On Sat, Sep 27, 2008 at 09:06:32AM +0300, Staffan Thomén wrote:
> On Fri, Sep 26, 2008 at 10:55:54AM +0200, Adam Hoka wrote:
> >
> > Jeff Rizzo wrote:
> >> Jean-Yves Migeon wrote:
> >>> Patrick Welche wrote:
> >>>> Is there a way of getting a NetBSD client to authenticate against an
> >>>> ldap
> >>>> server? (I can't see an nss-ldap / pam_ldap - is there something else?
> >>>> xxinit was mentioned earlier...)
> >>>>
> >>>> Cheers,
> >>> If you need PAM with ldap, try security/pam-ldap in pkgsrc.
> >>>
> >>
> >> If/when anyone gets this working, it would be really, really nice to
> >> have a step-by-step how-to somewhere. Maybe on the wiki?
> >>
> >> (I'm going to need to do this at some point, but no round tuits yet)
> >
> > I agree, that would be a really wanted guide.
> > As we have ldap libs in base system now, this should be an
> > out-of-the-box feature with minial setup.
>
> I've got a (test) setup running on a pair of machines here, if you want I'll
> try to put some guidelines down; provided I don't forget about it completely..
> This has been known to happen, feel free to poke me if I leave it too long ;-)
>
> Staffan
Here's what I wrote, and I realized I hate writing documentation because I can
never remember what I did ;-)
Hope you appreciate it...
$Id: netbsd-ldap-auth.txt 2 2008-09-29 12:36:44Z duck $
OpenLDAP Authentication on NetBSD
=================================
Author's note
-------------
This document really describes (what I remember of installing) my system,
with tidbits I've forgotten from various sources on the net. I can't guarantee
that following this document you'll get a working system, but I hope it will
provide some insights into how the thing is supposed to work.
Staffan Thom´en <duck%shangtai.net@localhost>
Server setup
------------
First things first, you'll need to set up an openldap server somewhere, this
is fairly straightforward, as it's available in pkgsrc. The tricky bit is
really configuring the ACL:s, since the openldap logs are incredibly hard to
read.
Generally it's probably a good idea to firewall it from outside and worry
about the ACL setup later if you want to do things like let other departments
to see your users or let the public see contact information for example.
An example config file is included in the package
(${LOCALBASE}/etc/opeldap/slapd.conf), and the only thing that really has to
be added is to include some schemas for user authentication:
cosine.schema
inetorgperson.schema
nis.schema
These are (in pkgsrc-2008Q2) installed in
${LOCALBASE}/share/examples/openldap/schema, and can just be included from
there, and tells the server which record keys (as in key-value pairs) it shall
accept.
And that really is it for the server bit. Next comes testing it out with a few
ldap commands.
The basic commands of talking directly with the ldap database are ldapadd,
ldapmodify and ldapsearch. These are in the openldap-client package, so you
won't have to install the entire server on a client machine.
Options you'll be using alot like -b (base) and -H (host URI) can
conveninently be stuck in a client configuration file,
${LOCALBASE}/etc/openldap/ldap.conf, which will save you time and aggravation
from having to type them all the time.
To talk to your ldap server, try running ldapsearch;
% ldapsearch -H ldap://my.server/
This really means dump everything, but since we've nothing in the database
it will respond with an error.
To set this database up for user authentication, we'll need to lay down some
structure. LDAP is generally a hierachial database of records with key-value
pairs. We'll first need to tell it about our organisation and then add a user.
Here we'll be using ldapadd, which reads a format called ldif. It is a flat
text format that looks something like this:
dn: cn=example,dc=org
objectClass: dcObject
objectClass: organization
objectClass: top
o: Example Organisation
dc: example
dn: ou=groups,dc=example,dc=org
objectClass: top
objectClass: organizationalUnit
ou: groups
dn: ou=people,dc=example,dc=org
objectClass: top
objectClass: organizationalUnit
ou: people
The text above define three records, they start with a distinguished name
of the record (dn:), which is a unique identifier for the record.
