01 OK Authenticated
successful</tt> in IMAP or <tt>235 Authentication
successful</tt> in SMPT. Go <a
href="#client_authentication_success">here</a>
<li>Another step in the authentication process is
necessary. This might look like <tt>+ HGHDS1HAFJ=</tt> in
IMAP or <tt>334
PENCeUxFREJoU0NnbmhNWitOMjNGNndAZWx3b29kLmlubm9zb2Z0LmNvbT4=</tt>
in SMTP. Note it could be an empty string such as <tt>+
\r\n</tt> in IMAP.
</ol>
Convert the continuation data to binary format (this may include base64 decoding it).
Perform another step in the authentication.
<tt><pre>
do {
result=sasl_client_step(conn, /* our context */
in, /* the data from the server */
inlen, /* it's length */
&client_interact, /* this should be unallocated and NULL */
&out, /* filled in on success */
&outlen); /* filled in on success */
if (result==SASL_INTERACT)
{
[deal with the interactions. See below]
}
} while (result==SASL_INTERACT);
if (result!=SASL_OK) [failure]
</pre></tt>
Format the output (variable out of lenght outlen) in the
protocol specific manner and send it across the network to
the server.
<br>
Goto <a href="#client_authentication_step">here</a>
<br><br><br>
<a name="client_authentication_success">Authentication Successful</a><br><br>
<p>Before we're done we need to call sasl_client_step() one
more time to make sure the server isn't trying to fool
us. Some protocols include data along with the last step. If
so this data should be used here. If not use a length of
zero.
<tt><pre>
result=sasl_client_step(conn, /* our context */
in, /* the data from the server */
inlen, /* it's length */
&client_interact, /* this should be unallocated and NULL */
&out, /* filled in on success */
&outlen); /* filled in on success */
if (result!=SASL_OK) [failure]
</pre></tt>
<p>Congradulations. You have successfully authenticated to the server.</p>
<p>Don't throw away the SASL connection object (sasl_conn_t* )
yet though. If a security layer was negotiated you will need
it to encode and decode the data sent over the network.</p>
<br>
Done with connection to server. Dispose of SASL connection
<tt><pre>
sasl_dispose(&conn);
</pre></tt>
Done with SASL forever (application quiting for example).
<tt><pre>
sasl_done();
</pre></tt>
<h3>sasl_client_init</h3>
<pre>
int sasl_client_init(const sasl_callback_t *callbacks);
</pre>
<ul>Paramaters:
<li>callbacks - List of callbacks. See Callbacks section
</ul>
This function initializes the SASL library. This must be called
before any other SASL calls. See the callbacks section for
complete description of callbacks.
<h3>sasl_client_new</h3>
<pre>
int sasl_client_new(const char *service,
const char *serverFQDN,
const sasl_callback_t *prompt_supp,
int secflags,
sasl_conn_t **pconn);
</pre>
<ul>Paramaters:
<li>service - the service name being used. This usually is the protocol name (e.g. "ldap")
<li>serverFQDN - Fully qualified domain name of server
<li>prompt_supp - List off callbacks specific to this connection
<li>secflags - security flags ORed together requested (e.g. SASL_SEC_NOPLAINTEXT)
<li>pconn - the SASL connection object allocated upon success
</ul>
This function creates a new SASL connection object. It should be
called once for every connection you want to authenticate for.
<h3>sasl_client_start</h3>
<pre>
int sasl_client_start(sasl_conn_t *conn,
const char *mechlist,
sasl_secret_t *secret,
sasl_interact_t **prompt_need,
char **clientout,
unsigned *clientoutlen,
const char **mech);
</pre>
<ul>Paramaters:
<li>conn - the SASL connection object gotten from sasl_client_new()
<li>mechlist - the list of mechanisms to try seperated by spaces
<li>secret - secret (optional)
<li>prompt_need - filled in when a SASL_INTERACT is returned
<li>clientout - filled in upon sucess with data to send to server
<li>clientoutlen - length of that data
<li>mech - filled in with mechanism being used
</ul>
This function starts an authentication session. It takes a list of
possible mechanisms (usually gotten from the server through a
capability command) and chooses the "best" mechanism to try. Upon
success clientout is filled in with data to send to the server.
