Posts Tagged security

What a difference a working resolver makes

The next phase in tidying up my user authentication environment in the lab was to enable SSL/TLS on the z/VM LDAP server I use for my Linux authentication (I’ll discuss the process on the DeveloperWorks blog, and put a link here).  Apart from being the right way to do things, LDAP authentication appears to require SSL or TLS in Fedora 15.

After I got the Fedora system working, I thought it would be a good idea to have other systems in the complex using SSL/TLS also.  The process was moderately painless on a SLES 10 system, but on the first SLES 11 system I went to YaST froze while saving the changes.  I (foolishly) rebooted the image, and it hung during boot.  Not fun.

After a couple of attempts to fix up what I thought were the obvious problems (each attempt involving logging off the guest, connecting its disk to another guest, mounting the filesystem, making a change, unmounting and disconnecting, and re-IPLing) with no success, I went into /etc/nsswitch.conf and turned off LDAP for everything I could find.  This finally allowed the guest to complete its boot — but I had no LDAP now.  I did a test using ldapsearch, which reported it couldn’t reach the LDAP server.  I tried to ping the LDAP server by address, which worked.  I tried to lookup the hostname of the LDAP server, and name resolution failed with the traditional “no servers could be reached” message.  This was odd, as I knew I’d changed it since it was pointing to the wrong DNS server before…  I could ping the DNS by address, and another system resolved fine.

I thought it might have been a configuration problem — I had earlier had trouble with systems not being able to do recursive DNS lookups through my DNS server.  I went to YaST to configure the DNS Server, and it told me that I had to install the package “bind”.  WHAT?!?!?  How did the BIND package get uninstalled from the system…

Unless…  It’s the wrong system…

I checked /etc/resolv.conf on a working system and sure enough I had the IP address wrong.  I was pointing at a server that was NOT my DNS server.  Presumably the inability to resolve the name of the LDAP server I was trying to reach is what made the first attempt to enable TLS for LDAP fail in YaST, and whatever preload magic SLES uses to enable LDAP authentication got broken by the failure.  Setting the right DNS and re-running the LDAP Client module in YaST not only got LDAP authentication working but got me a bootable system again.

A simple fix in the end, but I’d forgotten the power of the resolver to cause untold and unpredictable havoc.  Now, pardon me while I lie in wait for the YaST-haters who will no doubt come out and sledge me…  🙂

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RACF Native Authentication with z/VM

 In 2009 I was part of the team that produced the Redbook "Security for Linux on System z" (find it at  Part of my contribution was a discussion about using the z/VM LDAP Server to provide Linux guests with a secure password authentication capability.  I probably went a little overboard with screenshots of phpLDAPadmin, but overall I think it was useful.

I’ve come back to implement some of what I’d put together then, and unfortunately found…  not errors as such, but things I perhaps could have discussed in a little more detail.  I’ve been using the z/VM LDAP Server on a couple of systems in my lab but had not enabled RACF.  I realised I need to "eat my own cooking" though, so decided to implement RACF and enable the SDBM backend as well as switch to using Native Authentication in the LDBM backend.

Native Authentication provides a way for security administrators to present a standard RFC 2307 (or equivalent) directory structure to clients while at the same time taking advantage of RACF as a password or pass phrase store.  Have a look in our Redbook for more detail, but basically the usual schema is loaded into LDAP and records are created using the usual object classes like inetOrgPerson, but the records do not contain the userPassword attribute.  Instead of comparing a presented password against the field contained in LDAP, the z/VM LDAP Server (when Native Authentication is enabled) issues a RACROUTE call to RACF to have it check the password.

In my existing LDAP database, I had user records that were working quite successfully to authenticate logons to Linux.  My plan was simply to enable RACF, creating users in RACF with the same userid as the uid field in LDAP (I have access to a userid convention that fits RACF’s 8-character restriction, so no need to change it).  After going through the steps in the RACF program directory, and various follow-up tasks to make sure that various service machines would work correctly, I did the LDAP reconfiguration to get Native Authentication.

At this point I probably need to clarify my userid plan.  The documentation for Native Authentication in the TCP/IP Planning and Administration manual says that the LDAP server needs to be able to work out which RACF userid corresponds to the user record in LDAP to be able to validate the password.  It does this by either having the RACF userid explicitly specified using the ibm-nativeId attribute (the object class ibm-NativeAuthentication has to be added to the user object), or by matching the existing uid attribute with RACF.  This is what I hoped to be able to do; by using the same ID in RACF as I was already using in LDAP, I planned to not require the extra object class and attribute.  In the Redbook, because my RACF ID was different from the LDAP one I went straight to using the ibm-nativeId attribute and didn’t go back and test the uid method.

So, I gave it a try.  I had to disable SSH public-key authentication so that my password would actually get used, and once I did that I found that I couldn’t log on.  It didn’t matter whether I tried with my password or pass phrase, neither was successful.  I read and re-read all the LDAP setup tasks and checked the setup, but it all looked fine.  In one of those "let’s just see" moments, I decided to see if it worked with the ibm-nativeId attribute specified in uppercase…  and it did!

Okay, so it appeared that the testing of uid against a RACF id was case-sensitive.  I decided to try creating a different ID, with an uppercase uid, in LDAP to double-check.  Since phpLDAPadmin wouldn’t let me create an uppercase version of my own userid (since that would be non-unique), I created a different LDAP id to test:

[viccross@laptop ~]$ ssh MAINT@zlinux1
Could not chdir to home directory /home/MAINT: No such file or directory
/usr/X11R6/bin/xauth:  error in locking authority file /home/MAINT/.Xauthority

My MAINT user in LDAP has no ibm-nativeId attribute, so the only operational difference is the uppercase uid (the error messages are caused by the LDAP userid not having a home directory; I use a NFS shared home directory had I hadn’t bothered setting up the homedir for a test userid).

