Category Archives: System Administration

ulimits and my limits

  • May
  • 24

Even Linux systems have their limits!  One of the systems I manage started throwing a “Too many open files” error in /var/log/messages.  No error is good but this error is fixable.  A little tuning and the error goes away.  On this particular server we were running the Oracle Client and a Java Application, two very […]

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Copying a file to multiple locations

  • Jan
  • 17

I thought this was cool enough to share.  I have not had to use it however so I doubt it is all that useful 😉 xargs -n 1 cp -v foo.txt<<<“/tmp1/ /tmp2/ /tmp3/” Here we are copying one file named foo.txt to multiple directories called /tmp/1/, /tmp/2/, and /tmp/3 using xargs.  The xargs command construct […]

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Disable All User Login Access

  • Jan
  • 05

There are occasions when you want to make sure users can not log into a system.  This can be due to a maintenance period or I have used it as part of the process for retiring a system.  It is simple enough but I thought it worth documenting for prosperity. # cd /{root of user […]

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Yum Rollbacks

  • Dec
  • 21

Very occasionally installing a package or updates with yum can have unexpected results.  Fortunately yum provides the ability to roll back anything it installs.  Here is an example of what these commands look like: # yum history Loaded plugins: package_upload, product-id, search-disabled-repos, security, subscription-manager ID | Login user | Date and time | Action(s) | […]

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Subnet Cheat Sheet

  • Dec
  • 14

Subnet Mask Cheat Sheet Posted here because 95% of the networks are the same and when I encounter a different one I have to think about it…not any more! See RFC 1878 for more details & information. Addresses Hosts Netmask Amount of a Class C /30 4 2 255.255.255.252 1/64 /29 8 6 255.255.255.248 1/32 […]

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yum Invalid System Credential error

  • Sep
  • 30

I ran across the following yum error after migrating a system from being a client of Satellite 5.6 to Satellite 6.1.  First here is the error: # yum update Loaded plugins: package_upload, priorities, rhnplugin, search-disabled-repos, security, subscription-manager There was an error communicating with RHN. RHN Satellite or RHN Classic support will be disabled. Error Message: […]

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Working with Repositories

  • Sep
  • 29

Pulling packages from multiple sources can lead to problems.  If you are running rhel and have epel enabled an update could inadvertently pull down a newer version from the wrong repository.  This doesn’t always cause a problem, but it can.  If you need to tfind all the epel packages on your system, here is how […]

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Kerberizing RHEL Server

  • Jun
  • 13

Notes from Plone… yum install krb5-workstation pam_krb5 -y # if krb5.conf is present we should get a fresh copy mv /etc/krb5.conf /etc/krb5.conf.bak yum reinstall krb5-libs -y sed -ie ‘s/example.com/FQDN/g’ /etc/krb5.conf sed -ie ‘s/EXAMPLE.COM/FQDN/g’ /etc/krb5.conf fqdn=`hostname –fqdn`; echo ” ank -randkey host/$fqdn@FQDN ktadd -k /etc/krb5.keytab host/$fqdn@FQDN “; — OR — kadmin netid/admin@FQDN addprinc -randkey host/$fqdn ktadd […]

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Don’t Panic Kernel!

  • May
  • 17

You do your best to keep your systems running, but eventually something is going to happen (read you are going to do something) that leads to a Kernel Panic.  Every System Administrator fears a Kernel Panic at some point in their career, but it is never a good idea to panic over one!  Here are […]

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