How are Unix domain sockets related to networking?

Question

I understand that networking uses sockets as well but I am wondering if Unix domain (IPC) sockets somehow relate to network layer. If so, does this use network HW somehow? Does it work like a loopback? If not, then why are listed by netstat in Linux ?

Answer 1

Unix domain sockets share the same semantics as regular sockets but that's about it, the underlying code has nothing to do with the network layer. They provide an efficient means of communication for processes on the same host (IPC) where network-like semantics are desirable (listen, connect ...)

See What is the cost of establishing connection using Unix Domain sockets vs TCP sockets for example:

Messages sent through TCP sockets have to go all the way through the networking stack to the loopback interface (which is a virtual network interface device typically called "lo" on Unix-style systems), and then back up to the receiving socket. The networking stack code tacks on TCP and IP headers, makes routing decisions, forwards a packet to itself through "lo", then does more routing and strips the headers back off. Furthermore, because TCP is a networking protocol, the connection establishment part of it has all kinds of added complexity to deal with dropped packets. Most significantly for you, TCP has to send three messages just to establish the connection (SYN, SYN-ACK, and ACK).

In the case of unix sockets the kernel can bypass all this and just pass the data between the two processes so there is less overhead.

Googling around you might come across some benchmarks.