Is a loopback interface just another term for virtual interface?

Solution 1:

I guess you are moving a little bit too fast and mixing the terms a little.

To go over the basic meaning and utility of the loopback interface (from https://en.wikipedia.org/wiki/Loopback#Virtual_loopback_interface)

Implementations of the Internet protocol suite include a virtual network interface through which network applications can communicate when executing on the same machine. It is implemented entirely within the operating system's networking software and passes no packets to any network interface controller. Any traffic that a computer program sends to a loopback IP address is simply and immediately passed back up the network software stack as if it had been received from another device.

So from my point of view, the main purpose of the loopback interface(s) is to have an easy way to test the network stack of your system. If you have 2 applications running on the same system and you want to test if they can communicate with each other via the network, you won't need a router/switch (or whatever else would constitute the network), you just use software and a couple of commands to create some loopback interfaces and you route traffic through those.

The IPv4 address block 127.0.0.0/8 is actually reserved for this reason and usually the 'localhost' points to an address from that range. So pinging/sending traffic to 'localhost' is actually done using a loopback interface.

In networking

Also, (from https://en.wikipedia.org/wiki/Loopback#Management_interface), you can see that the use for a loopback interface can actually be different in some cases, as in Networking (and networking devices as router/switches/firewalls, etc.) is mostly used as a management interface. It is also used by various routing protocols and other services.

Networking devices do have software installed on them, just like an operating system, and can create multiple types of virtual interfaces, among them being the loopback. But there are other types of virtual interfaces, that have a different purpose (not for testing the network stack and sending traffic back or management), like the tunnel interfaces usually used for VPNs. Bridge interfaces are virtual interfaces as well.

In the end, all of these virtual interfaces are created with software (and they basically modify/change the network packets that you send/receive - source IP, source MAC etc.) and they do in fact use the physical interfaces to communicate over a network.

To conclude and to answer your questions, (almost) all loopback interfaces (check Spooler's comment of this post) are virtual interfaces, but not all virtual interfaces are (=have the purpose of) loopback interfaces.

Does this mean every interface of this virtual switch is a loopback interface?

No. The virtual switch could have a loopback interface, but all of its other interfaces probably simulate the behavior of a normal physical interface, that just sends the traffic to another interface to which is connected via a medium (cable, radio, etc.) which in this case is probably just software.

What about virtual interfaces created by VPNs? Can these be called loopback too?

No. VPNs usually use another type of virtual interface - a tunnel interface.

Solution 2:

The most important property of loopback interfaces is they are always up regardless of the state of other interfaces in the system. A loopback can only be down if you configure it to be down.

Virtual interfaces is a reasonable description, but one that might have a lot of semantic overlap with other types of interfaces.

I wouldn't describe vmware virtual switch interfaces as loopbacks. These interfaces may be down for a variety of operational status reasons, right? They also have a different purpose than loopbacks -- which are explicitly to provide an interface that is always up.