On 16/12/2016 08:21, Alexander Bokovoy wrote:

So you can have IPA masters with FQDNs in totally different DNS domains
than dictated by their Kerberos realm and --domain options.

That I understand - not only can the IPA masters have FQDNs in different DNS domains, but indeed the member machines of that realm as well.

What was unclear to me was whether "ipa-server-install --domain xxx" affects the content of the database being built (and therefore replicated later to the slaves), or is just something local to the host itself.

In the manpage for "ipa-client-install" it's much clearer: in that case, it says that --domain is the starting domain for LDAP server auto-discovery.

To clarify, there are several DNS auto-discovery mechanisms. Two of them are described in the MIT docs at

(1) Map hostname aaa.bbb.ccc to realm xxx.yyy.zzz

Look for TXT records for _kerberos.aaa.bbb.ccc, _kerberos.bbb.ccc, _kerberos.ccc. The TXT record gives the realm that this host belongs to.

(2) Realm xxx.yyy.zzz to Kerberos servers for that realm

Given realm xxx.yyy.zzz, look for in the DNS for SRV records for

This is all very clear.

Now, the manpage for ipa-client-install describes another one, which is where I get a bit fuzzy:


   DNS Autodiscovery
Client installer by default tries to search for _ldap._tcp.DOMAIN DNS SRV records for all domains that are parent to its hostname. For exam- ple, if a client machine has a hostname 'client1.lab.example.com', the installer will try to retrieve an IPA server hostname from _ldap._tcp.lab.example.com, _ldap._tcp.example.com and _ldap._tcp.com DNS SRV records, respectively. The discovered domain is then used to configure client components (e.g. SSSD and Kerberos 5 configuration) on
       the machine.

What it doesn't actually say (but I believe must be true) is that what it calls the "discovered domain" is in fact the *realm* to use. If so, effectively this is algorithm (2) in reverse: instead of using it for realm to SRV mapping, you hunt for a domain which contains the right SRV records and use this to infer your realm.

Is that right?

(Is this a mechanism modelled on Active Directory? Otherwise, I would have thought you could use MIT algorithm (1) to discover your realm)

After all, these are *flexibility* options. They are not supposed to
make sense in all combinations. Where they aren't making sense, you are
allowed to shoot yourself in your feet if you know what you are doing.

Absolutely, and I don't want to get this wrong and have to start again :-)

OK, I have a final question on the planning of realms and DNS.

As we've already said, in an IPA-only installation, the machines which are members of the realms can happily have hostnames which are unrelated to the realm name: e.g.

         | | |
machines <name>.foo.com
machines <name>.bar.com

A user in IPA.EXAMPLE.COM can login to host <name>.foo.com, either because their krb5.conf has a static domain->realm mapping, or there's a DNS entry: _kerberos.foo.com TXT "IPA.EXAMPLE.COM"

However, suppose I plan to end up with a trust to an Active Directory / Samba4 realm:

    | | |                      | | |
    users                    machines

I want to allow users in the AD.EXAMPLE.COM realm to login to machines in the IPA.EXAMPLE.COM realm.

Will this still work when the machines are in different DNS domains? Or at this point, am I forced to give all the machines hostnames of the form <name>.ipa.example.com ?

If the latter is true, it would be wise for me to start naming my hosts <name>.ipa.example.com in the first place.



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