Stupid /etc/hosts problems with service principal names

Andrew Bartlett abartlet at
Wed Apr 18 19:09:16 MDT 2012

On Wed, 2012-04-18 at 18:04 -0700, Richard Sharpe wrote:
> On Wed, Apr 18, 2012 at 5:37 PM, Andrew Bartlett <abartlet at> wrote:
> > On Wed, 2012-04-18 at 10:02 -0700, Richard Sharpe wrote:
> >> Hi folks,
> >>
> >> I recently saw a problem with Samba giving out what seemed like the
> >> wrong service principal name in the response to a Negotiate Protocol,
> >> but it came down to Samba trying to convert the hostname (short form)
> >> into an FQDN and name_to_fqdn calls gethostbyname, which, because of
> >> /etc/nsswitch, looks in /etc/hosts, and since we had an entry there
> >> that had not been changed after the domain join, came up with the
> >> wrong FQDN ...
> >>
> >> It seems to me that the correct thing here is not to put an entry for
> >> this machine in /etc/hosts (apart from localhost) relating to the
> >> hostname of the member server because it should be using DNS anyway,
> >> and if access to DNS is broken, lots of stuff is not going to work
> >> anyway.
> >>
> >> Any comments? Is this stuff that has been discussed before now?
> >
> > Which version is this?
> Samba 3.5.x
> > However, this is partly why in master we do not generate this principal
> > name in the NegProt, and in 3.6 we do not honour it by default in the
> > client.
> So, how does the client (Windows) find the principal name if we do not
> communicate it?

Clients should use cifs/hostname where hostname is the name it was asked
to connect to.  This avoids some man-in-the-middle attacks, and is an
important security feature.  

> > Which client is honouring the incorrect value (old Samba versions
> > would)?
> Well, the traces I have suggest that at least W2K03 uses the bad
> principal name and submit it to the KDC, getting back Principal Not
> Found. I will have to look again at the details but they are at work.
> I can probably post parts of a capture I have.

> In one case, someone seems to have defined a locally known principal
> name as well (on a Windows client) that matched the bogus principal
> name, and that caused authentication to not work at all. The client
> just kept asking for credentials. The ticket that was generated was
> bad, it seems.

Could it be that the client was connecting to one name, that was in DNS
or netbios, but was no the name registered with our KDC as a
servicePrincipalName and not the name we joined as?

Netbios aliases would do this in particular, if not also registered as a

Andrew Bartlett

Andrew Bartlett                      
Authentication Developer, Samba Team 

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