Contents

  1. Author's note
  2. Server setup
  3. Client Setup
  4. NSS
  5. PAM
    1. /etc/pam.d/sshd
    2. /etc/pam.d/su
    3. /etc/pam.d/system
  6. Securing your system
  7. ACL

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

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
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

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: openldap_authentication_on_netbsd.mdwn,v 1.2 2012/02/05 07:14:36 schmonz 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: openldap_authentication_on_netbsd.mdwn,v 1.2 2012/02/05 07:14:36 schmonz 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: openldap_authentication_on_netbsd.mdwn,v 1.2 2012/02/05 07:14:36 schmonz 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:

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.

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