Petter Reinholdtsen

On larger sites, it is useful to use a dedicated storage server for storing user home directories and data. The design for handling this in Debian Edu / Skolelinux, is to update the automount rules in LDAP and let the automount daemon on the clients take care of the rest. I was reminded about the need to document this better when one of the customers of Skolelinux Drift AS, where I am on the board of directors, asked about how to do this. The steps to get this working are the following:

1. Add new storage server in DNS. I use nas-server.intern as the example host here.
2. Add automoun LDAP information about this server in LDAP, to allow all clients to automatically mount it on reqeust.
3. Add the relevant entries in tjener.intern:/etc/fstab, because tjener.intern do not use automount to avoid mounting loops.

DNS entries are added in GOsa², and not described here. Follow the instructions in the manual (Machine Management with GOsa² in section Getting started).

Ensure that the NFS export points on the server are exported to the relevant subnets or machines:

root@tjener:~# showmount -e nas-server
Export list for nas-server:
/storage         10.0.0.0/8
root@tjener:~#

Here everything on the backbone network is granted access to the /storage export. With NFSv3 it is slightly better to limit it to netgroup membership or single IP addresses to have some limits on the NFS access.

The next step is to update LDAP. This can not be done using GOsa², because it lack a module for automount. Instead, use ldapvi and add the required LDAP objects using an editor.

ldapvi --ldap-conf -ZD '(cn=admin)' -b ou=automount,dc=skole,dc=skolelinux,dc=no

When the editor show up, add the following LDAP objects at the bottom of the document. The "/&" part in the last LDAP object is a wild card matching everything the nas-server exports, removing the need to list individual mount points in LDAP.

add cn=nas-server,ou=auto.skole,ou=automount,dc=skole,dc=skolelinux,dc=no
objectClass: automount
cn: nas-server
automountInformation: -fstype=autofs --timeout=60 ldap:ou=auto.nas-server,ou=automount,dc=skole,dc=skolelinux,dc=no

add ou=auto.nas-server,ou=automount,dc=skole,dc=skolelinux,dc=no
objectClass: top
objectClass: automountMap
ou: auto.nas-server

add cn=/,ou=auto.nas-server,ou=automount,dc=skole,dc=skolelinux,dc=no
objectClass: automount
cn: /
automountInformation: -fstype=nfs,tcp,rsize=32768,wsize=32768,rw,intr,hard,nodev,nosuid,noatime nas-server.intern:/&

The last step to remember is to mount the relevant mount points in tjener.intern by adding them to /etc/fstab, creating the mount directories using mkdir and running "mount -a" to mount them.

When this is done, your users should be able to access the files on the storage server directly by just visiting the /tjener/nas-server/storage/ directory using any application on any workstation, LTSP client or LTSP server.

This is a followup on my previous work on merging all the computer related LDAP objects in Debian Edu.

As a step to try to see if it possible to merge the DNS and DHCP LDAP objects, I have had a look at how the packages pdns-backend-ldap and dhcp3-server-ldap in Debian use the LDAP server. The two implementations are quite different in how they use LDAP.

To get this information, I started slapd with debugging enabled and dumped the debug output to a file to get the LDAP searches performed on a Debian Edu main-server. Here is a summary.

powerdns

Clues on how to set up PowerDNS to use a LDAP backend is available on the web.

PowerDNS have two modes of operation using LDAP as its backend. One "strict" mode where the forward and reverse DNS lookups are done using the same LDAP objects, and a "tree" mode where the forward and reverse entries are in two different subtrees in LDAP with a structure based on the DNS names, as in tjener.intern and 2.2.0.10.in-addr.arpa.

