Have you ever rebooted your computer only to see a black screen with a blinking cursor? If you have a USB drive attached, chances are the blinking cursor is caused by invalid bootstrap code in the Master Boot Record (MBR) on that drive which has caused the normal boot execution to stop without returning control to the BIOS. If you have physical access to the machine, simply remove the USB drive and/or change the boot order to pick the OS disk first.

If you have no physical access, things are a bit more tricky: This exact thing happened to me at work the other day. Unfortunately, it didn’t happen to my computer, but to a few dozen of our customer backup appliances during their scheduled upgrade/reboot. Now, while dozens out of over 60k isn’t that much, our customers rely on these devices, so it’s not acceptable to have them not boot properly.

In this short post, I’ll demonstrate how to reproduce the blinking cursor problem, and how to “fix” the MBR to ensure the computer still boots, regardless of the boot order.

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My laptop runs ZFS as its root file system (see this blog post) — meaning that I can snapshot my root file system and I can send it to another machine as a backup very easily. Unfortunately, while ZFS provides the raw functionality, there is no great tool to manage offsite backups and retention. To ease this pain, I wrote/forked and packaged a few helper scripts which I called zfsu, a collection of ZFS utilities.

It consists of the following tools: zfsu tx (aka zfstx) maintains a mirror of a ZFS pool over the network. zfsu ret (aka zfsret) is a simple script to apply local retention (destroy snapshots) of a file system and its snapshots. zfsu res (aka zfsres) is a script to resilver a slow mirror, e.g. a HDD disk if mirrored with a SSD.

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Debian packages and repositories are everywhere, yet many people don’t understand that creating them is actually pretty easy. While there are dozens of tutorials out there, none of them seemed to really show a good step-by-step. This is a quick tutorial on how to create a Debian package from scratch, and how to create a simple Debian repository.

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Greylisting is a very efficient technique for fighting spam and can reduce the spam messages in your mailbox by more than 90%. It uses the fact that most spammers only try delivering their spam-mails once, whereas real mail transfer agents (such as the ones regular e-mail service providers are using) try delivering each message up to 4-5 days before they give up.

I have always wondered why most ESPs don’t offer greylisting for their mailboxes, but only rely on less effective and resource-hungry post-retrieval filter methods. Unfortunately, my e-mail provider is one of them so that I get at least a couple of spam mails a day …

Luckily, it is very easy to set up your own mail relay with greylisting support, i.e. a mail server that simply forwards the mail to your real provider once it passes the greylist-filter.

This little tutorial describes how to set up Postfix and SQLgrey as mail relay.

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GCALDaemon (project doesn’t exist anymore, was at http://gcaldaemon.sourceforge.net/) is a great tool to synchronize many of Google’s services such as Google Calendar and Contacts with your local PC. Unfortunately, the installation on Ubuntu/Kubuntu and any other Linux distribution is still not the most comfortable. For this reason, I sat down some hours and packed the tool into a deb-package and additionally added a nice command line tool to simplify some of the basics.

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The good thing about the file sync tool Unison is, that it’s available for several operating systems. This is great for groups working on different systems (Mac, Linux and Windows) but want to share and synchronize files on a remote server.

Well, the bad thing about Unison on the other hand is, that its backwards compatibility is anything but great, so that you have to make sure that everybody in the team uses the same version. And this can be tricky depending on what system you are using.

My home system is Ubuntu Hardy, the remote server system is Debian Etch. Both come with Unison 2.16.13 which would be great if not Apple’s new Leopard brings the newest version 2.27.57. Long story short, I needed the newest version on Hardy and Etch.

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