Setting up DNS using bind
© 2009, 2010, 2011 Dennis Leeuw dleeuw at made-it dot com
License: GPLv2 or later
Kerberos makes use of the SRV records in DNS to look up its services. To make switching machines easier (when e.g. replacing the KDC) we use CNAMEs to point to the servers in a functional name, however you can not use CNAMEs in SRV records, so we have to use canonical names there. For quick reference we give an overview of what an SRV record looks like, see RFC 2782 for more information:
_service._proto.name ttl IN SRV priority weight port target
The different SRV records defined for kerberos use are:
$ORIGIN .
$TTL 86400 ; 1 day
@ IN SOA ns1.example.com. hostmaster.example.com. (
2009022507 ; serial
10800 ; refresh (3 hours)
900 ; retry (15 minutes)
604800 ; expire (1 week)
86400 ; minimum (1 day)
)
NS ns1.example.com.
A 192.168.1.1
$ORIGIN example.com.
_kerberos TXT "EXAMPLE.COM"
krb5 A 192.168.1.1
ns1 CNAME krb5
kerberos CNAME krb5
$ORIGIN _tcp.krb5.example.com.
_kerberos-adm SRV 0 0 749 krb5.example.com.
$ORIGIN _udp.krb5.example.com.
_kerberos SRV 0 0 88 krb5.example.com.
_kerberos-master SRV 0 0 88 krb5.example.com.
_kpasswd SRV 0 0 464 krb5.example.com.
A DNS service description for LDAP is very simple:
_ldap._tcp.example.com. SRV 0 0 389 ldap.example.net.
To support Active Directory in your bind DNS server you need a couple of extra entries. Below there is a little overview of what Active Directory expects, this overview only deals with the _ldap services but the same holds true for the other services.
An example list of entries generated by Microsoft Windows 2003 when doing an Active Directory installation looks like this:
_ldap._tcp.forest.example.com. 600 IN SRV 0 100 389 adf.forest.example.com. _gc._tcp.forest.example.com. 600 IN SRV 0 100 3268 adf.forest.example.com. _kerberos._udp.forest.example.com. 600 IN SRV 0 100 88 adf.forest.example.com. _kerberos._tcp.forest.example.com. 600 IN SRV 0 100 88 adf.forest.example.com. _kpasswd._tcp.forest.example.com. 600 IN SRV 0 100 464 adf.forest.example.com. _kpasswd._udp.forest.example.com. 600 IN SRV 0 100 464 adf.forest.example.com. _ldap._tcp.gc._msdcs.forest.example.com. 600 IN SRV 0 100 3268 adf.forest.example.com. _ldap._tcp.dc._msdcs.forest.example.com. 600 IN SRV 0 100 389 adf.forest.example.com. _kerberos._tcp.dc._msdcs.forest.example.com. 600 IN SRV 0 100 88 adf.forest.example.com. _ldap._tcp.pdc._msdcs.forest.example.com. 600 IN SRV 0 100 389 adf.forest.example.com. 34356b88-8d9f-457b-87ae-dd660c028489._msdcs.forest.example.com. 600 IN CNAME adf.forest.example.com. _ldap._tcp.f3d8bf35-d00a-48fd-aaf9-c7d1999227f3.domains._msdcs.forest.example.com. 600 IN SRV 0 100 389 adf.forest.example.com. _ldap._tcp.Default-First-Site-Name._sites.dc._msdcs.forest.example.com. 600 IN SRV 0 100 389 adf.forest.example.com. _kerberos._tcp.Default-First-Site-Name._sites.dc._msdcs.forest.example.com. 600 IN SRV 0 100 88 adf.forest.example.com. _ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.forest.example.com. 600 IN SRV 0 100 3268 adf.forest.example.com. _ldap._tcp.Default-First-Site-Name._sites.forest.example.com. 600 IN SRV 0 100 389 adf.forest.example.com. _gc._tcp.Default-First-Site-Name._sites.forest.example.com. 600 IN SRV 0 100 3268 adf.forest.example.com. _kerberos._tcp.Default-First-Site-Name._sites.forest.example.com. 600 IN SRV 0 100 88 adf.forest.example.com.
To make sure that our AD server also has this zone file we allow AD to transfer this file so our named.conf has the following setup:
zone krb5.example.com {
type master;
file internal/com.example.krb5.zone;
check-names ignore;
allow-transfer {
// ad server
192.168.1.2;
};
allow-update {
// test client
192.168.1.3;
};
notify no;
};
You will note that we have an entry for a client to do updates, this is so that the client can put its IP address in the zone file, AD relies on the ability to resolve client host names to IP addresses, and since we use DHCP to supply IP addresses to our clients, we need a way to do automatic DNS updates (RFC2136). The reverse is not needed, at least not that we have found.
It would be a lot of work to make sure your DNS is setup correctly with the above information. Lukily for us there are two ways to make life easier. The first one is to copy the \Windows\System32\Config\netlogon.dns file to your bind server and add the settings to DNS.
An even easier way is to make Active Directory do it automatically. Since we need dynamic DNS updates for the Windows clients anyway, this is the easiest route. Add to your named.conf the following section to your zone entry:
allow-update {
// Allow DNS updates from our network
192.168.1.0/24;
};
For security reasons this might not be the right solution, or you must trust all your employees and anybody that can connect a computer to your network. But it is beyond the scope of this document to handle all the security implications. Maybe I will add more documentation on this later on.
To test the setup use nsupdate:
nsupdate > server ns1.example.com > zone example.com > update delete bofh011.example.com. A > update add bofh011.example.com. 86400 A 192.168.1.15 > sendIf this works and your AD server is in the same network, it will happily add its own DNS entries.