In tree mode, the server is set up to use a LDAP subtree as its base, and uses a "base" scoped search for the DNS name by adding "dc=tjener,dc=intern," to the base with a filter for "(associateddomain=tjener.intern)" for the forward entry and "dc=2,dc=2,dc=0,dc=10,dc=in-addr,dc=arpa," with a filter for "(associateddomain=2.2.0.10.in-addr.arpa)" for the reverse entry. For forward entries, it is looking for attributes named dnsttl, arecord, nsrecord, cnamerecord, soarecord, ptrrecord, hinforecord, mxrecord, txtrecord, rprecord, afsdbrecord, keyrecord, aaaarecord, locrecord, srvrecord, naptrrecord, kxrecord, certrecord, dsrecord, sshfprecord, ipseckeyrecord, rrsigrecord, nsecrecord, dnskeyrecord, dhcidrecord, spfrecord and modifytimestamp. For reverse entries it is looking for the attributes dnsttl, arecord, nsrecord, cnamerecord, soarecord, ptrrecord, hinforecord, mxrecord, txtrecord, rprecord, aaaarecord, locrecord, srvrecord, naptrrecord and modifytimestamp. The equivalent ldapsearch commands could look like this:

ldapsearch -h ldap \
  -b dc=tjener,dc=intern,ou=hosts,dc=skole,dc=skolelinux,dc=no \
  -s base -x '(associateddomain=tjener.intern)' dNSTTL aRecord nSRecord \
  cNAMERecord sOARecord pTRRecord hInfoRecord mXRecord tXTRecord \
  rPRecord aFSDBRecord KeyRecord aAAARecord lOCRecord sRVRecord \
  nAPTRRecord kXRecord certRecord dSRecord sSHFPRecord iPSecKeyRecord \
  rRSIGRecord nSECRecord dNSKeyRecord dHCIDRecord sPFRecord modifyTimestamp

ldapsearch -h ldap \
  -b dc=2,dc=2,dc=0,dc=10,dc=in-addr,dc=arpa,ou=hosts,dc=skole,dc=skolelinux,dc=no \
  -s base -x '(associateddomain=2.2.0.10.in-addr.arpa)'
  dnsttl, arecord, nsrecord, cnamerecord soarecord ptrrecord \
  hinforecord mxrecord txtrecord rprecord aaaarecord locrecord \
  srvrecord naptrrecord modifytimestamp

In Debian Edu/Lenny, the PowerDNS tree mode is used with ou=hosts,dc=skole,dc=skolelinux,dc=no as the base, and these are two example LDAP objects used there. In addition to these objects, the parent objects all th way up to ou=hosts,dc=skole,dc=skolelinux,dc=no also exist.

dn: dc=tjener,dc=intern,ou=hosts,dc=skole,dc=skolelinux,dc=no
objectclass: top
objectclass: dnsdomain
objectclass: domainrelatedobject
dc: tjener
arecord: 10.0.2.2
associateddomain: tjener.intern

dn: dc=2,dc=2,dc=0,dc=10,dc=in-addr,dc=arpa,ou=hosts,dc=skole,dc=skolelinux,dc=no
objectclass: top
objectclass: dnsdomain2
objectclass: domainrelatedobject
dc: 2
ptrrecord: tjener.intern
associateddomain: 2.2.0.10.in-addr.arpa

In strict mode, the server behaves differently. When looking for forward DNS entries, it is doing a "subtree" scoped search with the same base as in the tree mode for a object with filter "(associateddomain=tjener.intern)" and requests the attributes dnsttl, arecord, nsrecord, cnamerecord, soarecord, ptrrecord, hinforecord, mxrecord, txtrecord, rprecord, aaaarecord, locrecord, srvrecord, naptrrecord and modifytimestamp. For reverse entires it also do a subtree scoped search but this time the filter is "(arecord=10.0.2.2)" and the requested attributes are associateddomain, dnsttl and modifytimestamp. In short, in strict mode the objects with ptrrecord go away, and the arecord attribute in the forward object is used instead.

The forward and reverse searches can be simulated using ldapsearch like this:

ldapsearch -h ldap -b ou=hosts,dc=skole,dc=skolelinux,dc=no -s sub -x \
  '(associateddomain=tjener.intern)' dNSTTL aRecord nSRecord \
  cNAMERecord sOARecord pTRRecord hInfoRecord mXRecord tXTRecord \
  rPRecord aFSDBRecord KeyRecord aAAARecord lOCRecord sRVRecord \
  nAPTRRecord kXRecord certRecord dSRecord sSHFPRecord iPSecKeyRecord \
  rRSIGRecord nSECRecord dNSKeyRecord dHCIDRecord sPFRecord modifyTimestamp

ldapsearch -h ldap -b ou=hosts,dc=skole,dc=skolelinux,dc=no -s sub -x \
  '(arecord=10.0.2.2)' associateddomain dnsttl modifytimestamp

In addition to the forward and reverse searches , there is also a search for SOA records, which behave similar to the forward and reverse lookups.

