kerberos: how the client knows the service name to request ticket to?

Question

Let's assume that the client wants to authenticate himself to a HTTP proxy. The proxy is configured with kerberos, and has clearly the service name HTTP/proxy.foo.bar set in it's configs. How does the client know which service name to request the ticket to ? Does it request the ticket to the domain name he's making request to (in this case it is proxy.foo.bar indeed), or does it receive the name in the authentication sequence, in a 407 reply in this case (which doest contain the negotiate challenge, but I just don't know if there's a way to look into it) ?

I'm trying to debug the kerberos errors on a proxy which suddenly stopped authenticating some clients. The thing is, that looking in the Wireshark, I see that the client is requesting a ticket not for a service name configured on a proxy (same name he's instructed to use), HTTP/proxy.foo.bar, but for a name that the proxy IP resolves to, HTTP/host.foo.bar (well, at least it's the name that the proxy resolves to, may be though the client gets it some other way), and TGS just cannot find one, thus an error happens.

Answer 1

So you’ve got two questions in here (you didn't ask how to actually solve the problem, to do that more details would be needed - see comments).

1. You asked "The proxy is configured with kerberos, and has clearly the service name HTTP/proxy.foo.bar set in it's configs. How does the client know which service name to request the ticket to?"

A. It works pretty much like this. The client types in a URL in the web browser or clicks on a hyperlink. It looks up the IP host in DNS domain which matches the host name in the URL. Then it goes to that IP host, looking for the service defined in the URL, in this case it is the HTTP service. If it receives an HTTP 401 Negotiate challenge (it's 401, not 407) from the web server, due to it being Kerberos-protected, it goes to its KDC and requests a Kerberos service ticket for HTTP/proxy.foo.bar, zips back to proxy.foo.bar and presents the ticket to that host for the HTTP service running on it. The host validates this ticket and if all is well and the client web browser renders the HTML. You've seen the Kerberos ticket ticket when you ran klist on the client. I don't have any web references for you, this is all off the top of my head.

1. You also asked “Does it request the ticket to the domain name he's making request to (in this case it is proxy.foo.bar indeed), or does it receive the name in the authentication sequence, in a 407 reply in this case (which doest contain the negotiate challenge, but I just don't know if there's a way to look into it) ?”

A. Your question was a bit hard to follow but if I am understanding you correctly, the answer is the web client requests a ticket as a result of the HTTP 401 Negotiate authentication challenge from the web server (see above).

There’s many diagrams sequencing this process on the web, including here: http://www.zeroshell.org/kerberos/Kerberos-operation/