<h3>sasl_client_step</h3>
<pre>
int sasl_client_step(sasl_conn_t *conn,
const char *serverin,
unsigned serverinlen,
sasl_interact_t **prompt_need,
char **clientout,
unsigned *clientoutlen);
</pre>
<ul>Paramaters:
<li>conn - the SASL connection object gotten from sasl_client_new()
<li>serverin - data from the server
<li>serverinlen - length of data from the server
<li>prompt_need - filled in with a SASL_INTERACT is returned
<li>clientout - filled in upon sucess with data to send to server
<li>clientoutlen - length of that data
</ul>
This step preforms a step in the authentication process. It takes
the data from the server (serverin) and outputs data to send to
the server (clientout) upon success. SASL_CONTINUE is returned if
another step in the authentication process is necesssary. SASL_OK
is returned if we're all done.
<a name="server_section"><h2>Server-only Section</h2></a>
<a name="server_typical"><h3>A typical interaction from the server's perspective</h3></a>
The server makes a few Cyrus SASL calls for initialization. When
it gets a new connection it should make a new context for that
connection immediatly. The client may then request a list of
mechanisms the server suppports. The client also may request to
authenticate at some point. The client will specify the mechanism
it wishes to use. The server should negotiate this authentication
and keep around the context afterwards for encoding and decoding
the layers.
<a name="server_code"><h3>How does this look in code?</h3></a>
Initialization (done once). The application name is used for
reading configuration information.
<tt><pre>
int result;
/* Initialize SASL */
result=sasl_server_init(callbacks, /* Callbacks supported */
"TestServer"); /* Name of the application */
</pre></tt>
This should be called for each new connection. It probably should
be called right when the socket is accepted. The service name is
used for PAM authentication if applicable.
<pre><tt>
sasl_conn_t *conn;
int result;
/* Make a new context for this connection */
result=sasl_server_new("smtp",
NULL, /* my fully qualified domain name;
NULL says use gethostname() */
NULL, /* The user realm used for password
lookups; NULL means default to serverFQDN
Note: This does not affect Kerberos */
NULL, /* Callbacks supported only for this connection */
SASL_SECURITY_LAYER, /* I support encryption layers;
otherwise pass 0 */
&conn);
</pre></tt>
When a client requests the list of mechanisms supported by the server. This particular call might produce the string: <i>"{PLAIN, KERBEROS_V4, CRAM-MD5, DIGEST-MD5}"</i>
<pre><tt>
result=sasl_listmech(conn, /* The context for this connection */
NULL, /* not supported */
"{", /* What to prepend the string with */
", ", /* What to seperate mechanisms with */
"}", /* What to append to the string */
&result_string, /* The produced string. Allocated by library */
&string_length, /* length of the string */
&number_of_mechanisms); /* Number of mechanisms in the string */
</pre></tt>
When a client requests to authenticate:
<tt><pre>
int result;
const char *errstr;
char *out;
unsigned outlen;
result=sasl_server_start(conn, /* context */
mechanism_client_chose,
clientin, /* the optional string the client gave us */
clientinlen, /* and it's length */
&out, /* allocated by library on success. Might not be NULL
terminated */
&outlen,
&errstr); /* error string filled in on failure */
if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
{
failure. Send client the protocol specific message that says authentication failed
}
if (result==SASL_OK)
{
client authentication suceeded. Send client the protocol specific message
to say that authentication is complete.
}
</pre></tt>
When a response is returned by the client. <i>clientin</i> is the
data from the client decoded from protocol specific format to a
string of bytes of length <i>clientinlen</i>. This step may occur
zero or more times. An application should be able to deal with it
occuring an arbitrary number of times.
<tt><pre>
int result;
result=sasl_server_step(conn,
clientin, /* what the client gave */
clientinlen, /* it's length */
&out, /* allocated by library on success.
