Monthly Archives: February 2015

How to find your CPU details from the command line

It’s often necessary to know what the exact type of CPU that’s installed on your system. For example, you may need to know if you’re dealing with a dual core or quad core system, or a 32/64 bit system. Only the CPU can tell you this.

Here’s how to find out the string you need for further investigation.

 

Windows

From the command line, execute the wmic command with the following parameters:

Thanks to Jonathan @ Next of Windows for this tip!

Windows also gives you an accurate result via the GUI: open Windows Explorer and head over to Computer – Properties:

Screen Shot 2015-02-25 at 19.04.01

Mac OS X

On the Mac you won’t get a very accurate result from the Apple Icon – About this Mac. It will tell you what CPU type you’re using, but not the exact model number.

To find that out, head over to Applications – Utilities – Terminal and enter the following command:

There. Much better than this:

Screen Shot 2015-02-25 at 19.15.09

Linux

You can take a look at the /proc/cpuinfo file which holds a plethora of information about your system’s CPU. So much in fact that it’s difficult to find what you’re looking for. Filtering the output of this file for ‘model name’ gives you an exact match:

 

Where can I find more information about my CPU?

Google is of course your friend when trying to find out more information about your processor, but there are two tools provided by Intel and AMD that may also be of help. Intel’s ARK website is particularly helpful:

  • http://ark.intel.com
  • http://products.amd.com/en-us/

How to find which package provides a command in yum

CentOS-LogoSometimes you know you need a package, but when you try to install it with yum you’ll get a message like “No matches found”. Yet you’re sure the package exists because you’ve used it before.

This can happen if the package in question is part of a set which installs multiple packages. The net-tools package springs to mind.

yum has a great option called whatprovides with which you can query what package you need to install to use a command. Let’s try it out!

Say I wanted to install mkfs.vfat which is not installed by default in CentOS. Simply trying to install it won’t work:

Of course it won’t. But I can ask yum what provides this package:

Well fantastic! The package I’m looking for is called dosfstools. Knowing that, I can simply use yum install dosfstools, and a few moments later mkfs.vfat will work as expected.

Thanks, yum!

How to view local websites on your iPad with Squid

squidI like developing and testing websites on my local network before they go live. On both Mac and PC it’s easy to tweak the /etc/hosts file so that the URL doesn’t point to a numeric IP, but instead to http://yourserver (or something equally catchy).

On iOS devices we can’t tweak that file unless we deal with the highly unethical practice of jailbreaking. Turns out there is an easier way to surf local websites on mobile devices, simply by using a Proxy Server such as Squid.

A Proxy Server is often used as a caching server or to disguise where a request is coming from. For example, surfers use proxies to pretend they’re visiting from a different country, or ISPs  use proxies to speed up data delivery in local areas. In simple terms, a proxy is fetching data on our behalf. Then we talk to the proxy and get the data from him. Think of a Proxy Server as a middle man in a network transaction.

To surf local websites on an iPad or iPhone, we can connect to our WiFi network with a proxy on a machine on which we CAN tweak the /etc/hosts file. Let me show you how this works.

For this example I’m using a development server on my local network. It’s a simple LAMP Stack running CentOS.

 

Installing and configuring Squid

We’ll install Squid on the development server. Websites are accessible via http://localhost but also as something more swish when the /etc/hosts file is tweaked, for example http://yourserver.

Squid is available via yum, and installation is simple:

The second line will start Squid on every server restart, just in case.

Squid should work out of the box on port 3128, but if you ever need to tweak this, you can do so in /etc/squid/squid.conf.

 

Configuring the iPad Connection

When you connect your iOS device to your local WiFi network it will do this without a Proxy Server by default. We’ll change this under Settings – WiFi, then tap the little info icon next to your active connection. It goes without saying that the development server needs to be on the same network as your iPad.

At the bottom of this page, under “http proxy”, select manual and add your development/proxy server’s numeric IP, and 3128 under Port. Leave authentication off. It should look like this:

image-2

 

Now any tweaked URL that works on your development server will work on the iOS Device too: visit http://yourserver to verify this.

Should Safari give you a problem, maybe due to its spurious caching technology, head over to Settings – Safari – Clear History and Website Data.

 

Notes

Squid will cache every request you make on your iOS device as long as your WiFi connection uses the proxy setting. Chances are that your development server isn’t going to deliver results as fast as non-cahced results would come in from your router – unless you surf the same slow website over and over.

Also, Squid will leave a record of every request that has been made in /var/log/squid/access.log. If you’re using such a setup in your office, you may need to tell users on the network that their requests will be logged.