A thing to note with the PowerDNS behaviour is that it do not specify any objectclass names, and instead look for the attributes it need to generate a DNS reply. This make it able to work with any objectclass that provide the needed attributes.

The attributes are normally provided in the cosine (RFC 1274) and dnsdomain2 schemas. The latter is used for reverse entries like ptrrecord and recent DNS additions like aaaarecord and srvrecord.

In Debian Edu, we have created DNS objects using the object classes dcobject (for dc), dnsdomain or dnsdomain2 (structural, for the DNS attributes) and domainrelatedobject (for associatedDomain). The use of structural object classes make it impossible to combine these classes with the object classes used by DHCP.

There are other schemas that could be used too, for example the dnszone structural object class used by Gosa and bind-sdb for the DNS attributes combined with the domainrelatedobject object class, but in this case some unused attributes would have to be included as well (zonename and relativedomainname).

My proposal for Debian Edu would be to switch PowerDNS to strict mode and not use any of the existing objectclasses (dnsdomain, dnsdomain2 and dnszone) when one want to combine the DNS information with DHCP information, and instead create a auxiliary object class defined something like this (using the attributes defined for dnsdomain and dnsdomain2 or dnszone):

objectclass ( some-oid NAME 'dnsDomainAux'
    SUP top
    AUXILIARY
    MAY ( ARecord $ MDRecord $ MXRecord $ NSRecord $ SOARecord $ CNAMERecord $
          DNSTTL $ DNSClass $ PTRRecord $ HINFORecord $ MINFORecord $
          TXTRecord $ SIGRecord $ KEYRecord $ AAAARecord $ LOCRecord $
          NXTRecord $ SRVRecord $ NAPTRRecord $ KXRecord $ CERTRecord $
          A6Record $ DNAMERecord
    ))

This will allow any object to become a DNS entry when combined with the domainrelatedobject object class, and allow any entity to include all the attributes PowerDNS wants. I've sent an email to the PowerDNS developers asking for their view on this schema and if they are interested in providing such schema with PowerDNS, and I hope my message will be accepted into their mailing list soon.

ISC dhcp

The DHCP server searches for specific objectclass and requests all the object attributes, and then uses the attributes it want. This make it harder to figure out exactly what attributes are used, but thanks to the working example in Debian Edu I can at least get an idea what is needed without having to read the source code.

In the DHCP server configuration, the LDAP base to use and the search filter to use to locate the correct dhcpServer entity is stored. These are the relevant entries from /etc/dhcp3/dhcpd.conf:

ldap-base-dn "dc=skole,dc=skolelinux,dc=no";
ldap-dhcp-server-cn "dhcp";

The DHCP server uses this information to nest all the DHCP configuration it need. The cn "dhcp" is located using the given LDAP base and the filter "(&(objectClass=dhcpServer)(cn=dhcp))". The search result is this entry:

dn: cn=dhcp,dc=skole,dc=skolelinux,dc=no
cn: dhcp
objectClass: top
objectClass: dhcpServer
dhcpServiceDN: cn=DHCP Config,dc=skole,dc=skolelinux,dc=no

The content of the dhcpServiceDN attribute is next used to locate the subtree with DHCP configuration. The DHCP configuration subtree base is located using a base scope search with base "cn=DHCP Config,dc=skole,dc=skolelinux,dc=no" and filter "(&(objectClass=dhcpService)(|(dhcpPrimaryDN=cn=dhcp,dc=skole,dc=skolelinux,dc=no)(dhcpSecondaryDN=cn=dhcp,dc=skole,dc=skolelinux,dc=no)))". The search result is this entry:

dn: cn=DHCP Config,dc=skole,dc=skolelinux,dc=no
cn: DHCP Config
objectClass: top
objectClass: dhcpService
objectClass: dhcpOptions
dhcpPrimaryDN: cn=dhcp, dc=skole,dc=skolelinux,dc=no
dhcpStatements: ddns-update-style none
dhcpStatements: authoritative
dhcpOption: smtp-server code 69 = array of ip-address
dhcpOption: www-server code 72 = array of ip-address
dhcpOption: wpad-url code 252 = text

