Cyrus SASL for System Administrators

This document covers configuring SASL for system administrators, specifically those administrators who are installing a server that uses the Cyrus SASL library.

What SASL is

SASL, the Simple Authentication and Security Layer, is a generic mechanism for protocols to accomplish authentication. Since protocols (such as SMTP or IMAP) use SASL, it is a natural place for code sharing between applications. Some notable applications that use SASL include Sendmail (versions 8.10.0 and higher) and Cyrus imapd (versions 1.6.0 and higher).

Applications use the SASL library to tell it how to accomplish the SASL protocol exchange, and what the results were.

SASL is only a framework: specific SASL mechanisms govern the exact protocol exchange. If there are n protocols and m different ways of authenticating, SASL attempts to make it so only n plus m different specifications need be written instead of n times m different specifications. With the Cyrus SASL library, the mechanisms need only be written once, and they'll work with all servers that use it.

Authentication and authorization identifiers

An important concept to become familiar with is the difference between an "authorization identifier" and an "authentication identifer".
userid (user id, authorization id)
The userid is the identifier an application uses to check allowable options. On my Unix system, the user "bovik" (the account of Harry Q. Bovik) is allowed to write to "/home/bovik" and it's subdirectories but not to "/etc".
authid (authentication id)
The authentication identifier is the identifier that is being checked. "bovik"'s password might be "qqqq", and the system will authenticate anyone who knows "qqqq" as "bovik". However, it's possible to authenticate as one user but act as another user. For instance, Harry might be away on vacation and assign one of his graduate students, Jane, to read his mail. He might then allow Jane to act as him merely by supplying her password and her id as authentication but requesting authorization as "bovik". So Jane might log in with an authentication identifier of "jane" and an authorization id of "bovik" and her own (Jane's) password.

Applications can set their own proxy policies; by default, the SASL library will only allow the same user to act for another (that is, userid must equal authid).


The Cyrus SASL library supports the concept of "realms". A realm is an abstract set of users and certain mechanisms authenticate users in a certain realm.

In the simplest case, a single server on a single machine, the realm might be the fully-qualified domain name of the server. If the applications don't specify a realm to SASL, most mechanisms will default to this.

If a site wishes to share passwords between multiple machines, it might choose it's authentication realm as a domain name, such as "CMU.EDU". On the other hand, in order to prevent the entire site's security from being compromised when one machine is compromised, each server could have it's own realm. Certain mechanisms force the user (client side) to manually configure what realm they're in, making it harder for users to authenticate.

A single site might support multiple different realms. This can confuse applications that weren't written in anticipation of this; make sure your application can support it before adding users from different realms into sasldb with saslpasswd.

The Kerberos mechanisms treat the SASL realm as the Kerberos realm. Thus, the realm for Kerberos mechanisms defaults to the default Kerberos realm on the server. They may support cross-realm authentication; check your application on how it deals with this.

Some authentication mechanisms, such as PLAIN and CRAM-MD5, do not support the concept of realms.

How SASL works

How SASL works is governed by what mechanism the client and server choose to use and the exact implementation of that mechanism. This section describes the way these mechanisms act in the Cyrus SASL implementation.

The PLAIN mechanism and sasl_checkpass() call

The PLAIN mechanism is not a secure method of authentication by itself. It is intended for connections that are being encrypted by another level. (For example, the IMAP command "STARTTLS" creates an encrypted connection over which PLAIN might be used.) The PLAIN mechanism works by transmitting a userid, an authentication id, and a password to the server, and the server then determines whether that is an allowable triple.

The principal concern for system administrators is how the authentication and password are verified. The Cyrus SASL library is flexible in this regard:

/etc/passwd is supported innately in the library. Simply set the configuration option "pwcheck_method" to "passwd".

/etc/shadow is somewhat trickier. If the servers that use SASL run as root (such as Sendmail) there's no problem: just set the "pwcheck_method" option to "shadow". However, many daemons do not run as root for additional security, such as Cyrus imapd. In order for these servers to check passwords, they either need a helper program that runs as root, or need special privileges to read /etc/shadow. The easiest way is to give the server the rights to read /etc/shadow by, for instance, adding the cyrus user to the "shadow" group and then setting "pwcheck_method" to "shadow".

It is also possible to write a special PAM module that has the required privileges; default PAM setups do not (to my knowledge) come with this.

Kerberos v4, if found by the configuration script at compile time, can be enabled for plaintext password checking by setting "pwcheck_method" to "kerberos_v4". This is different from the KERBEROS_V4 mechanism discussed below---this configuration option merely specifies how to check plaintext passwords on the server.

PAM, the pluggable authentication module, is the default way of authenticating users on Solaris and Linux. It can be configured to check passwords in many different ways: through Radius, through NIS, through LDAP, or even using the traditional /etc/passwd file. If you wish to use PAM for authentication and the Cyrus SASL library found the PAM library when it was configured at compilation time, it is the default (or set "pwcheck_method" to "PAM"). It uses PAM with the service name (for example, Sendmail uses "smtp" and Cyrus imapd uses "imap").

