On 21/05/2024 13.11, Djerk Geurts wrote:
Thank you, that’s really helpful, especially how to test.

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?

GSS-Proxy uses the power of Unix sockets. "GSS_USE_PROXY=yes" enables the interposer library in the client. When the interposer intercepts some GSS-API calls and forwards them to GSS-Proxy's Unix domain socket. The proxy daemon uses getsockopt() with SO_PEERCRED to get the euid and egid of the client from the Linux Kernel. In your case, it maps euid 998 to "service/3CXPBX" and "3cxpbx.keytab". In client keytab mode, GSS-Proxy then uses the SPN of the first keytab slot. The keytab contains the SPN:

# ktutil
ktutil:  rkt /var/lib/ipa/gssproxy/http.keytab
ktutil:  l
slot KVNO Principal
---- ---- ---------------------------------------------------------------------
   1    1 HTTP/server.ipa-hcc.t...@ipa-hcc.test
   2    1 HTTP/server.ipa-hcc.t...@ipa-hcc.test
   3    1 HTTP/server.ipa-hcc.t...@ipa-hcc.test
   4    1 HTTP/server.ipa-hcc.t...@ipa-hcc.test


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Christian Heimes
Principal Software Engineer, Identity Management and Platform Security

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