Might not be NULL terminated */
&outlen,
&errstr); /* error string sometimes filled in on failure */
if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
{
failure. Send client the protocol specific message that says authentication failed
}
if (result==SASL_OK)
{
client authentication suceeded. Send client the protocol specific message
to say that authentication is complete.
}
send data 'out' with length 'outlen' over the network in protocol
specific format
</pre></tt>
<a name="sasl_server_init"><h3>sasl_server_init</h3></a>
int sasl_server_init(const sasl_callback_t *callbacks,
const char *appname);
<ul>Parameters:
<li>callbacks - A list of callbacks supported by the application
(see Interaction and Callbacks section)
<li>appname - A string of the name of the application. This string
is what is used when loading configuration options.
</ul>
sasl_server_init() initializes the session. This should be the
first function called. In this function the shared library
authentication mechanisms are loaded.
<a name="sasl_server_new"><h3>sasl_server_new</h3></a>
int sasl_server_new(const char *service,
const char *serverFQDN,
const char *user_realm,
const sasl_callback_t *callbacks,
int secflags,
sasl_conn_t **pconn);
<ul>Parameters:
<li>service - The name of the service you are supporting. This
might be "acap" or "smtp". This is used by Kerberos mechanisms and
possibly other mechanisms. It is also used for PAM authentication.
<li>serverFQDN - This is the fully qualified domain name of the
server (i.e. your hostname); if NULL, the library calls
<tt>gethostbyname()</tt>.
<li>user_realm - The realm the connected client is in. The
Kerberos mechanisms ignore this parameter and default to the local
Kerberos realm. A value of NULL makes the library default,
usually to the serverFQDN; a value of "" specifies that the client
should specify the realm; this also changes the semantics of "@"
in a username for mechanisms that don't support realms.
<li>callbacks - Additional callbacks that you wish only to apply
to this connection.
<li>secflags - security flags. SASL_SECURITY_LAYER if your
application supports security layers. 0 otherwise.
<li>pconn - Context. Filled in on success.
</ul>
<a name="sasl_server_start"><h3>sasl_server_start</h3></a>
<pre>
int sasl_server_start(sasl_conn_t *conn,
const char *mech,
const char *clientin,
unsigned clientinlen,
char **serverout,
unsigned *serveroutlen,
const char **errstr);
</pre>
<ul>Paramaters:
<li>conn - The context for the connection
<li>mech - The authentication mechanism the client wishes to try
(e.g. <tt>KERBEROS_V4</tt>)
<li>clientin - Initial client challenge bytes. Note: some
protocols do not allow this. If this is the case passing NULL
is valid
<li>clientinlen - The length of the challenge. 0 is there is none.
<li>serverout - allocated and filled in by the function. These
are the bytes that should be encoded as per the protocol and
sent over the network back to the client.
<li>serveroutlen - length of bytes to send to client
<li>errstr - string possibly filled in on error
</ul>
This function begins the authentication process with a client. If
the program returns SASL_CONTINUE that means <tt>serverout</tt>
should be sent to the client. If SASL_OK is returned that means
authentication is complete and the application should tell the
client the authentication was sucessful. Any other return code
means the authentication failed and the client should be notified
of this.
<a name="sasl_server_step"><h3>sasl_server_step</h3></a>
<pre>
int sasl_server_step(sasl_conn_t *conn,
const char *clientin,
unsigned clientinlen,
char **serverout,
unsigned *serveroutlen,
const char **errstr);
</pre>
<ul>Paramaters:
<li>conn - The context for the connection
<li>clientin - Data sent by the client.
<li>clientinlen - The length of the client data. Note that this may be 0
<li>serverout - allocated and filled in by the function. These
are the bytes that should be encoded as per the protocol and
sent over the network back to the client.