Next, the entire subtree is processed, one level at the time. When all the DHCP configuration is loaded, it is ready to receive requests. The subtree in Debian Edu contain objects with object classes top/dhcpService/dhcpOptions, top/dhcpSharedNetwork/dhcpOptions, top/dhcpSubnet, top/dhcpGroup and top/dhcpHost. These provide options and information about netmasks, dynamic range etc. Leaving out the details here because it is not relevant for the focus of my investigation, which is to see if it is possible to merge dns and dhcp related computer objects.

When a DHCP request come in, LDAP is searched for the MAC address of the client (00:00:00:00:00:00 in this example), using a subtree scoped search with "cn=DHCP Config,dc=skole,dc=skolelinux,dc=no" as the base and "(&(objectClass=dhcpHost)(dhcpHWAddress=ethernet 00:00:00:00:00:00))" as the filter. This is what a host object look like:

dn: cn=hostname,cn=group1,cn=THINCLIENTS,cn=DHCP Config,dc=skole,dc=skolelinux,dc=no
cn: hostname
objectClass: top
objectClass: dhcpHost
dhcpHWAddress: ethernet 00:00:00:00:00:00
dhcpStatements: fixed-address hostname

There is less flexiblity in the way LDAP searches are done here. The object classes need to have fixed names, and the configuration need to be stored in a fairly specific LDAP structure. On the positive side, the invidiual dhcpHost entires can be anywhere without the DN pointed to by the dhcpServer entries. The latter should make it possible to group all host entries in a subtree next to the configuration entries, and this subtree can also be shared with the DNS server if the schema proposed above is combined with the dhcpHost structural object class.

Conclusion

The PowerDNS implementation seem to be very flexible when it come to which LDAP schemas to use. While its "tree" mode is rigid when it come to the the LDAP structure, the "strict" mode is very flexible, allowing DNS objects to be stored anywhere under the base cn specified in the configuration.

The DHCP implementation on the other hand is very inflexible, both regarding which LDAP schemas to use and which LDAP structure to use. I guess one could implement ones own schema, as long as the objectclasses and attributes have the names used, but this do not really help when the DHCP subtree need to have a fairly fixed structure.

Based on the observed behaviour, I suspect a LDAP structure like this might work for Debian Edu:

ou=services
  cn=machine-info (dhcpService) - dhcpServiceDN points here
    cn=dhcp (dhcpServer)
    cn=dhcp-internal (dhcpSharedNetwork/dhcpOptions)
      cn=10.0.2.0 (dhcpSubnet)
        cn=group1 (dhcpGroup/dhcpOptions)
    cn=dhcp-thinclients (dhcpSharedNetwork/dhcpOptions)
      cn=192.168.0.0 (dhcpSubnet)
        cn=group1 (dhcpGroup/dhcpOptions)
    ou=machines - PowerDNS base points here
      cn=hostname (dhcpHost/domainrelatedobject/dnsDomainAux)

This is not tested yet. If the DHCP server require the dhcpHost entries to be in the dhcpGroup subtrees, the entries can be stored there instead of a common machines subtree, and the PowerDNS base would have to be moved one level up to the machine-info subtree.

The combined object under the machines subtree would look something like this:

dn: dc=hostname,ou=machines,cn=machine-info,dc=skole,dc=skolelinux,dc=no
dc: hostname
objectClass: top
objectClass: dhcpHost
objectclass: domainrelatedobject
objectclass: dnsDomainAux
associateddomain: hostname.intern
arecord: 10.11.12.13
dhcpHWAddress: ethernet 00:00:00:00:00:00
dhcpStatements: fixed-address hostname.intern

One could even add the LTSP configuration associated with a given machine, as long as the required attributes are available in a auxiliary object class.

