Tuesday, February 12, 2008

SLES10 SP1 and pam session group errors

I was configuring a new backup server of our customer and I wanted to integrate it to the running LDAP infustructure. So I intented to configure it as a LDAP client and join it to the customer's LDAP server.

The backup server is based on SLES10 SP1 distribution and for such basic configuration tasks is equipped with YaST configurator. I can only recommend it if you don't want to waste time with simplicities! To configure a server as a LDAP client is really straightforward and you don't need to edit any config files like /etc/ldap.conf, /etc/nssswitch.conf or PAM config files and to remember exactly what to write where. Just fill in the proper options like address of LDAP server, LDAP base DN, if to use SSL/TLS, LDAP protocol version and confirm it. The screenshot ilustrates these options.


But sometimes troubles happen. After finishing the above process, remotely of course, everything seems to work as I expected. I was able to see LDAP users what was the main goal. Perfect!

The first problem which I noticed was that I wasn't able to connect to the server remotely via ssh once more. Debbuging of the connection didn't helped me. Why?!? Good question!

I had to inspect the server locally and I found the following errors in messages:
  • sshd[8351]: Accepted publickey for root from A.B.C.D port 59203 ssh2
  • sshd[8353]: pam_warn(sshd:session): function=[pam_sm_open_session] service=[sshd] terminal=[/dev/pts/0] user=[xxx] ruser=[] rhost=[yyy]
  • error: PAM: pam_open_session(): Cannot make/remove an entry for the specified session
The errors are related to the sshd service and they were the result of unsuccesful connection. Another malfunctioned service was crond service and its errors were identical.

It is clear that something is wrong with PAM configuration of the services. In SLES10 and others distros the PAM modules are used for authentication, account and session processing of the most services. This behaviour of sshd daemon is affected with one option in the /etc/ssh/sshd_config config file:
  • UsePAM yes
So I decided to try turning it off to make me sure I'm going in the right direction. The sshd service started working again but I wasn't still sure what's wrong with it. What should I do with the crond service to bypass the PAM modules?

I realized that the only thing I had changed before was the LDAP client configuration. I tried to bring the system to the previous state without it but it didn't helped. That means that when I had configured the LDAP client with YaST some operations weren't successful. Unfortunately, as I mentioned the LDAP client configuration is straightforward and you need to change only a few config files. Of course, you need to have installed the required packages with binaries and libraries.

I took a look over the configuration files and they seemed to be perfect. Only the /etc/pam.d/common-session file didn't contain any lines. This file is common for all other PAM config files and it is inluded from them. So, how to check its contents? Remember to use the rpm command in such situations. To check the validity of the pam package I run:
  • rpm -V pam
It showed me that the file had to be changed:
  • S.5....T c /etc/pam.d/common-session
The config file was different from the original one. The difference were these two missing lines:
  • session required pam_limits.so
  • session required pam_unix2.so
Finally, I tried to replace the modified file with the one from the installation source, put the support of PAM modules back and checked the services. They started to work again.

What's the result? Don't forget to use the strong tools like rpm and remember that simple things can go wrong too.

1 comment:

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