What is loopback interface in linux

How do I enable loopback in Linux?

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.

What is the command for loopback?

At the command line, type these commands: For IPv4: PING ‘127.0. 0.1’ or PING LOOPBACK. For IPv6: PING ‘::1’ or PING IPV6-LOOPBACK.

How do I check for loopback in Linux?

How to Perform Loopback Testing in UNIX and Linux Using the DPA Interface

1. Press [Enter] while highlighting the desired Digi product entry.
2. The next screen represents the port signal activity for port 1 or channel 1.
3. Press the letter T to start the loopback test.

Is a loopback address?

A loopback address is a special IP address, 127.0. 0.1, reserved by InterNIC for use in testing network cards. This IP address corresponds to the software loopback interface of the network card, which does not have hardware associated with it, and does not require a physical connection to a network.

How do I find the loopback interface?

To display the list of loopback interfaces which have been assigned IP addresses, enter the show ip command. In the show ip command output, information about configured loopback interfaces is displayed below other IP configuration parameters, such as packet TTL and ARP age-out values, and VLAN IP configurations.

Why is Ping loopback address?

In a TCP/IP network, the loopback IP address is 127.0. 0.1, and pinging this address will always return a reply unless the firewall prevents it. The loopback address allows a network administrator to treat the local machine as if it were a remote machine. See ping, loopback plug and localhost.

Why do we need loopback interface?

A loopback interface has many uses. Loopback interface’s IP Address determines a router’s OSPF Router ID. A loopback interface is always up and allows Border Gateway Protocol (BGP) neighborship between two routers to stay up even if one of the outbound physical interface connected between the routers is down.

Why is ping loopback address?

Why 127 is the loopback address?

The class A network number 127 is assigned the “loopback” function, that is, a datagram sent by a higher level protocol to a network 127 address should loop back inside the host. No datagram “sent” to a network 127 address should ever appear on any network anywhere.

How do I set up loopback?

To configure the primary loopback interface IP address:

1. Select Network > Configuration.
2. Select the Loopback tab.
3. Select the Enable check box.
4. (Optional) In the Interface Description text box type a description for this interface.
5. In the IP Address text box, type the IPv4 address and subnet mask.
6. Click OK.

Why is 127 called loopback address?

The class A network number 127 is assigned the “loopback” function, that is, a datagram sent by a higher level protocol to a network 127 address should loop back inside the host. 0 and 127 were the only reserved Class A networks by 1981. 0 was used for pointing to a specific host, so that left 127 for loopback.

How does the loopback interface in Linux work?

Which is the best command line tool for loopback?

How does the CLI work in loopback 4?

Can a loopback device be used by root?

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

What are loopback and localhost? Why do they exist, what are they used for, and how would I use them?

1 Answer 1

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 have all your web documents and could examine them file by file. You may be able to load the files in your browser too, though with server-side active content, it won’t work the way it does when someone accesses it normally.

So if you want to experience the same site others do, the best course is usually to connect to your own server. The loopback interface facilitates that.

Addresses on Loopback

For IPv4, the loopback interface is assigned all the IPs in the 127.0.0.0/8 address block. That is, 127.0.0.1 through 127.255.255.254 all represent your computer. For most purposes, though, it is only necessary to use one IP address, and that is 127.0.0.1 . This IP has the hostname of localhost mapped to it.

Thus, to log in as bob via SSH to the SSH server running on your own machine, you would use:

Like other network adapters, the loopback device shows up in the output of ifconfig . Its name is lo .

ek@Del:~$ ifconfig lo lo Link encap:Local Loopback inet addr:127.0.0.1 Mask:255.0.0.0 inet6 addr: ::1/128 Scope:Host UP LOOPBACK RUNNING MTU:16436 Metric:1 RX packets:50121 errors:0 dropped:0 overruns:0 frame:0 TX packets:50121 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:0 RX bytes:4381349 (4.3 MB) TX bytes:4381349 (4.3 MB) 

An Example: CUPS

One common, production (i.e., not just diagnostic) use of localhost on Ubuntu is to perform advanced printer configuration. In a web browser, go to:

CUPS runs a web server on port 631, and this can be used to configure printing, regardless of what GUI you are running (or even if you are not running a GUI at all).

If you try connecting to http://127.0.0.1:631 , this will work too. However, if you try to connect to http://127.0.0.2 , it will not. All the 127.*.*.* addresses identify your computer on the loopback interface, but a server program can decide to bind just to a specific IP address.

A Notable Difference from Windows

If you come from a Windows background, you might expect loopback to itself be a synonym of localhost (and thus to be able to ping loopback , connect to servers on loopback , and so forth). That behavior is peculiar to Windows.

