Confusion on SSH and PAM

[Rakhesh Sasidharan]

Christian Baer wrote:

> On Tue, 25 Sep 2007 15:56:22 +0400 (GST) Rakhesh Sasidharan wrote:
>
>> Any ideas or nudges in the right direction as to why this is happening?
>> Looks like I've understood the interaction between SSH and PAM wrong here,
>> so would appreciate some enlightenment.
>
> I'm not sure if I can offer any enlightenment here, but you can have my 2
> cents. :-)

I don't mind enlightenment that can be got for 2 cents! :-)

> This is one of these things with computer logic. :-) You have told the
> sshd that a root login vai PAM is not ok, only via private key. PAM is
> activated just the same (and probably works for other users). The login
> follows a certain order...
>
>
> 1 Ask for username
> 2 Did we get a key? If not, goto 5
> 3 Is the key ok? If not, goto 5
> 4 Let user login, exit authentification
> 5 Is PAM globally on? If not exit
> 6 Ask for password
> 7 Is the password ok? If not goto 6 max 2 times, after that exit
> 8 Let user login, exit

... snip ...

> Your problem seems to be from steps 5 to 7. After the authentification by
> key fails, the sshd just goes to the next step, which is the password. For
> security reasons, the communication "inside" is a bit brief. PAM only gets
> the answer "not authenticated" and because the reason isn't an issue, the
> user is asked for the password again. The point is that the sshd just
> refuses your login each time, because a password just isn't enough.

I see. I thought the interaction between SSHD and PAM was that SSHD tells 
PAM to authenticate on its behalf, PAM replies with a PASS/ FAIL depending 
on the final result of its modules, and SSHD allows/ disallows based on 
this result. But from what you say, I get the impression that SSHD can ask 
PAM to re-try even if PAM replies with a PASS ... that's kind of futile, 
isn't it? Why doesn't SSHD just take the PASS result and deny the user 
straightaway instead of making PAM retry twice?

Here's something else that I tried. There's a PAM module for CAPTCHA. 
(http://www.semicomplete.com/projects/pam_captcha/ in case someone's 
interested). I modified my PAM config to include that too before the 
pam_unix module.

-----8<---------
auth      required      pam_nologin.so                  no_warn
auth      requisite     /usr/local/lib/pam_captcha.so   math randomstring
auth      required      pam_unix.so                     try_first_pass
account   required      pam_login_access.so
account   required      pam_unix.so
session   required      pam_permit.so
password  required      pam_unix.so 
-----8<---------

Following our previous logic, shouldn't pam_captcha get invoked, verify 
through CAPTCHA, pass onto pam_unix to get password, pass result to SSH, 
fail, and restart with pam_captcha and pam_unix for 2 more times? But it 
does not happen that way! Instead, now, pam_captcha does the looping for 2 
more times, and even after successfully entering the CAPTCHA strings root 
login is denied. Strange. pam_unix is not even called for the password!

When PAM is used to authenticate for SSHD, is it not that PAM goes through 
all its modules and *then* passes the result to SSH? Or are there any 
subtler interactions ... each module passes its result to SSH and their 
behaviour is influenced by SSHD's reply?

> I know, crappy algorithem that remindes of BASIC a bit. In this case it
> should do the job, though. Please forget that the word "goto" exists in
> other languages too (even Java). :-)
>

:-)

Regards,

 				- Rakhesh
                                 http://rakhesh.net/