Entries Tagged 'Linux' ↓

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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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:

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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SSH (Secure SHell) is possibly the best way to remotely access a Unix system – it's very secure thanks to automatic encryption of all the traffic, and it's also quite universal because you can do all sorts of things: access remote command line shell, forward graphics session output, establish network tunnels and set up port redirections. Today I'm going to show you how to get started with SSH in Ubuntu.

Installing SSH server in Ubuntu

By default, your system will have no SSH service enabled, which means you won't be able to connect to it remotely using SSH protocol (TCP port 22). This means that installing SSH server will be one of the first post-install steps on your system.

The most common SSH implementation is OpenSSH server, and that's exactly what you want to install.

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I had to download a piece of software today for one of the servers which I haven't used in a while. A question of confirming the 64bit CPU capability came up, and I realized that I never mentioned it here on Unix Tutorial.

Some of you probably remember the uname command which also shows you similar information, but uname confirms the running kernel of your OS and not the CPU capability: if you're booted into 32bit mode, it will not help you to recognize the 64bit potential of your system.

Obtaining CPU information from /proc/cpuinfo

Most Linux distros will have the special /proc/cpuinfo file which contains a textual description of all the features your processors have. This is a very useful file – depending on your task it may help you identify any features of your processors, as well as confirm the overall number of CPUs your system has installed.

Most commonly, the following information is obtained from /proc/cpuinfo:

• processor model name and type
• processor speed in Mhz
• processor cache size
• instruction flags supported by CPU

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Hi everyone, I'm finally back from my holidays, and simply cannot wait to share some more Unix tips with all of you!

Today I'll talk a bit more about yet another way of learning version information about your Linux OS: the /proc/version file. I mentioned it briefly in one of the previous posts, but would like to finish the explanations.

What you can learn from /proc/version

This file will not show you the name of the actual OS release, but will instead give you specifics about the version of Linux kernel used in your distribution, and confirm the version of a GCC compiler used to build it.

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Hi all, today I'm going to teach you not one, but two really cool things in one post! First, I'll introduce you to advanced memory usage stats available on Linux systems through /proc/meminfo file, and then I'll explain the basics of using the watch command.

Memory usage with /proc/meminfo

As you know, quite a few Unix-like systems use the so-called pseudo file systems like /proc. It's not a real filesystem, but just a convenient representation of processes managed by your Unix OS. In Linux systems, this directory also contains quite a few files allowing you to access various information about your system. /proc/meminfo is one of such files, it gives you access to most of the memory usage stats.

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I've just been asked a question about changing the ownership of files from one Unix user to another, and thought it probably makes sense to have a quick post on it.

File ownership in Unix

Just to give you a quick reminder, I'd like to confirm that every single file in Unix belongs to some user and some group. There simply isn't a way to create a file without assigning ownership. I've briefly touched the topic of confirming file ownership in Unix before, so today I will simply build on that and show you how to change ownership of files.

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I've been upgrading Ubuntu installations quite a few times recently, and thought it makes sense to post a really short how-to if you ever want to upgrade your Ubuntu distro from the command line.

Use apt-get to upgrade Ubuntu

The procedure for upgrading one Ubuntu release to another one is pretty straightforward. There are some rules though:

1. Never attempt to skip a release or two when upgrading
2. Never do a few Ubuntu release upgrades in a row without reboots in between
3. Always backup the files you change
4. Always have an install CD for your current Ubuntu release around

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Yesterday in my post on numeric userids instead of usernames, I touched briefly the problem of recovering the username if you only know the userid it once had. Today I would like to show you another option which may be available to you when it comes to recovering the usernames of removed users by their userid.

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