NAME

libsaslc, saslc.d, saslc_alloc, saslc_end, saslc_init, saslc_sess_init, saslc_sess_end, saslc_sess_getprop, saslc_sess_setprop, saslc_sess_cont, saslc_sess_decode, saslc_sess_encode, saslc_sess_getmech, saslc_sess_strerror, saslc_strerror — Simple Authentication and Security Layer client library

LIBRARY

Simple Authentication and Security Layer client library (libsaslc, -lsaslc)

SYNOPSIS

#include <saslc.h>

saslc_t *
saslc_alloc(void);

int
saslc_end(saslc_t *ctx);

int
saslc_init(saslc_t *ctx, const char *appname, const char *cfgpath);

saslc_sess_t *
saslc_sess_init(saslc_t *ctx, const char *mechs, const char *secopts);

void
saslc_sess_end(saslc_sess_t *sess);

const char *
saslc_sess_getprop(saslc_sess_t *sess, const char *key);

int
saslc_sess_setprop(saslc_sess_t *sess, const char *key, const char *value);

int
saslc_sess_cont(saslc_sess_t *sess, const void *in, size_t inlen, void* *out, size_t *outlen);

ssize_t
saslc_sess_decode(saslc_sess_t *sess, const void *in, size_t inlen, void* *out, size_t *outlen);

ssize_t
saslc_sess_encode(saslc_sess_t *sess, const void *in, size_t inlen, void* *out, size_t *outlen);

const char *
saslc_sess_getmech(saslc_sess_t *sess);

const char *
saslc_sess_strerror(saslc_sess_t *sess);

const char *
saslc_strerror(saslc_t *ctx);

DESCRIPTION

The libsaslc library offers a client interface for the Simple Authentication and Security Layer (SASL). The library is heavily influenced by its use with postfix(1).

FUNCTIONS

The following functions are available in the library.

saslc_alloc()

The saslc_alloc() function allocates and returns a new saslc context. The context is uninitialized: see saslc_init(). Returns NULL on error.

saslc_end(ctx)

The saslc_end() function destroys and deallocate resources used by the context ctx. The context shouldn't have any sessions assigned to it. Returns 0 on success and -1 if the context has active sessions and cannot be deallocated.

saslc_init(ctx, appname, cfgpath)

The saslc_init() function initializes the saslc context ctx. Based on the application name appname, it also parses the configuration files as indicated by cfgpath, sets up the context and mechanism dictionaries, and creates mechanism list for the context. If cfgpath is NULL, it checks the environment variable SASLC_CONFIG for a location and if that is not found it uses the default path /etc/saslc.d. Returns 0 on success and -1 on failure.

saslc_sess_init(ctx, mechs, secopts)

The saslc_sess_init() function creates new session assigned to the ctx context. The function chooses the mechanism to use for authentication from the mechs list taking into account the requirements from the secopts list. Both lists may be space or comma delimited. The first matching mechanism from the mechs list is used. See CONFIGURATION below for the supported mechanisms. The valid security options are

“noanonymous”

reject anonymous mechanisms

“noplaintext”

reject plaintext mechanisms

“nodictionary”

reject mechanisms prone to dictionary attack

“noactive”

reject mechanisms prone to active non-dictionary attacks

“mutual”

require mutual authentication mechanisms

Unknown security options are ignored. Returns a session handle or NULL on error or no match.

saslc_sess_end(sess)

The saslc_sess_end() function ends the sasl session sess. It destroys and deallocates all internal resources. This does not fail.

saslc_sess_getprop(sess, key)

The saslc_sess_getprop() function gets the property indicated by the key from the saslc dictionaries. Dictionaries are searched in following order: session sess dictionary, context dictionary (global configuration), and mechanism dictionary. Returns the property value or NULL if the property is not found.

saslc_sess_setprop(sess, key, value)

