What is the loopback device and how do I use it?

The person who works in Linux / or *nix environment should know what's loop-back.
It's a "software implementation" of network device or any hardware (please see below for other context), i.e. with network class, and that's the fundamental definition of the loop-back.

Why it's called Loopback in context with network?
All the traffic to the localhost or lo (i.e. is sent back or loop back to local machine.
Let us see following command:

sunil@thakares:~$ ifconfig

Output of above command is as follows: (I've modified MAC address)

First Ethernet Card:
eth0 Link encap:Ethernet HWaddr xx:xx:xx:xx:xxxx
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)

****(Here my eth0, i.e. LAN card not in use so, RX...

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You don’t have to hold up a microphone to your computer’s speakers to record its audio. Even if you don’t have a Stereo Mix option on your PC, you can easily record the sound coming from any Windows PC.

This can all be done with software. No, you don’t even need to connect your computer’s audio output to its audio input with an audio cable to do this — although that is an option.

Option 1: Stereo Mix

Stereo Mix is sometimes called “What U Hear.” It’s a special recording option that may be provided by your sound drivers. If it is, you can select it (instead of a microphone or audio line-in input) and force any application to record the same sound that your computer is outputting from its speakers or headphones.

On modern versions of Windows, Stereo Mix is generally disabled by default — even if your sound drivers support it. Follow our instructions to enable the Stereo Mix audio source on Windows. Use any audio-recording program and select...

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If you use applications that use the ALSA API for audio I/O and not some intermediate such as GStreamer or PulseAudio, you can still force most of them to route their audio to/from JACK. There are two ways of approaching this.

Using an ALSA Loopback device and JACK alsa_in/alsa_out clients

This more complex but probably more robust approach is well-documented in this document.

Using the ALSA JACK PCM plugin

The simpler approach has its drawbacks: if an application stops playing audio, it will disappear from the JACK world, which can be quite inconvenient. Some people have also found it quite buggy or unstable in some situations. For basic audio playback, it works quite well but when used with a more demanding application or one with more of its own “quirks” (e.g. VirtualBox), this approach isn’t recommended.

It requires an ALSA “plugin” that is not installed by default on many Linux distributions, and the name of the package containing it will...

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On Windows XP, Audacity relies on your computer sound device providing an input for recording computer playback, typically called "Stereo Mix" or "What U Hear". This input can be chosen in Audacity's Device Toolbar. If there is no such input, you can use other methods such as a loopback cable. On Windows Vista, Windows 7 or Windows 8, sound devices often lack a "Stereo Mix" or similar input, or it must be specially enabled in the Windows "Sound" Control Panel. On these operating systems, you can also choose Windows WASAPI host and the (loopback) input choice in Device Toolbar. This will record computer playback even if the computer sound device lacks its own input to do this.

Choosing the recording device in Audacity

In Device Toolbar (pictured below) or in Devices Preferences, choose "MME" or "Windows DirectSound" in the Audio Host box. In the Recording Device box, look if there is an input meant for recording computer playback. This input could be called "Stereo...

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Jeffery Hicks

Jeffery Hicks is a multi-year Microsoft MVP in Windows PowerShell, Microsoft Certified Professional and an IT veteran with 25 years of experience specializing in automation. He works today as an author, trainer and consultant.

Latest posts by Jeffery Hicks (see all)

Starting the Add Hardware Wizard ^

At first you would think to merely go to Devices under Control Panel and add the adapter. Sadly, no. At least not that I could find. The best approach seems to be to launch the Add Hardware Wizard. Click Windows Key+R and run hdwwiz.

Starting the Hardware Wizard

Click Next.

Manually install hardware

Select the option to manually install hardware and proceed.

Select network adapters

Scroll down to the network adapters section and select it as I did in the screenshot above. Click Next.

Microsoft KM-TEST Loopback Adapter

Select Microsoft for the...

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Unexpected results testing serial loopback using echo and cat. Thanks to the second comment by Bruce, I was able to figure out the problem on my own. After running stty - a - F /dev/tty. Download Par Juegos Popcap Games Registration code Patch Georgia Blairsville .

S1, there were 3 options I found to contribute to the problem: "echo", "onlcr", and "icrnl". Since this serial port is looped back to itself, here is what happened after running echo "hi" > /dev/tty.

Unix & Linux Stack Exchange is a question and answer site for users of Linux. Unexpected results testing serial loopback using echo and cat. How do I test that two connected serial ports really transmit data? Hello, I want to perfrom loopback test on serial port using a loopback connector. I need to write a code using qt 4.7.4. But I am totally new to. The Audio LoopBack Test uses an audio loopback cable to sample the output waveform at the microphone input. The sampled waveform is then saved for post-analysis....

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This page explains how to enhance a read-only LiveCD by adding persistent file storage on another drive. In more recent Ubuntu versions, you can instead use the usb-creator tool to create a bootable USB flash drive from a Live CD, and its user interface has an option to add persistent storage on the same USB drive; see Installation/FromUSBStick.

Live CDs and LiveUsbs provide a highly portable, secure, dependable and familiar environment that is easy to use on almost any machine.

However, LiveCd sessions become even more useful when a persistent image can retain data and settings (even installed programs and packages) when the machine gets switched off. All the data can be easily pocketed and kept safe, perhaps to be physically carried to another machine.

The advantages over Cloud Computing are; that it does not require a working internet connection, it gives better privacy and security as it does not place your data in other peoples possession.


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The loopback device is a special, virtual network interface that your computer uses to communicate with itself. It is used mainly for diagnostics and troubleshooting, and to connect to servers running on the local machine.

