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  • 30 November 1999

Archives For 30 November 1999

Using GNU Screen to manage terminal sessions

29 April 2012 — 1 Comment

Whether you’re using Mac OSX, Windows or Linux, we’re all using a so-called “window manager”. Most are graphical user interfaces; and that’s a good thing 🙂 But as a sysadmin I need to manage many servers. Servers without a graphical user interface. So, how to handle that?

One could just ssh into a server when you need to do work on a given server. Depending on the terminal program you use, you might be able to have multiple sessions at the same time, preferably in tabs. It looks like this when 3 tabs are open:

Although this works well, it has one drawback for me: it only works on one computer. And when you turn off your computer everything is gone and disconnected. Since I’m working on multiple computers (desktop/laptop), multiple OS’es (OSX, Ubuntu) and multiple places (Work, Home), this no longer worked for me and I started looking for a better solution.

GNU Screen to the rescue! GNU Screen is a full-screen window manager, but terminal based. That is, it works in interactive shells such as a ssh session and is able to keep running while disconnected.

Starting a screen is easy:

screen -S screenname

You can attach and detach a session when needed. To detach, press Ctrl+a+d. To reattach enter:

screen -r screenname

This means that no matter on what computer I login, on any place, I always am able to attach a running screen. It looks like this:

Note the bar at the bottom where the tabs are. You can even give them a name!

GNU Screen can be a bit obscure to configure. After googling a lot and some help from co-workers, I have now configured GNU Screen as you can see on the above image. Configuration is read from the .screenrc file in your homedir. In the image above you can see my .screenrc file.

Commands in GNU Screen are all prefixed by a control command, the default is Ctrl+a. This means that all commands you type will be entered in the terminal you’re connected to, except commands followed directly after you press Ctrl+a. In the manual page you’ll find C-a, which is short for Ctrl+a.

When you want to create a new tab, enter Ctrl+a+c. To change from one tab to the other, you press Ctrl+a+2 to go to tab #2. Ctrl+a+spacebar brings you to the next tab. To name that tab, enter Ctrl+a+A, etc. It takes a bit of time to get used to it, but for me it works very well.

GNU Screen is a window manager, so apart from multiple tabs, you can even split the window to host multiple screens next to each other. To split vertically you enter Ctrl+a+|. Ctrl+a+tab brings you to the newly created space. You then create a new tab there, like Ctrl+a+c. It looks like this:

Like I said, Ctrl+a+tab switches between the left and right screen. In each you can call any tab that is below with Ctrl+a+tabnumner, or toggle between them using the spacebar.

Need to go away and want to protect your screens? Enter Ctrl+a+x and you’re screens will be locked. You need to enter the password of the connected user to unlock.

This is just a quick introduction of what is possible with screen. Have a look at the man page:

man screen

or use Google to get some more configuration examples. Have fun!

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HOWTO enable RPMForge repository in CentOS

18 April 2012 — 2 Comments

Since I’m working with CloudStack I’m also working with CentOS. As I’ve been working with Debian for over 10 years, it sometimes takes some extra time to get things done. Like when you want to install a package that is not in the default repo’s.

This is how to enable the RPMForge repository:

Start to import the key:

rpm --import http://apt.sw.be/RPM-GPG-KEY.dag.txt

Then download the RPMForge release file:

wget http://apt.sw.be/redhat/el6/en/x86_64/rpmforge/RPMS/rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm

Then all you need to do is install this rpm:

rpm -i rpmforge-release-0.5.2-2.el6.rf.x86_64.rpm

You only need to do this once. Now the packages from RPMForge can be installed using yum. Have fun!

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HOWTO create php5-PECL Debian packages

5 April 2012 — Leave a comment

 

The PHP Extension Community Library (PECL) has many great extensions for php5. Unfortunately, most are not packaged for Debian. Of course it is pretty easy to install it by hand. For example the ‘pecl_http’ extension:

pecl install pecl_http
echo "extension=http.so" > /etc/php5/conf.d/http.ini

For this to work you need some -dev packages to be installed. php5-dev and build-essential are the minimum required.

I have two problems with this method:
1. Too many -dev packages will be installed on production servers. I want to have as less packages installed as possible;
2. The software is not easily installable and upgradable;

I’ve yet another requirement: I need to distribute my package in a local Debian repository so it can be automatically installed. Therefore I need to have the right metadata with my new package. The method I describe here does create all files you need to upload the package to a Debian Repository.

Start to create an empty directory for our new package:

mkdir php-pecl-http_1.7.4-1

Then go into this new directory and download the PECL package you want to package for Debian.

pecl download pecl_http

Using ‘dh-make-pecl’ you can create a Debian source package like this:

dh-make-pecl --only 5 pecl_http-1.7.4.tgz

The next step is fixing a problem with ‘pecl_http’. The generated .so file is called http.so and not pecl_http.so. You can skip this step when creating other packages.

sed -i 's/PECL_PKG_NAME=pecl-http/PECL_PKG_NAME=http/1' debian/rules
mv debian/pecl-http.ini debian/http.ini
sed -i 's/pecl-http.so/http.so/1' debian/http.ini

If you want, you can edit the changelog. For example when you want to customize the version number.

vim debian/changelog

Now it’s time to build the package. Look closely to the output when you get an error. You might need some dependencies like php5-dev and build-essential.

