openssh and pam_ldap

[Michael Thomas]

I've been fighting with this problem for a couple of days now and 
thought it was time to appeal to the experts...

I'm using RHEL4 with pam_ldap to authenticate users who do not have 
local accounts.  This works.  I'm also using pam_check_host_attr in 
ldap.conf to limit who is allowed to log into a host based on their host 
attribute in ldap.  This also works, mostly.

When users log in using their ldap password, that is, when ssh prompts 
them for their password, the host-based access control provided by 
pam_check_host_attr works.  Users who don't have the proper ldap host 
attribute are denied access.

But as soon as a user puts a public key in ~/.ssh/authorized_keys, 
pam_check_host_attr stops working.  Regardless of what is in their ldap 
host attribute, openssh lets them in.  I want to allow (and encourage) 
the use of authorized_keys files for logging into the machines, but I 
also want to retain control of which machines they can log into by using 
the host attribute in their ldap entry.

I've tried changing ChallengeResponseAuthentication to 'yes', but then 
users are always prompted for their password.  I've also tried adding 
the 'debug' option to pam_ldap.so and directing all *.debug messages in 
syslog to /var/log/debug, but I don't see anything appear from pam_ldap.

Is it possible to allow ssh key-based authentication, but still prohibit 
logins based on the ldap host attribute?

--Mike

sshd_config contains:

Protocol 2
SyslogFacility AUTHPRIV
ChallengeResponseAuthentication no
GSSAPIAuthentication yes
GSSAPICleanupCredentials yes
UsePAM yes
X11Forwarding yes
Subsystem       sftp    /usr/libexec/openssh/sftp-server

/etc/pam.d/system-auth contains:

#%PAM-1.0
# This file is auto-generated.
# User changes will be destroyed the next time authconfig is run.
auth        required      /lib/security/$ISA/pam_env.so
auth        sufficient    /lib/security/$ISA/pam_unix.so likeauth nullok
auth        sufficient    /lib/security/$ISA/pam_ldap.so use_first_pass
auth        required      /lib/security/$ISA/pam_deny.so

account     required      /lib/security/$ISA/pam_unix.so broken_shadow
account     sufficient    /lib/security/$ISA/pam_succeed_if.so uid < 100 
quiet
account     [default=bad success=ok user_unknown=ignore] 
/lib/security/$ISA/pam_ldap.so
account     required      /lib/security/$ISA/pam_permit.so

password    requisite     /lib/security/$ISA/pam_cracklib.so retry=3
password    sufficient    /lib/security/$ISA/pam_unix.so nullok 
use_authtok md5 shadow
password    sufficient    /lib/security/$ISA/pam_ldap.so use_authtok
password    required      /lib/security/$ISA/pam_deny.so

session     required      /lib/security/$ISA/pam_mkhomedir.so 
skel=/etc/skel/ umask=0022
session     required      /lib/security/$ISA/pam_limits.so
session     required      /lib/security/$ISA/pam_unix.so
session     optional      /lib/security/$ISA/pam_ldap.so