userguide:checking_iso_file_integrity [NethServer Wiki]

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====== Checking NethServer's ISO Image File Integrity ======

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This is an optional but recommended step.\\ In this guide, the terms //message digest//, //hash//, and //checksum// are used interchangeably (although some difference may exist).
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When downloading and storing a file, data corruption can happen; verifying its integrity is a good practice to ensure you got an unmodified copy of the original file. File checksums are committed to this purpose.

In simple terms, a //[[https://en.wikipedia.org/wiki/Checksum|checksum]]// or //hash sum// is like a signature or fingerprint that identifies digital data. A copy of a file getting the same computed checksum as the original indicates there is a very high probability the data has not been accidentally altered or corrupted((Security flaws where identified in MD5 and SHA-1 algorithms; they are still useful for error checking, where data tampering is not a primary concern, but the use of stronger hash functions would be desirable.)).

Checking NethServer's ISO image file takes almost no time and prevents further installation problems. Checksums for the ISOs can be found at the [[https://www.nethserver.org/getting-started-with-nethserver/|download page]].

The following examples show different ways to check the integrity of a file. 
Bear in mind the checksums and ISO names mentioned in the examples may differ from the current NethServer release.


===== File Integrity Check from an On-Line Service =====

Online file integrity checkers have the benefit of being multi-platform, installation-free, and easy to use.

There are multiple (free and paid) on-line services offering file integrity checks. Most of them make use of a web form to upload the file to their servers, where the file hash is computed and then shown to the user. Our advice is not to use them, due to privacy concerns (if uploading files with sensitive information), file size upload limits, and possible data upload corruption which defies its own purpose.

There are some on-line services that do the file verification on the client-side, without uploading the file to their servers. One of them is [[http://www.toolsley.com/hash.html|Toolsley Hash & Validate]] browser-based service, which can be used from GNU/Linux, OS X, Windows and other Operating Systems.

{{ youtube>QwAqRn_XDew?medium }}\\

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===== File Integrity Check from a Graphical User Interface Program =====

Graphical programs use to provide an intuitive way to accomplish specific tasks, thus making it suitable for most users.

Few Operating Systems integrate a graphical file checksum mechanism, therefore third-party tools might be installed to accomplish the task.

The next examples show how to use some file integrity checkers on different Operating Systems.

**DownThemAll! Firefox web browser add-on** (works on GNU/Linux, OS X, Windows)\\ //[[https://addons.mozilla.org/firefox/addon/downthemall/|DownThemAll!]]// Firefox add-on is an Open Source download manager that integrates into [[https://www.mozilla.org/firefox/new/|Mozilla Firefox]] web browser. Its built-in checksum (hash verification) support allows you to download a file and verify its integrity in the same step, without leaving the web browser.

In short, the operation process has these steps:

  - Get the file checksum/hash from the original source\\ 
  - Choose the file to be downloaded (NethServer's ISO image)\\ 
  - Choose //DownThemAll!// as the download manager\\ 
  - Paste the checksum in the appropriate //DownThemAll!// text field\\ 
  - Start the download

{{ youtube>ochWBOdayK8?medium }}\\

Once the file has been downloaded, //DownThemAll!// will verify it against the hash provided previously. In case of a hash mismatch, a dialog will appear informing you of the failure and will give you the option to retry the download, keep the file, or delete the file.

{{ :howto:downthemall-hash-mismatch.png |DownThemAll! hash mismatch notification}}\\


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**MultiHasher** (works on Windows)\\ //[[http://www.abelhadigital.com/multihasher|MultiHasher]]// is a freeware file hash calculator. It has an installable version and a portable one. //MultiHasher// can be integrated into Windows Explorer context menu.

You can also add the files and folders that you want to view their hashes by dragging them into the main window program, or by using the menus.

Its association with the most common file extensions used to distribute hashes makes it very simple to use. You just have to download NethServer's ISO image file and its computed file hash (i.e. MD5, SHA1...) on the same folder. Once the files are in place, opening the checksum file will bring up the //MultiHasher// program, that in turn will verify the integrity of the ISO image file against it.

{{ youtube>9MkJ8LiB_g8?medium }}\\

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**GtkHash** (works on GNU/Linux)\\ //[[http://gtkhash.sourceforge.net/|GtkHash]]// is //Free Software// for computing message digests or checksums. It can be installed from the Software/Package Manager of most GNU/Linux distributions.

Its use is very simple:

  * Open //GtkHash// program\\ 
  * Select the file to be verified\\ 
  * Paste the checksum of the file\\ 
  * Press the [Hash] button

There are additional packages you can install to integrate //GtkHash// into multiple file managers like //Nautilus//, //Nemo//, and //Thunar//.

