SysAdmin

NIC Discovery on CentOS

When adding a NIC to CentOS, one normally expects to see the device appear in /etc/udev/rules.d/70-persistent-net.rules. But things don’t always go your way.

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PowerShell Hell

Every programming language has it’s idiosyncrasies, and sometimes it seems like language authors go out of the way to be different. Enough that a seasoned polyglot can benefit from a cheat sheet for keeping track of even the simplest things like if/then/else syntax (elif? elsif? elseif? else if?).

Coming from a bash/Perl background, I am beginning to like Windows PowerShell. The object pipeline capability brings the familiarity of Unix pipes with a more modern object-oriented approach. But you must take the bad with the good, and there is some bad. There is a special hell reserved for the developer of this particular ‘feature’ that I was wrestling with today…

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CentOS/RHEL GnuTLS Dependents

Last week’s announced GnuTLS bug is a serious one, but do you know the ramifications? Many articles painted Linux with a broad brush, claiming Red Hat, Debian, etc were deeply impacted. While it is true that the GnuTLS library is included in all distros (including CentOS and other RHEL flavors), it may not be widely used.

GnuTLS is licensed under LGPL. The alternative OpenSSL library is licensed under a combined BSD(SSLeay) and Apache 1.0 license. Some distros (notably Debian-based) don’t appear to like the licensing complexity that OpenSSL brings, so GnuTLS may be preferred. But many RHEL packages do not seem to be as fearful.

The following command will show all packages that are dependent on GnuTLS. (Which is not to be confused with “yum deplist” for dependencies.)

repoquery --whatrequires --installed --recursive gnutls

For CentOS and others of similar ilk, it is likely that you’ll find that OpenSSL has more dependents than GnuTLS.

PS: a gnutls patch has already been released, so get a jump and install it. Much is made about the need for dependent apps to regression-test, but don’t wait. If all the patch did was correctly fix the goto logic that left a truck-sized hole, I hardly think the fix could be any worse.


Lost Network When Upgrading To ESXi 5.1

After upgrading from ESXi 5.0 to 5.1, I was presented with a curious “change has been detected” message during VM startup. It asked whether the VM had been recently “Moved” or “Copied”. I was leaning towards “Moved”, except for the fact that it also said “when in doubt, select ‘Copied'”. Big mistake picking ‘Copied’.

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Keeping Track of iSCSI LUNs as Multipath Devices

The Dell MD3200i is a very flexible mid-level iSCSI SAN. With 12 bays (up to 3TB NearLine SAS in each) and dual quad-port controllers, it offers excellent redundancy, capacity, and expandability. The Windows or Linux MD Storage Manager provides a decent GUI from which to define disk groups of various RAID styles and virtual volumes within each disk group for future filesystems.

But for all the labeling controls that MDSM offers for groups and volumes, the LUN mappings (to allocate volumes to iSCSI IP connections) are the real exposed labels. No matter how anal-retentive you get with group/volume hierarchy, all the ‘naming’ that the iSCSI initiators will ever see are the LUN numbers 0-31.

In an iSCSI system, there could be multiple ways to reach LUNs (via multiple network interfaces). For this reason, multipath support is used to provide route management and redundancy. On CentOS, filesystems are accessed via the multipath /dev/mapper device naming, mapped to the ‘physical’ iSCSI LUN. How are these mappings managed? ‘mpatha‘, ‘mpathb‘, ‘mpathc‘, etc are assigned first-come first-served to LUNs in the order they are discovered. This can make it difficult when adding multiple LUNs at the same time, because it is not entirely obvious which /dev/mapper/mpath device corresponds to which partition (despite all that effort you employed in creatively naming your MD3200i groups and volumes).

Because of my momentary confusion in regard to this LUN mapping, I thought that I would document the iSCSI implementation steps as performed on CentOS 6:

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An Overview of Network Monitoring

Every fledgling network administrator eventually comes face-to-face with the question “how do I monitor my network?”. But “monitoring a network” means different things to different people, and if you ask a crowd of experienced network managers for tool suggestions, it is highly likely that you will get a few misleading recommendations simply because the what-to-measure is not in alignment with your objectives.

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Exchange 2007 OAB Subtleties

For quite some time, we had been dealing with Outlook Offline Address Book problems. The OAB would go in and out of visibility for both Outlook 2003 (Public Folder distribution) and Outlook 2007 (Autodiscover method); both RPC and Proxy; both Cached Mode vs not. Errors seen whenever the problem was present included: (continue reading…)


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