[Dovecot] User not found when using shadow for passdb

[John J. Stimson III]

I am getting "user unknown" when trying to connect to the dovecot
server using IMAP.  The client gets an authentication failed message
and does not download mail.

The host system uses shadow passwords in /etc/shadow.  I would like to
use the same passwords for IMAP sessions, so I have set the passdb
driver to shadow.

The system is Slackware, which does not use PAM.

Here is the dovecot version and configuration output:

# dovecot -n
# 2.1.17: /usr/local/etc/dovecot/dovecot.conf
# OS: Linux 3.6.5 i686 Slackware 13.1.0
auth_debug_passwords = yes
auth_verbose = yes
namespace inbox {
  inbox = yes
  location =
  mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix =
}
passdb {
  args = blocking=no
  driver = shadow
}
service auth {
  unix_listener auth-userdb {
    user = root
  }
}
ssl_cert = </etc/ssl/certs/dovecot.pem
ssl_key = </etc/ssl/private/dovecot.pem
userdb {
  args = blocking=no
  driver = passwd
}

Here are the dovecot auth_passwd_debug level log messages when the
client tries to connect:

May  2 09:05:07 harlie dovecot: auth: Debug: Loading modules from directory: /usr/local/lib/dovecot/auth
May  2 09:05:07 harlie dovecot: auth: Debug: auth client connected (pid=22181)
May  2 09:05:07 harlie dovecot: auth: Debug: client in: AUTH^I1^IPLAIN^Iservice=imap^Isecured^Isession=ABCDEFGHIJKLMNOP^Ilip=96.229.223.7^Irip=10.0.0.181^Ilport=993^Irport=51898^Iresp=zxywvutsrqponmlkji==
May  2 09:05:07 harlie dovecot: auth: Debug: shadow(john,10.0.0.181,<ABCDEFGHIJKLMNOP>): lookup
May  2 09:05:07 harlie dovecot: auth: shadow(john,10.0.0.181,<ABCDEFGHIJKLMNOP>): unknown user
May  2 09:05:09 harlie dovecot: auth: Debug: client passdb out: FAIL^I1^Iuser=john
May  2 09:05:09 harlie dovecot: imap-login: Disconnected (auth failed, 1 attempts in 2 secs): user=<john>, method=PLAIN, rip=10.0.0.181, lip=96.229.223.7, TLS, session=<ABCDEFGHIJKLMNOP>

The user john is a real account on the host machine and has entries in
both /etc/passwd and /etc/shadow.

Help?

-- 

john at idsfa.net                                              John Stimson
http://www.idsfa.net/~john/                              HMC Physics '94