Using ssh-keygen

The ssh-keygen command allows you to regenerate a public key using the -y flag. Using the -t flag you can tell it whether the key is rsa or dsa.

Is my key RSA or DSA?

Chances are it doesn’t matter; ssh-keygen will try to guess based on the input key. However, if you don’t know your key encryption, it’s probably rsa since that’s the default. The filename will also typically tell you, since it’s usually either id_rsa or id_dsa. And even beyond that, if you look at the text in the file, it should be present there as well.

Regenerating a Public Key

Here’s an example:

# ssh-keygen -y
Enter file in which the key is (/home/username/.ssh/id_rsa):
ssh-rsa AAAAB3NzaC1yc2EAAAABIwAAAQEAr3T65FaSononqBjGEZXMg8x0U3ZjYvZxAUZQA7H27VtPgrn9FhsP8Jn+sp0zOi2nFjDsbXWM5L6OPVg1N0OHpiNcg7I
lrc83GiqVGg2AWeHWWolnOwXIsrfwybVcS6ZSCGbGKVWWL5VB/mt/zzF5WD6bhU+TZXYLq8fZC4sa0sapqVccubKw2YbjA53n0wKxrYLfOjP1k56EfkHzm4n7fmlyFi
3kaCvPo31yaMD3zIVJnl/4wMntnnxqFkG7mEtQ29ngkc5ocgRvSbNNvD9IFNvL/9BqlUtiOUcV790cdoLyd0o1mFV8sGPY3zsL6l3lTkjYDmSXTTnxavjHEudC5w==

BAM! There’s your public key!

Is this safe?

Yes, regenerating your key is completely safe and there’s no reason why you wouldn’t want to do this. Public keys are public, just as their name implies. You can give your public key to anyone.

The private key is the one you should keep to yourself and safe-guard. You should be the only one with your private key, and there’s no reason you should not be able to get a copy of your own public key.

–

Now save that public key somewhere safe so you don’t have to generate it every time!

Happy authenticating!

Well, as it turns out, your PHP installation must be configured to support SSH before this option will show up in your WordPress dashboard. In this article I’ll explain how to set this up using just a few simple commands or a plugin.

Option 1: Install libssh2 and use the built-in WordPress SSH/SFTP Support

The upside of following this guide instead of installing the plugin is you’ll be installing libssh2, which means you’re enabling SSH/SFTP support across all PHP applications. You’re also be enabling and using the built-in WordPress support for SSH/SFTP, which is updated and supported with each new release. I personally followed this method, so I can also confirm that it works first-hand. The downside to this guide, of course, is that it takes a bit longer.

Step 1: See if it’s enabled already

Chances are SSH/SFTP is not enabled already, but it doesn’t hurt to check. If SSH is not enabled, your WordPress connection info page should look like this when you go to install updates:

Notice the distinct lack of an SSH/SFTP option. If this is what your page looks like, continue with the instructions below!

Step 2: Install all the development tools you’ll need

We’ll be installing the PHP package for libssh2, which will be compiled from source; this requires some development tools. Run these commands to install the tools you need:

$ sudo yum groupinstall "Development Tools"
$ sudo yum install kernel-devel kernel-headers

Step 3: Install libssh2

You’ll need libssh2 and some php development tools, so make sure those are installed by running the following command:

$ sudo yum install php-devel php-pear libssh2 libssh2-devel

Step 4: Use PECL to install SSH2 for PHP

PECL is the PHP Extension Community Library and we can use it to install SSH for PHP, like so:

$ sudo pecl install -f ssh2

After running this command, you’ll notice that it stops at a line and waits for input from you. It will look like this:

..
........done: 22,740 bytes
5 source files, building
running: phpize
Configuring for:
PHP Api Version:         20041225
Zend Module Api No:      20060613
Zend Extension Api No:   220060519
libssh2 prefix? [autodetect] :

There’s no need to enter a specific prefix; just press the [enter] key to move forward and it will autodetect the prefix.