For a while now, I have wanted to find a way to change the DNS and DHCP services in Debian Edu to use the same LDAP objects for a given computer, to avoid the possibility of having a inconsistent state for a computer in LDAP (as in DHCP but no DNS entry or the other way around) and make it easier to add computers to LDAP.

I've looked at how powerdns and dhcpd is using LDAP, and using this information finally found a solution that seem to work.

The old setup required three LDAP objects for a given computer. One forward DNS entry, one reverse DNS entry and one DHCP entry. If we switch powerdns to use its strict LDAP method (ldap-method=strict in pdns-debian-edu.conf), the forward and reverse DNS entries are merged into one while making it impossible to transfer the reverse map to a slave DNS server.

If we also replace the object class used to get the DNS related attributes to one allowing these attributes to be combined with the dhcphost object class, we can merge the DNS and DHCP entries into one. I've written such object class in the dnsdomainaux.schema file (need proper OIDs, but that is a minor issue), and tested the setup. It seem to work.

With this test setup in place, we can get away with one LDAP object for both DNS and DHCP, and even the LTSP configuration I suggested in an earlier email. The combined LDAP object will look something like this:

  dn: cn=hostname,cn=group1,cn=THINCLIENTS,cn=DHCP Config,dc=skole,dc=skolelinux,dc=no
  cn: hostname
  objectClass: dhcphost
  objectclass: domainrelatedobject
  objectclass: dnsdomainaux
  associateddomain: hostname.intern
  arecord: 10.11.12.13
  dhcphwaddress: ethernet 00:00:00:00:00:00
  dhcpstatements: fixed-address hostname
  ldapconfigsound: Y

The DNS server uses the associateddomain and arecord entries, while the DHCP server uses the dhcphwaddress and dhcpstatements entries before asking DNS to resolve the fixed-adddress. LTSP will use dhcphwaddress or associateddomain and the ldapconfig* attributes.

I am not yet sure if I can get the DHCP server to look for its dhcphost in a different location, to allow us to put the objects outside the "DHCP Config" subtree, but hope to figure out a way to do that. If I can't figure out a way to do that, we can still get rid of the hosts subtree and move all its content into the DHCP Config tree (which probably should be renamed to be more related to the new content. I suspect cn=dnsdhcp,ou=services or something like that might be a good place to put it.

If you want to help out with implementing this for Debian Edu, please contact us on debian-edu@lists.debian.org.

Vagrant mentioned on IRC today that ltsp_config now support sourcing files from /usr/share/ltsp/ltsp_config.d/ on the thin clients, and that this can be used to fetch configuration from LDAP if Debian Edu choose to store configuration there.

Armed with this information, I got inspired and wrote a test module to get configuration from LDAP. The idea is to look up the MAC address of the client in LDAP, and look for attributes on the form ltspconfigsetting=value, and use this to export SETTING=value to the LTSP clients.

The goal is to be able to store the LTSP configuration attributes in a "computer" LDAP object used by both DNS and DHCP, and thus allowing us to store all information about a computer in one place.

This is a untested draft implementation, and I welcome feedback on this approach. A real LDAP schema for the ltspClientAux objectclass need to be written. Comments, suggestions, etc?

# Store in /opt/ltsp/$arch/usr/share/ltsp/ltsp_config.d/ldap-config
#
# Fetch LTSP client settings from LDAP based on MAC address
#
# Uses ethernet address as stored in the dhcpHost objectclass using
# the dhcpHWAddress attribute or ethernet address stored in the
# ieee802Device objectclass with the macAddress attribute.
#
# This module is written to be schema agnostic, and only depend on the
# existence of attribute names.
#
# The LTSP configuration variables are saved directly using a
# ltspConfig prefix and uppercasing the rest of the attribute name.
# To set the SERVER variable, set the ltspConfigServer attribute.
#
# Some LDAP schema should be created with all the relevant
# configuration settings.  Something like this should work:
# 
# objectclass ( 1.1.2.2 NAME 'ltspClientAux'
#     SUP top
#     AUXILIARY
#     MAY ( ltspConfigServer $ ltsConfigSound $ ... )

