Problems with GSSAPI and LDAP

Juha Koho juha.koho at trineco.fi
Tue Oct 11 10:10:33 UTC 2016 

On 2016-10-11 11:03, Aki Tuomi wrote:
> On 11.10.2016 11:56, Juha Koho wrote:
>> 
>> On 2016-10-11 10:00, Aki Tuomi wrote:
>>> On 11.10.2016 10:43, Juha Koho wrote:
>>>> 
>>>> On 2016-10-11 09:18, Aki Tuomi wrote:
>>>>> On 11.10.2016 10:13, Juha Koho wrote:
>>>>>> Hello,
>>>>>> 
>>>>>> I have a Dovecot 2.2.25 set up with OpenLDAP back end. I was
>>>>>> trying to
>>>>>> set up a GSSAPI Kerberos authentication with the LDAP server but 
>>>>>> with
>>>>>> little success. Seems no matter what I try I end up with the
>>>>>> following
>>>>>> error message:
>>>>>> 
>>>>>> dovecot: auth: Error: LDAP: binding failed (dn
>>>>>> (imap/host.example.com at EXAMPLE.COM)): Local error, SASL(-1): 
>>>>>> generic
>>>>>> failure: GSSAPI Error: Unspecified GSS failure.  Minor code may
>>>>>> provide more information (No Kerberos credentials available 
>>>>>> (default
>>>>>> cache: FILE:/tmp/dovecot.krb5.ccache))
>>>>>> 
>>>>>> I have set the import_environment in dovecot.conf:
>>>>>> 
>>>>>> import_environment = TZ CORE_OUTOFMEM CORE_ERROR LISTEN_PID
>>>>>> LISTEN_FDS
>>>>>> KRB5CCNAME=FILE:/tmp/dovecot.krb5.ccache
>>>>>> 
>>>>>> And these in LDAP configuration:
>>>>>> 
>>>>>> dn = imap/host.example.com at EXAMPLE.COM
>>>>>> sasl_bind = yes
>>>>>> sasl_mech = gssapi
>>>>>> sasl_realm = EXAMPLE.COM
>>>>>> sasl_authz_id = imap/host.example.com at EXAMPLE.COM
>>>>>> 
>>>>>> I have tried with different values in dn and sasl_authz_id and 
>>>>>> also
>>>>>> leaving them out completely but I always end up with the error
>>>>>> message
>>>>>> above. Using simple bind without GSSAPI works just fine.
>>>>>> 
>>>>>> The credentials cache file exists and is valid for the principal
>>>>>> imap/host.example.com at EXAMPLE.COM. The file is owned by dovecot 
>>>>>> user
>>>>>> so it shouldn't be a permission problem either.
>>>>>> 
>>>>>> GSSAPI in OpenLDAP works but I suppose it is irrelevant here since
>>>>>> the
>>>>>> connection attempt never reaches the LDAP server due to the error. 
>>>>>> I
>>>>>> also have similar setup for Postfix and it works fine.
>>>>>> 
>>>>>> Any ideas what to try next?
>>>>>> 
>>>>>> Best regards,
>>>>>> Juha
>>>>> 
>>>>> Can you provide klist output for the cache file? Also, it should be
>>>>> readable by dovenull user, or whatever is configured as
>>>>> default_login_user.
>>>> 
>>>> 
>>>> Here's the klist output of the cache file:
>>>> --
>>>> Ticket cache: FILE:/tmp/dovecot.krb5.ccache
>>>> Default principal: imap/host.example.com at EXAMPLE.COM
>>>> 
>>>> Valid starting       Expires              Service principal
>>>> 10/11/2016 09:26:25  10/11/2016 21:26:25
>>>> krbtgt/EXAMPLE.COM at EXAMPLE.COM
>>>>         renew until 10/12/2016 09:26:25
>>>> ---
>>>> 
>>>> That I didn't know that also dovenull must have access to the cache
>>>> but I tried also setting 0644 permissions to the cache file with no
>>>> luck. So permissions shouldn't be the issue...
>>>> 
>>>> Juha
>>> 
>>> Your ccache has no ticket for imap/host.example.com at EXAMPLE.COM
>>> 
>>> please use kinit to acquire one.
>> 
>> 
>> Now I'm confused. The cache file is created by kinit using the 
>> command:
>> 
>> sudo -u dovenull kinit -c FILE:/tmp/dovecot.krb5.ccache -k -t
>> /path/to/keytab imap/host.example.com
>> 
>> After that:
>> 
>> $ sudo -u dovenull klist /tmp/dovecot.krb5.ccache
>> Ticket cache: FILE:/tmp/dovecot.krb5.ccache
>> Default principal: imap/host.example.com at EXAMPLE.COM
>> 
>> Valid starting       Expires              Service principal
>> 10/11/2016 10:47:47  10/11/2016 22:47:47  
>> krbtgt/EXAMPLE.COM at EXAMPLE.COM
>>         renew until 10/12/2016 10:47:47
>> 
>> Also, I can use the cache file with ldapsearch just fine by running
>> the following:
>> 
>> sudo -u dovenull KRB5CCNAME=FILE:/tmp/dovecot.krb5.ccache ldapsearch
>> -Y GSSAPI -ZZ -H ldap://ldap.example.com/ -b dc=example,dc=com
>> 
>> After the ldapsearch has succeeded the klist output is the following:
>> 
>> $ sudo -u dovenull klist /tmp/dovecot.krb5.ccache
>> Ticket cache: FILE:/tmp/dovecot.krb5.ccache
>> Default principal: imap/host.example.com at EXAMPLE.COM
>> 
>> Valid starting       Expires              Service principal
>> 10/11/2016 10:47:47  10/11/2016 22:47:47  
>> krbtgt/EXAMPLE.COM at EXAMPLE.COM
>>         renew until 10/12/2016 10:47:47
>> 10/11/2016 10:49:32  10/11/2016 22:47:47
>> ldap/ldap.example.com at EXAMPLE.COM
>>         renew until 10/12/2016 10:47:47
>> 
>> 
>> Which is what I expected. Isn't this basically what dovecot does (or
>> should do) or did I miss something?
>> 
>> Juha
> 
> Dovecot won't acquire service tickets for you. It requires that you 
> have
> ticket for imap/imap.example.com at EXAMPLE.COM in the cache or keytab.
> 
> The default principal is used when *CONNECTING* to a service, but you
> are *ACCEPTING* a service, so you need a service principal.
> 
> Aki