So if you’re concerned about any of these aspects, simply switch the proxy off in your WiFi settings.

 

  • http://www.squid-cache.org
  • http://www.ravelrumba.com/blog/ipad-http-debugging/
  • https://www.digitalocean.com/community/tutorials/how-to-install-squid-proxy-on-centos-6

How to block Spam Trackbacks in WordPress

wordpress-iconTrackbacks are a great way for other blogs to notify your blog about a link back to you. Many blogging platforms support this feature, including WordPress.

But sometimes it’s very obvious that those trackbacks aren’t coming from a legitimate source, especially when you get several dozen of them every day from the same source.

No one loves you that much.

The most recent two examples are semalt.com and buttons-for-website.com, the latter can’t even properly mix a plural with a singular. But that’s not for here.

To make sure those trackbacks don’t bother our WordPress site anymore, we can add a bit of code to your re-reite rule file. If your host is using Apache then this will be your .htaccess file, famously in use for Pretty Permalinks and some cache plugins.

A typical .htaccess file is either empty or contains a block of code courtesy of WordPress. It’s a simple text file. If we add this little snippet to the bottom of the file, friendly trackbacks from semalt.com will no longer notify our website:

This rather strange looking code is a rewrite rule. It says “if you encounter a link or a visitor from semalt.com, then forbid them access to anywhere on this site”.

Notice the backslash, followed by the domain extension in semalt\.com. This is necessary to escape the dot character, otherwise Apache would interpret it as an instruction. In our other example, buttons-for-website.com, we need to deal with the slashes in the domain name in the same way:

You can stack these rules in your .htaccess file and add as many as you like for your very own Trackback Spammers. Simply replace the URL in the code with your own, escaping special characters as seen above (a special character is anything that isn’t “a to z” or “0 to 9”).

Note that these rules do not prevent such websites from linking to you. However as soon as someone from the offending website clicks a link to your website, they will be denied access. On the other hand, when the same visitor would type in your URL, or come from a different website, they will be able so see your content without problems.

  • http://httpd.apache.org/docs/2.0/misc/rewriteguide.html
  • http://httpd.apache.org/docs/2.2/rewrite/flags.html

How to enable automatic logins in CentOS 6 and GNOME

CentOS-LogoI was researching auto login options for CentOS today. I thought those would come in handy when GNOME is used as a standard desktop, so that the computer starts straight into the desktop environment without the need to provide a password.

It’s also a handy feature to have if the machine lives in another room and needs GNOME to login to the wireless network when I issue a remote restart.

Turns out there are two parts to the puzzle: providing auto logins and removing a pesky Keyring Dialogue that appears to come up when those are enabled. Let’s tackle both of them here.

I’m using CentOS 6.6 with GNOME here. In principle this works in CentOS 7 too, but there’s a much easier way to accomplish the same thing. See my CentOS 7 article for details: https://wpguru.co.uk/2015/08/how-to-enable-automatic-user-logins-in-centos-7-and-gnome/

Enabling Auto Logins in CentOS 6

Head over to the trusty command line and edit /etc/gdm/custom.conf. The file already contains several sections, all of which are empty by default.

In the daemon section, add the following values (replacing youruser with your actual user name):

Thanks to Keith Wright on the CentOS Forum for this tip.

Now when you restart your system, youruser is automatically logged in when GNOME starts.

Disabling the Keyring Dialogue

However, you may experience a Keyring Dialogue which will ask for your root password every time after a restart. This isn’t much better than the login screen. This may or may not happen, depending on your current configuration:

Photo Feb 15, 8 47 34 AM

One article I found suggested to head over to System – Preferences – Startup Applications and simply remove the Keyring Daemon from the list. Turns out this doesn’t actually make a difference, so don’t do that (although it doesn’t seem to have an adverse effect either):

Remove-Keyring

The real solution comes courtesy of Jim and iiSeymour: http://superuser.com/questions/43132/why-do-i-need-to-enter-a-password-for-the-default-keyring-to-unlock

All we have to do is

– head over to the Network Connections Icon at the top of the screen
– and right-click it
– select Edit Connections…
– pick your current wireless connection and select Edit
– provide the root password (only necessary this once)
– check the tick box Available to all users

Photo Feb 15, 8 59 55 AM

Photo Feb 15, 9 00 10 AM

Photo Feb 15, 9 00 30 AM

In CentOS 7, choose the little gear icon to bring up a window. You can find the available to all users tick box in the Identity section on the left hand side.

And that’s it! On subsequent logins, GNOME will now start with youruser already logged in and your wireless network connected.

Wait! My system boots into the Command Line interface, even though I have GNOME installed. What gives?