The saslc_sess_setprop() function sets the property indexed by key to the value value in the session sess dictionary. If the property already exists in the session dictionary, then the previous value is replaced by the new value. If value is NULL, then any previous value in the session dictionary is removed. Returns 0 on success or -1 on failure.

saslc_sess_cont(sess, in, inlen, out, outlen)

The saslc_sess_cont() function performs one step of the sasl authentication. It reads inlen bytes of input data (from the server) from the in buffer and stores outlen bytes of output data in out (for the server). The user is responsible for freeing memory allocated for out. It returns 0 if the authentication process is completed, 1 if another step is required, and -1 on error. Note that the completion of authentication process does not mean the client is authenticated; that is determined by the server.

saslc_sess_decode(sess, in, inlen, out, outlen)

The saslc_sess_encode() and saslc_sess_decode() functions are used to provide the integrity (“auth-int”) and confidentiality (“auth-int”) layers for mechanisms that provide them. They encode and, respectively, decode inlen bytes of data from the in buffer using the method negotiated during authentication. On error they return -1. Otherwise, they return the number of bytes consumed from in and output outlen bytes of data in the out buffer. The user is responsible for freeing memory allocated for out. If outlen is 0, more data is needed before anything can be output. Unused input data is stored internally for use in subsequent calls.

When decoding, the internal buffers can only be flushed by providing the missing packet data and it is an error to call ssalc_sess_decode() with inlen = 0. The first call of saslc_sess_decode() in a session must begin at the start of a packet. Subsequent calls need not be aligned on packet boundaries.

saslc_sess_encode(sess, in, inlen, out, outlen)

As described above, saslc_sess_encode() encodes inlen bytes of data from the in buffer. Note that unlike when decoding, the internal buffer may be flushed through the encoder by calling saslc_sess_encode() with inlen = 0. In this case, saslc_sess_encode() returns the number of bytes that were flushed from the internal buffer.

saslc_sess_getmech(sess)

The saslc_sess_getmech() function returns the name of the mechanism used in the session sess. The function does not fail.

saslc_sess_strerror(sess)

The saslc_sess_strerror() returns the error message associated with the session sess.

saslc_strerror(ctx)

The saslc_strerror() function operates as saslc_sess_strerror(), but instead returns the error message string for the last error in the context ctx. Neither function will ever return NULL.

CONFIGURATION

The library uses three types of dictionaries: context (or global), session, and mechanism, and they are searched in that order by saslc_getprop() and the first matching entry is taken. The context and mechanism dictionaries are loaded from configuration files, while the session dictionary is loaded by the caller via saslc_setprop().

The configuration file <cfgpath>/<appname>/saslc.conf is used for the configuration context. The <cfgpath>/<appname>/mech/<mechanism>.conf file is used for the mechanism configuration. The <cfgpath> is /etc/saslc.d by default, but this may be overridden by the environment variable SASLC_CONFIG, which in turn may be overridden by saslc_init(). The <appname> is saslc by default, but may also be overridden by saslc_init(). Finally, the <mechanism> is the mechanism in use by the session as returned by saslc_sess_getmech(). Note that this name is case sensitive. The currently supported mechanisms are

ANONYMOUS

See RFC 2245 and RFC 4505.

CRAM-MD5

See RFC 2195.

DIGEST-MD5

See RFC 2831.

EXTERNAL

See RFC 2222 section 7.4 and RFC 4422 appendix A.

GSSAPI

See RFC 2222 section 7.2 and RFC 4752. This requires GSS, Heimdal, or MIT Kerberos.

LOGIN

Non-standard, but common.

PLAIN

See RFC 2595 and RFC 4616.

If any of the mechanism files are missing they are silently ignored, unless debugging is enabled.

The configuration files consists of lines of the form:

# comment line 
<key>    <value>    [# comment]

The <key> is a string beginning with an alpha character (isalpha(3)) followed by any number of alpha numeric (isalnum(3)) or underscore ‘_’ characters; this is case sensitive. The <value> is a number or a quoted string. More than one <key> and <value> pair may occur on a single line, but they may not be broken across lines. A ‘#’ character (outside a quoted string) indicates that the rest of the line is a comment.