The final test was to change the contents of the ibm-nativeId attribute in my LDAP user record to lower-case — and it broke my login.  So that would seem to indicate that the user check against RACF is case sensitive wherever LDAP gets the userid from.  I’m going to have a look through documentation to see if there’s something I need to change, but this looks like something to be aware of when using Native Authentication.

I also noticed that I didn’t describe the LDAP Server SSL/TLS support in the Redbook, but that’s a post for another day…

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Security blows

I was about to post about how pleased I was with Synergy in helping me tidy up my desktop clutter (by removing a keyboard and mouse from the surface). Ironically, I’m instead posting about a problem with the configuration that will cause me to throw it out and look for something else. Why the title? Because the default configuration of a Linux distribution nowadays has given me no way to fix this ridiculously simple problem without powering off the running PC, VMware guests and all.

The problem is that Synergy and the VMware console don’t play well together (I could have sworn that when I first started using Synergy I had no trouble with it, but there are a few hits around that describe problems like I’ve now hit). The problems people are reporting are that keys like Shift and Ctrl are not passed to the VM (some described here and here).

My problem is slightly different: the screen of my Synergy client (the one that’s running VMware) locked while a VMware guest had focus. Now, the Shift and Ctrl keys are not picked up by gnome-screensaver to unlock the screen. Even the real keyboard attached directly via USB doesn’t work. Big problem, for the following reasons:

* Thanks to password strength rules enforced on the Linux build I use, my password now has a Shift-obtained punctuation character.
* I can’t switch to a virtual console, since that requires Ctrl (e.g. Ctrl-Alt-F1).

Okay, so the keyboard doesn’t work. This client machine just happens to be a tablet PC, and I had hacked gnome-screensaver (to display the onscreen keyboard to allow the screen to be unlocked in tablet mode). I grabbed the pen and tapped out my password, but it *still* didn’t work: even the output of the virtual keyboard gets the Shift modifier dropped. Hmm… Starting to fume now.

Never mind, I’ll connect via the network…

* Fedora does not start SSH by default (okay, yes, and I didn’t make sure it gets started after I’d finished the install).
* There is no remote desktop (VNC server, XDMCP) configured.
* The shiny web-based management interface on VMware Server 2.0 only listens on (or is being blocked by the Fedora firewall).

So with no way to get access to the machine to try and fix it, a power-off is the only solution. Some readers are probably thinking “boo-hoo, diddums had to kill-switch his widdle poota, how tewwible,” but I hate having to do that; not because the system doesn’t recover, but it’s “problem resolution, Windows-style”.

Even though the real problem was between Synergy and VMware, I’m blaming the (perceived) need for security since without that I wouldn’t have a cryptic password that I can’t enter without Shift and a system I can’t administer over the network. Red Hat and Fedora doing everything in their power to ensure I don’t fall prey to nasty Internet fiends (rich analogies to governmental nannying, but that’s probably over-thinking things).

So in summary: Synergy is great, just as long as you’re not using VMware console and have a password with punctuation or uppercase… Remember to have your SSH or other network access enabled before you play!

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Active Directory accounts on Linux

Never thought I could get this excited about something to do with a Windows server!  But there it is — one of my SLES 9 test servers is now supporting logons from a user account stored in Active Directory, with no Samba in sight!

Before you say ANYTHING, this is not an indication that the Crossed Wires campus is switching to the evil side.  Any experienced Linux sysadmin will tell you that working with Windows systems can’t be avoided — and in some cases, welcomed (after all it’s better to have one or two Linux boxes in a sea of Windows than no Linux boxes at all).  My main customer at work is essentially a Windows shop, but their main file servers are Linux on zSeries, which means that me as a Linux guy needs to know more than I thought I would want to know about bringing Linux and Windows together.

So they are doing a migration to Microsoft Active Directory, and the Linux systems need to be integrated into the AD setup.  To our architects, Linux Windows integration equals Samba — they never bothered to look at making use of AD’s LDAP component to create a model that Linux can handle natively, instead of the (to me) less-than-optimal Winbind (don’t get me wrong, Winbind works, it just imposes some operational issues that I’d sooner do without, like SID-[UG]ID mapping, for instance).

So I proposed that the solution be updated to utilise LDAP, through the use of Microsoft’s own Services for Unix (SFU).  I was told “yeah, dunno why it wasn’t designed that way, would be the best way to do it, but no”.  Sigh.

So I decided to stick to my guns and set up something to show that it would work exactly as I said it would.  And I have!  I’ve worked around some inaccurate information on the ‘Net, some incomplete documentation from Microsoft, and some finger-checks on my part, to be able to show The Right Way to anyone who cares…  Yep, sometimes the useless thing is just worth doing.  🙂

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Veejoe goes LDAP

We bit the bullet here at the new Ellendale data centre.  LDAP authentication!  Works like a bought one.

Coinciding with the relocation of the prime server from Rubicon DC to Ellendale DC, we’ve implemented LDAP authentication for Linux and Mac OS X clients.  There’s also automounted home directories to boot!  It went quite smoothly, all things considered.

Now will come the dreaded data reorganisation (there’s about 1.5TB of storage across all the Crossed Wires machines).  Also, I’ve been running Samba 3 for a while so there’s probably not much reason to keep putting off integrating the Windows boxes into LDAP as well…

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