You can tell your system in which mode to start. To do this, edit your /etc/inittab file:

How to fix ProFTP Handshake Trouble in Plesk

Plesk-LogoI fixed a problem this morning which wouldn’t let the latest version of FileZilla v3.10.1.1 connect to one of my client’s servers anymore.

This had not been a problem in the past.

The connection itself worked, but FileZilla failed due to a problem with the TLS Certificate. Here’s the error:

Turns out that FileZilla have made a few changes and deprecated the insecure RC4 algorithm in FTP over TLS. Since ProFTP didn’t know the path to the server certificates, TLS failed and hence no connection was possible.

Thankfully there was an easy fix for this, courtesy of this Parallels Knowledge Base article: http://kb.sp.parallels.com/en/2207

To add the default Plesk certificates to the server, all I had to do was tweak the ProFTP config file at /etc/proftpd.conf and add the following at the bottom:

In this example the Server Certificate section contains the default path to Plesk’s certificates, but feel free to substitute them if yours are stored elsewhere.

There’s no need to restart xinetd because ProFTP creates a new process for every new connection, which will then include the new configuration. NOw FileZilla can connect without a hitch, only displaying the new Server Certificate the first time it is encountered:

Screen_Shot_2015-02-14_at_07_50_05

Note that this issue no longer occurs with newer installations of Plesk. This particular instance of Plesk has seen many updates since version 10.4, hence the tweak was necessary.

  • http://kb.sp.parallels.com/en/2207
  • https://filezilla-project.org
  • http://superuser.com/questions/874981/recieved-tls-alert-from-the-server-handshake-failed-40

How to copy a CentOS ISO to USB on Mac OS X

CentOS-LogoWindows users have a great free tool called ISO2USB which efficiently transfers ISO images to a USB stick. Mac users don’t have such a luxury – at least I haven’t found one yet.

Instead we can make use of a command line tool named dd which can do this for us. It needs a few parameters though, and in this article we’ll look at what those are. The following will work in both Mavericks and Yosemite, with ISOs from CentOS 6.5 and above. Our operation will result in a bootable USB stick.

First, head over to a CentOS Mirror and download your favourite ISO image. Next, have a USB stick handy and insert it into your Mac. Now open Terminal – it’s under Applications – Utilities, or do a Spotlight search to find it.

For this example I’m assuming that the image file is called centos.iso and that it’s in your Downloads directory. Let’s enter that directory by issuing the following command:

The second command lists all files, and your ISO image should be one of them.

Find out what device your USB stick is

So far so good, we’re in the right location. The dd command needs to know which file you want to copy (see above), and the device corresponding to your USB stick. It’s easy to get confused at this point. Let me explain this:

Devices are things attached to the system, such as hard disk, memory cards and USB sticks. Each device can hold a number of partitions which is where your data is stored. Depending on how many storage devices are attached to your system, you’ll get a specific address for each device. For example, /dev/disk0 is your internal Mac hard disk in which you’ll find three partitions.

Since our ISO image will overwrite all partitions on the USB stick, we need to know what the system knows our USB stick as. Type the following to get a list of what’s attached where:

Looks like I’ve got 5 storage devices on my system. The one at the bottom is my USB stick, an old 4GB model currently formatted with FAT32. Your layout will be different, so keep an eye on the SIZE parameter. If your currently formatted stick is named you can identify it that way too (mine is called C64).

The device I want to use here is /dev/disk4. Note that when we get to work, everything on your USB stick will be erased when we copy the ISO over. The dd command will not warn you before this happens.

Unmount your USB stick

Before we can continue we need to make sure your USB stick isn’t mounted to OS X. If it was formatted with a filesystem that your Mac can read, then you’ll see your stick as an orange icon on the desktop. But since the dd command will do a low level format with a different filesystem, OS X needs to let go of our stick. If we don’t do this, you’ll get a “Resource busy” message in the next step.

To unmount your stick, type the following:

That orange icon should disappear from the desktop, and you will no longer see your USB stick in the Finder either. Leave it attached though – do not eject it.

Copy the ISO image over

Now let’s give the dd command something to do. Since we’re in the correct directory already, type the following (you will be prompted for your password):

Amend /dev/disk4 with your own device. Feel free to specify the direct path to your image file in place of centos.iso (no wildcards I’m afraid).

This step can take a long time, during which you won’t get any feedback whatsoever. That’s normal, even though it looks like your session hangs. It’s all good. As long as you leave the Terminal window open, feel free to do other things with your Mac and leave it running.