"cn=example,dc=org" is the root of this organisation, with a common name (cn)
example and a domain component (dc) of org.
Next come the objectClass lines which tells us that this is domain component
object, an organisation object and a top-level object.
We then have an organisation (o:) line which is a descriptive text and finally
a domain component line (dc:) which is the stored value for the dc (same as in
the distinguished name).
Following this are two records which define something called in ldap terms
organisational units, and as you see from the dn:, essentially two branches
of the main tree. They are here to be used for the user groups (yes, like
/etc/groups) and the actual users.
If you want you can just stick all of this in one file (even the user below)
and add it with ldapadd -f file.ldif, this will create the initial structure
of your database.
Adding a group and then a user user is no more difficult, you just have to
fill out the right fields.
dn: cn=ldapusers,ou=groups,dc=example,dc=org
objectClass: top
objectClass: posixGroup
cn: ldapusers
gidNumber: 101
memberUid: bill
memberUid: george
A group named ldapusers with the number 101, and the secondary users bill
and george (these are of course not required).
dn: uid=test,ou=people,dc=example,dc=org
objectClass: top
objectClass: posixAccount
objectClass: inetOrgPerson
uid: test
uidNumber: 2000
gidNumber: 101
o: Example Organisation
cn: Test User
givenName: Test
sn: User
gecos: Test User,3b,+358800128128,+35801234567
loginShell: /bin/ksh
homeDirectory: /home/test
mail: test%example.org@localhost
displayName: El Magnifico Test User
A user with the uid test, belonging to group ldapusers (101); o: is the same
as the root record above and the others apart from sn (surname) is fairly
obvious.
The GECOS field contains comma separated values, apparently it's pulled
straight into the client system.
The fields actually required by the schemes are:
uid
uidNumber
gidNumber
cn
sn
homeDirectory
LDAP can store multiple roots and each user entry for example can be more than
just the login information, as above it also mentions email, phone numbers and
so on for our test user, and it can also include binary data like a mugshot
and them playing the corporate theme on banjo. As far as authentication is
concerned, we've got what we want though.
So far so good, this should not cause much trouble to set up, I believe I've
covered everything required; the thing I had most problem with in relation to
the database itself was that it was so unstructured, you have to provide all
the structure yourself.
Client Setup
------------
In order to log in on a NetBSD system we need to provide two things,
a way for the system to authenticate you and a way for it to find out what
your group, user id, etc. is.
The first part of this, authentication is taken care of by PAM (or possibly
by some BSD auth scheme, but this is not yet implemented as far as I know.)
The second part is done via libc and the NSS subsystem.
In order to do this, we need to provide some plugins for either system that
enables LDAP support in them. The plugins are in pkgsrc and are called
security/pam-ldap
and
databases/nss_ldap
Once these are installed, you can either link them, copy or use in place from
${LOCALBASE}/lib and ${LOCALBASE}/lib/security into /usr/lib and
/usr/lib/security respectively. I prefer to use symbolic links because then
any upgrade I make will automatically have the latest version already in place.
Before we go any further, I'd like to introduce some security into the mix;
up til now we've talked to the ldap server without any limitations and what's
called anonymous binds, i.e. not logged in.
XXX can anonymous binds actually write to a db without ACLs?
This is an ldap user, just like the test user outlined above, since the ldap
database can authenticate against itself. (You don't have to, but I haven't
explored the other possibilities such as SASL)
So we'll create a user called nss
dn: cn=nss,dc=example,dc=org
objectClass: top
objectClass: inetOrgPerson
o: Example Organisation
cn: nss
sn: manager
We'll attach a password so that not just anyone can connect, and also change
our LDAP configuration slightly so that we use encrypted passwords.
userPassword: {SSHA}w5aocfmGgZqq3h8AjvaZiw8WKdrRTjTi
To generate this password I use (bundled with openldap-server) slapdpasswd
% slappasswd -h "{SSHA}"
And in slapd.conf add
passsword-hash {SSHA}
And of course you'll need to change the secret for the rootpw into something
encrypted.
