Authenticating Users
Introduction
The Cyrus IMAP Server uses the Cyrus SASL library to authenticate
users. Please refer to the Cyrus SASL documentation for specific
details on SASL. This section focuses specifically on the server
processes distributed with Cyrus IMAPd.
Authentication Mechanisms
At this writing, the underlying Cyrus SASL library supports a variety
of SASL mechanisms, including CRAM-MD5, DIGEST-MD5, KERBEROS_V4, and
GSSAPI. The Cyrus IMAP, POP, and LMTP servers also support STARTTLS
using client-side certificates and the EXTERNAL authentication method.
GSSAPI is specific to Kerberos version 5. Additionally, STARTTLS client
side certificates have not been extensively tested.
When STARTTLS is enabled, the PLAIN SASL mechanism (if installed) also
becomes available. This is because one should not pass a clear text
password over the wire unless the connection is encrypted.
The IMAP protocol also supports a way for users to authenticate without
using SASL (the specification). This is via the 'LOGIN' command (not to
be confused by the LOGIN SASL mechanism). The IMAP LOGIN command (as
with PLAIN) will send your password in clear-text to the server. In
this case, the password is still verified through the Cyrus SASL
library, though no SASL mechanism actually performs a negotiation.
The POP server is capable of APOP authentication, but this requries
that Cyrus SASL be compiled --with-checkapop, and also that you are
using an auxprop backend for your password store (e.g. the sasldb
auxprop plugin).
Authentication Recommendations
* If you are running a mail server on a single machine, we recommend
that you configure the system to use CRAM-MD5 or DIGEST-MD5. We
have not provided utilities for you to let users change their
passwords but either we or someone else might provide that feature.
* If you have more than one mail server, we recommend that you
configure the system to use GSSAPI and Kerberos5.
* If you have some other authentication mechanism that requires the
clear text password, you should use saslauthd.
saslauthd is something specific to the Cyrus SASL libraries. While
it is less generic than PAM, it is much simpler to configure. The
IMAP server simply sends a userid and a corresponding password down
a Unix domain pipe. Then, saslauthd takes that userid and password
and tries to authenticate with it -- using whatever authentication
you use -- and simply returns "yes" or "no" as to whether or not
the password was correct.
It is possible to configure saslauthd to check these passwords via
a PAM mechanism, /etc/passwd, or other possibilities.
PAM stands for pluggable authentication modules and the purpose is
to provide a common API which applications can use to obtain
authentication for a user. You can think of PAM as a complementary
layer under the SASL layer. See
http://www.kernel.org/pub/linux/libs/pam/FAQ for more information
on PAM. By using a PAM module, all the other applications on your
system can take advantage of it -- for example, login, xlock, etc.
Keep in mind that when you use PLAIN or LOGIN you should encrypt
the stream so a user's password can not be trivially sniffed off of
the network.
Configuring Authentication
Cyrus SASL has a number of options that can be configured by the
application. To configure these via imapd.conf, simply prefix the
appropriate option name with sasl_ (e.g. pwcheck_method becomes
sasl_pwcheck_method).
/etc/sasldb2
The easiest method for authenticating users is to use the libsasl
authentication database and create users using the "saslpasswd2"
utility. Set "sasl_pwcheck_method: auxprop", and be sure that the SASL
sasldb auxprop module is installed (it is, by default). Make sure Cyrus
can read "/etc/sasldb2":
chown cyrus /etc/sasldb2*
Shadow Passwords
If you want to authenticate users from "/etc/shadow", things are
considerably more complicated, since the cyrus user cannot read the
shadow password file. Additionally, this will not allow you to use
shared secret mechanisms. To do this, it is necessary to configure
libsasl with saslauthd support, and set "sasl_pwcheck_method:
saslauthd". The SASL library will then make calls to an external
utility running as root to authenticate users.
Kerberos
Configuring Kerberos v4
Cyrus IMAP supports Kerberos v4 if the SASL library was compiled with
KERBEROS_V4 support.
You'll have to create a Kerberos v4 identity for the server and add the
server's key to the "srvtab" file. The file must be readable by the
cyrus user. The server's Kerberos identity is "imap.HOST@REALM", where
"HOST" is the first component of the machine's host name and "REALM" is
the machine's Kerberos realm.
1. Here is a sample session, creating a srvtab file for the host named
"foobar":
ksrvutil -f /var/imap/srvtab add
Here is the information "ksrvutil" requests. Respond by filling in
values or by pressing RETURN. In this example, the host name is
"foobar" and the realm is "ANDREW.CMU.EDU".
Name: imap
Instance: foobar
Realm: ANDREW.CMU.EDU
Version number:
New principal: imap.foobar@ANDREW.CMU.EDU; version 0
Is this correct? (y,n) [y]
Password:
Verifying, please re-enter Password:
Key successfully added.
Would you like to add another key? (y,n) [y] n
2.
If you plan to install Kerberized POP, create the Kerberos identity
"pop.HOST@REALM" and add the key to the "srvtab" file. Likewise, if
you plan on using LMTP over TCP, create the Kerberos identity
"lmtp.HOST@REALM" and add the key to the "srvtab" file.
3.
Make the "srvtab" file owned by the cyrus user:
chown cyrus /var/imap/srvtab
4. Add the option srvtab option to /etc/imapd.conf:
srvtab: /var/imap/srvtab
5. Test using imtest -m KERBEROS_V4. imtest will attempt to authorize
as the current Unix user regardless of the current ticket's held.
Override this with the -u option.
Troubleshooting Kerberos_V4 problems
Run the program "krbck" (found in the imap directory) as the cyrus user
on the IMAP server. This program will diagnose some common Kerberos v4
configuration errors.
Configuring Kerberos v5
Cyrus IMAP supports Kerberos v5 if the SASL library was compiled with
GSSAPI support.
You'll have to create a Kerberos v5 identity for the server. Kerberos
v5 keys are generally stored in "/etc/krb5.keytab".
1. Add the "imap/hostname" key using "kadmin".
2. Let the cyrus user read "/etc/krb5.keytab": user:
chown cyrus /etc/krb5.keytab
3. Test using imtest -m GSSAPI. imtest will attempt to authorize as
the current Unix user regardless of the current ticket's held.
Override this with the -u option.