• But you canadd any name including loopback to your /etc/hosts file, with 127.0.0.1 as its address, and it will act like localhost .

Other Meanings of «Loopback»

The general concept of loopback is a mechanism through which a message or signal ends up (or loops) back to where it started.

So there are a few other ways loopback is use in Ubuntu that should not be confused with the loopback device in networking.

Loop Mounts

To mount a disk image in Ubuntu, you could run:

sudo mount -o loop image.iso /media/label

This is usually called a loop device (and not a loopback device), but the term loopback file interface is occasionally used.

This has nothing to do with the loopback device in networking.

Sound

Pulseaudio and other sound systems provide a mechanism to «connect» line-in to line-out, so that audio input is echoed back to your speakers/headphones. Pulseaudio’s loopback module facilitates this.

Here, it is correct to use the term loopback, but like loop mounts, this also has nothing to do with the loopback device in networking. (And nothing to do with loop mounts, either.)

Further Reading

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How is the loopback network interface implemented?

The loopback network interface «is a virtual network device implemented entirely in software,» according to another question on Unix.SE. But how is this implementation actually accomplished, and how do you manipulate it? As far as I know, IPs are marked as local in the local routing table:

ip route show table local 

Sending to an IP marked as local will trigger the loopback device. But is this detection done purely by routing tables or also by some other kernel operations? My ultimate goal is to manipulate the loop-device configuration, so that sending datagrams between two WLAN adapters on my laptop will result in real traffic / datagrams in the network and not in local loops. That is:

Interface 1 -> WLAN -> Interface 2 

Interface 1 -> LOOP -> Interface 2 

2 Answers 2

On a very basic level, it’s a device that’s implemented purely in software. Similar to the way that a VM is entirely software, so is the loopback (although using quite different mechanisms).

Ordinarily when an interface sends a packet it eventually is put to the wire and flushed. The loopback on the other hand, instead of being put to the wire the outgoing packet is moved to the incoming queue for that interface and then processed like any incoming packet.

While netfilter is evaluating a packet there are two times when a routing decision is made. Once for incoming packets (e.g., was this packet received on the right interface?) and once for outgoing packets (e.g., what interface should this packet be transmitted on?). This routing decision is when the kernel checks the packet against the local routing table.

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How does the loopback interface work

I have enabled the ip_forward (net.ipv4.ip_forward = 1) My Question： ping 10.0.3.2 works，but why? tcpdump can’t get packets on enp0s3 ，but does get packets on lo . The default route is lo ; why does ping 10.0.3.2 work? Why can’t I get packets on enp0s3 ?

2 Answers 2

The loopback interface is a virtual interface. The only purpose of the loopback interface is to return the packets sent to it, i.e. whatever you send to it is received on the interface. It makes little sense to put a default route on the loopback interface, because the only place it can send packets to is the imaginary piece of wire that is looped from the output of the interface to the input. There is nothing that can change this behaviour of the loopback interface, that’s what it is coded to do.

When you ping 10.0.3.2, the reply does not come from some external device, but from the loopback interface itself. When you add an address on the loopback interface with e.g.

sudo ip addr add 10.0.3.1/24 dev lo 

a route to 10.0.3.0/24 is added. You can see this with

ip route show table local 

local 10.0.3.0/24 dev lo proto kernel scope host src 10.0.3.1 

should show up. This routing table entry tells that a packet sent to any address between 10.0.3.1 and 10.0.3.254 is sent via the lo interface, from which it is immediately returned.

EDIT: clarification as a response to the comment below.

Here is what happens when you ping 10.0.3.2: the kernel gets an IP packet for delivery with a destination address 10.0.3.2. Just like with any packet to be delivered, the kernel consults the routing table. In this case the matching entry is this: local 10.0.3.0/24 dev lo proto kernel scope host src 10.0.3.1 , which says the packet should be delivered via the lo interface with the source address 10.0.3.1.

Now, because the packet was given to the lo interface, the loopback interface does what it normally does: it takes the packet off the send queue and puts it on the receive queue. From the kernel’s point of view, we have now received an incoming packet ready for consumption by a server process listening on a socket. (In the case of ping, the kernel processes it internally.) We have now received a «remote» ICMP packet with a destination address of 10.0.3.2, which is arguably not one of our local addresses, but it was delivered to the loopback interface nonetheless.

Next, the kernel sends a response to the ping: an ICMP response packet with the addresses reversed: 10.0.3.2 as source address and 10.0.3.1 as destination. This is delivered via the loopback interface back to the ping program, which shows that we got a reply from 10.0.3.2.

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