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Quite recently I came across a very interesting issue: while configuring passwordless SSH (it's public key based, so depending on you have it configured it may not be completely passwordless) access to some of my VPS servers, I found that the same keypair just wouldn't work on one of the servers.

Not only that, but the behaviour was quite bizzare: upon my first attempt to connect the public key would get rejected and a regular password would be requested by the ssh session. But once I successfully logged in with my password, any subsequent ssh connections would happily authenticate by my public key and would let me in without a problem.

Those of you using home dir encrypiton in Ubuntu are probably smiling right now! But becase I have never consciously configured or used this feature, it took me a good few hours to troubleshoot the issue and come up with the fix.

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It's been so unbelievably long since my last post on this blog that I decided to start this year with a non-technical welcome.

I have great plans for UnixTutorial in 2012, and would welcome any opportunity to share knowledge and experience with all of the readers and new visitors
of this blog.

Here's just a few of the things I plan to do:

• UnixTutorial members area – long time coming, this area of the website will finally make a proper debut in the next few months. I'll be announcing the next round of email subscriptions shortly, so don't miss out if you're still interested
• A series of UnixTutorial eBooks – eventually a balanced collection of free and paid material in PDF and Kindle formats (polls to decide which topis are in demand will follow shortly)
• Broader coverage of Unix topics – this year I expect to write a lot more about Mac OS and AIX systems
• New WordPress theme and quite likely a mobile copy of the website (let me know what devices you have, I own  iPhone and iPad so will do initial testing)
• Completion and expansion of the Basic Unix Commands and Advanced Unix Commands sections
• More Unix book reviews and recommendations
• Reviews of latest Unix-like OS releases
• Even more Questions and Answers

If you expect to see even more – now would be a really good time to let me know by leaving a comment. Thanks and stay tuned!

As you probably know, TTL (time-to-live) parameters can be set not only for the whole DNS zone you're managing, but for each individual zone entry. A common example could be a higher TTL for the zone as a whole, while having much smaller TTLs for critical servers like web or MX servers.

Not everyone is aware that dig command is very useful for entry-specific DNS research. This post just shows you a very simple example.

dig to confirm TTL for a DNS entry

When using dig, we're usually after a specific section of its output. Consider this simple query (your output may slightly vary):

srv1# dig www.google.com

This query targets a specific DNS record – namely the www. one, rather than a whole google.com domain.

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There comes a time (a couple of times a year, actually) when you may want to upgrade your Ubuntu distro (read here for instructions on confirming your version of Linux: Find Out Linux Version)

Once that's done, you can use do-release-upgrade for a hassle free upgrade.

IMPORTANT: are you can see, I've used a really old Ubuntu server with 8.10, hence your procedure for upgrading more recent Ubuntu versions may be slightly different. For example, later upgrades will warn you if you're doing a release upgrade over ssh.

What do-release-upgrade is and when you should use it

do-release-script is a Python script which automates the process of updating multiple packages. It relies upon Ubuntu's core package management functionality.

Apart from downloading and installing updated versions of packages found on your system, this command attempts to take care of all the necessary Ubuntu-release related file changes.

Step 1: Run do-release-upgrade

Once you type the do-release-upgrade command name and press Enter, you should see how vital information about packages currently installed is being collected:

# do-release-upgrade
Checking for a new ubuntu release Done
Upgrade tool signature Done
Upgrade tool Done
downloading
extracting 'jaunty.tar.gz'
authenticate 'jaunty.tar.gz' against 'jaunty.tar.gz.gpg'
Reading cache
Checking package manager
Reading package lists: Done
Reading state information: Done
Updating repository information
Done http://archive.ubuntu.com jaunty Release.gpg
Done http://archive.ubuntu.com jaunty-updates Release.gpg
Done http://security.ubuntu.com jaunty-security Release.gpg
Done http://us.archive.ubuntu.com jaunty-backports Release.gpg
Done http://security.ubuntu.com jaunty-security Release
…
Checking package manager
Reading package lists: Done
jaunty-security/multiverse
Packages: 98  2
Reading state information: Done
Reading state information: Done
Reading state information: Done
Calculating the changes

2. Confirming what upgrading will do

This is your last change to change your mind. All the necessary information about your current Ubuntu release is collected, and now you're presented with the exact upgrade details: how many packages will be removed, how many new ones will be installed, how many will be upgraded. You also are given details about the required amount of data to be downloaded should you decide to proceed with the upgrade;

Do you want to start the upgrade?

1 package is going to be removed. 23 new packages are going to be installed. 420 packages are going to be upgraded.

