+
+
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.
+
+