Step 5: Configure PHP to load the SSH2 extension

Now that the SSH2 extension has been installed, you just have to tell PHP to load it. Configure PHP to load the extension by running the following commands:

$ sudo touch /etc/php.d/ssh2.ini
$ echo 'extension=ssh2.so' | sudo tee -a /etc/php.d/ssh2.ini

Step 6: Restart Apache

You’re almost done! Just restart the web server to have your changes take effect:

$ sudo /etc/init.d/httpd restart

Step 7: Verify the extension is loaded

Finally, check to see if the extension has been loaded by running php -m to see the installed PHP modules:

$ php -m | grep ssh2
ssh2

If you don’t see the ssh2 output from the above command, then something went wrong! Try following the steps again carefully to see what you might have missed.

Finish Line

You’re all done! You can now use SFTP to install and update plugins in WordPress. If you did everything correctly, your WordPress connection info page should now look like this:

* Thanks to Jacob for the original guide to installing libssh2 for PHP under CentOS.

Option 2: Install a Plugin for WordPress

Update: User ‘diamondback’ pointed out that there’s also a WordPress plugin for this. I haven’t used it myself, but it utilizes phpseclib, which is an implementation of SSH/SFTP entirely in PHP.

The upside to using a plugin is WordPress plugins are incredibly easy to install and uninstall. Additionally, the plugin developer could incorporate features that the standard WordPress installation does not have. The downside to using this plugin is this enables SSH/SFTP in WordPress only! It will not enable it in any other PHP applications. On top of that, you now have to rely on the author to update his plugin for new versions of WordPress instead of relying on the built-in WordPress support, which you get with the libssh2 installation.

Step 1: Install the plugin

Just install the SSH SFTP Updater Support plugin. You’re done!

Disclaimer: I haven’t used this plugin and do not endorse it in any way.

Enjoy your SSH/SFTP support! Happy blogging!

Installing build tools in Debian/Ubuntu

In Debian/Ubuntu, you can install the typical build tools by installing the package build-essentials, which is just a pseudo-package that downloads all the popular development packages:

# apt-get install build-essentials

Installing build tools in CentOS

Since I prefer CentOS as my server platform, I also occasionally need to install packages using yum.

To install the common build tools using yum in CentOS you’ll want to install the group package “Development Tools”, which is similar to build-essentials in Debian/Ubuntu flavors of linux. You’ll probably also want to install kernel-devel and kernel-headers if they’re not already installed:

# yum groupinstall "Development Tools"
# yum install kernel-devel kernel-headers

This should give you a copy of make, gcc, gdb, and all those other tools you were looking for.

Happy hacking!

Why?

xargs takes a file list as input and performs some arbitrary operation on it. This is obviously useful for scripting and bulk file operations. Why use xargs? Well, quite frankly, it’s faster than using find -exec because it splits the file list into sublists and calls the command once for every sublist instead of calling the command once for every file. Also, because it uses sublists you never run into problems when trying to run commands with very large argument lists (it is actually possible to run a command and have bash reply that the maximum line length is exceeded). Below is example usage.

Example: fixing permissions

Using xargs we can easily correct permissions recursively. For instance, let’s improve our web security by locking down write privileges on our directories and files to only the owner:

find . -type d -print0 | xargs -0 -I {} chmod 755 {}
find . -type f -print0 | xargs -0 -I {} chmod 644 {}

Here’s what’s happening:

• we use find and -type d to locate all directories under the current directory
• we use -print0 so find will print the directories and separate the directory names by null characters instead of newlines; delimiting this way improves xargs ability to handle special characters in file names
• then we pipe this list to xargs and use -0 to tell xargs the list is null-character delimited
• we also use -I {} to tell xargs to replace any occurrence of {} with each filename as it runs the following command
• ultimately the command chmod 755 {} tells xargs to chmod 755 each directory

Of course we then do the same by running chmod 644 on all files under the current directory.

Efficiency

Using xargs this way is faster than running these equivalent find commands:

find . -type d -exec chmod 755 '{}' \;
find . -type f -exec chmod 644 '{}' \;

Using find this way will run chmod individually for each directory and file, which is less efficient. The general rule of thumb is to always opt for xargs.