The PAM authentication for SASL only affects the plaintext authentication it does. It has no effect on the other mechanisms, so it is incorrect to try to use PAM to enforce additional restrictions beyond correct password on an application that uses SASL for authentication.

This stores passwords in the SASL secrets database, the same database that stores the secrets for shared secret methods. Its principal advantage is that it means that the passwords used by the shared secrets mechanisms will be in sync with the plaintext password mechanisms. However, system built-in routines will not use sasldb.

Note that to set plaintext passwords in sasldb, you need to configure "saslpasswd" to do so. The "saslpasswd" uses the same configuration files like any SASL server. Make /usr/lib/sasl/saslpasswd.conf contain the line "pwcheck_method: sasldb" to instruct "saslpasswd" to create plaintext secrets in addition to the normal secrets.

write your own
Last, but not least, the most flexible method of authentication for PLAIN is to write your own. If you do so, any application that calls the "sasl_checkpass()" routine or uses PLAIN will invoke your code. The easiest place to modify the plaintext authentication routines is to modify the routine "_sasl_checkpass()" in the file lib/server.c to support a new method, and to add that method to lib/checkpw.c. Be sure to add a prototype in lib/saslint.h!

Shared secrets mechanisms

The Cyrus SASL library also supports some "shared secret" authentication methods: CRAM-MD5 and it's successor DIGEST-MD5. These methods rely on the client and the server sharing a "secret", usually a password. The server generates a challenge and the client a response proving that it knows the shared secret. This is much more secure than simply sending the secret over the wire proving that the client knows it.

There's a downside: in order to verify such responses, the server must keep password equivalents in a database; if this database is compromised, it is the same as if every user's password for that realm is compromised.

The Cyrus SASL library stores these secrets in the /etc/sasldb database. Depending on the exact database method used (gdbm, ndbm, or db) the file may have different suffixes or may even have two different files ("sasldb.dir" and "sasldb.pag"). It is also possible for a server to define it's own way of storing authentication secrets. Currently, no application is known to do this.

The principle problem for a system administrator is to make sure that sasldb is properly protected; only the servers that need to read it to verify passwords should be able to. If there are any normal shell users on the system, they must not be able to read it.

Managing password changes is outside the scope of the library. However, system administrators should probably make a way of letting user's change their passwords available to users. The "saslpasswd" utility is provided to change the secrets in sasldb. It does not affect PAM, /etc/passwd, or any other standard system library; it only affects secrets stored in sasldb.

Finally, system administrators should think if they want to enable "auto_transition". If set, the library will automatically create secrets in sasldb when a user uses PLAIN to successfully authenticate. However, this means that the individual servers, such as imapd, need read/write access to sasldb, not just read access. By default, "auto_transition" is set to false; set it to true to enable. (There's no point in enabling this option if "pwcheck_method" is "sasldb".)

Kerberos mechanisms

The Cyrus SASL library also comes with two mechanisms that make use of Kerberos: KERBEROS_V4, which should be able to use any Kerberos v4 implementation, and GSSAPI (tested against MIT Kerberos 5 and Heimdal Kerberos 5). These mechanisms make use of the kerberos infrastructure and thus have no password database.

Applications that wish to use a kerberos mechanism will need access to a service key, stored either in a "srvtab" file (Kerberos 4) or a "keytab" file (Kerberos 5). Currently, the keytab file location is not configurable and defaults to the system default (probably /etc/krb5.keytab).

The Kerberos 4 srvtab file location is configurable; by default it is /etc/srvtab, but this is modifiable by the "srvtab" option. Different SASL applications can use different srvtab files.

A SASL application must be able to read its srvtab or keytab file.

How to set configuration options

The Cyrus SASL library comes with a built-in configuration file reader. However, it is also possible for applications to redefine where the library gets it's configuration options from.

The default configuration file

By default, the Cyrus SASL library reads it's options from /usr/lib/sasl/App.conf (where "App" is the application defined name of the application). For instance, Sendmail reads it's configuration from "/usr/lib/sasl/Sendmail.conf" and the sample server application included with the library looks in "/usr/lib/sasl/sample.conf".

A standard Cyrus SASL configuration file looks like:

srvtab: /var/app/srvtab
pwcheck_method: kerberos_v4

Application configuration

Applications can redefine how the SASL library looks for configuration information. Check your application's documentation for specifics.

For instance, Cyrus imapd reads its sasl options from it's own configuration file, /etc/imapd.conf, by prepending all SASL options with "sasl_": the SASL option "pwcheck_method" is set by changing "sasl_pwcheck_option" in /etc/imapd.conf. Check your application's documentation for more information.

Compiling and installing the library

Unfortunately, since SASL is very flexible (allowing administrators to upgrade and install new authentication plugins without recompiling any applications) its flexibility can also make it a chore to compile.

I'll have some sage advice here when I find some.