<li>serveroutlen - length of bytes to send to client. Note that this may be 0
<li>errstr - string possibly filled in on error
</ul>
This function preforms a step of the authentication. This may need
to be called an arbitrary number of times. If the program returns
SASL_CONTINUE that means <tt>serverout</tt> should be sent to the
client. If SASL_OK is returned that means authentication is
complete and the application should tell the client the
authentication was sucessful. Any other return code means the
authentication failed and the client should be notified of this.
<a name="sasl_listmech"><h3>sasl_listmech</h3></a>
<pre>
int sasl_listmech(sasl_conn_t *conn,
const char *user,
const char *prefix,
const char *sep,
const char *suffix,
char **result,
unsigned *plen,
unsigned *pcount);
</pre>
<ul>Paramaters:
<li>conn - The context for this connection
<li>user - Currently not implemented
<li>prefix - The string to prepend
<li>sep - The string to seperate mechanisms with
<li>suffix - The string to end with
<li>result - Resultant string
<li>plen - Number of characters in the result string
<li>pcount - Number of mechanisms listed in the result string
</ul>
This function is used to create a string with a list of SASL
mechanisms supported by the server. This string is often needed
for a capability statement.
<a name="sasl_checkpass"><h3>sasl_checkpass</h3></a>
<pre>
int sasl_checkpass(sasl_conn_t *conn,
const char *user,
unsigned userlen,
const char *pass,
unsigned passlen,
const char **errstr);
</pre>
<ul>Parameters:
<li>conn - The context for this connection
<li>user - The user trying to check the password for
<li>userlen - The user length
<li>pass - The password
<li>passlen - The password length
<li>errstr - Possibly filled in on failure
</ul>
This checks a plaintext password <i>pass</i> for user <i>user</i>
Some protocols have legacy systems for plaintext authentication
where this might be used. Note that this has no relationship to
the PLAIN mechanism but it does check the password the same way in
the Cyrus SASL library.
<a name="common_section"><h2>Common Section</h2></a>
<a name="callbacks_interactions"><h3>Callbacks and Interactions</h3></a>
When the application starts and calls sasl_client_init() you must
specify for what data you support callbacks and/or
interactions. These are for the library getting information needed
for authentication from the application. This is needed for things
like authentication name and password. If you do not declare
supporting a callback you will not be able to use mechanisms that
need that data. A callback is for when you have the information
before you start the authentication. The SASL library calls a
function you specify and your function fills in the requested
information. For example if you had the userid of the user already
for some reason. An interaction is usually for things you support
but will need to ask the user for
(e.g. password). sasl_client_start() or sasl_client_step() will
return SASL_INTERACT. This will be a list of sasl_interact_t's
which contain a human readable string you can prompt the user
with, a possible computer readable string, and a default
result. The nice thing about interactions is you get them all at
once so if you had a GUI application you could bring up a dialog
box asking for authentication name and password together instead
of one at a time.
<p><b>Memory management:</b> Any data passed by the application to
the library will be copied by the library. The application is
responsible for freeing any memory allocated in the callbacks, if
any. Interactions have the same memory management requirements as
callbacks.
<p><b>Apparent random exception:</b> The secret returned from
SASL_CB_PASS should be allocated with <tt>malloc()</tt> and will
be freed by the library.
<p>For a detailed description of what each of the callback types are
see the sasl.h file. Here are some brief explanations:
<ul>
<li> SASL_CB_AUTHNAME - the name of the user authenticating
<li> SASL_CB_USER - the name of the user acting for.