Note that the traffic between the ldap client and the server is still not (that
is if you've been following this document) encrypted so this might be best to
perform locally.
This user will be used for ACL filtering later.
Next we'll need to configure the LDAP part of the plugins, a convenience here
is that since both the plugins are made by the same people, they can share a
configuration file. They will look for ${LOCALBASE}/etc/nss_ldap.conf and
${LOCALBASE}/etc/pam_ldap.conf, but linking them to the same file will let you
have just one place to configure (and protect for your ldap user password)
The important bits in this file is the base setting and the uri for your
ldap server:
base dc=example,dc=org
uri ldap://my.server/
Next we need to tell it who it should contact the ldap database as:
binddn cn=nss,dc=example,dc=org
bindpw unencrypted-password
And if you want to be able to change passwords as root without knowing the
user's password in advance (with passwd, using ldapmodify you can still just
set it, if you bind with the credentials to do it (see ACLs).)
I haven't mentioned this user before, it's the database's root user, allowed
to do anything;
rootbinddn cn=root,dc=example,dc=org
The password for this will not be in this file, but in a separate file called
${LOCALBASE}/etc/nss_ldap.secret or for pam; ${LOCALBASE}/etc/pam_ldap.secret
*) not sure about this, but my system has both, linked together
Finally we will set the password exchange method to exop;
pam_password exop
This is the OpenLDAP extended method and while the passwords will still be sent
in the clear, they are encrypted with the database's scheme in the database.
So while you can use ldapsearch to get the data (though ACLs can prevent this
if properly set up) it will still only be a hash.
That's it for configuring the plugins so far.
NSS
---
The next change we will need to do is to enable the ldap module in
nsswitch.conf:
Change
group: files
...
passwd: files
To
group: files ldap
...
passwd: files ldap
This will enable you to have local accounts as well as ldap users.
You could test this out now, by running the getent program;
% getent group
Will present you with a list of all the groups in the system, with the ldap
group 'ldapusers' we created earlier tacked on to the end of the list.
% getent passwd
And this will show you the user list, with the ldap user 'test' at the end.
PAM
---
PAM keeps it's configuration files in /etc/pam.d/, these are divided into
individual files per each pam service in the system; most are just including
system but some need special attention.
On my system I have the following changes from the stock netbsd setup:
