Think about this situation: the user logging in is also spoofing the AS-REP. Then the user knows his password (it could be anything). The key thing is: the client must be able to use a secret key to verify the authentication process. Our PAM_KRB5:pam_sm_authenticate() returns PAM_IGNORE if it cannot verify the AS exchange because of a missing keytab. It returns PAM_SUCCESS if it can very a successful AS exchange. It returns an error under other circumstances. Various options can change this behaviour. Nico On Tue, Sep 18, 2001 at 07:22:47PM +0530, SRIDHAR BANDI wrote: > 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 > > regards > bandi > > > 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 > > > > > > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > > > > > > Pam-list@redhat.com > > > > https://listman.redhat.com/mailman/listinfo/pam-list > > > -- > > -- > > -DISCLAIMER: an automatically appended disclaimer may follow. By posting- > > -to a public e-mail mailing list I hereby grant permission to distribute- > > -and copy this message.- > > > > Visit our website at http://www.ubswarburg.com > > > > This message contains confidential information and is intended only > > for the individual named. If you are not the named addressee you > > should not disseminate, distribute or copy this e-mail. 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