Don’t run this as root, but add ‘-rfakeroot’ as oridinary user.

dpkg-buildpackage -rfakeroot

You will find your new package one level below:

ls -la ../

drwxr-xr-x 3 remi staff   4096 Apr  5 16:58 .
drwxr-xr-x 7 remi staff   4096 Apr  5 18:23 ..
-rw-r--r-- 1 remi staff   1134 Apr  5 16:08 channel.xml
-rw-r--r-- 1 remi staff 174503 Apr  5 16:08 pecl_http-1.7.4.tgz
-rw-r--r-- 1 remi staff 146352 Apr  5 16:38 php5-pecl-http_1.7.4-1_amd64.deb
drwxr-xr-x 5 remi staff   4096 Apr  5 16:56 php-pecl-http-1.7.4
-rw-r--r-- 1 remi staff   1516 Apr  5 16:38 php-pecl-http_1.7.4-1_amd64.changes
-rw-r--r-- 1 remi staff   4334 Apr  5 16:58 php-pecl-http_1.7.4-1.diff.gz
-rw-r--r-- 1 remi staff    769 Apr  5 16:38 php-pecl-http_1.7.4-1.dsc
-rw-r--r-- 1 remi staff   4332 Apr  5 16:57 php-pecl-http_1.7.4-1.md.0.diff.gz
lrwxrwxrwx 1 remi staff     19 Apr  5 16:09 php-pecl-http_1.7.4.orig.tar.gz -> pecl_http-1.7.4.tgz

The *.deb *.changes *.diff.gz *.dsc and *.orig.tar.gz files are needed for uploading to a repository. I’m not covering setting up your own Debian Repository. Don’t worry, you can always install the .deb directly like this:

dpkg -i php5-pecl-http_1.7.4-1_amd64.deb

To install from your Debian Repository:

apt-get update
apt-get install php5-pecl-http

In my configuration management (Puppet) I can now ensure this package is installed at all times at all nodes that run php5. Furthermore, I can compile and package on a dedicated compile vm and have as clean as possible production vm’s!

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DevOps: The new way of doing sysadmin and development

25 March 2012 — Leave a comment

Today while I was running I realized I’ve been doing a lot of coding lately. Coding our new infrastructure to be exact. I remembered Kris Buytaert’s talk about DevOps back in February when I was in Antwerp. One of the key statements is that there are ‘sysadmin coders’ and not ‘sysadmins’ and ‘coders’. The only way to achieve great results, is when these two work together and communicate with each other. The idea’s behind this are called DevOps. IT Operations starts using code to manage configurations and infrastructure instead of doing it by hand over and over again. Thanks to CloudStack and Puppet this is now possible. Ideally, you would not have two groups, but one. Stephen Nelson-Smith from jedi.be describes it like this:

So, the Devops movement is characterized by people with a multidisciplinary skill set – people who are comfortable with infrastructure and configuration, but also happy to roll up their sleeves, write tests, debug, and ship features. These are people who making connections, because they can – because they have feet in multiple camps, they can be ambassadors, peace makers, facilitators and communicators. And the point of the movement is to identify these, currently rare, people and encourage them, compare ideas, and start to identify, train, recruit and popularize this way of doing IT.

More on Stephen’s blog..

I didn’t realize back then what this would mean because I was focused on CloudStack and the tools around it. But it is not only about the tools, it’s the way you look at managing infrastructure and development. What we’re doing looks like DevOps but we’re not there yet ;-). In the coming weeks I’ll spend some more time reading about DevOps to see how we can implement this in our organization. Because I really believe this is the way to go..

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HOWTO automatically reboot a Linux system after a kernel panic

21 March 2012 — Leave a comment

Remote Linux systems usually have to be available all the time. Although Linux is rock solid and stable, an occasional crash can occur. For example when there’s a problem with hardware or software. The last thing you want, is that a Linux server got a kernel panic and then waits forever for someone to reboot it.

Of course there’s monitoring, and there are APC’s too, which can reboot the server. This is like pulling the plug. It usually takes some time for monitoring to notify a sysadmin and then for the sysadmin to reboot the server. And, when using IPMI devices (especially the older ones that share a network connection with the server), a kernel panic could make them unavailable, too. I’ve had that on many occasions. Then you end up driving to the data center or call someone to reset the server. I hate that 😉

In our setup, most servers are in clusters. This means losing one server should not be a problem. But you still want a server to be available again as soon as possible, to be able to handle future problems.

There’s a way to configure Linux to reboot automatically, say 10, seconds after a kernel panic occurs. This will quickly and automatically have the server up again. Should rebooting not help, then bad luck and there’s probably some failing hardware part. You’ve then to drive to the data center anyway.

So, how to do that? Well, there are several options to set this parameter. To test this out, use this command:

/sbin/sysctl -w kernel.panic=10

Note that this setting will not survive a reboot. If you want it to remain active, add this line to /etc/sysctl.conf:

kernel.panic=10

To check the current setting, issue:

cat /proc/sys/kernel/panic

It is wise to implement some sort of monitoring on reboots or uptime. You definitely want to read the logfiles and find out what exactly happened that led to the kernel panic.

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