The Purpose of Loopback

When a network interface is disconnected--for example, when an Ethernet port is unplugged or Wi-Fi is turned off or not associated with an access point--no communication on that interface is possible, not even communication between your computer and itself. The loopback interface does not represent any actual hardware, but exists so applications running on your computer can always connect to servers on the same machine.

This is important for troubleshooting (it can be compared to looking in a mirror). The loopback device is sometimes explained as purely a diagnostic tool. But it is also helpful when a server offering a resource you need is running on your own machine.

For example, if you run a web server, you...

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A Loopback Device is a mechanism used to interpret files as real devices. The main advantage of this method is that all tools used on real disks can be used with a loopback device.

Note: This article only covers UNIX environments (including Cygwin). For information on how to use loopback devices on Windows, see diskpart.

Loopback Device under Linux

The linux loopback device can be used by root only, and needs to be enabled in the kernel before use.

Floppy Disk Images With GRUB and EXT2

First, lets create an empty image.

dd if=/dev/zero of=floppy.img bs=512 count=2880

Now, lets set it up for mounting.

losetup /dev/loop0 floppy.img

Now lets make it EXT2 formatted.

mkfs -t ext2 /dev/loop0


mount -t ext2 /dev/loop0 /mnt/myfloppy

Create GRUB directory.

cd /mnt/myfloppy mkdir grub

Copy GRUB's second stage files. (GRUB stage[12] could also be located in /usr/lib/grub/)

cp /lib/grub/i386-pc/stage[12]...
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A loopback test allows you to send and receive data from the same serial port to verify that the port is operational. To perform this test, you need to temporarily connect the proper pins to allow signals to be sent and received on the same port.

1. Loopback Test

A loopback test can be used to troubleshoot serial communications. It can show problems in the serial port, the cable, or the software generating the messages without having to connect to third party hardware. By connecting the proper wires, a loopback test can verify the operation of serial communication. Loopback tests can be used for RS-232, RS-422, and RS-485 serial communication.

For more information on RS-232, RS-422, and RS-485 see the Serial Communication Overview. Note that not all serial devices, especially those with many connections on one card, allow hardware flow control on every serial output of the device.

To perform a loopback test for RS-232, the TXD pin must be connected...

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There really is no such thing as a ethernet loopback plug. This is actually used on data communication lines.

You have done the test that proves you have a issue with the ethernet port on the PC...ie you plug a different pc into the same cable and it works.

If you were to get lucky you may have just bent one of the wires inside the port, you can take a flashlight and look to see if all the wires look straight.

Pretty much after this you are stuck I suspect. Most modern equipment everything is part of the main board of the machine. The only thing you could hope to replace would be the physical port but you should be able to see any damage. More likely you have some damage in some chip which does not pay to even think about repairing.

I would get a new board that goes into a PCI slot or maybe a USB based ethernet...

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In computer networking, localhost is a hostname that means this computer. It is used to access the network services that are running on the host via its loopback network interface. Using the loopback interface bypasses any local network interface hardware.

The local loopback mechanism is useful for testing software during development, independently of any networking configurations. For example, loopback may be used to direct a locally running web browser to http://localhost to display a local html file.

On most computer systems, localhost resolves to the IP address, which is the most commonly used IPv4 loopback address, and to the IPv6 loopback address ::1.[1]

The name localhost is also a reserved top-level domain name, set aside to avoid confusion with the definition as a hostname.[2] The IETF standards restrict domain name registrars from assigning the name localhost in registration procedures, such as for second-level domains.


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The /dev directory contains entries for the physical devices that may or may not be present in the hardware. Appropriately enough, these are called device files. As an example, the hard drive partitions containing the mounted filesystem(s) have entries in /dev, as df shows.

Among other things, the /dev directory contains loopback devices, such as /dev/loop0. A loopback device is a gimmick that allows an ordinary file to be accessed as if it were a block device. This permits mounting an entire filesystem within a single large file. See Example 17-8 and Example 17-7.

A few of the pseudo-devices in /dev have other specialized uses, such as /dev/null, /dev/zero, /dev/urandom, /dev/sda1 (hard drive partition), /dev/udp (User Datagram Packet port), and /dev/tcp.

For instance:

To manually mount a USB flash drive, append the following line to /etc/fstab.

(See also

Example A-23


Checking whether a disk is in the...

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Copy the SAIO scripts for resetting the environment:

mkdir -p $HOME/bin cd $HOME/swift/doc; cp saio/bin/* $HOME/bin; cd - chmod +x $HOME/bin/*

Edit the $HOME/bin/resetswift script

The template resetswift script looks like the following:

#!/bin/bash swift-init all stop # Remove the following line if you did not set up rsyslog for individual logging: sudo find /var/log/swift -type f -exec rm -f {} \; sudo umount /mnt/sdb1 # If you are using a loopback device set SAIO_BLOCK_DEVICE to "/srv/swift-disk" sudo mkfs.xfs -f ${SAIO_BLOCK_DEVICE:-/dev/sdb1} sudo mount /mnt/sdb1 sudo mkdir /mnt/sdb1/1 /mnt/sdb1/2 /mnt/sdb1/3 /mnt/sdb1/4 sudo chown ${USER}:${USER} /mnt/sdb1/* mkdir -p /srv/1/node/sdb1 /srv/1/node/sdb5 \ /srv/2/node/sdb2 /srv/2/node/sdb6 \ /srv/3/node/sdb3 /srv/3/node/sdb7 \ /srv/4/node/sdb4 /srv/4/node/sdb8 sudo rm -f /var/log/debug /var/log/messages /var/log/rsyncd.log /var/log/syslog find /var/cache/swift* -type f -name *.recon -exec rm -f {}...
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