LDAPSERVER=$(debian-edu-ldapserver)
if [ "$LDAPSERVER" ] ; then
    LDAPBASE=$(debian-edu-ldapserver -b)
    for MAC in $(LANG=C ifconfig |grep -i hwaddr| awk '{print $5}'|sort -u) ; do
	filter="(|(dhcpHWAddress=ethernet $MAC)(macAddress=$MAC))"
	ldapsearch -h "$LDAPSERVER" -b "$LDAPBASE" -v -x "$filter" | \
	    grep '^ltspConfig' | while read attr value ; do
	    # Remove prefix and convert to upper case
	    attr=$(echo $attr | sed 's/^ltspConfig//i' | tr a-z A-Z)
	    # bass value on to clients
	    eval "$attr=$value; export $attr"
	done
    done
fi

I'm not sure this shell construction will work, because I suspect the while block might end up in a subshell causing the variables set there to not show up in ltsp-config, but if that is the case I am sure the code can be restructured to make sure the variables are passed on. I expect that can be solved with some testing. :)

If you want to help out with implementing this for Debian Edu, please contact us on debian-edu@lists.debian.org.

Update 2010-07-17: I am aware of another effort to store LTSP configuration in LDAP that was created around year 2000 by PC Xperience, Inc., 2000. I found its files on a personal home page over at redhat.com.

Since my last post about available LDAP tools in Debian, I was told about a LDAP GUI that is even better than luma. The java application jXplorer is claimed to be capable of moving LDAP objects and subtrees using drag-and-drop, and can authenticate using Kerberos. I have only tested the Kerberos authentication, but do not have a LDAP setup allowing me to rewrite LDAP with my test user yet. It is available in Debian testing and unstable at the moment. The only problem I have with it is how it handle errors. If something go wrong, its non-intuitive behaviour require me to go through some query work list and remove the failing query. Nothing big, but very annoying.

For a laptop, centralized user directories and password checking is a bit troubling. Laptops are typically used also when not connected to the network, and it is vital for a user to be able to log in or unlock the screen saver also when a central server is unavailable. This is possible by caching passwords and directory information (user and group attributes) locally, and the packages to do so are available in Debian. Here follow two recipes to set this up in Debian/Squeeze. It is also possible to set up in Debian/Lenny, but require more manual setup there because pam-auth-update is missing in Lenny.

LDAP/Kerberos + nscd + libpam-ccreds + libpam-mklocaluser/pam_mkhomedir

This is the traditional method with a twist. The password caching is provided by libpam-ccreds (version 10-4 or later is needed on Squeeze), and the directory caching is done by nscd. The directory lookup and password checking is done using LDAP. If one want to use Kerberos for password checking the libpam-ldapd package can be replaced with libpam-krb5 or libpam-heimdal. If one is happy having a local home directory with the path listed in LDAP, one can use the pam_mkhomedir module from pam-modules to make this happen instead of using libpam-mklocaluser. A setup for pam-auth-update to enable pam_mkhomedir will have to be written until a fix for bug #568577 is in the archive. Because I believe it is a bad idea to have local home directories using misleading paths like /site/server/partition/, I prefer to create a local user with the home directory in /home/. This is done using the libpam-mklocaluser package.

These packages need to be installed and configured

libnss-ldapd libpam-ldapd nscd libpam-ccreds libpam-mklocaluser

The ldapd packages will ask for LDAP connection information, and one have to fill in the values that fits ones own site. Make sure the PAM part uses encrypted connections, to make sure the password is not sent in clear text to the LDAP server. I've been unable to get TLS certificate checking for a self signed certificate working, which make LDAP authentication unsafe for Debian Edu (nslcd is not checking if it is talking to the correct LDAP server), and very much welcome feedback on how to get this working.