You can use xargs for several other things, but this example is my most frequent use-case. Happy hacking!

During the installation Yii uses a ‘Requirement Checker’ webpage that verifies you have the correct PHP version and the necessary plugins. If you’re like me, you like to see all your boxes turn green just to be sure you can get the full functionality out of your apps and frameworks instead of having to debug stuff like this later and going down a rabbit hole.

Install All The Packages

To make the best use of Yii (and several other things), you’ll need these packages: GD, mcrypt, MySQL, PDO, PEAR, APC, Memcache, PgSQL, SOAP, and XML. To get proper packages for these extensions on CentOS 5, I recommend adding the IUS Community Repository. Here’s a one-liner you can run with sudo or as root after you’ve set up that repo:

yum install php53u php53u-cli php53u-common php53u-devel php53u-gd php53u-mcrypt php53u-mysql php53u-pdo \
php53u-pear php53u-pecl-apc.x86_64 php53u-pecl-memcache.x86_64 php53u-pgsql.x86_64 php53u-soap.x86_64 php53u-xml.x86_64

Have fun with Yii!

Wget

Surprise! Wget supports FTP and can download recursively! You could download the full contents of an FTP server like this:

wget -r 'ftp://user:pass@host'

Or just grab a single directory like this:

wget -r 'ftp://user:pass@host/dir'

Cool, huh?

In preparation for the Pentesting with Backtrack course (the course you take before applying for the OSCP exam), I’ll be installing Backtrack 5 in VirtualBox. Continue reading to learn how.

Installing Backtrack in Virtual Box

1. Download the latest BackTrack from the Backtrack website
2. Download and install the latest VirtualBox from the VirtualBox website
3. Open VirtualBox Manager
4. Click New
5. Name your VM anything you want; I chose to name mine bt5. Select Linux as the OS and Linux 2.6 as the version.

Installing BackTrack in VirtualBox; click to enlarge

6. Select the RAM size; I chose 2048
7. Select Create new hard disk
8. Use VDI
9. You can choose to use either a dynamic or static disk; I personally chose static and allocated 8GB
10. Once this is done, you can select your new VM and click the Start button in VirtualBox
11. The new set up guide should appear and it will ask you to select your CD/DVD drive. Choose the BackTrack ISO you downloaded as the CD/DVD drive and VirtualBox will boot into BackTrack’s live disc.
12. You will see the splash screen below. Just type startx to boot into Gnome or KDE.

BackTrack splash screen; click to enlarge

13. Double-click the Install BackTrack link on the desktop to start the installation process
14. Select your geographical location and click Forward. Same for the keyboard layout.

BackTrack location setup; click to enlarge

15. The next screen allows you to configure the partitioning layout. The assumption is that you are deleting the whole drive and installing BackTrack. This should be fine since you’re installing within a VM, but you can configure it if you would like.
16. Accept the installation summary and click Install.

BackTrack installation summary; click to enlarge

17. BackTrack is now installing! Wait patiently; this can take a while depending on how much RAM you allocated and how fast your PC is!

Installing BackTrack to the hard drive; click to enlarge

18. You’re done installing! Restart!
19. When BackTrack boots, use the username root and the password toor to login. Change the password using passwd.
20. You may have noticed you’re at a duller black screen instead of the cool splash screen you had during the live session. If you think this is hardcore, then good for you! If you liked the splash screen better then use fix-splash to restore it!

21. If you fixed the splash screen, go ahead and reboot again using shutdown -r now.

Finish Line

Congratulations! You’re all done! Go forth and prosper! Don’t forget you can still use startx to get back to your desktop environment if you want to use the GUI tools.

Footnote: If you need more detail about archiving and compressing, check out the guide!

tar cvf archive.tar /home/user/directory

tar cjvf archive.tar.bz2 /home/user/directory

tar czvf archive.tgz /home/user/directory

zip -r archive.zip /home/user/directory

tar xjvf archive.tar.bz2

tar xzvf archive.tgz

unzip archive.zip

tar tjvf archive.tar.bz2

tar tzvf archive.tgz

unzip -l archive.zip

Happy hacking!

