This guide gives a tutorial on the use of the Cyrus SASL library
for a client or server application. It complies with versions including
and after 2.0.0. The following pages should only be
considered a guide, not the final word on programming with the
Cyrus SASL library. Consult the header files in the distribution in
the case of ambiguities.
What is SASL?
SASL stands for Simple Authentication Security Layer and is
explained in RFC
2222. That document is very difficult to understand however
and it should be unnecessary to consult it.
Background
How did the world work before SASL?
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.
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.
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.
SASL to the rescue!
SASL hopefully solves all these problems. In practice it makes many of them easier to deal with.
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.
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.
Cyrus SASL version 2 supports a much improved API over version 1,
that allows for much smarter and faster memory allocation for the
mechanisms as well as the applications. It is also provides for
several new types of plugins to allow for greater overall flexibility.
Unfortunately, though similar, this new API is completely incompatible
with the old API, and applications will need to be rewritten.
Briefly
What is the Cyrus SASL library good for?
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.
What does the Cyrus SASL library do?
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.
What doesn't the Cyrus SASL library do?
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: + [base64'ed data]\r\n. LDAP
just sends data in binary via bind requests. The Cyrus SASL library
has utility base64 encode and decode routines to help with this.
Client-only Section
A typical interaction from the client's perspective
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]
For every network connection, make a new SASL connection:
/* The SASL context kept for the life of the connection */
sasl_conn_t *conn;
/* 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, NULL, /* Local and remote IP
address strings
(NULL disables mechanisms
which require this info)*/
NULL, /* connection-specific
callbacks */
0, /* security flags */
&conn); /* allocated on success */
/* check to see if that worked */
if (result!=SASL_OK) [failure]
Next get the list of SASL mechanisms the server supports. This
is usually done through a capability command. Format the list
as a single string separated by spaces.
Feed this string into SASL to begin the authentication process.
sasl_interact_t *client_interact=NULL;
const char *out, *mechusing;
unsigned outlen;
do {
result=sasl_client_start(conn, /* the same context from
above */
mechlist, /* the list of mechanisms
from the server */
&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]
Note that you do not need to worry about the allocation and freeing
of the output buffer out. This is all handled inside of the mechanism.
It is important to note, however, that the output buffer is not valid
after the next call to sasl_client_start or
sasl_client_step.If this is successful 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).
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=
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 || result == SASL_CONTINUE);
if (result!=SASL_OK) [failure]
Format the output (variable out of length outlen) in the
protocol specific manner and send it across the network to
the server.
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.
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]
Congratulations. You have successfully authenticated to the server.
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.
sasl_dispose(&conn);
If you are done with SASL forever (application quiting for example):
sasl_done();
int sasl_client_init(const sasl_callback_t *callbacks);
int sasl_client_new(const char *service,
const char *serverFQDN,
const char *iplocalport,
const char *ipremoteport,
const sasl_callback_t *prompt_supp,
unsigned secflags,
sasl_conn_t **pconn);
int sasl_client_start(sasl_conn_t *conn,
const char *mechlist,
sasl_interact_t **prompt_need,
const char **clientout,
unsigned *clientoutlen,
const char **mech);
int sasl_client_step(sasl_conn_t *conn,
const char *serverin,
unsigned serverinlen,
sasl_interact_t **prompt_need,
const char **clientout,
unsigned *clientoutlen);
int result;
/* Initialize SASL */
result=sasl_server_init(callbacks, /* Callbacks supported */
"TestServer"); /* Name of the application */
This should be called for each new connection. It probably should
be called right when the socket is accepted.
sasl_conn_t *conn;
int result;
/* Make a new context for this connection */
result=sasl_server_new("smtp", /* Registered name of service */
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, NULL, /* IP Address information strings */
NULL, /* Callbacks supported only for this connection */
0, /* security flags (security layers are enabled
* using security properties, separately)
&conn);
When a client requests the list of mechanisms supported by the server. This particular call might produce the string: "{PLAIN, KERBEROS_V4, CRAM-MD5, DIGEST-MD5}"
result=sasl_listmech(conn, /* The context for this connection */
NULL, /* not supported */
"{", /* What to prepend the string with */
", ", /* What to separate mechanisms with */
"}", /* What to append to the string */
&result_string, /* The produced string. */
&string_length, /* length of the string */
&number_of_mechanisms); /* Number of mechanisms in
the string */
When a client requests to authenticate:
int result;
const 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, /* The output of the library.
Might not be NULL terminated */
&outlen);
if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
[failure. Send protocol specific message that says authentication failed]
else if (result==SASL_OK)
[authentication succeeded. Send client the protocol specific message
to say that authentication is complete]
else
[send data 'out' with length 'outlen' over the network in protocol
specific format]
When a response is returned by the client. clientin is the
data from the client decoded from protocol specific format to a
string of bytes of length clientinlen. This step may occur
zero or more times. An application must be able to deal with it
occurring an arbitrary number of times.
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);
if ((result!=SASL_OK) && (result!=SASL_CONTINUE))
[failure. Send protocol specific message that says authentication failed]
else if (result==SASL_OK)
[authentication succeeded. Send client the protocol specific message
to say that authentication is complete]
else
[send data 'out' with length 'outlen' over the network in protocol
specific format]
This continues until authentication succeeds. When the connection is
concluded, make a call to sasl_dispose as with the client
connection.
int sasl_server_start(sasl_conn_t *conn,
const char *mech,
const char *clientin,
unsigned clientinlen,
const char **serverout,
unsigned *serveroutlen);
int sasl_server_step(sasl_conn_t *conn,
const char *clientin,
unsigned clientinlen,
const char **serverout,
unsigned *serveroutlen);
int sasl_listmech(sasl_conn_t *conn,
const char *user,
const char *prefix,
const char *sep,
const char *suffix,
const char **result,
unsigned *plen,
unsigned *pcount);
int sasl_checkpass(sasl_conn_t *conn,
const char *user,
unsigned userlen,
const char *pass,
unsigned passlen);
Memory management:As in the rest of the SASLv2 API, whoever allocates the memory is responsible for freeing it. In almost all cases this should be fairly easy to manage, however a slight exception where the interaction sasl_interact_t structure is allocated and freed by the library, while the results are allocated and freed by the application.
For a detailed description of what each of the callback types are see the sasl.h file. Here are some brief explanations:
/* 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);
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).
Also, sometimes you will receive a get "realm: Information not
available" message, or similar; this is due to the fact that some
mechanisms do not support realms and therefore never set it.
Cyrus imapd v1.6.0 or later
This uses Cyrus SASL prior to v2 only. We should update this section
when Cyrus Sasl moves to SASL v2
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 "prot_setsasl()" call.
Cyrus imapd also sets a SASL_CB_PROXY_POLICY callback,
which should be of interest to many applications.
imtest, from cyrus imapd 1.6.0 or later
This uses Cyrus SASL prior to v2 only. We should update this section
when Cyrus Sasl moves to SASL v2
imtest 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.
Miscellaneous Information
Empty exchanges
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 "inlength" passed in is 0.
Note especially that the distinction between the empty string "" and the lack of a string (NULL) is extremely important in many cases (most notably, the client-send first scenario), and the application must ensure that it is passing the correct values to the SASL library at all times.