I was introduced to the bash builtin /dev/tcp by warg the other day on x-tab#chat. He explained a basic use of this device by demonstrating how to download wget’s compressed tarball. The download process itself can be done with pure bash, but some post-processing of the downloaded file must be done to remove HTTP headers. I document warg’s application of /dev/tcp here because I found the idea fascinating and want this documentation for myself .

Connecting and Downloading

To read about the /dev/tcp builtin for yourself, check out the following:

$ info '(bash) Redirections'

With the exec line we initiate the connection, allocating a file descriptor and storing the numeic file descriptor into the HTTP_FD variable. Then, with the echo line, we send an HTTP request through the descriptor to the server. After sending the request, we process the server’s response with the sed line which skips over the HTTP headers sent by the server and stows the results into wget-latest.tar.gz. Note that this last command will sit around for a while. It is with this command that the builk of the data transfer is performed. And, since you’re using shell redirections to download the file, you cannot see the download progress. Instead, wait for the command to complete. This also involves waiting for the server to time out your connection since it supports pipelining. After this process is completed, the wget-latest.tar.gz file is as your disposal.

$ WGET_HOSTNAME='ftp.gnu.org'
$ exec {HTTP_FD}<>/dev/tcp/${WGET_HOSTNAME}/80
$ echo -ne 'GET /gnu/wget/wget-latest.tar.gz HTTP/1.1\r\nHost: '\
    ${WGET_HOSTNAME}'\r\nUser-Agent: '\
    'bash/'${BASH_VERSION}'\r\n\r\n'  >&${HTTP_FD}
$ sed -e '1,/^.$/d' <&${HTTP_FD} >wget-latest.tar.gz

Now you have a wget source tarball on your machine. As long as you have tar and a compiler on the machine, you are well on your way to downloading stuff using a self-compiled wget. In the commands above, you may replace “gz” with “bz2″ or “lzma” for smaller downloads if the machine you’re using has bzip2 or xz-utils installed. And, of course, it should not be too hard to repurpose the above code to download a particular version of wget or even a completely unrelated software package.

Please feel free to point out problems with this approach or give pointers on porting this to other environments .

I have a Canon HF200 video camera and it outputs HD video, but it’s in .MTS file format. This is a pain because there are no good free video converters or editors for .MTS files. But using Avidemux, an open-source video converter, I was able to edit and convert my video to .mp4 while keeping my sanity and my money.

If you know your way around avidemux, pay attention to black colored text.

If you need extra help along the way, or I have something pointless to say, it will be in gray.

1 – Open the File

The first thing you have to do is import your video into Avidemux by one of two methods:

1) file –> open –> your-video.MTS

2) drag and drop the video file into avidemux

A dialog will appear saying, “This looks like MPEG, Do you want to index it?”… say “yes”

Avidemux will then tell you that it has detected H.264, say “yes”  to it’s other mode.

2 – Set Encoding Options

For the Video encoder you should use MPEG-4 AVC

For Audio I recommend AAC (Faac) and a bitrate of 192, (I’ll tell you right now AC3 does NOT work with youtube).

Use mp4 for the encapsulation (not avi like in the above pic).

Configure –> General

Use CRF (Constant Rate Factor) around 16 – 20. A CRF of 16 produces a larger file size; the higher the number, the less the quality and file size.

Configure –> Motion

Use UHS (Uneven Hexagonal Search) and SR (Subpixel Refinement) of 9 for best results.

3 – Fix Frame Rate Issues

Here is where everyone has the problem of “Why is the video half speed!?”

Well, that’s because avidemux doesn’t like you, and that Avidemux decided to treat the predictive and reference frames differently.

The way I fixed this was to double the video frame rate:

Main menu –> Video –> Frame Rate

According to my TI-89, 29.97 * 2 = 59.94 (or I just did it in my head, but think the 89 is a powerful tool, especially if you’re an electrical engineer)

Well… now your video is the right speed… but why is the file so large?

Go to video filters, transform –> Resample fps (back to 29.97).

Yeah, I know, I went extra wide screen with my video, so I feel like a ninja when I watch it, big deal.

I used other filters as well to reduce the resolution and trim the unneeded parts of the video to save space.

4 – Save the Video

File –> Save –> Save Video

Make sure you use the .mp4 extension.