Sorry, all this Kerberos stuff is quite new to me and I'm still a bit 
confused... :) What I still fail to understand is why would I need the 
service principal in the cache since I'm trying to set dovecot to use 
GSSAPI when connecting to the LDAP back end for passdb and userdb 
lookups.

My imap users can connect to Dovecot using GSSAPI without problems. This 
isn't the issue. Dovecot being the client to the LDAP service is the 
issue.

But anyway, after adding the ticket for 
imap/host.example.com at EXAMPLE.COM in the cache the error still remains:

dovecot: auth: Error: LDAP: binding failed (dn 
imap/host.example.com at EXAMPLE.COM): Local error, SASL(-1): generic 
failure: GSSAPI Error: Unspecified GSS failure.  Minor code may provide 
more information (No Kerberos credentials available (default cache: 
FILE:/tmp/dovecot.krb5.ccache))

$ sudo -u dovenull klist /tmp/dovecot.krb5.ccache
Ticket cache: FILE:/tmp/dovecot.krb5.ccache
Default principal: imap/host.example.com at EXAMPLE.COM

Valid starting       Expires              Service principal
10/11/2016 11:00:50  10/11/2016 23:00:50  krbtgt/EXAMPLE.COM at EXAMPLE.COM
         renew until 10/12/2016 11:00:50
10/11/2016 11:19:09  10/11/2016 23:00:50  imap/host.example.com@
         renew until 10/12/2016 11:00:50
10/11/2016 11:19:09  10/11/2016 23:00:50  
imap/host.example.com at EXAMPLE.COM
         renew until 10/12/2016 11:00:50

Juha

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