NOTE: Currently, no escaping is supported in strings, so they may not contain quotes. Numbers must be between 0 and LLONG_MAX, inclusive. Any base supported by strtoll(3) is allowed.

PROPERTIES

Most of the control of the library behavior is done via setting various properties in the context or mechanism dictionaries via the configuration files or in the session dictionary with saslc_setprop(). The following properties are currently used, as defined in saslc.h:

SASLC_PROP_AUTHCID (“AUTHCID”)

The authentication name (or username) to authenticate with. Used by all mechanisms except EXTERNAL.

SASLC_PROP_AUTHZID (“AUTHZID”)

The authorization string to use. By default, this string is empty. Used by the DIGEST-MD5, EXTERNAL, and PLAIN mechanisms.

SASLC_PROP_BASE64IO (“BASE64IO”)

If true ("true", "yes", or nonzero), then input and output strings are base64 encoded. Any other value is false and the input and output strings are not base64 encoded. By default, this is assumed true. Used by all mechanisms.

SASLC_PROP_CIPHERMASK (“CIPHERMASK”)

The mask of ciphers to use with the DIGEST-MD5 mechanism when using the “auth-conf” QOP. By default all supported ciphers are used, but they may be limited by a comma delimited list of cipher names. The recognized cipher names for DIGEST-MD5 are:

3des

Triple-DES Cipher in CBC "two keys" mode with 112 bit key

aes

AES Cipher in CBC mode with 128 bit key

des

DES Cipher in CBC mode with 56 bit key

rc4

RC4 Cipher with 128 bit key

rc4-40

RC4 Cipher with 40 bit key

rc4-56

RC4 Cipher with 56 bit key

The default value is “des,3des,rc4,rc4_40,rc4_56,aes”. (Note that “aes” is not part of the official standard.) Used by the DIGEST-MD5 mechanism.

SASLC_PROP_DEBUG (“DEBUG”)

If true, then enable debug messages. This is implemented as a global variable so it will affect all sessions. If set via saslc_sess_setprop(), it should be set before the first call to saslc_sess_cont(). (Also see the environment variable SASLC_ENV_DEBUG in the ENVIRONMENT section below.)

SASLC_PROP_HOSTNAME (“HOSTNAME”)

The fully qualified domain name of the server host. Used by the DIGEST-MD5 and GSSAPI mechanisms.

SASLC_PROP_MAXBUF (“MAXBUF”)

The size of the decode buffer. This info is sent to the server so that it doesn't send packets that won't fit in the decode buffer when decoded. Used by the DIGEST-MD5 and GSSAPI mechanisms.

SASLC_PROP_PASSWD (“PASSWD”)

The password to authenticate with. Used by the CRAM-MD5, DIGEST-MD5, LOGIN, and PLAIN mechanisms.

SASLC_PROP_QOPMASK (“QOPMASK”)

The mask of QOP (quality of protection) to use with the DIGEST-MD5 and GSSAPI mechanisms. By default all supported QOP values are allowed, but they may be limited by a comma delimited list of QOP values. The recognized QOP values are:

auth

authentication only

auth-int

authentication with integrity

auth-conf

authentication with confidentiality

so the default value of the mask is “auth,auth-int,auth-conf”. Used by the DIGEST-MD5 and GSSAPI mechanisms.

SASLC_PROP_REALM (“REALM”)

A comma delimited list of possible realms to use for authentication. The format of each element in the list is “[<hostname>:]<realm>”. The user specified realm is the first realm in the list with a matching hostname or, if none is found, the first realm in the list with no hostname. If the server provides a list of realms, the one matching the user specified realm is selected. If no match is found or if the user didn't provide a realm, the first realm provided by the server is selected. If the server doesn't provide any realms, use the user specified realm if there is one, or the hostname if not. This is useful when the server provides multiple realms or no realm. Used by the DIGEST-MD5 mechanism.