(for example postman delivering mail for tmartin might have an
AUTHNAME of postman and a USER of tmartin)
<li> SASL_CB_PASS - password for AUTHNAME
<li> SASL_CB_GETREALM - Realm of the server
</ul>
An example of a way to handle callbacks:
<pre>
/* callbacks we support. This is a global variable at the
top of the program */
static sasl_callback_t callbacks[] = {
{
SASL_CB_GETREALM, NULL, NULL /* we'll just use an interaction if this comes up */
}, {
SASL_CB_USER, NULL, NULL /* we'll just use an interaction if this comes up */
}, {
SASL_CB_AUTHNAME, &getauthname_func, NULL /* A mechanism should call getauthname_func
if it needs the authentication name */
}, {
SASL_CB_PASS, &getsecret_func, NULL /* Call getsecret_func if need secret */
}, {
SASL_CB_LIST_END, NULL, NULL
}
};
static int getsecret_func(sasl_conn_t *conn,
void *context __attribute__((unused)),
int id,
sasl_secret_t **psecret)
{
[ask the user for their secret]
[allocate psecret and insert the secret]
return SASL_OK;
}
static int getauthname_func(void *context,
int id,
const char **result,
unsigned *len)
{
if (id!=SASL_CB_AUTHNAME) return SASL_FAIL;
[fill in result and len]
return SASL_OK;
}
in the main program somewhere
sasl_client_init(callbacks);
</pre>
<a name="example_section"><h2>Example applications that come with the Cyrus SASL library</h2></a>
<a name="sample_client"><h3><tt>sample-client</tt> and <tt>sample-server</tt></h3></a>
The sample client and server included with this distribution were
initially written to help debug mechanisms. They base64 encode all
the data and print it out on standard output.
<p>Make sure that you set the IP addresses, the username, the
authenticate name, and anything else on the command line (some
mechanisms depend on these being present).
<p>Also, sometimes you will receive a get "<tt>realm: Information not
available</tt>" message, or similar; this is due to the fact that some
mechanisms do not support realms and therefore never set it.
<a name="cyrus_imapd"><h3>Cyrus imapd v1.6.0 or later</h3></a>
The Cyrus IMAP server now incorporates SASL for all its
authentication needs. It is a good example of a fairly large server
application. Also of interest is the prot layer, included in
libcyrus. This is a stdio-like interface that automatically takes
care of layers using a simple "<tt>prot_setsasl()</tt>" call.
<p>Cyrus imapd also sets a <tt>SASL_CB_PROXY_POLICY</tt> callback,
which should be of interest to many applications.
<a name="imtest"><h3><tt>imtest</tt>, from cyrus imapd 1.6.0 or later</h3></a>
<tt>imtest</tt> is an application included with Cyrus imapd. It is a
very simple IMAP client, but should be of interest to those writing
applications. It also uses the prot layer, but it is easy to
incorporate similar support without using the prot layer.
<a name="random_things"><h2>Miscelaneous Information</h2></a>
<a name="empty_exchanges"><h3>Empty exchanges</h3></a>
Some SASL mechanisms intentionally send no data; an application should
be prepared to either send or receive an empty exchange. The SASL
profile for the protocol should define how to send an empty string;
make sure to send an empty string when requested, and when receiving
an empty string make sure that the "<tt>inlength</tt>" passed in is 0.
<a name="not_implemented"><h2>What's not implemented</h2></a>
Some parts of this API are not implemented by this implementation. A
brief outline of these features (and what might come of them) is in
this section.
<a name="credentials"><h3>Credentials</h3></a>
None of the modules support passing credentials. In the future, we
hope to add credential passing to the modules that support it (most
likely the Kerberos modules). Thus, an application that specifies
<tt>SASL_SEC_PASS_CREDENTIALS</tt> will not receive any mechanisms.
The functions <tt>sasl_cred_install()</tt> and
<tt>sasl_cred_uninstall()</tt> do nothing.
<p>It's likely that the credential API will change, and it's also
likely that you'll need the Cyrus SASL library on both sides of the
connection to make it work.
<a name="secrets"><h3>Secrets</h3></a>
<tt>sasl_client_auth()</tt> is unimplemented. It is unclear what this
is intended for. If we implement it, it will probably be for
generating secrets for fast reauthentication.
<p>The "<tt>secret</tt>" parameter to <tt>sasl_client_start()</tt> is
unused. It is likely that we will use this for fast reauthentication.
<a name="idle"><h3>Idle</h3></a>
While the implementation & plugins correctly implement the idle calls,
none of them currently do anything.
<hr>
<address><a href="mailto:tmartin+@andrew.cmu.edu">Timothy L Martin</a></address>
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