Because nscd do not have a default configuration fit for offline caching until bug #485282 is fixed, this configuration should be used instead of the one currently in /etc/nscd.conf. The changes are in the fields reload-count and positive-time-to-live, and is based on the instructions I found in the LDAP for Mobile Laptops instructions by Flyn Computing.

	debug-level		0
	reload-count		unlimited
	paranoia		no

	enable-cache		passwd		yes
	positive-time-to-live	passwd		2592000
	negative-time-to-live	passwd		20
	suggested-size		passwd		211
	check-files		passwd		yes
	persistent		passwd		yes
	shared			passwd		yes
	max-db-size		passwd		33554432
	auto-propagate		passwd		yes

	enable-cache		group		yes
	positive-time-to-live	group		2592000
	negative-time-to-live	group		20
	suggested-size		group		211
	check-files		group		yes
	persistent		group		yes
	shared			group		yes
	max-db-size		group		33554432
	auto-propagate		group		yes

	enable-cache		hosts		no
	positive-time-to-live	hosts		2592000
	negative-time-to-live	hosts		20
	suggested-size		hosts		211
	check-files		hosts		yes
	persistent		hosts		yes
	shared			hosts		yes
	max-db-size		hosts		33554432

	enable-cache		services	yes
	positive-time-to-live	services	2592000
	negative-time-to-live	services	20
	suggested-size		services	211
	check-files		services	yes
	persistent		services	yes
	shared			services	yes
	max-db-size		services	33554432

While we wait for a mechanism to update /etc/nsswitch.conf automatically like the one provided in bug #496915, the file content need to be manually replaced to ensure LDAP is used as the directory service on the machine. /etc/nsswitch.conf should normally look like this:

passwd:         files ldap
group:          files ldap
shadow:         files ldap
hosts:          files mdns4_minimal [NOTFOUND=return] dns mdns4
networks:       files
protocols:      files
services:       files
ethers:         files
rpc:            files
netgroup:       files ldap

The important parts are that ldap is listed last for passwd, group, shadow and netgroup.

With these changes in place, any user in LDAP will be able to log in locally on the machine using for example kdm, get a local home directory created and have the password as well as user and group attributes cached.

LDAP/Kerberos + nss-updatedb + libpam-ccreds + libpam-mklocaluser/pam_mkhomedir

Because nscd have had its share of problems, and seem to have problems doing proper caching, I've seen suggestions and recipes to use nss-updatedb to copy parts of the LDAP database locally when the LDAP database is available. I have not tested such setup, because I discovered sssd.

LDAP/Kerberos + sssd + libpam-mklocaluser

A more flexible and robust setup than the nscd combination mentioned earlier that has shown up recently, is the sssd package from Redhat. It is part of the FreeIPA project to provide a Active Directory like directory service for Linux machines. The sssd system combines the caching of passwords and user information into one package, and remove the need for nscd and libpam-ccreds. It support LDAP and Kerberos, but not NIS. Version 1.2 do not support netgroups, but it is said that it will support this in version 1.5 expected to show up later in 2010. Because the sssd package was missing in Debian, I ended up co-maintaining it with Werner, and version 1.2 is now in testing.

These packages need to be installed and configured to get the roaming setup I want

libpam-sss libnss-sss libpam-mklocaluser

The complete setup of sssd is done by editing/creating /etc/sssd/sssd.conf.

[sssd]
config_file_version = 2
reconnection_retries = 3
sbus_timeout = 30
services = nss, pam
domains = INTERN

[nss]
filter_groups = root
filter_users = root
reconnection_retries = 3

[pam]
reconnection_retries = 3

[domain/INTERN]
enumerate = false
cache_credentials = true

id_provider = ldap
auth_provider = ldap
chpass_provider = ldap

ldap_uri = ldap://ldap
ldap_search_base = dc=skole,dc=skolelinux,dc=no
ldap_tls_reqcert = never
ldap_tls_cacert = /etc/ssl/certs/ca-certificates.crt

I got the same problem here with certificate checking. Had to set "ldap_tls_reqcert = never" to get it working.

With the libnss-sss package in testing at the moment, the nsswitch.conf file is update automatically, so there is no need to modify it manually.

If you want to help out with implementing this for Debian Edu, please contact us on debian-edu@lists.debian.org.