SASLC_PROP_SECURITY (“SECURITY”)

A comma delimited list of extra security option flags that will be “or”-ed together with those passed to saslc_sess_init(). Since these flags are used to choose the session mechanism, they are only effective if they are in the context configuration file. (See the CONFIGURATION section and the saslc_sess_init() function.)

SASLC_PROP_SERVICE (“SERVICE”)

The service being used, e.g., smtp, imap, etc. Used by the DIGEST-MD5 and GSSAPI mechanisms.

SASLC_PROP_SERVNAME (“SERVNAME”)

A comma delimited list of possible service names with elements of the form “[<hostname>:]<serv-name>” and with the same rules as for the SASLC_PROP_REALM list. This should only be used if the client uses a DNS name for the service that is different from the FQDN of the server. For example, the service name example.com might resolve (via SRV or MX records) into a set of other DNS names, one of which, mail3.example.com, is the FQDN of the server. (See RFC 2831 section 2.1.2 “serv-name”.) Used by the DIGEST-MD5 mechanism.

The defines in saslc.h should be used in code, but their values need to be used in the config files.

ENVIRONMENT

The following environment variables (defined in saslc.h) affect the behavior of the library:

SASLC_ENV_CONFIG (“SASLC_CONFIG”)

If the environment variable SASLC_CONFIG is set it overrides the default configuration file location of /etc/saslc.d. This may be overridden by saslc_init().

SASLC_ENV_DEBUG (“SASLC_DEBUG”)

If set, turn on debugging messages. This turns on debugging as early as possible and is a global setting.

GSSAPI AND KERBEROS

The following is a minimal (Heimdal) Kerberos 5 setup for use with an smtp server that has been configured to support SASL with the GSSAPI mechanism. It assumes that Kerberos and the smtp server will both run on server.my.domain and that the client is on client.my.domain. It also assumes that the smtp server runs as user postfix and group mail, and that it is not chrooted.

On server.my.domain run the following script as root and then start the Kerberos server kdc(8). You will be prompted for a master password for Kerberos and a password for the postfix principal.

#/bin/sh 

cat <<- EOF >> /etc/krb5.conf 
[libdefaults] 
	default_realm = MY.DOMAIN 
[realms] 
	MY.DOMAIN = { 
		kdc = server.my.domain 
		admin_servers = server.my.domain 
	} 
[domain_realm] 
	.my.domain = MY.DOMAIN 
EOF 

mkdir /var/heimdal 
chown root:wheel /var/heimdal 
chmod 755 /var/heimdal 

kstash 
kadmin -l init --realm-max-ticket-life=unlimited \ 
               --realm-max-renewable-life=unlimited \ 
               MY.DOMAIN 
kadmin -l add  --max-ticket-life="1 day" \ 
               --max-renewable-life="1 week" \ 
               --expiration-time=never \ 
               --pw-expiration-time=never \ 
               --attributes="" \ 
               postfix 
kadmin -l add  --random-key \ 
               --max-ticket-life="1 day" \ 
               --max-renewable-life="1 week" \ 
               --expiration-time=never \ 
               --pw-expiration-time=never \ 
               --attributes="" \ 
               smtp/server.my.domain 
kadmin -l ext -k /etc/krb5.keytab smtp/server.my.domain 
chown root:mail /etc/krb5.keytab 
chmod 640 /etc/krb5.keytab

Note that the keytab /etc/krb5.keytab must be readable by the smtp server or authentication will fail. The location of this keytab file may be changed with the environment variable KRB5_KTNAME. If postfix is the smtp server, note the import_environment parameter (see postconf(5)).