/etc/pam.d/sshd
# $NetBSD: sshd,v 1.5.2.2 2005/03/19 17:45:49 tron Exp $
#
# PAM configuration for the "sshd" service
#
# auth
auth required pam_nologin.so no_warn
auth sufficient pam_ldap.so
auth sufficient pam_krb5.so no_warn try_first_pass
# pam_ssh has potential security risks. See pam_ssh(8).
#auth sufficient pam_ssh.so no_warn try_first_pass
auth required pam_unix.so no_warn try_first_pass
# account
account required pam_krb5.so
account sufficient pam_ldap.so
account required pam_login_access.so
account required pam_unix.so
# session
# pam_ssh has potential security risks. See pam_ssh(8).
#session optional pam_ssh.so
session sufficient pam_ldap.so
session required pam_permit.so
# password
password sufficient pam_krb5.so no_warn try_first_pass
password sufficient pam_ldap.so
password required pam_unix.so no_warn try_first_pass
/etc/pam.d/su
# $NetBSD: su,v 1.5 2005/03/01 16:28:46 christos Exp $
#
# PAM configuration for the "su" service
#
# auth
auth sufficient pam_ldap.so
auth sufficient pam_rootok.so no_warn
auth sufficient pam_self.so no_warn
auth sufficient pam_ksu.so no_warn try_first_pass
auth requisite pam_group.so no_warn group=wheel
root_only fail_safe
#auth sufficient pam_group.so no_warn
group=rootauth root_only fail_safe authenticate
auth required pam_unix.so no_warn
try_first_pass nullok
# account
account required pam_login_access.so
account include system
# session
session required pam_permit.so
/etc/pam.d/system
# $NetBSD: system,v 1.6 2005/03/03 02:12:32 christos Exp $
#
# System-wide defaults
#
# auth
auth sufficient pam_ldap.so
auth sufficient pam_krb5.so no_warn try_first_pass
auth required pam_unix.so no_warn
try_first_pass nullok
# account
account sufficient pam_ldap.so
account required pam_krb5.so
account required pam_unix.so
# session
session sufficient pam_ldap.so
session required pam_lastlog.so no_fail no_nested
# password
password sufficient pam_ldap.so
password sufficient pam_krb5.so try_first_pass
password sufficient pam_unix.so try_first_pass
password required pam_deny.so prelim_ignore
The last bit here with pam_deny, is a bit special, it is what enables you to
change passwords for both local users and those in the ldap database with the
passwd command. pam_deny with the prelim_ignore flag is needed, else pam will
will fail in the preliminary phase (it is always run trough twice) and you will
not be able to change passwords.
In order to use this you need to patch your pam_deny
(/usr/src/lib/libpam/modules/pam_deny.c) with the patch by
Edgar Fuß <ef%math.uni-bonn.de@localhost>:
http://mail-index.netbsd.org/tech-userlevel/2007/08/29/0001.html
The original message describing the problem is here:
http://mail-index.netbsd.org/tech-userlevel/2007/08/25/0006.html
Securing your system
--------------------
As far as the document goes now, this setup is unprotected in that anyone
listening in to the packets travelling trough your network would be able to
find the unencrypted messages of your ldap users. Not a happy thought.
So we'll want to enable SSL encryption of the traffic between your clients and
the server.
In order to do this you will need to create an SSL certificate for your server
and also distribute it to the client machines, so that they will be able to
certify the authenticity of the server.
We'll also need to configure slapd to use it, I put my keys in the /etc/openssl
hierachy, since it seemed made for it.
TLSCipherSuite HIGH:MEDIUM:+SSLv2
TLSCertificateFile /etc/openssl/certs/openldap.pem
TLSCertificateKeyFile /etc/openssl/private/openldap.pem
TLSCACertificateFile /etc/openssl/certs/openldap.pem
And we'll also have to change the way slapd is started, so add this to your
/etc/rc.conf
slapd_flags="-h ldaps://"
Note that this will make slapd answer only to ldaps!
Next we'll need to change the clients setup so that they will use ldaps.
Enable ssl in ${LOCALBASE}/etc/{nss_,pam_}ldap.conf;
ssl on
Next if you're like me using the ${LOCALBASE}/etc/openldap/ldap.conf file,
telling the client libs where to find the cert file is enough, we don't have
to put it in the nss/pam config:
URI ldaps://my.server
TLS_CACERT /etc/openssl/certs/openldap.pem
If you can still use getent, encryption is happening. You can of course also
tcpdump your network traffic to see what's going on.
ACL
---
I left access control lists of the server to the last, because they are the
easiest to get wrong and often cause problems that you might attribute to other
things in the various setups.
The syntax is fairly straightforward;
acceess to [something] by [someone] [access]
The order is important; if something matches, later tests will not be run.
The one I use looks like this:
#
# Protect passwords from prying eyes
#
access to attrs=userPassword
by dn="cn=nss,dc=example,dc=org" write
by anonymous auth
by self write
by * none
#
# set read-only attributes
#
access to attrs=uidNumber,gidNumber,uid,homeDirectory
by dn="cn=nss,dc=example,dc=org" write
by self read
by * read
#
# For all else, let the user edit his own entry and everyone else watch
#
access to *
by dn="cn=nss,dc=example,dc=org" write
by self write
by * read
Note that access to the user password can be set to auth; so that the database
can authenticate a user without letting them see the password hash using an
anonymous bind.
--
Staffan Thomen 1024D/7C7E2EF8
Sed quis custodiet ipsos Custodes?
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