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.
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
(versions 8.10.0 and higher) and
(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
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)
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 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
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"
- /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
- 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
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".)
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
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.
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
A standard Cyrus SASL configuration file looks like:
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.