• AuthDatabase
• CheckPassword

CheckPassword

Checkpassword is an authentication interface originally implemented by qmail. Checkpassword combines both the password database and user database lookups into a single checkpassword lookup, which makes the standard implementation unsuitable for a standalone userdb. With Dovecot extensions it's also possible to use checkpassword as a userdb.

Typically you'll use prefetch as the userdb, but it's not required that you use the checkpassword script's userdb capabilities. You can still use for example static userdb if you're using only a single UID and GID, and your home directory fits into a template.

Deliver

If your checkpassword script doesn't support Dovecot extensions, you can't use it as a user database. This means that if you wish to use LDA, you can't use the -d parameter to do userdb lookups. There are two ways to solve this:

1. Use another userdb which does the lookup for deliver, for example SQL or static. Add this userdb after the prefetch userdb.

2. Use a script to look up the user's home directory and run deliver without -d parameter. For example:

#!/bin/sh

# <<Lookup user's home directory here.>>

# If users have different UIDs/GIDs, make sure to also change this process's UID and GID.
# If you want to override any settings, use dovecot-lda's -o parameter
# (e.g. dovecot-lda -o mail_location=maildir:~/Maildir).

export HOME
exec /usr/local/libexec/dovecot/dovecot-lda

Checkpassword Interface

The interface is specified in http://cr.yp.to/checkpwd/interface.html. However here's a quick tutorial for writing a script:

• Read <username> NUL <password> NUL from fd 3.

• Verify the username and password.
  • If the authentication fails, exit with code 1. This makes Dovecot give "Authentication failed" error to user.
    • This error is returned both for password mismatch and also if the user doesn't exist at all. Internally Dovecot maps this as password mismatch.
  • If you encounter an internal error, exit with code 111. This makes Dovecot give "Temporary authentication failure" error to user.
• If the authentication succeeds, you'll need to:

  • Set user's home directory to $HOME environment. This isn't required, but highly encouraged.

  • Set $USER environment variable. If the user name was changed (eg. if you lowercased "Username" to "username"), you can tell about it to Dovecot by setting $USER to the changed user name.

  • Change the process's effective UID and GID to the user's UNIX UID and GID.

    • Alternatively you could set userdb_uid and userdb_gid environments and add them to EXTRA environment (see below for Dovecot extensions).

  • Your program received a path to checkpassword-reply binary as the first parameter. Execute it.

Qmail-LDAP

Note that auth_imap that comes with qmail-ldap is not compatible with this interface. You can get a patch that adds auth_dovecot functionality to qmail-ldap here. Or you can use auth_pop instead, but you may need to pass aliasempty to let auth_pop find the Maildir, so it is recommended to write a /var/qmail/bin/auth_dovecot wrapper (don't forget to chmod +x it) around auth_pop.

#!/bin/sh
QMAIL="/var/qmail"
if [ -e $QMAIL/control/defaultdelivery ]; then
    ALIASEMPTY=`head -n 1 $QMAIL/control/defaultdelivery 2> /dev/null`
else
    ALIASEMPTY=`head -n 1 $QMAIL/control/aliasempty 2> /dev/null`
fi
ALIASEMPTY=${ALIASEMPTY:-"./Maildir/"}
exec $QMAIL/bin/auth_pop "$@" $ALIASEMPTY

you can also use this wrapper to pass LOGLEVEL environmental variable to auth_pop.

Dovecot Extensions

If you wish to return extra fields for Dovecot, set them in environment variables and then list them in EXTRA environment variable. The userdb extra fields can be returned by prefixing them with userdb_. For example:

userdb_quota_rule=*:storage=10000
userdb_mail=mbox:$HOME/mboxes
EXTRA=userdb_quota_rule userdb_mail

Dovecot also sets some environment variables that the script may use:

• SERVICE: contains eg. imap, pop3 or smtp

• TCPLOCALIP and TCPREMOTEIP: Client socket's IP addresses if available

• MASTER_USER: If master login is attempted. This means that the password contains the master user's password and the normal username contains the user who master wants to log in as.

• AUTH_*: All of the auth variables are available as AUTH_<long name> extra fields. For example %{cert} is in AUTH_CERT. (v2.0.16+)

Checkpassword as userdb

Dovecot calls the script with AUTHORIZED=1 environment set when performing a userdb lookup. The script must acknowledge this by changing the environment to AUTHORIZED=2, otherwise the lookup fails. Other than that, the script works the same way as a passdb checkpassword script. If user doesn't exist, use exit code 3.

Checkpassword with passdb lookups (v2.1.2+)

Normally checkpassword answers to questions "is user X's password Y?" This doesn't work with non-plaintext auth mechanisms, or when Dovecot wants to do a non-authenticating passdb lookup (e.g. for LMTP proxy). These passdb credentials lookups can be implemented the same way as a userdb lookup (i.e. change the AUTHORIZED environment).

• AUTHORIZED=1 is set, just like for userdb lookup

• When doing a non-plaintext authentication:

  • CREDENTIALS_LOOKUP=1 environment is set

  • The password scheme that Dovecot wants is available in SCHEME environment (e.g. SCHEME=CRAM-MD5)

  • If a password is returned, it must be returned as password={SCHEME}secret.

• When doing a passdb lookup, e.g. a proxy which doesn't really want the password, just the passdb extra fields:

  • Neither CREDENTIALS_LOOKUP nor SCHEME is set.

  • FIXME: Unfortunately it looks like you currently can't easily differentiate a passdb lookup from userdb lookup!
• If user doesn't exist, use exit code 3.

• If you get an error about checkpassword exiting with code 0, you didn't execute the checkpassword-reply binary as you should have (which exits with code 2 on success)

Example

The standard way:

passdb {
  driver = checkpassword
  args = /usr/bin/checkpassword
}
userdb {
  driver = prefetch
}
# If you want to use deliver -d and your users are in SQL:
userdb {
  driver = sql
  args = /etc/dovecot/dovecot-sql.conf.ext
}

Using checkpassword only to verify the password:

passdb {
  driver = checkpassword
  args = /usr/bin/checkpassword
}
userdb {
  driver = static
  args = uid=vmail gid=vmail home=/home/%u
}

Performance

The CheckPassword backend is not suited for heavy traffic. Especially if the script spawned have to launch an entire language interpreter.

If your user database is only accessible with custom code an alternative might be using the Dict AuthDatabase over a UNIX socket.