Today while starting up JDownloader I noticed a strange behavior. The program prompted to pick a language first, and then showed up with an empty download queue. That was unfortunate since I had some downloads left in the queue from the other day.

I first thought that I had just installed a second instance of JDownloader, but discovered a moment later that the new installation seemed to have overwritten the database file containing the file download queue.

I knew that JDownloader was creating backups of the database in regular intervals. I checked the backup folder of the JDownloader program folder to find out that the program had saved more than a dozen database backups in the past months.

This was interesting in itself as it can be seen as a privacy issue by some users as it is possible to restore the download queue with the help of the backups.

The last database backup file is database.zip. To restore it you need to do the following. Quit the JDownloader program. Make sure it is completely terminated and not sitting minimized in the system tray.

Open the database.zip file and extract the contents (database.properties and database.script) to the main JDownloader/config/ directory. Replace the two database files listed there.

This restores the download queue. You may still be missing a few downloads though depending on the time and day the last backup was created by the file downloader. In my case, the last backup had been created four days ago which meant that I lost a few file downloads that I added after that date.

The Firefox add-on Save File To changes this by offering options to configure multiple user-defined folders where files can be saved to comfortably. It can be used to configure folders for web page, link, image and file downloads separately.

The add-on adds options to each file type by default to display the recently used folders as well as the computer desktop. New folders are added easily with a click on Add in the add-on options. It is also possible to edit or remove existing entries from the menu.

These options are then displayed on a right-click in the Firefox web browser. The contents may be different based on the user configuration. It is for instance possible to display a menu for image downloads, and a different one for general file downloads.

Additional preferences are available under the Main tab. It is possible to edit the number of recent folders that the add-on remembers, clear the recent folder listing automatically when Firefox closes or to always open the save file dialog window.

Another benefit of the add-on is that the standard file browser is bypassed if the add-on is used to save files in the web browser.

Save File To is a handy Firefox add-on for downloaders who like to store their files and downloads in different folders based on their type. The add-on can be installed directly at the Mozilla Firefox add-on repository.

Retrying the download yielded the same results. The first thought was that this was related to a single file or server, that acted up strangely.

But the problems persisted and after trying to download a dozen different files, all with a file size of 50 to 200 Megabytes, it became clear that Google Chrome was the problem.

Examples of files are the Emsisoft Emergency Kit with a size of 103 Megabytes, some Mediafire downloads with a size of about 180 Megabytes each, and downloads over at Softpedia, like Kaspersky’s Small Office Security.

All downloads stopped at approximately the same size, 36.7 or 36.8 Megabytes. File extensions were also different, including archive formats like zip or executables like exe.

There does not seem to be a fix for this issue yet, and it is not clear if all Chrome versions are affected or only the latest developer release, version 6.0.466.0.

All downloads were completed successfully in other web browsers like Firefox, which seems to confirm that this is a Chrome-only problem. This also is the workaround for users who experience the same download problems.

The test system was running Windows 7 Pro, 64-bit. Have you been experiencing the same troubles? Let us know in the comments.

Update: The issue has been fixed in the latest Chrome dev release.

When someone puts a file up on a server for download, how does the host or the end-user know, for sure, the file they are about to download (or are serving up) is the valid file? What if someone hacked into the server and replaced the file with a bogus file that contained malicious code? It’s happened before and it will happen again. Fortunately there is a way to avoid downloading invalid files – checking the md5 hash. The only problem is that this method only works if the host and user knows how to use md5 tools. In this tutorial you will learn how to add an md5 checksum to a file and how to run a check on a file you have downloaded.

What is md5 and checksum?

Before we continue with the actual steps, you might benefit from knowing exactly how the process of checksumming works. MD5 stands for Message Digest algorithm 5, which is a cryptographic 128 bit hash function and serves as a “fingerprint” for a digital file. A checksum is a fixed-size datum that is computed from a block of data. When it is crucial for a piece of data (such as a download) to be valid, the datum is compared to the original block the datum was computed from to check for a match. When an md5 checksum matches, the user/host can be certain the file is valid. When the md5 checksum does not match, a red flag should immediately go up and the original block of data should be discarded. If a file changes by so much as a byte, the checksum will fail.

For most users these tasks are handled from the command line. There are GUI tools available (such as GtkHASH) that can tackle the same tasks. But for the purposes of this tutorial we will stick with the command line tool.

Creating an md5 sum

For those who plan on hosting files for download, you will want to know how to create an md5 sum. This is very simple. Open up a terminal and change to the directory holding the file you want to work with. Say, for example, you want to create an md5 on the file /var/www/files/download.tgz. To do this you would change to the /var/www/files directory and issue the following command:

md5 download.tgz

The above command will output something like:

It all begins at a website or server. This is the starting point and it might be a good idea to start validating that server before even thinking about downloading files from there. This can be done manually by performing some searches in search engines but also automatically with browser add-ons or plugins like Web of Trust, McAfee’s Site Advisor and a plethora of other respected programs including local security software that can also check websites and servers.

The second step involves downloading the file to the local computer system. There is not a lot that can be done here in this step. The only defense are security software programs that are installed on the computer system that should scan the file and report back to the user if they believe it to be malicious. Cautious users can also use one of the many online virus scanners to upload the file and scan it online. Services like Virus Total scan the files with more than a dozen different up to date antivirus engines resulting in a more precise analysis of the file.

Another option is to check the hash values of the downloaded files to make sure that they have not been tampered with. This only makes sense if the developer is displaying the values on a trusted website.

It is pretty safe to assume that the file is safe and can be executed on the computer system if it did pass the tests. There is however a last step that can be done to add the extra mile of security: Virtualization. Programs like Sandboxie or VMWare Player make it possible to execute programs in a closed environment for testing purposes. The benefit of this approach is that they cannot harm the rest of the computer system if they should be malicious.

Did we leave something out? Let us know in the comments.