Footnote: If you know what you’re doing already, you might like the cheat-sheet better!

We’ll start with compressing a single file because that’s the easiest thing to do and we’ll work our way to archiving and compressing directories.

Gzip Files

Creating a gzipped file is as simple as typing gzip followed by the filename:

$ gzip bank_statement

This will output a file with a gz extension indicating it has been compressed: bank_statement.gz. Simple enough.

Decompressing a gzipped file is just as simple using the gunzip command followed by the filename:

$ gunzip bank_statement.gz

This will output the original file without the gz extension indicating it has been decompressed: bank_statement.

Bzip2 Files

Creating a bzipped file works identically to gzip; simply type bzip2 followed by the filename:

$ bzip2 bank_statement

This will output a file with a bz2 extension indicating it has been compressed: bank_statement.bz2.

Decompressing a bzipped file works just like you would expect. Use the bunzip2 command followed by the filename:

$ bunzip2 bank_statement.bz2

This will output the original file without the bz2 extension indicating it has been decompressed: bank_statement.

Tar Archives

Creating a tar archive allows you to group multiple files or directories into a single file. Windows users might find this similar to a zip file. One difference is that tar simply groups files together but does not compress them the way zip does. To create a tar archive we will use the tar command, the filename of the archive to create, and the files or directories you want to archive.

Archiving a Single File

To archive a file you would use a command similar to this:

$ tar cf bank_statement.tar /home/user/bank_statement

This will output a file named bank_statement.tar that contains /home/user/bank_statement. The flags used here are:

• c: create an archive
• f: specifies that the argument following is the filename for the archive

The file to archive doesn’t have to be the full path; you can just as easily use these commands to change to the /home/user directory and tar the bank_statement:

$ cd /home/user
$ tar cf bank_statement.tar bank_statement

Archiving Multiple Files or Directories

Typically you wouldn’t want to tar a single file though. It’s more useful to tar multiple files or directories like the following respective commands:

$ tar cf some_important_documents.tar bank_statement tax_document
$ tar cf all_important_documents.tar /home/user/important_documents/

The first command will output a file named some_important_documents.tar that contains both bank_statement and tax_document, while the second command will output a file named all_important_documents.tar that contains the entire contents of the important_documents directory. Now we’re getting somewhere!

Unarchiving

Unarchiving a tar file uses the tar command followed by the archive file name. Instead of the c flag, we use x instead:

$ tar xf some_important_documents.tar

This will create the original files that were stored in the tar archive; in this case both bank_statement and tax_document would be output to the file system. The new flags used in that command are:

• x: extract an archive

Remember that the f is used to specify the archive filename.

Gzipped Tar Archives

Creating a gzipped tar archive is very similar to creating a zip file in the Windows world because you achieve a compressed archive file that contains many other files. To create a tar archive and simultaneously gzip it, pass the z flag to tar:

$ tar czf all_important_documents.tar.gz /home/user/important_documents/

This will output the file all_important_documents.tar.gz which is a gzip compressed archive of the entire contents of the important_documents directory. The new flags used in that command are:

• z: apply gzip when creating an archive and gunzip when extracting an archive

Decompressing and extracting a gzipped tar archive is easy once you know how to create one. To reverse the process just swap the c flag for the x flag just like before:

$ tar xzf all_important_documents.tar.gz

This will create the original files that were stored in the tar archive; in this case the important_documents directory would be output to the file system.

Bzipped Tar Archives

Creating a bzipped tar archive works just like the gzipped tar archive; just pass the j flag to tar instead of the z flag:

$ tar cjf all_important_documents.tar.gz /home/user/important_documents/

This will output the file all_important_documents.tar.gz which is a bzip2 compressed archive of the entire contents of the important_documents directory. The new flags used in that command are:

• j: apply bzip2 when creating an archive and bunzip2 when extracting an archive

Decompressing and extracting a bzipped tar archive works the same, of course:

$ tar xjf all_important_documents.tar.gz

This will create the original files that were stored in the tar archive; in this case the important_documents directory would be output to the file system.