{{ youtube>6aCu9GKI298?medium }}\\

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**7-Zip** (works on Windows)\\ Althought //[[http://www.7-zip.org/|7-Zip]]// is a file archiver, it adds a //CRC, SHA...// context menu entry to ISO image files. This menu allows you to compute the hashes and visually compare them against the original ones. //7-Zip// is //Open Source// software.

{{ :howto:iso-image-file-integrity-check-using-7zip-checksuming-hashing.jpg?direct&420 |7-Zip context menu and hashing}}

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===== File Integrity Check from the Command-Line =====

Operating Systems like GNU/Linux and OS X have built-in command-line tools to compute and compare file checksums. Therefore, there is no need to install third-party tools.

To run any of the following commands in GNU/Linux or OS X, open a Terminal window, and navigate where the file is (or type the full file path).

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When typing a command, you can use tab completion by pressing the [⭾ Tab] Key.\\ Partial text will be completed. If the typed text matches multiple options, they will be shown.
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You can also type a command followed by a space, then drag and drop the file into the terminal window. This avoids the need to manually type the file path.
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**Getting and comparing MD5 checksums**\\ In both, GNU/Linux and Mac OS X Operating Systems, you can use the ''%%openssl%%'' command to get the MD5 hash of a file:

<code>openssl md5 path-to-file</code>
Example:

<code>openssl md5 nethserver-7.9.2009-x86_64.iso</code>
This command will output a computed file hash that can be manually compared against the original one:

<code>MD5(nethserver-7.9.2009-x86_64.iso) = 6a32d2738aafdcf4e124c2822e5c668d</code>
On **Mac OS X**, you can also use the ''%%md5%%'' command to get the same result:

<code>md5 nethserver-7.9.2009-x86_64.iso</code>
On **GNU/Linux**, you can also get the MD5 file hash using the ''%%md5sum%%'' command,

<code>md5sum nethserver-7.9.2009-x86_64.iso
6a32d2738aafdcf4e124c2822e5c668d  nethserver-7.9.2009-x86_64.iso</code>
and if you have downloaded the original MD5 file hash to the same folder as NethServer's ISO image, you can use ''%%md5sum%%'' command to check its integrity:

<code>md5sum -c path-to-file</code>

or

<code>md5sum --check path-to-file</code>

Example:
<code>md5sum --check nethserver-7.9.2009-x86_64.iso.md5
nethserver-7.9.2009-x86_64.iso: OK</code>
**Getting and comparing SHA1 checksums**\\ In both, GNU/Linux and Mac OS X Operating Systems, you can use ''%%openssl%%'' or ''%%shasum%%'' command to get the SHA1 hash of a file:

<code>openssl sha1 path-to-file</code>

<code>shasum path-to-file</code>

''%%openssl%%'' example:

<code>openssl sha1 nethserver-7.9.2009-x86_64.iso
SHA1(nethserver-7.9.2009-x86_64.iso)= 9e3e67f13328163cfd383d1e0fcb7288eed282b8</code>
''%%shasum%%'' example:

<code>shasum nethserver-7.9.2009-x86_64.iso
9e3e67f13328163cfd383d1e0fcb7288eed282b8  nethserver-7.9.2009-x86_64.iso</code>
If you have downloaded the original SHA1 file hash to the same folder as NethServer's ISO image, you can check its integrity with the ''%%shasum%%'' command:

<code>shasum -c path-to-file</code>
or

<code>shasum --check path-to-file</code>
Example:

<code>shasum --check nethserver-7.9.2009-x86_64.iso.sha1
nethserver-7.9.2009-x86_64.iso: OK</code>
On **GNU/Linux**, you can also use the ''%%sha1sum%%'' command in place of ''%%shasum%%''.

**Getting and comparing hashes computed with other message digest algorithms**\\ If you need to check the integrity of a file against a hash computed with other algorithms, you can do so with the appropriate command:

  * ''%%openssl%%'' supports a wide range of message digest algorithms. For further details, refer to its manual.\\ 
  * ''%%shasum%%'' defaults to SHA1 message digest algorithm, but the used algorithm can be explicitly set with the ''%%-a%%'' and ''%%--algorithm%%'' options. Refer to the ''%%shasum%%'' manual to get more information.\\ 
  * On GNU/Linux, there are specific commands for other algorithms, like the ''%%sha224sum%%'', ''%%sha256sum%%'', ''%%sha384sum%%'', and ''%%sha512sum%%'' commands. Refer to your Operating System manual.

Alternative programs might exist on each Operating System.

{{tag>ht_installation userguide before_installation}}