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Useful crontab short cuts
While reading through crontab(5) this morning I came across this useful nugget of information: "These special time specification "nicknames" are supported, which replace the 5 initial time and date fields, and are prefixed by the @ character" This translates into the following known time periods: So instead of writing: You can specify hourly to accomplish the same thing: @hourly command_to_run Or if you want to run something when a machine is rebooted you can add this: @reboot echo "" | mail -s " was recently rebooted" root Several of these short cuts are rather handy and it makes things easier to read IMHO. Gotta love short cuts!
$ read more →How Redhat 5 and Centos 5 detect hardware at boot
With the introduction of RHEL6 our beloved kudzu was removed from Redhat Enteprise Linux (it's been gone from Fedora for quite some time). If you're not familiar with kudzu, RHEL5 and below use it to detect new hardware when a system is bootstraped. All of the functionality that was part of kudzu is now handled by the kernel and udev, though a lot of sites will need to support RHEL 5 systems for years and years to come. I was curious how kudzu detected new hardware, so I started reading through the kudzu man page and source code…
$ read more →Kudzu removed from RHEL6
With the introduction of RHEL6 the kudzu hardware hardware manager was removed. All of the functionality that was once a par of kudzu has been integrated into the kernel and udev, as evidenced by this e-mail correspondence with one of Redhat's support engineers: "Kudzu is removed from rhel6. The kernel should be taking care of module loading from this point onwards. When it enumerates the device through its own methods or udev…
$ read more →Figuring out if a Linux executable supports a configuration directive
While debugging a problem last week I needed to quickly see if a version of bind supported the edns-udp-size configuration directive. Since RHEL3, RHEL4 and RHEL5 ship with different releases of Bind, I needed to see when this directive was introduced (EDNS was introduced in Bind 9, and the directive appears to have been there from the start). While I could have read through a crap load of documentation and release notes, I decided an easier approach would be to run nm against all of the binaries and libraries that come with the bind package. Low and behold it was: Both the RHEL4 and RHEL5 named binary contained the configuration directive, though RHEL3 did not (RHEL3 ships with Bind 8)…
$ read more →One way to avoid tcpdump packets dropped by kernel messages
I have been knee deep this week debugging a rather complex DNS issue. I'll do a full write up on that next week. While I was debugging the issue I needed to fire up tcpdump to watch the DNS queries from one of my authoritative servers to various servers on the Internet. What I noticed when I fed the data into wireshark were periods of time with no data, and I wasn't quite sure why at first…
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