On client.my.domain copy the keytab file from server.my.domain:/etc/krb5.keytab to /etc/krb5.keytab. Setup the /etc/saslc.d configuration directory (see CONFIGURATION above). Add the line

AUTHCID		"postfix"

to the file /etc/saslc.d/postfix/mech/GSSAPI.conf so that the postfix principal will be used for authentication. Enable SASL in the smtp client. Assuming the smtp client is postfix, you will need to add the following to the /etc/postfix/main.cf file to do this:

smtp_sasl_auth_enable = yes 
smtp_sasl_type = saslc 
smtp_sasl_mechanism_filter = GSSAPI 
relayhost = [server.my.domain]:submission

Here we have assumed the submission port is the port the server is listening to. Finally, as root, run the command

su -m postfix -c kinit

to obtain a ticket for the postfix user with the postfix credential and you should be good to go!

FILES

/etc/saslc.d

EXAMPLES

The following code fragments illustrate the possible use of the functions described above.

int 
decode_stream(saslc_sess_t *sess, int fdin, int fdout) 
{ 
	uint8_t buf[BUFSIZE]; 
	uint8_t *in; 
	void *out; 
	size_t inlen, outlen; 
	ssize_t n, rval; 

	for (;;) { 
		if ((rval = read(fdin, buf, sizeof(buf))) == -1) 
			return -1; 
		if (rval == 0) 
			break; 
		in = buf; 
		inlen = rval; 
		while (inlen > 0) { 
			rval = saslc_sess_decode(sess, in, inlen, &out, 
			    &outlen); 
			if (rval == -1) 
				return -1; 
			if (outlen > 0) { 
				n = write(fdout, out, outlen); 
				free(out); 
				if (n == -1) 
					return -1; 
			} 
			in += rval; 
			inlen -= rval; 
		} 
	} 
	return 0; 
} 

int 
encode_stream(saslc_sess_t *sess, int fdin, int fdout) 
{ 
	uint8_t buf[BUFSIZE]; 
	uint8_t *in; 
	void *out; 
	size_t inlen, outlen; 
	ssize_t n, rval; 

	for (;;) { 
		if ((rval = read(fdin, buf, sizeof(buf))) == -1) 
			return -1; 
		if (rval == 0) 
			break; 
		in = buf; 
		inlen = rval; 
		while (inlen > 0) { 
			rval = saslc_sess_encode(sess, in, inlen, &out, 
			    &outlen); 
			if (rval == -1) 
				return -1; 
			if (outlen > 0) { 
				n = write(fdout, out, outlen); 
				free(out); 
				if (n == -1) 
					return -1; 
			} 
			in += rval; 
			inlen -= rval; 
		} 
	} 
	/* flush internal encoder buffer */ 
	if (saslc_sess_encode(sess, NULL, 0, &out, &outlen) == -1) 
		return -1; 
	if (outlen > 0) 
		if (write(fdout, out, outlen) == -1) 
			return -1; 
	return 0; 
}

COMPATIBILITY

There exist other SASL client library implementations including Cyrus SASL (http://asg.web.cmu.edu/sasl/sasl-library.html) and GNU SASL (http://www.gnu.org/software/gsasl/).

STANDARDS

RFC 2195, RFC 2222, RFC 2245, RFC 2595, RFC 2831, RFC 4422, RFC 4505, RFC 4616, RFC 4752.

HISTORY

The libsaslc library appeared in NetBSD 6.0.

CAVEATS

The API was heavily influenced by its use with postfix(1).

Currently the ANONYMOUS, LOGIN, PLAIN, CRAM-MD5, DIGEST-MD5, and GSSAPI mechanisms have been tested and shown to work for authentication with a postfix(1) SMTP server using the cyrus-sasl library. LOGIN, PLAIN, CRAM-MD5, and DIGEST-MD5 have also been tested and shown to work with a postfix(1) SMTP server using a dovecot backend for authentication. The DIGEST-MD5 and GSSAPI specs also provide for integrity and confidentiality layers via the saslc_sess_encode() and saslc_sess_decode() routines, but these have not yet been tested against any servers.