You have to download a total of 248M. This download will take about 7 minutes with your connection.

Fetching and installing the upgrade can take several hours. Once the download has finished, the process cannot be cancelled.

Continue [yN]  Details [d]

Ready? Press y for yes!

3. Downloading all the packages

Just like with apt-get, you will now see the progress of downloading all the updated packages for your Ubuntu OS. At the bottom of the screen you will see the overall completeness of the download (22% in my example), the current download speed (598kB/s in my case) and the ETA:

Done http://archive.ubuntu.com jaunty-updates/main libbz2-1.0 1.0.5-1ubuntu1.1
Done http://archive.ubuntu.com jaunty/main libdb4.7 4.7.25-6ubuntu1
Done http://archive.ubuntu.com jaunty/main libncursesw5 5.7+20090207-1ubuntu1
Done http://archive.ubuntu.com jaunty-updates/main libssl-dev 0.9.8g-15ubuntu3.6
Done http://archive.ubuntu.com jaunty-updates/main libssl0.9.8 0.9.8g-15ubuntu3.6
Done http://archive.ubuntu.com jaunty/main python2.6 2.6.2-0ubuntu1
[23%] 598kB/s 5min17s

4. Upgrade

Once package are downloaded, they will get installed once by one, with package-specific questions asked for software like postfix or apache.

5. Reboot

To finalize the distro upgrade, you will need to do a reboot. Once completed, you should have a shine next release available.

Recommended books:

I've recently decided to give Mac OS X a try. For the past week or so I've been spending a good few hours a day working in Snow Leopard installed on a MacBook Pro borrowed from a friend.

While Mac OS is unlike any Unix-like operating system I've managed so far, there are certainly some of similarities. I can honestly say that I'm enjoying the Mac Book Pro so far, and hope to discover most of the differences compared to my previous Unix-like desktop which is Ubuntu 9.10.

Mounting NFS on MAC OS X

One thing which I noticed immediately was that out of the box it was impossible to mount any NFS shares from my Ubuntu NAS server. Any attempt to mount a remote filesystem would give me an error like this:

mbp:~ root# mount nasbox:/try /mnt
mount_nfs: /mnt: Operation not permitted

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Hi everyone, and thanks for staying with Unix Tutorial for so long! My first weeks of the 2010 have been pretty busy, but I've been quietly upgrading the blog engine and components of the Unix Tutorial Members Area to get things ready for the long-awaited second module of the Introduction to Unix course.

If you're an existing member and have a topic which you think belongs to the Operating Systems Basics module, please leave a comment.

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If you have a Facebook account and would like to get in touch with other visitors of this blog to discuss Unix, here's a great way to get started: join the Unix Tutorial Facebook page!

See you all there, and feel free to share your suggestions – either here or on the wall of the Unix Tutorial page.

Since it may be a while before I'm ready to use the IPv6 on my systems, I've been disabling IPv6 on most servers so far. And since there's a particularly elegant way of doing this in Red Hat Linux, I think it's worth sharing.

How to confirm if IPv6 is running on your system

IPv6 is implemented as a kernel module, so you can use the lsmod command to confirm if it's currently running on your Red Hat system:

$ lsmod | grep ip
ipv6                  410913  36

If lsmod doesn't return anything, it confirms that your system isn't running IPv6.

Prevent IPv6 from getting started by modprobe

As you probably know, modprobe command is used for probing modules upon system boot. Probing simply means a module is loaded and an attempt is made to start it up. With any luck, the module starts successfully and its functionality becomes available to the Linux kernel.

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Since it's Blog Action Day 2009 today, I'd like to remind all the readers of my blog how climate change can be prevented by following really simple rules.

Since Unix Tutorial is a technical blog, I'll try and stay as technical as possible within the topic.

Virtualize to consume less energy, get rid of old hardware

Old servers required a much bigger commitment in the past: not only did they cost a fortune, but they also needed a lot of space and required a lot of power. These days, 1u or 2u server solution can easily outperform a computing system which used to take a whole cabinet in your datacentre. And since the cost of supporting old hardware only increases with each year, it makes a lot of sense to simply but a new server to replace the old infrastructure.

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Hi everyone, it seems to be really long since my last technical post here – too much stuff happening at work, plus I'm studying for a CCNA exam.

If you're in need of quick help – drop me a message on Twitter – I'm UnixTutorial there. I can't promise a prompt reply, but at least this way you'll have some interactivity.

I see how many people leave questions in comments to my posts, and I don't always have the time to reply – so feel free to send me an email if you really need my help.