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SASL Programmer's GuideNOTE: This is a work in progress. Any contributions would be very appreciated
ContentsIntroductionAbout this GuideThis 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.
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.
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 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.
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.
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.
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.
How does this look in codeInitialize the library. (done once).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: 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]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. 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]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). 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= Check Results Next, read what the server sent back. It can be one of three things:
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]Format the output (variable out of lenght outlen) in the protocol specific manner and send it across the network to the server. Goto here Authentication Successful 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.
Congradulations. 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. Done with connection to server. Dispose of SASL connection sasl_dispose(&conn);Done with SASL forever (application quiting for example). sasl_done(); sasl_client_initint sasl_client_init(const sasl_callback_t *callbacks);
sasl_client_newint sasl_client_new(const char *service, const char *serverFQDN, const sasl_callback_t *prompt_supp, int secflags, sasl_conn_t **pconn);
sasl_client_startint 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);
sasl_client_stepint sasl_client_step(sasl_conn_t *conn, const char *serverin, unsigned serverinlen, sasl_interact_t **prompt_need, char **clientout, unsigned *clientoutlen);
Server-only SectionA typical interaction from the server's perspectiveThe 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.How does this look in code?Initialization (done once). The application name is used for reading configuration information.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. The service name is used for PAM authentication if applicable. 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);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 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 */When a client requests to authenticate: 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. }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 should be able to deal with it occuring 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, &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 sasl_server_initint sasl_server_init(const sasl_callback_t *callbacks, const char *appname);
sasl_server_newint sasl_server_new(const char *service, const char *serverFQDN, const char *user_realm, const sasl_callback_t *callbacks, int secflags, sasl_conn_t **pconn);
sasl_server_startint sasl_server_start(sasl_conn_t *conn, const char *mech, const char *clientin, unsigned clientinlen, char **serverout, unsigned *serveroutlen, const char **errstr);
sasl_server_stepint sasl_server_step(sasl_conn_t *conn, const char *clientin, unsigned clientinlen, char **serverout, unsigned *serveroutlen, const char **errstr);
sasl_listmechint sasl_listmech(sasl_conn_t *conn, const char *user, const char *prefix, const char *sep, const char *suffix, char **result, unsigned *plen, unsigned *pcount);
sasl_checkpassint sasl_checkpass(sasl_conn_t *conn, const char *user, unsigned userlen, const char *pass, unsigned passlen, const char **errstr);
Common SectionCallbacks and InteractionsWhen 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.Memory management: 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. Apparent random exception: The secret returned from SASL_CB_PASS should be allocated with malloc() and will be freed by the library. 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); Example applications that come with the Cyrus SASL librarysample-client and sample-serverThe 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.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 also sets a SASL_CB_PROXY_POLICY callback,
which should be of interest to many applications.
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.
The "secret" parameter to sasl_client_start() is
unused. It is likely that we will use this for fast reauthentication.
Timothy L Martin Last modified: Wed Apr 5 15:24:47 EDT 2000 |