The last few days I have been looking into the status of the LDAP directory in Debian Edu, and in the process I started to miss a GUI tool to browse the LDAP tree. The only one I was able to find in Debian/Squeeze and Lenny is LUMA, which has proved to be a great tool to get a overview of the current LDAP directory populated by default in Skolelinux. Thanks to it, I have been able to find empty and obsolete subtrees, misplaced objects and duplicate objects. It will be installed by default in Debian/Squeeze. If you are working with LDAP, give it a go. :)

I did notice one problem with it I have not had time to report to the BTS yet. There is no .desktop file in the package, so the tool do not show up in the Gnome and KDE menus, but only deep down in in the Debian submenu in KDE. I hope that can be fixed before Squeeze is released.

I have not yet been able to get it to modify the tree yet. I would like to move objects and remove subtrees directly in the GUI, but have not found a way to do that with LUMA yet. So in the mean time, I use ldapvi for that.

If you have tips on other GUI tools for LDAP that might be useful in Debian Edu, please contact us on debian-edu@lists.debian.org.

Update 2010-06-29: Ross Reedstrom tipped us about the gq package as a useful GUI alternative. It seem like a good tool, but is unmaintained in Debian and got a RC bug keeping it out of Squeeze. Unless that changes, it will not be an option for Debian Edu based on Squeeze.

A while back, I complained about the fact that it is not possible with the provided schemas for storing DNS and DHCP information in LDAP to combine the two sets of information into one LDAP object representing a computer.

In the mean time, I discovered that a simple fix would be to make the dhcpHost object class auxiliary, to allow it to be combined with the dNSDomain object class, and thus forming one object for one computer when storing both DHCP and DNS information in LDAP.

If I understand this correctly, it is not safe to do this change without also changing the assigned number for the object class, and I do not know enough about LDAP schema design to do that properly for Debian Edu.

Anyway, for future reference, this is how I believe we could change the DHCP schema to solve at least part of the problem with the LDAP schemas available today from IETF.

--- dhcp.schema    (revision 65192)
+++ dhcp.schema    (working copy)
@@ -376,7 +376,7 @@
 objectclass ( 2.16.840.1.113719.1.203.6.6
        NAME 'dhcpHost'
        DESC 'This represents information about a particular client'
-       SUP top
+       SUP top AUXILIARY
        MUST cn
        MAY  (dhcpLeaseDN $ dhcpHWAddress $ dhcpOptionsDN $ dhcpStatements $ dhcpComments $ dhcpOption)
        X-NDS_CONTAINMENT ('dhcpService' 'dhcpSubnet' 'dhcpGroup') )

I very much welcome clues on how to do this properly for Debian Edu/Squeeze. We provide the DHCP schema in our debian-edu-config package, and should thus be free to rewrite it as we see fit.

If you want to help out with implementing this for Debian Edu, please contact us on debian-edu@lists.debian.org.

The state of standardized LDAP schemas on Linux is far from optimal. There is RFC 2307 documenting one way to store NIS maps in LDAP, and a modified version of this normally called RFC 2307bis, with some modifications to be compatible with Active Directory. The RFC specification handle the content of a lot of system databases, but do not handle DNS zones and DHCP configuration.

In Debian Edu/Skolelinux, we would like to store information about users, SMB clients/hosts, filegroups, netgroups (users and hosts), DHCP and DNS configuration, and LTSP configuration in LDAP. These objects have a lot in common, but with the current LDAP schemas it is not possible to have one object per entity. For example, one need to have at least three LDAP objects for a given computer, one with the SMB related stuff, one with DNS information and another with DHCP information. The schemas provided for DNS and DHCP are impossible to combine into one LDAP object. In addition, it is impossible to implement quick queries for netgroup membership, because of the way NIS triples are implemented. It just do not scale. I believe it is time for a few RFC specifications to cleam up this mess.

I would like to have one LDAP object representing each computer in the network, and this object can then keep the SMB (ie host key), DHCP (mac address/name) and DNS (name/IP address) settings in one place. It need to be efficently stored to make sure it scale well.

I would also like to have a quick way to map from a user or computer and to the net group this user or computer is a member.

Active Directory have done a better job than unix heads like myself in this regard, and the unix side need to catch up. Time to start a new IETF work group?