That’s all, enjoy your well-compressed .mp4 video!

function getCurrentFile() {
  var filename = document.location.href;
  var tail = (filename.indexOf(".", (filename.indexOf(".org")+1)) == -1) ? filename.length : filename.lastIndexOf(".");
  return (filename.lastIndexOf("/") >= (filename.length - 1)) ? (filename.substring( filename.substring(0, filename.length - 2).lastIndexOf("/")+1, filename.lastIndexOf("/"))).toLowerCase() : (filename.substring(filename.lastIndexOf("/")+1, tail)).toLowerCase();
}

This function returns the current filename (minus the file extension) in all lower-case. Feel free to modify it to suit your needs. It also works properly even when a URL does not have a file extension (such as with some asp-based sites), or when there is no filename present (returns the folder name). This function is great for highlighting menu items based on which page the user is currently browsing. If you don’t understand some of the syntax, note that I’m using javascript’s conditional syntax as shorthand for if/else statements.

The problem: you have a computer sitting behind a firewall. You want to access it from a different location, but you don’t have the ability to forward any ports to it. The answer: SSH tunneling.

The Solution

Using an SSH tunnel, you can reverse-forward ports from one computer to another. To do this, you will need a computer running linux and sshd to reverse-forward the ports to. It is very convenient if this is the computer you will be using to access the remote machine. Otherwise, additional steps must be taken.

The Setup

The easiest way to set up and maintain a reverse port-forwarding tunnel is with ohnobinki’s insurgent script. The script allows you to specify a remote host and the ports you want to reverse-forward. To start off, create a new user on your system, such as insurgent. Log in or start a shell as this user. Assuming you have mercurial installed, run:

hg clone https://ohnopublishing.net/hg/insurgent

Now cd to the newly created insurgent/bin directory. Finally, place the contents of insurgent/share/contab.txt into your crontab (use crontab -e to edit your crontab).

Now you simply need to configure the script. To do so, open insurgent.sh in your favorite editor, and update the REMOTE_HOST and other variables. The format for ports is [remoteport]:hostname:[localport] (ssh(1) ). I recommend starting with reverse-fowarding SSH (port 22), a vnc session (590x where x is the VNC display number), and nfs.

If you have not done so already, you need to set up passwordless public key authentication for the new insurgent user.

You’re Done!

If you’ve gotten this far, you may be ready to go. You should be able to access any port on your insurgent box via the corresponding port on your local box. Have any problems? Drop some comments below or pop into irc.ohnopub.net#protofusion and speak to ohnobinki or normaldotcom.

Image credit: star6. Used under Creative Commons CC BY-SA 2.0

By default, when you exit most IRC clients (including pidgin), the quit message will look something like “normaldotcom has quit (Quit: normaldotcom).” The preferred way to exit out of a channel or an IRC session is using the /quit IRC command. It’s pretty simple, just type “/quit leaving to do something in real life with real people.” This will notify others of your departure, and your reasoning for doing so. It’s just a lot nicer and a lot less annoying. Practice proper IRC etiquette and use /quit !

Classify the problem: Are your fans loud or do they make scraping noises?

Scraping Noises

Scraping noises are often caused by bad bearings, whether it be an oil-sleeve bearing or a ball bearing. Take the label off of your fan, if you see a bearing surrounding the shaft, then you do NOT have an oil-sleeve bearing.

Oil sleeve bearings: Place one drop of light oil (sewing machine oil works quite well) on the spindle, replace the label, and let the fan run for a few minutes. Do not over-oil. If the fan is still noisy, remove the clip from the end of the shaft (if present) and detach the fan blades from the coil/case assembly. Ensure that the spindle and sleeve are clean, then re-assemble and oil.

Ball bearings: Place 1-2 drops of oil on the bearing(s). This may have limited success. If it fails, you can try flushing the bearings with oil, drying with paper towel, then putting on a single drop of oil.

Loud

Have loud fans? Chances are you need to just replace them or throttle them.

Throttling: Check out your bios for settings related to “Quiet n Cool” (for AMD CPU fan throttling) and PWM settings for other 3-pin fans on your system. Don’t have the options or 3-pin fans? Throttling can be done manually with rheostats and resistors, just make sure you don’t over heat your system.

Replacement: Check out eBay or your local junk drawer for any fans that may work as decent replacements. Feel free to use overly large fans on small heatsinks, such as northbridges; if positioned correctly, these may render decent results. Warning: Don’t try this with graphics card heatsinks, as they get much hotter than the average northbridge under load. For case fans, try and get larger (120mm) fans if your case accommodates them. Larger == less rpm == less noise + good airflow.

Any questions, comments, or criticism? Feel free to leave some comments.