Zip Files and Archives

Creating a zip file will seem suprisingly simple after reading about tar since it handles the compression automatically without you having to pick one.

Zipping a Single File

To zip a single file you just use the zip command followed by the archive filename and then the file you want to zip:

$ zip bank_statement.zip bank_statement

This will output a file named bank_statement.zip that contains bank_statement.

Zipping Multiple Files

To zip multiple files you just add more filenames after the archive name:

$ zip some_important_documents.zip bank_statement tax_document

This will output a file named some_important_documents.zip that contains both bank_statement and tax_document.

Zipping Directories

To zip a directory you can use the -r flag:

$ zip -r all_important_documents.zip important_documents

This will output a file named all_important_documents.zip that contains the entire contents of the important_documents directory.

Extracting Zip Files

To unzip a zip file, just use unzip followed by the filename:

$ unzip all_important_documents.zip

This will create the original files that were zipped; in this case the important_documents directory.

Additional Tips

And that’s all you need to know about archiving and compressing in linux (well, maybe not everything)! Here a few more tips that I find useful day-to-day:

• Before unzipping a file, try using the -l flag to view the contents of the archive first:

  $ unzip -l unknown_zip_archive.zip

• Before untarring a file, try using the t flag to view the contents of the archive first:

  $ tar tzf unknown_gzip_tar_file.tar.gz

• You might see gzipped tar files with the extension tar.gz or simply tgz. There is no difference, they are both gzip compressed tar files.
• You might see bzipped tar files with the extension tar.bz2, tb2, or tbz2. There is no difference, they are all bzip2 compressed tar files.
• For more information when tarring and untarring archives (such as what files are going into the archive), try using the v flag to enable verbose output:

  $ tar cvzf backup.tar.gz /home/user

Most users know that to install a package from an RPM, you simply use the rpm command. But a bit of perusing on the internet will show that people use various flags when installing packages:

• rpm -i
• rpm -ivh
• rpm -U
• rpm -Uvh

In this post we’ll examine the difference and whether or not you should care.

The simple truth is that the only flags that are truly important to most users are -i and -U.

Using -i

When using -i you tell the rpm package manager to install the package. For example to install htop version 0.8.3:

# rpm -i htop-0.8.3-1.el5.rf.x86_64.rpm

This command only installs htop version 0.8.3. It does not care if you have other versions of htop installed. If you do not have any other versions of htop installed this is probably the correct command to use. If you do have other versions of htop installed, this is probably NOT what you want to do.

Using -U

When using -U you tell the rpm package manager to upgrade the package. For example to upgrade htop to version 0.9:

# rpm -U htop-0.9-1.el5.rf.x86_64.rpm

This command not only installs htop version 0.9, but also removes any other versions of htop that are installed. The great thing about using -U is that even if you don’t have other versions of the software installed, it will still install the version you requested. In this way you can think of -U as a more careful version of -i. In nearly every practical case you want to use -U!

But what about -v and -h?

The -v and -h flags are just to print more verbose information regarding the installation or upgrade. These are optional, but useful parameters. Below is the information from the man page:

-v Print verbose information - normally routine progress messages will be displayed.
-h, --hash Print 50 hash marks as the package archive is unpacked. Use with -v|--verbose for a nicer display.

How can I learn more about rpm?

If you want to learn how to use other features of rpm, I would suggest using the man page by typing this at terminal:

# man rpm

Man pages aren’t nearly as scary as people make them out to be. They provide plenty of helpful information and they’re always the first place to turn when looking for help with a command.

If you want more information about the RPM Package Manager or the RPM format, you might try Wikipedia’s article first or even Google.

—

In conclusion, any good admin should understand every command he or she uses at least in some detail. But for the common case, rpm -U or rpm -Uvh should serve you well.

Happy hacking.