For the 3CX service I do indeed need to add the GSS_USE_PROXY=yes, but as a side note, I’ll need to work out which service needs it as there are many daemons that make up 3CX. Anyway, this is on my todo list.

What I need to do first is create a Service Principal for this service to use. The GSS proxy config references the local uid, but how does it match the SPN? I guess I’m not clear on the service name as the host and REALM parts are straight forward. Is the username used for this? If so the SPN should be phonesystem/FQDN@REALM, as the uid is a number, so can I assume that the local machine uses the local account name belonging to the uid for this?

Djerk Geurts

On 21 May 2024, at 12:35, Christian Heimes <cheimes@redhat.com> wrote:
On 21/05/2024 12.15, Djerk Geurts via FreeIPA-users wrote:

Hi all,

Judging by my online searches, I’m far from the first to ask the question, but I’m keft with holes in my understanding of Kerberos and how services can authenticate via Kerberos (keytab).

I’m switching from sec=sys to sec=krb5p and either way struggle with local services which must place files on an NFS share for backup purposes. Using sec=sys things just work but the uid/gid numbers get matched locally and this often worked fine (when local services used the same aid/gid. But this doesn’t scale well, so I’m looking for ways to deal with this.

One way is to create a user in FreeIPA with the name of the service (for example bhsvc for Nakivo backup), and then adjust the uid on the local server to the IPA issued one, which is quick. But requires finding any file with the old id and changing it to the new one, which can be time consuming.

As the nfs client is a 3CX server, which don’t do well when manually configured as 3CX treat them as appliances. (God forbid someone might want to centrally manage these beast…); I would prefer not to change the uid of the local system account (phonesystem) to an IPA assigned one.

What are my options?

Despite finding how to configure gssproxy, I don’t yet understand how a daemon running as a certain user is mapped to an SPN with related keytab. Creating an SPN in IPA is easy, but how does the nfs-client know that a local system account should use/fetch a keytab for a certain SPN?

I could just manually set the uid of the local user on the nfs server, but while this worked with sec=sys, I don’t think this works with sec=krb5. So an option is to revert to sec=system, but I’d prefer not to.

The gssproxy config I created for the 3cxpbx daemon(s):

user@3cx04:~$ cat /etc/gssproxy/00-3cxpbx.conf

[service/3CXPBX]

mechs = krb5

cred_store = ccache:/var/lib/gssproxy/clients/krb5cc_3cxpbx

cred_store = client_keytab:/var/lib/gssproxy/clients/3cxpbx.keytab

cred_usage = initiate

euid = 998

gss-proxy maps an execution context to a configuration and keytab. In your case, it maps the context by effective uid. gss-proxy then uses the SPN of the first entry of the keytab to acquire a TGT with that keytab.
By the way, did you set the environment variable "GSS_USE_PROXY=yes" for your daemons, too? The easiest way is to use a systemd override with e.g. "systemctl edit yourdaemon.service", then add:

[Service]
Environment=GSS_USE_PROXY=yes
You can test the gssproxy configuration by switching to the user (sudo, su), then running "GSS_USE_PROXY=yes ipa ping". If everything is working correctly, then ipa CLI will automatically acquire a TGT for you. Don't try this with kinit, it doesn't use GSS-API.

Christian
-- 
Christian Heimes
Principal Software Engineer, Identity Management and Platform Security

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