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Re: verify TGT in pam-kerberos

Thank you so much for the clarification but I have a small doubt in that ,

  When rlogind (for example)  is the server who makes as [AS_REQ] to the KDC on
behalf of the client (user) then the KDC to issues the [AS_REP] ( without any
preauthentication if no preauth are present) that contains the ticket(TGT) and
the encrypted data that can be decrypted by clients password, on receiving the
[AS-REP] the server (rlogind) will try to decrypt the data that is obtained
from the KDC , So the client is said to be authenticated if he knows the
password which can decrypt the data in [AS-REP] . Now if the KDC is spoofed
then the password of the clients also need to be known to the KDC so that the
client's password can be used to decrypt the packet as its at the server
(rlogind) end that the decryption  takes place. So I still have a doubt that,
will the spoofing of KDC be caught by doing verify_krb_v5_tgt() .
please help me out .

I could think of a problem that is addressed with this is that when both the
user and the KDC are spoofed this will work , but even this will fail in the
case when the keytab file is not present for the serveras it will ignore the
case and allows the user in.

thanks you so much for the help


Nicolas Williams wrote:

> Also, one might want to use pam_krb5 with no keytab, as a convenience
> kinit. Thus, if there's no keytab pam_krb5:auth returns PAM_IGNORE, but
> it does fetch the TGT and its setcred() will create the ccache.
> Nico
> On Tue, Sep 18, 2001 at 09:04:35AM -0400, Nicolas Williams wrote:
> > AS replies can be spoofed as nothing in them authenticates the KDC to
> > the client doing the AS request. Therefore something must be done to
> > authenticate the AS reply or else you can't use Kerberos for password
> > validation. The thing to do is this: use the TGT from the AS reply to
> > get a TGT from the TGS (the KDC) for talking to a service corresponding
> > to the client and for which the client knows the current key -- if the
> > TGS response can be validated against such a key then the client knows
> > that the AS reply must is valid, that it came from the correct KDC and
> > was not spoofed.
> >
> > Cheers,
> >
> > Nico
> >
> >
> > On Tue, Sep 18, 2001 at 03:03:14PM +0530, SRIDHAR BANDI wrote:
> > >   I am sorry if I had hit the wrong list .
> > >   can someone enlighten me about the PAM Kerberos authentication (
> > > downloaded from fcusack) validating  the TGT that it obtained with the
> > > host service principal's keytab entry (using the funciton
> > > verify_krb_v5_tgt) , I dont understand the purpose of doing this because
> > > the server machine is the one who gets the TGT on behalf of the client(
> > > principal ) and its the one who is handling the host entry also . And
> > > this call fails if the keytab file exists and there is no valid entry
> > > for the host service principal and the authentication will be successful
> > > if there is a valid entry for host principal or there is no keytab file
> > > at all .
> > >
> > > thanks in advance for the help .
> > > bandi
> > >
> > >
> > >
> > >
> > >
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