The domain argument of the socket(2) specifies a communication domain; this selects the protocol family which will be used for communication. These families are defined in <sys/socket.h>. The currently understood formats include:
Local communication For further information, see unix(7).
IPv4 Internet protocols. For further information, see ip(7).
Amateur radio AX.25 protocol. For further information, see ax25(4).
Reliable Datagram Sockets (RDS) protocol (since Linux 2.6.30). RDS over RDMA has no relation to AF_SMC or AF_XDP. For further information see rds(7), rds-rdma(7), and Documentation/networking/rds.txt in the Linux kernel source tree.
Socket interface over IrDA (moved to staging in Linux 4.14, removed in Linux 4.17). For further information see irda(7).
Generic PPP transport layer, for setting up L2 tunnels (L2TP and PPPoE). See Documentation/networking/l2tp.txt in the Linux kernel source tree for details.
Legacy protocol for wide area network (WAN) connectivity that was used by Sangoma WAN cards (called "WANPIPE"); removed in Linux 2.6.21.
Logical link control (IEEE 802.2 LLC) protocol, upper part of data link layer of ISO/OSI networking protocol stack (since Linux 2.4); has no relation to AF_PACKET. See chapter 13.5.3. Logical Link Control in Understanding Linux Kernel Internals (O'Reilly Media, 2006) and IEEE Standards for Local Area Networks: Logical Link Control (The Institute of Electronics and Electronics Engineers, Inc., New York, New York, 1985) for details. See also
Nokia cellular modem IPC/RPC interface (since Linux 2.6.31). See Documentation/networking/phonet.txt in the Linux kernel source tree for details.
IEEE 802.15.4 WPAN (wireless personal area network) raw packet protocol (since Linux 2.6.31). See Documentation/networking/ieee802154.txt in the Linux kernel source tree for details.
Ericsson's Communication CPU to Application CPU interface (CAIF) protocol (since Linux 2.6.36). See Documentation/networking/caif/Linux-CAIF.txt in the Linux kernel source tree for details.
Interface to kernel crypto API (since Linux 2.6.38).  See Documentation/crypto/userspace-if.rst in the Linux kernel source tree for details.
VMWare VSockets protocol for hypervisor-guest interaction (since Linux 3.9); has no relation to AF_IUCV and AF_SMC. For further information, see vsock(7).
KCM (kernel connection multiplexor) interface (since Linux 4.6). See Documentation/networking/kcm.txt in the Linux kernel source tree for details.
Qualcomm IPC router interface protocol (since Linux 4.7).
SMC-R (shared memory communications over RDMA) protocol (since Linux 4.11), and SMC-D (shared memory communications, direct memory access) protocol for intra-node z/VM quest interaction (since Linux 4.19); has no relation to AF_RDS, AF_IUCV or AF_VSOCK. See