I would like to develop a web interface to allow users of a Linux system to do certain tasks related to their account. I decided to write the backend of the site using Python and mod_python on Apache. To authenticate the users, I thought I could use python_pam to query the PAM service. I adapted the example bundled with the module and got this:

# out is the output stream used to print debug
def auth(username, password, out):
    def pam_conv(aut, query_list, user_data):
        out.write("Query list: " + str(query_list) + "
")

        # List to store the responses to the different queries
        resp = []

        for item in query_list:
            query, qtype = item

            # If PAM asks for an input, give the password
            if qtype == PAM.PAM_PROMPT_ECHO_ON or qtype == PAM.PAM_PROMPT_ECHO_OFF:
                resp.append((str(password), 0))

            elif qtype == PAM.PAM_PROMPT_ERROR_MSG or qtype == PAM.PAM_PROMPT_TEXT_INFO:
                resp.append((  , 0))

        out.write("Our response: " + str(resp) + "
")
        return resp

    # If username of password is undefined, fail
    if username is None or password is None:
        return False

    service =  login 
    pam_ = PAM.pam()
    pam_.start(service)

    # Set the username
    pam_.set_item(PAM.PAM_USER, str(username))

    # Set the conversation callback
    pam_.set_item(PAM.PAM_CONV, pam_conv)

    try:
        pam_.authenticate()
        pam_.acct_mgmt()
    except PAM.error, resp:
        out.write("Error: " + str(resp) + "
")
        return False
    except:
        return False

    # If we get here, the authentication worked
    return True 

My problem is that this function does not behave the same whether I use it in a simple script or through mod_python. To illustrate this, I wrote these simple cases:

my_username = "markys"
my_good_password = "lalala"
my_bad_password = "lololo"

def handler(req):
 req.content_type = "text/plain"
 req.write("1- " + str(auth(my_username,my_good_password,req) + "
"))
 req.write("2- " + str(auth(my_username,my_bad_password,req) + "
"))
 return apache.OK

if __name__ == "__main__":
 print "1- " + str(auth(my_username,my_good_password,sys.__stdout__))
 print "2- " + str(auth(my_username,my_bad_password,sys.__stdout__))

The result from the script is :

Query list: [( Password:  , 1)]
Our response: [( lalala , 0)]
1- True
Query list: [( Password:  , 1)]
Our response: [( lololo , 0)]
Error: ( Authentication failure , 7)
2- False

but the result from mod_python is :

Query list: [( Password:  , 1)]
Our response: [( lalala , 0)]
Error: ( Authentication failure , 7)
1- False
Query list: [( Password:  , 1)]
Our response: [( lololo , 0)]
Error: ( Authentication failure , 7)
2- False

I don t understand why the auth function does not return the same value given the same inputs. Any idea where I got this wrong ? Here is the original script, if that could help you.

Thanks a lot !

EDIT: All right I found the error. I was running the script as root. mod_python was running the script as the webserver s user. Only root has the right to read shadow. I am not sure how I will circumvent this, but at least now I know what is the problem !