In the above example I was copying a 190MB image and it took just over four minutes. I had an abysmally slow USB stick mind you – something to keep in mind when you want to transfer a large “everything ISO” image onto a stick from 10 years ago: you’re not doing yourself a favour, it’ll take hours to copy, and just as long to boot from later.

Should you want to terminate your dd session during this time (and use a faster USB stick for example), simply hit CTRL+C and you’ll be returned to the command line.

If all went well you’ll receive a summary message at the end. Now you’re ready to boot from your stick into the wonderful world of CentOS.

  • http://wiki.centos.org/HowTos/InstallFromUSBkey
  • http://linux.die.net/man/1/dd
  • http://www.myiphoneadventure.com/os-x/create-a-bootable-centos-usb-drive-with-a-mac-os-x

How to set firewall rules from a GUI in CentOS

CentOS-LogoSick and tired of countless command line statements to set your firewall rules? Me too. No matter what I try, I never get the results quite right. There’s always some switch I forget and ultimately something isn’t working.

For years I was thinking, “there has to be an easier way, like there is in Plesk”?

And today I found that there is: a rather un-obvious tool called system-config-firewall. It’s a godsend and works in CentOS 6 with iptables, and in CentOS 7 with firewalld.

Installation

To make use of it, install the following two packages:

The first one is a version that runs under Gnome and KDE, and second one works on the command line.

The Command Line Version

You can invoke the command line version by running

and it will present you with the following interface. You may need to switch the firewall off temporarily, but the tool will tell you if that’s necessary:

Screen Shot 2015-02-10 at 22.33.26

Here’s how to use the interface:

  • use the cursor keys to move up and down
  • use the SPACE bar to select items
  • use TAB to choose the next option
  • and once selected, hit RETURN

Screen Shot 2015-02-10 at 22.33.52

 

system-config-firewall has several built-in presets, such as DNS, FTP, Mail, standard and secure http ports and many others. If you need to open a specific port, hit Add on the “other” screen and define both the port and the protocol. In this example I’m opening port 3306 for incoming MySQL traffic:

Screen Shot 2015-02-10 at 22.39.30

Step forward through all available options, or select Close to move back to the first screen. Make sure the Firewall Enabled option is ticked, then hit OK and all your rules will be saved.

The Desktop Version

If you have Gnome or KDE installed, you can invoke the Desktop Version from the command line like this:

In addition, there should also be a handy menu item under System – Administration – Firewall which will start the same thing.

Screen Shot 2015-02-10 at 22.48.05

The options are much the same, perhaps a little easier on the eye and easier to select. In addition you have a Wizard which will let you start your firewall rules with a clean slate (great if you’ve been previously poking around on the command line, potentially messing things up).

Screen Shot 2015-02-10 at 22.48.53

Thousand thanks to all the developers who have written this tool: Thomas Woerner, Chris Lumens, Florian Festi, Brent Fox and many others.

  • https://bugs.centos.org/view.php?id=7831
  • http://www.cyberciti.biz/faq/linux-web-server-firewall-tutorial/
  • http://wiki.centos.org/HowTos/Network/IPTables

FIXED: WordPress refuses to send you a Password Reset Link

Screen Shot 2015-02-10 at 21.33.07

I ran into an interesting problem today: on a CentOS 6 server a colleague of mine wanted to reset her WordPress password via the handy link provided in the login dialogue. But rather than sending an email, WordPress got back to her with the following error message:

The e-mail could not be sent.
Possible reason: your host may have disabled the mail() function.

Intrigued I had a look at the server. To my surprise sendmail was installed, and emails could be sent from the command line as well as from PHP scripts. But not from WordPress. What was going on?

Examining the logs I came across the following error message:

followed by a Web Server 500 error caused by the password reset link. Interesting.

Turns out it was an old acquaintance of mine, someone who has been spoiling the broth on many occasions: SELinux.

SELinux can prevent Apache from sending mail when enabled, but lucky for us there’s a quick way to fix this, courtesy of manyon over at the Simple Machines Forum.

To test if SELinux is preventing mail from being sent, try this test from the command line:

It will return with httpd_can_sendmail –> on or off, and if your server is set to the latter then mail can’t be sent.

To change this, execute the following, switching this bool to on:

Note that this can take a minute or two (literally) because the entire SELinux policy needs to be recompiled. Be patient, your server isn’t hanging.

Once changed, make sure to restart Apache:

Now WordPress can send the password reset link. #result

  • http://www.simplemachines.org/community/index.php?topic=446074.0
  • https://wordpress.org/support/topic/wordpress-cant-send-mail-for-forgot-password-and-new-account
  • http://unix.stackexchange.com/questions/67732/why-setsebool-command-of-selinux-taking-so-long-and-slow