Re: DFS referrals

[Marcus Moeller <marcus.moeller-OI3hZJvNYWs@public.gmane.org>]

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Hi again,

>>>>>>> On Mon, 29 Jul 2013 14:50:03 +0200
>>>>>>> Marcus Moeller <marcus.moeller-OI3hZJvNYWs@public.gmane.org> wrote:
>>>>>>>
>>>>>>>> [  124.607810] fs/cifs/cifssmb.c: negprot rc 0
>>>>>>>> [  124.607814] fs/cifs/connect.c: Security Mode: 0xf Capabilities:
>>>>>>>> 0x8001f3fc TimeAdjust: -7200
>>>>>>>> [  124.607817] fs/cifs/sess.c: sess setup type 4
>>>>>>>> [  124.607826] fs/cifs/cifs_spnego.c: key description =
>>>>>>>> ver=0x2;host=d.ethz.ch;ip4=82.130.70.6;sec=krb5;uid=0xaf05;creduid=0xaf05;user=mam4tst;pid=0x61a
>>>>>>>>
>>>>>>>>
>>>>>>>> [  124.803185] fs/cifs/sess.c: ssetup freeing small buf
>>>>>>>> ffff88022c31a000
>>>>>>>> [  124.803195] CIFS VFS: Send error in SessSetup = -126
>>>>>>>> [  124.803203] fs/cifs/connect.c: CIFS VFS: leaving
>>>>>>>> cifs_get_smb_ses (xid = 5) rc = -126
>>>>>>>> [  124.803212] fs/cifs/fscache.c:
>>>>>>>> cifs_fscache_release_client_cookie:
>>>>>>>> (0xffff88022a1b6000/0xffff88022a6430f0)
>>>>>>>> [  124.803368] fs/cifs/connect.c: CIFS VFS: leaving cifs_mount (xid
>>>>>>>> = 4) rc = -126
>>>>>>>> [  124.803374] CIFS VFS: cifs_mount failed w/return code = -126
>>>>>>>
>>>>>>> The only failure I see is the one above, and that's because it failed
>>>>>>> to upcall for the correct key. Are you sure you have krb5 creds as
>>>>>>> that
>>>>>>> user?
>>>>>>
>>>>>> Yes, creds are there and it also works when mounting from one of the
>>>>>> servers directly.
>>>>>>
>>>>>> Only mounting using the domainname does not work.
>>>>>>
>>>>>>
>>>>>>>> [  131.324798] fs/cifs/cifssmb.c: negprot rc 0
>>>>>>>> [  131.324804] fs/cifs/connect.c: Security Mode: 0xf Capabilities:
>>>>>>>> 0x8001f3fc TimeAdjust: -7200
>>>>>>>> [  131.324808] fs/cifs/sess.c: sess setup type 4
>>>>>>>> [  131.324821] fs/cifs/cifs_spnego.c: key description =
>>>>>>>> ver=0x2;host=d.ethz.ch;ip4=172.31.65.62;sec=krb5;uid=0xaf05;creduid=0xaf05;user=mam4tst;pid=0x62c
>>
>>>>>>>> [  131.384335] fs/cifs/transport.c: For smb_command 115
>>>>>>>> [  131.384344] fs/cifs/transport.c: Sending smb: smb_len=1666
>>>>>>>> [  131.387043] fs/cifs/connect.c: RFC1002 header 0xf9
>>>>>>>> [  131.387055] fs/cifs/misc.c: checkSMB Length: 0xfd,
>>>>>>>> smb_buf_length: 0xf9
>>>>>>>> [  131.387095] fs/cifs/transport.c: cifs_sync_mid_result: cmd=115
>>>>>>>> mid=2 state=4
>>>>>>>> [  131.387100] fs/cifs/misc.c: Null buffer passed to
>>>>>>>> cifs_small_buf_release
>>>>>>>
>>>>>>> Here' the upcall for a similar set of creds worked fine. The only
>>>>>>> thing
>>>>>>> that seems to have changed in the key description is the IP address.
>>>>>>>
>>>>>>> Do you have cifs.upcall set up to use the --trust-dns flag? If so,
>>>>>>> why?
>>>>>>
>>>>>> A relict from the past. I have removed it from the config. Thanks for
>>>>>> pointing out.
>>
>> Sorry, I was wrong. Without the -t option I am not even able to mount it
>> at all. The man page states a few words on that parameter, but I am
>> still not sure how it works when -t is not set.
>>
>> With -t set, the initial problem with the domain lookup works, when
>> reverse DNS is configured propably.
>>
>
> Ok, that makes sense then. The problem here is that the kernel needs to
> know what service principal name to use when contacting the server, and
> I suspect your krb5 configuration is not quite right.
>
> It looks like you're doing something like:
>
>      mount //d.ethz.ch/dfs /mnt/dfs -o sec=krb5...
>
> ...at this point, what happens is that the kernel needs to get a krb5
> service ticket to talk to the CIFS service on the host.
>
> What it typically does is take the hostname in the UNC that you're
> trying to mount, prepend it with "cifs/" and then try to get a service
> ticket for that. In your case, it'll look something like this:
>
>      cifs/d.ethz.ch-ofn1FrHcITAsyahpCud6bTnlAmrJQu31@public.gmane.org
>
> ...now, typically if that fails, we'll give up. Trying to do anything
> else is not considered safe since it's vulernable to DNS spoofing.
>
> If however, you add the '-t' flag to cifs.upcall, that tells it to try
> and guess the hostname part of that principal by reverse resolving it in
> DNS. It takes the IP address to which you are connecting, does a
> reverse DNS lookup and then uses that in the SPN.
>
> This is less safe, since if your DNS server is compromised someone
> could redirect you to a malicious server, and your client wouldn't be
> able to trivially detect that. So it in effect waters down krb5
> security.
>
> The correct fix is to ensure that the server(s) to which you are
> connecting have the ability to accept SPNs for the "hostnames" to which
> you want to connect. That means that you need to add SPNs for
> cifs/d.ethz.ch and ensure that the server will accept them to talk to
> its cifs service.
>
> Alternately, you can continue to use the '-t' flag and ensure that each
> possible server accepts principals for the hostnames to which their IP
> addresses reverse-resolve, with the caveat that its less safe than
> doing that the "right way".
>
> As to how to add these principals and make the server accept them...it
> depends on the server.
>
> Clear as mud?

Hehe, thanks for pointing that out. One thing I am not yet aware of is 
where the SPN cifs/d.ethz.ch has to be set? On the DFS Servers and/or on 
the servers which hold the shares? The latter ones are EMC and the DFS 
Servers are 2008R2.

Greets
Marcus




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