Cyrus SASL for System Administrators

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What SASL is

Applications use the SASL library to tell them 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

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). See your application's documentation for details about changing the default proxy/authorization policies.

Realms

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 your databases.

To add users of different realms to sasldb, you can use the -u option to saslpasswd2. The SQL plugin has a way of integrating the realm name into the query string with the '%r' macro.

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.

Realms will be passed to saslauthd as part of the saslauthd protocol, however the way each saslauthd module deals with the situation is different (for example, the LDAP plugin allows you to use the realm to query the server, while the rimap and PAM plugins ignore it entirely).

Realms are represented in a username string by any text followinhg the '@' sign. So, usernames like rjs3@ANDREW.CMU.EDU, is user 'rjs3' in the realm 'ANDREW.CMU.EDU'. If no realm is provided, the server's FQDN is assumed (likewise when specifying a realm for saslpasswd2).

How SASL works

The PLAIN mechanism, sasl_checkpass(), and plaintext passwords

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

The LOGIN mechanism (not to be confused with IMAP4's LOGIN command) is an undocumented, unsupported mechanism. It's included in the Cyrus SASL distribution for the sake of SMTP servers that might want to interoperate with old clients. Do not enable this mechanism unless you know you're going to need it. When enabled, it verifies passwords the same way the PLAIN mechanism does.

Shared secrets mechanisms

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

Put another way, you cannot use saslauthd with these methods. If you do not wish to advertise these methods for that reason (i.e. you are only using saslauthd for password verification), then either remove the non-plaintext plugins (those other than login and plain) from the plugin directory, or use the mech_list option to disable them.

For simplicity sake, the Cyrus SASL library stores plaintext passwords only in the /etc/sasldb2 database. These passwords are then shared among all mechanisms which choose to use it. 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.

This point is important, so we will repeat it: sasldb stores the plaintext versions of all of its passwords. If it is compromised so are all of the passwords that it stores.

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 "saslpasswd2" 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 "auxprop", and the sasldb plugin is installed, since you'll be transitioning from a plaintext store to a plaintext store)

Kerberos mechanisms

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.

The OTP mechanism

OTP via OPIE

OPIE uses its own "opiekeys" database for storing the data necessary for generating the server challenges. The location of the opiekeys file is configurable in SASL; by default it is /etc/opiekeys, but this is modifiable by the "opiekeys" option.

Auxiliary Properties

Note that this means that if your password database is in a SASLdb, and you wish to use it for plaintext password lookups through the sasldb, you will need to set the sasl option pwcheck_method to be auxprop.

How to set configuration options

The default configuration file

By default, the Cyrus SASL library reads it's options from /usr/lib/sasl2/App.conf (where "App" is the application defined name of the application). For instance, Sendmail reads it's configuration from "/usr/lib/sasl2/Sendmail.conf" and the sample server application included with the library looks in "/usr/lib/sasl2/sample.conf".

Application configuration

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_method" in /etc/imapd.conf.

Cyrus SASL for System Administrators

What SASL is

Authentication and authorization identifiers

Realms

How SASL works

The PLAIN mechanism, `sasl_checkpass()`, and plaintext passwords

Shared secrets mechanisms

Kerberos mechanisms

The OTP mechanism

OTP via OPIE

Auxiliary Properties

How to set configuration options

The default configuration file

Application configuration

Troubleshooting

Cyrus SASL for System Administrators

What SASL is

Authentication and authorization identifiers

Realms

How SASL works

The PLAIN mechanism, sasl_checkpass(), and plaintext passwords

Shared secrets mechanisms

Kerberos mechanisms

The OTP mechanism

OTP via OPIE

Auxiliary Properties

How to set configuration options

The default configuration file

Application configuration

Troubleshooting

The PLAIN mechanism, `sasl_checkpass()`, and plaintext passwords