I am happily using Kerberos authentication for my AD domain users. In fact the 
driving force was less prompts for my Mac users - Safari and some other 
browsers don't support Kerberos, so I also have a fallback for NTLM auth, but 
they are much happier using Kerberos (in firefox) and I don't take nearly so 
many calls... Plus there's one less auth req between my dcs and squid.

AFAIK winbind is used for your NTLM and Samba config but not for Kerberos 
authentication directly. 

Process for AD domain is: 
Get your time, network, samba, winbind and Kerberos settings configured and 
join squid server to the domain
Kinit a user
Create a dummy computer account, add the SPNs and export the keytab using 
msktutil 
Klist -k /locationto-the-keytab file  i.e. /etc/squid/HTTP.keytab. This will 
confirm you have exported the keys properly.
Ensure permissions on the keytab allow squid to use it
Update the init.d/squid startup to use the keytab
Update squid.conf to use the squid_kerb_auth helper


>> Are the kerberos-tickets persistent, or do I have to renew them periodically?
Host Kerberos tickets are by default 10 hours. They will renew automatically 
providing the user (for example) is valid and the SPNs are ok.. and the KVNO 
doesn't change for the auth account/keytab.

>> What happens, if this account will locked out? Is then the squid-access 
>> denied?
Locked out account won't matter, you are authenticating your users against AD 
not the domain account you created.

>>Can someone help me with this? Are there some other examples, which describes 
>>a promptless login (SSO) with plain kerberos?
Squid wiki howto on Keberos has the basics, although that example uses Samba to 
create and export the keytab. I have found this to cause problems as Samba 
periodically changes the computer account in AD and thus the KVNOs get out of 
sync, hence the dummy account.

Search this list for squid_kerb_auth, msktutil and Kerberos for more info and 
help


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